Development of Smart Energy Glass: Switchable Infrared...

Luc De Heyn

Infrared ReflectorDevelopment of Smart Energy Glass: Switchable

Academiejaar 2010-2011Faculteit Ingenieurswetenschappen en ArchitectuurVoorzitter: prof. dr. ir. Jan Van CampenhoutVakgroep Elektronica en Informatiesystemen

Master in de ingenieurswetenschappen: fotonicaMasterproef ingediend tot het behalen van de academische graad van

Begeleider: Pieter VanbrabantPromotoren: prof. Jeroen Beeckman, Casper van Oosten

Development of Smart Energy GlassSwitchable Infrared Reflector

Luc De Heyn

Supervisor(s): Jeroen Beeckman, Casper van Oosten

Abstract— This article is about the implementation of a switchable in-frared reflector (SIR) in the existing product of Smart Energy Glass (SEG),a new type of smart window based on liquid crystals. Three models areproposed to fulfill the switchable reflection in the infrared. The models areinvestigated using computer aided simulations and experiments in the labusing a spectrophotometer. Using the polarisation rotation properties ofa twisted nematic liquid crystal (TNLC), both the simulations and experi-ments predict a good switching ability for perpendicular angle of incidence.This switching ability decreases for increasing angle of incidence.

Keywords— Liquid crystal, polarisation, smart windows, switchable in-frared reflector, reflection

I. INTRODUCTION

THE world market energy consumption keeps increasing[2], but one of the energy objectives of the EU impliesa decrease in energy consumption [3]. As 40% of the energyconsumption in the EU goes to the building sector, energy ef-ficient buildings could be the key. SEG, a product of Peer+,addresses this problem. It is able to control the incident lightto high and low absorption using the LC guest host effect withabsorbing dyes. It can also generate electricity using the con-cept of switchable luminescent concentrators (SLC). However,the SEG does not use the near infrared spectrum of light. Thiswork describes an implementation to obtain a switchable reflec-tion in the infrared for SEG by sandwiching the existing systembetween two reflective polarizers. With the SIR, less IR lightenters the building so one can save on cooling costs. Recent re-search also showed an increase in electricity generation when aSLC is sandwiched between circular polarizers [1].

II. THE MODELS

The suggested models that have the ability to switch in thenear infrared are called model circular, model linear and modelqwp (quarter-wave plate), as shown in figure 1. These threemodels use a reflective polarizer that reflect only one polarisa-tion of light. Model linear (circular) uses linear (circular) reflec-tors that reflect one polarisation of light: x (RCP). The other po-larisation, y (LCP), is transmitted in the homeotropic state: a re-flection of 50% is obtained. In the TN configuration, the y polar-isation (LCP) is transformed to its orthogonal component by theTNLC using polarisation rotating property (half wave plate re-tardation), resulting in 100% reflection. Model qwp uses circu-lar reflectors and a quarter wave plate between the reflectors andthe liquid crystal to transform circular polarised light to a linearpolarisation. Model qwp uses the polarisation rotating propertyof the TN configuration to obtain full reflection. The orientationof the quarter wave plates is not the same βtop = −π/4 andβbottom = +π/4. The liquid crystal in TN configuration willbe used for two different purposes: as a retardation plate (model

Fig. 1. Concept of the SIR in homeotropic (left) and TN (right) state

circular) and as a polarisation rotator (model linear and modelqwp). However, both options do have some flaws that prevent aperfect switchable infrared reflector.

A. Liquid crystal as retardation plate

The birefringence of the liquid crystal enables the possi-bility to create a retardation plate with a retardation Δφ =2π(ne−no)d

λ . When correctly tuned, a λ/2-plate (HWP) can becreated that transforms any polarisation to its orthogonal com-ponent (e.g. right circular to left circular). The SEG impliesthree restrictions to the SIR: a minimal thickness (d > 15μm),a minimal twist (φ ≥ π/2) and a birefringence Δn ≈ 0.133.Using these values, the created HWP is a higher order waveplate, which is smallband. It is not possible to create a broad-band HWP due to these restrictions. It is investigated that thetwist of the LC layer (necessary for the absorption effect of theSEG) has a negative influence on the retardation effect. Thehigher the twist, the worse the hwp-effect. Therefore, the LClayer will have a twist that is as small possible (φ = π/2). Thethickness variations also have a severe impact on the retardationeffect. Small variations (up to 10%) are possible, and the devi-ation results in a redshifted retardation spectrum by 60nm. Theaforementioned reasons indicate that model circular, that usesthe LC as a retardation plate, will not be the perfect candidatefor the SIR.

B. Liquid crystal as a polarisation rotator

Model circular and model qwp use the TNLC for its polari-sation rotating property: if a linear polarisation is incident par-allel to the first director of the TNLC, the linear polarisationwill tend to follow the twist. The efficiency of this rotation isgiven by the transmission between two perfect parallel polar-izers, with u = 2dΔnλ , see (1). This transmission should below (high reflection), and it is dependent on the optical pathdifference dΔn as can be seen in figure 2. In figure 2, thepolarisation after a TNLC is plotted using the Stokes S1 pa-rameter. The input is y polarised (S1 = −1) and x polarisedlight is expected (S1 = +1), for varying thickness of LC layer(d = 15μm, 25μm, 100μm). When a thicker (d = 100μm)TNLC layer is used, the output polarisation is highly x polarised

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Fig. 2. Stokes S1 parameter for y polarised light incident on a TNLC, output ismainly S1 ≈ 1 indicating x polarised light.

(S1 = +1). When a thin (d = 15μm)cell is used, the fluctua-tions around the x-polarisation are higher. In the latter case, theS1,min = 0.91 indicating the the output polarisation is mainlyx-polarised and that the effect is rather small. For longer wave-lengths (IR), the effect is worse. If one needs higher quality, ahigher optical path difference (e.g. thicker cell) is a good solu-tion.

Tparallelpolarizers =1

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sin2(π2√1 + u2)

1 + u2(1)

III. RESULTS

The three models are investigated using computer aided simu-lations with Matlab and experiments in the lab. The simulationspredicted the outcome of the experiments, indicating the qualityof the simulation model. In the first part of the investigation, thevisible spectrum was used to clearly see the effects of the layerson the polarisation of light.

A. Model Circular

The simulation result of model circular are presented in figure3: the retardation effect causes the fluctuations in the TN config-uration. This model will not be investigated in the NIR becauseit will never work perfectly.

Fig. 3. Transmission spectra for model Circular. Left: LC is in TN configura-tion. Right: random polarised light results in high (homeotropic) and lowtransmission (TN). Average transmissions are THomeotropic = 50% andTTN = 29%.

B. Model Linear

Model Linear is not investigated using simulations because ofthe unknown exact structure of the linear reflectors. The reflec-tors were only available in the visible spectrum, so the modelcould not be tested in the NIR. In figure 4, the transmission ofmodel linear is presented for different angles of incidence. Theswitching ratio is for 0◦,30◦,45◦ and 60◦ angle of incidence is

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Fig. 4. Experimental transmission of model Linear in the high (dotted) and low(full line) transmission state for angles of incidence 0◦,30◦,45◦ and 60◦ .

4.1, 3.5, 2.3, 1.7 respectively: a decrease for increasing angle ofincidence.

C. Model QWP

The simulation results of model qwp in the visible spectrumpredict good results, so its NIR behavior is investigated exper-imentally. The results are presented in figure 5. The qwp usedfor this experiment has dΔn = 140nm, it is designed to workperfect for λ = 560nm and not in the NIR. However, the resultsare still very promising: the switching ratio is 3.1, 2.6, 1.3 and1.4 for 0◦,30◦,45◦ and 60◦ respectively.

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Fig. 5. Experimental transmission of model QWP in the high (dotted) and low(full line) transmission state for angles of incidence 0◦,30◦,45◦ and 60◦ .

IV. CONCLUSIONS

Using the linear polarisation rotation property of the liquidcrystal results in a high quality switching behavior. Model linearis only investigated in the visible spectrum due to the absence oflinear reflectors for the NIR spectrum. Model qwp was investi-gated in the visible and NIR spectrum. Both models show highswitching ratio’s.

REFERENCES[1] Michael G. Debije et al., Effect on the output of a luminescent solar con-

centrator on application of organic wavelength-selective mirrors, AppliedOptics, vol. 49, pp.745-751, Feb. 2010.

[2] U.S. Energy Information Administration, International Energy Outlook,July 2010

[3] Energy Efficiency, http://ec.europa.eu/energy/efficiency/buildings/buildings-en.htm/

Ontwikkeling van Smart Energy GlassSchakelbare Infrarood Reflector

Luc De Heyn

Supervisor(s): Jeroen Beeckman, Casper van Oosten

Abstract— Dit artikel bespreekt de implementatie van een schakelbareinfrarood reflector (SIR) in Smart Energy Glass (SEG), een raam gebaseerdop vloeibare kristallen. Er worden drie modellen voorgesteld die schakel-bare infrarood reflectie kunnen realiseren. De modellen worden onder-zocht met gebruik van simulaties en experimenten. Indien de polarisatie-rotatie eigenschap van een twisted nematic vloeibaar kristal (TNLC) ge-bruikt wordt, tonen zowel de simulaties als de experimenten een goede scha-kelbare reflectie voor loodrechte inval van licht. Het contrast tussen hogeen lage reflectie daalt voor lichtinval onder een grotere hoek.

Keywords— Vloeibaar kristal, polarisatie, smart windows, schakelbareinfrarood reflector, reflectie

I. INTRODUCTIE

VOLGENS [2] blijft het wereldwijde energieverbruik stij-gen, maar een van de energiedoelstellingen van de EU stelteen daling van dit energieverbruik [3]. Aangezien 40% van hettotale energiegebruik in de EU verbruikt wordt door de gebou-wen sector, zouden energie-efficiente gebouwen de sleutel kun-nen zijn tot het behalen van deze doelstelling. SEG, een pro-duct van Peer+, is een oplossing voor meer energie-efficientegebouwen. SEG kan enerzijds de transmissie van het zichtbarespectrum controlleren door het LC guest host effect met absor-berende dyes, anderzijds kan SEG ook energie opwekken doorhet concept van schakelbare luminescente concentrators (SLC).Dit werk beschrijft een implementatie van de SIR voor het SEGdoor het bestaande systeem te plaatsen tussen twee IR reflec-tieve polarisators. Door de SIR kan minder IR licht het gebouwbinnen en kan er bespaard worden op koelingkosten. Recent on-derzoek toont ook een stijging in efficientie van de SLC zodat ermeer energie opgewekt wordt, wanneer een SLC geplaatst wordttussen twee circulaire polarisators [1].

II. DE DRIE MODELLEN

De voorgestelde modellen die de schakelbare infrarood re-flectie eigenschap bezitten, zijn model circulair, model lineairen model qwp, schematisch voorgesteld in figuur 1. Deze mo-dellen gebruiken reflectieve polarisatoren die een polarisatie vanhet licht reflecteren. Model lineair (circulair) gebruikt lineaire(circulaire) reflectoren die de x-polarisatie (RCP) reflecteren.De andere polarisatie, y (LCP), wordt doorgelaten in de ho-meotrope toestand zodat een reflectie van 50% behaald wordt.In de TN configuratie wordt de y-polarisatie (LCP) omgevormdtot zijn orthogonale component door de TNLC, gebruik makendvan het polarisatie-rotatie (retardatie) effect, zodat een reflec-tie van 100% behaald wordt. Model QWP gebruikt circulairerelectoren en een kwart-golflengteplaatje tussen de reflectorenen het vloeibaar kristal om de circulaire polarisatie om te vor-men naar een lineaire polarisatie. De orientatie van de kwart-golflengteplaatjes is :βtop = −π/4 and βbottom = +π/4. Het

Fig. 1. Concept van de SIR in de homeotrope en TN configuratie

vloeibaar kristal in de TN configuratie zal gebruikt worden voortwee eigenschappen: enerzijds als een retardatieplaat (modelcirculair) en anderzijds als een polarisatie rotator (model line-air en model qwp). Beide eigenschappen werken echter nietperfect.

A. Vloeibaar kristal als een retardatieplaat

De dubbelbreking van een vloeibaar kristal maakt het mo-gelijk om een retardatieplaat te maken met faseretardatie vanΔφ = 2π(ne−no)dλ . Met de juiste waarden voor de parameters,is het mogelijk om een λ/2-retardatieplaat (HWP) te maken zo-dat elke polarisatie wordt omgevormd tot zijn orthogonale com-ponent. Het SEG zorgt voor drie restricties op de LC laag: eenminimale dikte (d > 15μm), een minimale twist (φ ≥ π/2) eneen dubbelbreking van Δn ≈ 0.133. Indien deze eigenschap-pen gebruikt worden, is de HWP niet van nulde orde maar vaneen hogere orde HWP en wordt deze smalbandig. Er werd on-derzocht dat de twist van de LC laag (nodig voor het absorptieeffect van de SEG) een negatieve invloed heeft op het retardatie-effect. Hoe groter de twist, des te slechter het hwp-effect. Hier-door zal de LC laag een zo klein mogelijke twist krijgen in hetfinale ontwerp van model circulair (φ = π/2). De variaties inde dikte van de LC laag hebben ook een zware impact op hetretardatie-effect. Kleine variaties (tot 10%) zijn mogelijk en re-sulteren in een roodverschuiving van het retardatie-effect van60nm. Bovengenoemde redenen zorgen ervoor dat model cir-culair, dat gebruik maakt van de LC laag als retardatieplaat, geengoede SIR zal zijn.

B. Vloeibaar kristal als een polarisatie rotator

Model circular en model qwp gebruiken het TNLC voor zijnpolarisatie rotatie eigenschap: als het invallend licht lineair ge-polariseerd is, parallel aan de eerste director van het TNLC, danzal het de rotatie van het LC volgen. De efficientie van dezerotatie van het licht, kan beschreven worden door de transmis-sie van een TN cel tussen twee perfecte parallelle polarisatoren,(1), met u = 2dΔnλ . Deze transmissie moet zo laag mogelijkzijn (hoge reflectie) en is afhankelijk van het optische padleng-teverschil dΔn, zoals weergegeven in figuur 2. In deze figuurwordt de polarisatie (Stokes parameter S1) weergegeven na een

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TNLC met als input de y-polarisatie, waarbij de dikte varieert(d = 15μm, 25μm, 100μm). Als een dikkere cel gebruikt wordt(d = 100μm), is de output inderdaad sterk x-gepolariseerd.Hoe dunner de cel, hoe groter de fluctuaties omheen perfect x-gepolariseerd licht. Nochtans is S1,min = 0.91, zodat het lichtnog steeds sterk x-gepolariseerd blijft. Voor IR golflengtes heeftdit effect een grotere impact. Indien betere kwaliteit gewenst is,moet een groter optisch padlengteverschil gebruikt worden.

Tparallelpolarizers =1

2

sin2(π2√1 + u2)

1 + u2(1)

III. RESULTATEN

De drie modellen worden onderzocht met behulp van com-putersimulaties en experimenten in het lab. De simulaties voor-spellen de uitkomst van de experimenten, wat de hoge kwaliteitvan het simulatiemodel aantoont. In het onderzoek werd eersthet zichtbaar spectrum gebruikt om de effecten van de verschil-lende lagen op de polarisatie van het licht beter te begrijpen.Daarna werd er overschakeld naar het infrarode spectrum.

A. Model Circulair

De simulatieresultaten van model circulair, figuur 3, vertonensterke fluctuaties door het smalbandige retardatie-effect. Hetmodel wordt daarom niet verder onderzocht in het NIR.

Fig. 3. Transmissie spectra voor model circulair. Links: LC in TN configuratie.Rechts: random gepolariseerd licht resulteert in hoge (THomeotroop =50%) and lage transmissie (TTN = 29%).

B. Model Lineair

Model lineair wordt niet onderzocht met simulaties aange-zien de exacte structuur van de lineaire reflectors niet gekend is.De reflectoren waren enkel beschikbaar in het zichtbare spec-trum, zodat het model niet getest kon worden in het nabije in-frarood. In figuur 4 wordt de transmissie van model lineair ge-toond voor verschillende hoeken van het invallende licht. Hetcontrast voor hoeken 0◦,30◦,45◦ en 60◦ bedraagt respectievelijk4.1, 3.5, 2.3, 1.7: een daling voor grotere hoek.

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Fig. 4. Experimentele transmissie van model lineair in de hoge (.) en lage (:)transmissie toestand voor licht onder hoek van 0◦,30◦,45◦ en 60◦ .

C. Model QWP

De simulatieresultaten van model qwp voorspellen een goedcontrast. De experimentele resultaten staan in figuur 5. Hetkwart-golflengteretardatie plaatje (dΔn = 140nm) is ontwor-pen voor het zichtbare spectrum en werkt daarom niet meerperfect in het infrarood. Toch zijn de resultaten veelbelovend:het contrast bedraagt 3.1, 2.6, 1.3 and 1.4 voor respectievelijk0◦,30◦,45◦ en 60◦.

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Fig. 5. Experimentele transmissie van model QWP in de hoge (.) and lage (-)transmissie toestand voor licht invallend onder een hoek van 0◦,30◦,45◦ en60◦ .

IV. CONCLUSIES

Het gebruik van de lineaire polarisatierotatie eigenschap vanhet vloeibaar kristal resulteert in een schakelgedrag van hogekwaliteit (hoog contrast). Model lineair werd enkel onderzochtin het zichtbare spectrum, omdat NIR reflectors niet aanwezigwaren, en heeft een zeer sterk contrast. Model qwp werd zowelin het zichtbare als infrarode spectrum onderzocht met simula-ties en experimenten en vertoont ook een sterk contrast.

REFERENCES[1] Michael G. Debije et al., Effect on the output of a luminescent solar con-

centrator on application of organic wavelength-selective mirrors, AppliedOptics, vol. 49, pp.745-751, Feb. 2010.

[2] U.S. Energy Information Administration, International Energy Outlook,July 2010

[3] Energy Efficiency, http://ec.europa.eu/energy/efficiency/buildings/buildings-en.htm/

Acknowledgments

I would like to acknowledge everybody who helped me during this master thesis. First of all, Iwould like to thank my supervisor at Peer+: Casper, a good motivator, with great people skills.Your comments were always right and to the point. I also thank my coworkers at Peer+, Teun andJeroen for the pleasant moments in the office, lab or coffee room. I would also want to expressmy gratitude towards Prof. Dick Broer for using his circular reflectors and Lieven Penninck, forthe use of his simulation files. I would also like to express my gratitude to the people in Ghent:Prof. Kristiaan Neyts, Prof. Jeroen Beeckman, for giving me the opportunity to do my researchoutside the University of Ghent. Last, but not least, I thank my family and of course Sarah, forthe great support and motivation!

Permission for usage

The author gives permission to make this master dissertation available for consultation and tocopy parts of this master dissertation for personal use. In the case of any other use, the limita-tions of the copyright have to be respected, in particular with regard to the obligation to stateexpressly the source when quoting results from this master dissertation.

Toelating tot bruikleen

De auteur geeft de toelating deze masterproef voor consultatie beschikbaar te stellen en delen vande masterproef te kopiëren voor persoonlijk gebruik. Elk ander gebruik valt onder de beperkin-gen van het auteursrecht, in het bijzonder met betrekking tot de verplichting de bron uitdrukkelijkte vermelden bij het aanhalen van resultaten uit deze masterproef.

Luc De Heyn, June 2011

Development of Smart Energy Glass: Switchable Infrared ReflectorLuc De Heyn

Promotoren: prof. Jeroen Beeckman, Casper van Oosten

Begeleider: Pieter Vanbrabant

Masterproef ingediend tot het behalen van de academische graad vanMaster in de ingenieurswetenschappen: fotonica

Academiejaar 2010-2011

Faculteit Ingenieurswetenschappen en ArchitectuurVoorzitter: prof. dr. ir. Jan Van CampenhoutVakgroep Elektronica en Informatiesystemen

Summary

This work describes the implementation of the switchable infrared reflector in Smart EnergyGlass. The switchable infrared reflector uses polarisation effects of light in order to obtain re-flection. The concept of the switchable infrared reflector is transformed to three models: modelCircular, model Linear and model QWP. The models are investigated using computer aided sim-ulations and experiments in the lab. The near infrared switching in reflection is obtained for thethree models, although model Linear and model QWP are more suitable for the implementationof the switchable infrared reflector in the Smart Energy Glass.

Keywords

Liquid crystal, polarisation, smart windows, switchable infrared reflector

Contents

1 Introduction 11.1 Switchable glazing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 Smart Energy Glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3 Switchable Infrared Reflector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.4 The goal and structure of this master thesis . . . . . . . . . . . . . . . . . . . . . . . . 6

2 The working principle of the SIR 72.1 The working principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2 The models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.3 Reflective polarisers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.4 Retardation plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3 Modelling and optical measurements 183.1 The computer model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.2 Lab results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4 Experimental and simulation results 214.1 Numerical analysis of the performance of TNLC layer . . . . . . . . . . . . . . . . . 224.2 Model Circular . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254.3 Model Linear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.4 Model QWP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

5 Discussion, conclusions & outlook 375.1 Mode Circular . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375.2 Model Linear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385.3 Model QWP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

A Polarisation 39A.1 Polarisation of light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39A.2 Poincaré Sphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

B The fabrication of a liquid crystal cell 42

Bibliography 44

List of Figures 46

List of abbreviations

CLC Cholesteric Liquid Crystal

CP Circular Polarised

LCP Left Circular Polarisation

LC Liquid Crystal

LP Linear Polarised

IR InfraRed

ne Extraordinary refractive index

no Ordinary refractive index

p Pitch

QWP Quarter Wave Plate

RCP Right Circular Polarisation

SEG Smart Energy Glass

SIR Switchable Infrared Reflector

TN Twisted Nematic

Chapter 1

Introduction

As indicated in the International Energy Outlook 2010 [1], the world marketed energy consump-tion will keep increasing, as can be seen in figure 1.1. The building sector accounts for 20%− 30%of global energy use and emissions [2]. Large portions of this energy goes to heating, cooling andlighting, see figure 1.2. These three factors have a large impact on global energy consumption andtherefore on greenhouse gases emission (depending on energy source). The cost of this energyconsumption is not only environmental, but it is also a huge economic cost. In order to radicallyreduce the energy consumption in the building sector, all losses and uses must be addressed.

Figure 1.1: World marketed energy consumption, 1990-2035 (quadrillion Btu)[1]

In the EU, buildings are responsible for 40% of the energy consumption and 36% of the CO2emissions. The Directive on the energy performance of buildings (EPBD) is the main legislativeinstrument at EU level to achieve energy performance in buildings. The EU Climate and Energyobjectives include 20% reduction of the greenhouse gases emissions and a 20% energy saving by2020 [3]. The key to these objectives is the energy performance of buildings.

As buildings are widely present, every small energy reduction on individual building basiscan have a large impact. Energy saving is a worldwide problem, and the number of products thatcontrol the energy consumption, increase every day.

Energy efficient windows are one solution to this problem. The current solution is to use

1.1 Switchable glazing 2

Figure 1.2: Energy consumption in residential and commercial buildings

low-e (low-emissivity) windows. They use a variety of spectrally selective and infrared reflec-tive coatings. However, these windows cannot change their properties from summer to winterand therefore the transmission level of the coating is optimized with a ’best average’ solution.Additional solar control is necessary like exterior or interior sun shading. Switchable glazingaddresses this problem: it combines the adjustable transmission of a sunshade in a glazing el-ement. Many technologies can already fulfill this switching behavior: electrochromic windows,suspended particle devices, gasochromic glazing, . . .

1.1 Switchable glazing

An overview of the most important switchable glazing technologies is given in this section.

1.1.1 Electrochromic windows

Electrochromic windows use the electrochromic effect to darken windows. The function of thewindows stems from the transport of hydrogen or lithium ions from a storage layer to an elec-trochromic layer and is based on a reduction/oxidation reaction. The presence of these ions atthe electrochromic layer changes the optical properties, causing it to absorb the visible spectrumof light (figure 1.3). When a voltage is applied, the ions will move away from the storage layerto the electrochromic layer in order to darken the window. Reversing the voltage, results in theopposite effect with a bright transmission.

The advantage of this technology is the high modulation of light transmission at low voltages.On top of that, the voltage needs only to be applied for changing the properties and not tomaintain a transmission state. The main drawback is the low switching speed and the cost.

1.1 Switchable glazing 3

Figure 1.3: The princple of the electrochromic window in a bright (left) and dark (right) state [4].

1.1.2 Suspended particle devices

Suspended particle devices, see figure 1.4, are made of charged rod shaped particles. Without anelectric field, the orientation is random which enhances absorption (a). Applying an electric field,the random orientation becomes aligned (b) so light can pass through. Suspended particle deviceshave switching speeds of hundreds of milliseconds, typical transmission ranges are 20% − 60%.

Figure 1.4: The principle of a suspended particle device in operation: no voltage (a) and withapplied voltage (b) [5].

1.1.3 Gasochromic glazing

Gasochromic glazing is based upon an active layer of tungsten oxide. It is able to switch from ahigh transparent state to a dark blue, low transmission coloured state when hydrogen interactswith the active WO3 material. The interaction of the gasochromic film to a low concentrationof hydrogen in a carrier gas of argon or nitrogen, results in a blue colour which decreases thevisible and total solar energy transmission. Exposing the gasochromic film to low concentrationof oxygen, the film bleaches and a transparent state is obtained [6].

1.2 Smart Energy Glass 4

1.2 Smart Energy Glass

The previous technologies offer two states: high or low transparency, transparent or private. TheSmart Energy Glass (SEG) provides three states: a bright state, a dark state and a privacy state.The SEG adds another functionality: it can generate electricity in order to be self sustainable. Thetechnology is developed at the Technical University of Eindhoven (TU/e) by Peer+.

1.2.1 Controlling transmission

Figure 1.5: The transmission state of SEG can be controlled to a dark, bright and privacy state.These states are obtained by different orientations of the LC layer.

The transmission of light is controlled by a LC guest-host dye layer. By applying differentvoltages over this layer, the orientation of the directors of the LC can be changed in three differ-ent states. As the chemical structure of the dye molecules is similar to the LC molecules, the dyemolecules will align themselves with the LC directors. A schematical representation of the direc-tor structure of each state is presented in figure 1.5. When unpolarised light is incident on the dyematerial, a polarisation dependent absorption occurs. Introducing a twist in the LC guest-hostdye layer, the orientation of the dyes will be different through the LC layer so the absorptionbecomes more polarisation independent.

When a high electric field is applied, the directors align themselves along this field, resultingin a low absorption state as the absorption axis of the dye is parallel to incident light. At anintermediate voltage, the configuration is more complex, a diffraction grating is formed, resultingin a privacy state. The grating results in a periodic varying index, and scattering occurs. Whenno voltage is applied, the directors become aligned in a planar helical structure, light is incidentperpendicular on the absorption axis of the dye and strong absorption is obtained.

1.3 Switchable Infrared Reflector 5

1.2.2 Generating electricity

The SEG is able to generate electricity using the concept of the luminescent solar concentrators,see figure 1.6. Light is incident on the dichroic fluorescent dyes. These dyes absorb the light,and re-emit the light in all directions. Some of this light is trapped inside the waveguide and isguided towards the edges where a photovoltaic cell converts the photons to electricity [7].

Figure 1.6: The SEG generates electricity using the concept of solar luminescent concentrators.

1.3 Switchable Infrared Reflector

The SEG switches the transmission in the visible spectrum (λ = 400nm − 700nm) by changingthe absorption properties of the LC layer. Adding the NIR to this wavelength range would bebeneficial to the SEG as less energy would enter the building. If the absorbing dyes not onlyabsorb and re-emit the VIS spectrum, but also the NIR spectrum, more light can be used forthe luminescent solar concentrators, and more energy can be generated. However, no efficientfluorescent dyes exist for > 700nm absorption. Therefore, the solar radiation for λ > 700nmcannot be used for generating electricity. As the NIR spectrum cannot be used for generatingenergy and we want to control the transmission through the window, the last option is to reflectthis part of the spectrum.

Adding a switching ability in the infrared would greatly increase the energy savings relatedto air conditioning. Reflecting the IR spectrum of incident sunlight, less energy can enter thebuilding through the windows. This reduces the amount of energy entering the building andtherefore there is a saving on cooling costs. On top of that, recent developments have proven thatadding reflective polars above and below the solar luminescent concentrator improves its energygenerating abilities [8]. Thus without affecting the transmission levels in the visible spectrum, wewill add switching ability in the NIR. This is the concept of the switchable infrared reflector (SIR).

The SEG with a SIR ability will be used for glazing of buildings. The angle of incidence fordirect sunlight varies over a wide range during the day, but changes during the year. At noon,the inclination of the sun is the lowest in December (16◦) and the highest in June (60◦) for TheNetherlands.

1.4 The goal and structure of this master thesis 6

1.4 The goal and structure of this master thesis

The goal of this master thesis is to obtain the best implementation for the SIR. First of all, theconcept of the SIR will be transformed in three different models. These three models will beinvestigated thoroughly using simulation software and tests in the lab. In chapter 2, the theoryof the SIR will be explained and the models are introduced. Chapter 3 explains how the modelsare implemented in software and fabricated. Chapter 4 discusses the performance of the threemodels under varying conditions. The conclusions of this work are written in chapter 5 togetherwith some recommendations.

Chapter 2

The working principle of the SIR

First of all, the working principle of the switchable infrared reflector (SIR) will be explained. Thenthe concept will be transformed into three models. These models use special layers: a reflectivepolariser, a retardation plate and a twisted nematic liquid crystal layer. These special layers willbe investigated. A good understanding of these layers is crucial for the following chapters.

2.1 The working principle

The aim of the SEG combined with the SIR is to provide a glazing that allows controlled trans-mission of electromagnetic radiation of different wavelength ranges. The SEG controls the trans-mission in the visible spectrum (400nm − 700nm) by tuning the absorption. The SIR controls thetransmission in the infrared (IR) spectrum (750nm − 1200nm) by tuning the reflection. This aimcan be achieved by sandwiching the controllable glazing consisting of guest-host liquid crystalsystem with dichroic dyes between two reflective polarisers. The reflective polarisers reflect onepolarisation of the electromagnetic radiation and transmit the other polarisation. The schematicpresentation of this design is presented in figure 2.1. The top and bottom layer represent the re-flective polarisers that sandwich the glass plates and they reflect the same polarisation. Betweenthe glass plates is the liquid crystal with dichroic dyes. As explained in section 1.2, the SEGcontrols the transmission properties by changing the state of the LC. On the left the LC is in ahomeotropic state, on the right the LC is in a twisted nematic state (TN).

In order to understand the working principle of this design, the reader should be familiarwith the concept of polarisation of light. More information about the polarisation of light can befound in appendix A.

In short, light can be described using the electric field vector in an orthonormal basis. Lightcan thus be split in two polarisations that are orthogonal. In order to be general, the polarisationsare called pol1 and pol2. We will now describe the transmission of light through the stack offigure 2.1. For this explanation we assume that light is perpendicularly incident on the stackdirected from top to bottom. The SIR is intended for sunlight radiation and as direct sunlightis randomly polarised, it is for 50% pol1 polarised and 50% pol2 polarised. The light is incident

2.1 The working principle 8

Figure 2.1: The concept for the implementation of the SIR. The liquid crystal is in a homeotropicstate (left) and a twisted nematic state (right).

on the first layer, the top reflective polariser, which reflects 1 polarisation for example pol1. Theother polarisation pol2 will be transmitted and arrives at the liquid crystal (LC) layer.

When the LC is in a homeotropic state (left in figure 2.1), pol2 will not be changed as it feelsa non-birefringent material. Pol2 will continue through the stack and goes through the bottomreflective polariser which reflects the same polarisation as the upper polariser: pol1. As a result,1 polarisation is transmitted and 1 polarisation is reflected: there is a 50% transmission in thehomeotropic state.

When the LC is in a twisted nematic (TN) state (right in figure 2.1), pol2 will feel a birefrin-gent material. When correctly tuned, the LC can act as a half wave plate that transforms anypolarisation into its orthogonal polarisation. The LC layer transforms pol2 into pol1, as pol1 andpol2 are orthogonal basis vectors. This pol1-light is now incident on the lower reflective polariserwhich is a reflective layer for pol1. The reflected light goes through the LC layer, it becomes againpol2-light and it is transmitted by the upper reflective polariser. The two polarisations are nowreflected, the LC in a TN state results in 0% transmission.

2.1.1 Integration of SEG and SIR

As the SIR is constructed around the SEG, the state of the SIR (high/low reflection) depends onthe state of the SEG (high/low absorption). The SIR can be configured in two ways, depending onthe reflectors. In the section above, the working principle is explained using two reflectors thatreflect the same polarisation. Using this configuration, the different absorption and reflectionstates are summarized in table 2.1.

TN HomeotropicVIS A = high A = lowNIR R = 100% R = 50%

Table 2.1: Absorption and reflection effect in the visible (VIS) and near infrared (NIR) depend onthe state of the window if the same polarisation is reflected by the two polarisers.

It is however also possible to use two polarisers that reflect each an orthogonal polarisation,and as can be seen in table 2.2, the reflective states are now different. In a TN configuration,

2.2 The models 9

the reflection is low and in the homeotropic configuration, the reflection is high. In the rest ofthis work, two polarisers are used that reflect the same polarisation: when the SEG is in a darkstate (high VIS absorption), the SIR will be in a high reflective state (high NIR reflection). Inthis configuration, almost no radiation can enter the building through the window when it isswitched to the TN state. It will never be possible to switch between 100% and 0% reflectionusing reflectors, as the first reflector always will reflect one polarisation of light.

TN HomeotropicVIS A = high A = lowNIR R = 50% R = 100%

Table 2.2: Absorption and reflection effect in the visible (VIS) and near infrared (NIR) depend onthe state of the window if the two polarisers reflect an opposite polarisation.

2.2 The models

The concept in section 2.1 can be realized in different ways. For this master thesis, three modelsare presented: model Circular, model Linear and model QWP. Model Circular and model Linearare explained using figure 2.2. Model QWP is explained using figure 2.3.

2.2.1 Model Circular

The first model is called model circular and uses circular polarisation properties. The reflectivepolarisers will reflect one circular polarisation. In the twisted nematic state (right), a LC layer ofthe correct thickness d and birefringence Δn will act as a retardation plate of half a wavelength.This retardation plate is used to transform left circular polarisation (LCP) into right circularpolarisation (RCP) and vice versa. The polarisation transmitted by the upper polariser is thusreflected by the lower polariser as the LC layer transforms this polarisation to its orthogonalcounterpart. In this way, both polarisations of light are reflected: a reflection of 100% is obtained.When the LC layer is in the homeotropic configuration, it will act as a non-birefringent materialand will not transform any polarisation, transmitting 50% of the light.

Figure 2.2: Model Linear and model Circular in homeotropic (left) and twisted nematic (right)state

2.2 The models 10

2.2.2 Model Linear

Model Linear is the second model and uses the linear polarisation properties of light: the reflec-tive polarisers reflect one linear polarisation. The LC layer in the TN configuration can be usedfor its polarisation rotating properties. If the optical anisotropy of the LC is higher than the helicaltwist velocity (2.1), the LC is in the Mauguin regime causing any linear polarisation to follow therotation of the LC. As the LC is in a TN configuration, the linear polarised light will be rotatedover π/2. In the homeotropic state, the TN LC will not change the polarisation and there is atransmission of 50%.

p >λ

Δn(2.1)

2.2.3 Model QWP

The third model is a combination of model Linear and model Circular. This model uses circularreflective polarisers and two extra quarter wave plates (QWP) that sandwich the LC layer, seefigure 2.3. The two QWP have another orientation: βQWPtop = −π/4 and βQWPbottom = +π/4.The value of β is dependent on the direction of the beam, the mentioned values are chosen for atop-to-down direction. When random polarised light is incident, the RCP-part will be reflected bythe polariser while LCP will be transformed by the QWPtop into a y-linear polarisation (LP). Whenthe LC is in Mauguin regime (2.1), this y-LP will follow the twist of π/2 and is thus transformedinto its orthogonal component: the x-LP. The QWPbottom will transform x-LP into RCP that willbe reflected by the bottom circular reflective polar. The reflected light is still RCP and will betransformed by the QWPbottom to x-LP light (βbottom = −π/4 as the direction of the beam is nowupwards). The TNLC will transform x-LP to y-LP, which will be transformed by the QWPtop toLCP. This LCP is transmitted by the upper reflective polariser and there is no transmission in thisstate. In the homeotropic state, the LCP transmitted by the top reflector becomes y-LP by the firstQWP and is transformed again to LCP by the bottom QWP, so it will be transmitted by the lowerreflector. In the homeotropic state, there is a transmission of 50%. As this model uses two QWP,this model is called model QWP in the rest of this work.

Figure 2.3: Model QWP

2.3 Reflective polarisers 11

2.3 Reflective polarisers

The three models make use of reflective polarisers which reflect one polarisation of light andtransmit the orthogonal. Two models make use of circular reflective polariser, one model uses alinear reflective polariser. This section describes how these polarisers work.

2.3.1 The working principle

Light can be described as an electromagnetic wave, and when trying to solve a diffraction problemat a periodic medium, one has to solve the vectorial Maxwell equations with the correct boundryconditions. The solution to this problem is beyond the scope of this work, but can be found inthe lecture notes of the Microphotonics course [9]. An arbitrary wave propagating in an uniformmedium can be described as a set of plane waves which propagate independently. In a periodicmedium however, the plane waves will be scattered. Due to the periodic nature, there exists avery specific coupling between certain plane waves of the set: this is the so called Bragg condition.

The set of waves that are reflected due to Bragg reflection, can be described using the centralwavelength (λ0) and the wavelength band (Δλ), see equations 2.2 and 2.3. p, Δn and n representrespectively the pitch, the difference between the extraordinary (ne) and ordinary (no) refractiveindex and the average of ne and no [10].

λ0 = np (2.2)

Δλ = Δnp (2.3)

It is also possible to make Bragg reflection polarisation dependent. An anisotropic mediumis used, where the refractive index modulation only occurs in one direction. In that case, thereflection occurs for light polarised equal to the polarisation of the refractive index change. Inorder to reflect right circular polarised light (RCP), one needs right-handed circular orientedindex change. This particular index change is found in cholesteric liquid crystals (CLC). Toreflect linear polarised light, for example x−polarised light, one needs a periodic index changein the 1x direction and no periodic index change in the 1y direction.

2.3.2 Circular reflective polarisers

Cholesteric liquid crystals (CLC) are liquid crystals that tend to align themselves in a helicalstructure with the helical axis perpendicular to the director, as can be seen in figure 2.4. Thehelical pitch p is the length over which the LC directors rotate 2π. An interesting feature of CLCis that they reflect light of the same handedness of the helix. Most CLC have a right-handed helixand reflect RCP-light, while the LCP-light is transmitted. The reflection band is defined by 2.2and 2.3. Outside the reflection band, both polarisations are transmitted.

A typical birefringence of LC is 0.12, resulting in a reflection band at 700nm of about 75nm,which is too small to be viable for the SIR: a broadband reflection is needed. A lot of research

2.3 Reflective polarisers 12

Figure 2.4: Helical alignment of cholesteric liquid crystals (CLC).

has already been executed on the wide-band reflective polarisers from CLC, because of theirimportance in practical applications. There are a few ways to obtain broadband reflection inCLC, the common techniques use a multi-stacking approach or a gradient-pitch approach [11].

The introduction of a gradient pitch in the cholesteric helix was first realized by D.J. Broer.Adding a small amount of a dye, with absorption maximum close to that of the photoinitiatorand with a larger extinction coefficient, they ensured a gradient in ultraviolet intensity throughthe thickness of the CLC sample. This resulted in faster polymerization rate at the top of thesample to enhance a gradient pitch. They realized a 15μm thick film of CLC with a gradient pitchrange of ptop = 0.27μm at the top and pbottom = 0.45μm at the bottom of the film, resulting in areflection band over the visible spectrum [12]. Another study obtained a gradient pitch by thediffusion between adjacent LC layers with a different pitch during orientation. The obtained pitchgradient was first restored by rapid cooling and further stabilized by photopolymerization [13].

Another way of introducing a wide reflection band is to stack multiple chiral films with asmall adjacent reflection bands to cover a certain spectrum. This technique is easier in terms offabrication. An example is shown in figure 2.5.

Figure 2.5: Broadband reflection can be obtained by stacking multiple CLC layers with sligthlyoverlapping reflection bands[14].

2.4 Retardation plate 13

2.3.3 Linear reflective polarisers

To obtain a linear reflective polariser, the periodic index change needs to be along one axis, whilethe orthogonal axis has no index change through the thickness of the sample. Figure 2.6 showslinear reflection along the x-polarisation. Stacking multiple films on top of each other, it possibleto create a broadband linear reflector.

Figure 2.6: Reflection of the x-polarisation [11].

2.3.4 Reflective polarisers: angular dependence

As the SIR will be used to reflect direct sunlight, and the sun changes position during the day,it is important to know the angular dependency of the reflected wavelength range. Based onBragg’s and Snell’s law, the central wavelength λ0 of the reflection band can be calculated atoblique incidence using equation 2.4 [15]. In this equation, θ is the angle of incidence in air

and arcsin(

sin(θ)n

)is the angle of light in the medium. This equation implies a blueshift for the

reflected wavelength range.

λ0(θ) = λ0 cos(

arcsin(

sin (θ)n

))(2.4)

2.4 Retardation plate

A retardation plate is an optical device that converts the polarisation of light by resolving it intotwo orthogonal linear polarisations and introducing a phase shift between them. This phase shiftstems from the different velocities of the two polarisations inside the retardation plate. A retar-dation plate is made out of a material of which the optical properties depend on the propagationdirection of light as well as on the polarisation of light. A birefringent material is often used as apolarisation converter.

In this work, the retardation plate is used in two ways. First of all, the LC layer in a TNconfiguration will act as a retardation plate for model Circular in order to convert LCP into RCPand vice versa. Secondly, model QWP also uses retardation plates to convert circular polarised(CP) light into linear polarised (LP) light and vice versa.


The best way to describe the retardation in an uniaxial material is through the Jones formalism.For an uniaxial anisotropic material with an in-plane optical axis, the Jones matrix can be writtenas 2.5, with φi = k0nid, k0 = 2π/λ0 and d the thickness of the plate. The total retardation (Δφ)of the plate is given by 2.6. Using equation 2.7, it is easy to relate the path difference δ and thephase retardation Δφ.

[e−jφo 0

0 e−jφe

]= e−jφo

[1 00 e−jΔφ

](2.5)

Δφ = k0(ne − no)d = 2π(ne − no)dλ

(2.6)

Δφ2π

=δ

λ(2.7)

The wave nature of light imposes different retardations for different wavelengths. This meansthat for any integer number of wavelength retardation, the polarisation state remains unchanged.It is obvious that the retardation is dependent on the wavelength. The birefringence howeveris also wavelength dependent although it has less impact, especially in the IR-spectrum. In theapplication of the SIR, a broadband wavelength polarisation converter is needed. As 2.6 is validfor multiple values of M2π, multiple wavelengths satisfy the condition. In order to be smallband,low order wave plates are needed (M ≤ 2). A retardation plate is characterized with threeparameters: the birefringence Δn, the thickness d and the angle of the optical axis β.

2.4.1 Quarter wave plate

A quarter wave plate (QWP) introduces a path difference δ = (2M + 1) λ4 . When visible x-polarised light is incident on a retardation plate with dΔn = 140nm the plate will act as a QWPfor λ = 4 × 140nm = 560nm. If β = +π/4, the x-polarised light will thus be converted to aRCP wave, as can be seen in figure 2.7. The S3 parameter hits a maximum at λ = 560nm and atthe same time S1 = 0, the characteristics of a RCP wave. In table 2.3 the influence of a QWP issummarized for different combinations of input polarisations and β-angles.

2.4.2 Half wave plate

A half wave plate (HWP) introduces a path difference of δ = (2M + 1) λ2 , transforming any po-larisation to its orthogonal counterpart. On the Poincaré sphere, this transformation is visualizedas an inversion through the center of the sphere. In our experiments, we would like the TNLC toact as a HWP.

The dispersion of the birefringence of the LC is shown in figure 2.8. The wavelength depen-dence of the birefringence is quite important in the visible spectrum, but in the IR spectrum thisbirefringence is in a first approximation constant (ΔnIR ≈ 0.133). The HWP-effect should be ob-tained in the near IR, with a central wavelength λ0 = 900nm. The thickness of the layer shouldbe d = λ04Δn = 1.691μm in order to have a zeroth order, and thus broadband, HWP-effect. Thisthickness is difficult to realize, and as the SIR will be integrated with the SEG, it is bounded to


Figure 2.7: The effect of a quarter wave plate (qwp) when x-polarised light is incident withβ = π/4.

the SEG-configuration: d > 15μm. It is possible to use another LC with a smaller birefringence,but a birefringence of Δn ≈ 0.0167 would be needed, which was not available for experiments.

In figure 2.9 the HWP effect is presented. The input is a LCP wave that is incident on a platewith thickness d = 15μm and a birefringence equal to the used LC. The S3 parameter indicatesthe degree of circular polarisation, when S3 = 1 the HWP effect is obtained as the output is nowRCP.

2.4.3 Polarisation rotation property of a liquid crystal.

A twisted liquid crystal exhibits a peculiar optical property. If a linear polarisation is incidentalong one of the principal axis at the surface, so it does not feel any birefringence, the polarisationwill follow the rotation of the directors of the LC along the propagation through the LC layer. Ifthe LC is in a 90◦ twisted configuration, the linear polarisation will have rotated over π/2. If alinear polarisation is rotated of π/2, it is transformed to its orthonogal counterpart (an inversionthrough the center of the Poincaré sphere). This means that the TNLC layer can be seen as a

400 450 500 550 600 650 700 750 8000.1

0.15

0.2

Wavelength (nm)

Bire

frin

genc

e

Figure 2.8: The birefringence of the LC used in the experiments ("MLC 6653").


β input output

−π/4 x LCPy RCP

RCP xLCP y

0 x xy y

RCP +π/4LCP −π/4

π/4 x RCPy LCP

RCP yLCP x

Table 2.3: The influence of a QWP with varying β on different input polarisations.

half wave plate. It is important to note that this effect is achromatic over a wide wavelengthrange, much wider than a zeroth order wave plate. This effect takes place when the LC layer isin Mauguin regime, see equation 2.1.

The efficiency of the rotation can be expressed by the transmission of a TN cell between twoparallel perfect polarisers, 2.9. After the TNLC, the light is not exactly linear polarised, butslightly elliptical. In section 4.1.2, this polarisation rotating effect will be further explained.

Tparallelpolarisers =12

sin2(π2√

1 + u2)1 + u2

(2.8)

u =2dΔn

λ(2.9)


Figure 2.9: The retardation effect of a liquid crystal for λ = 400nm − 800nm presented using tworepresentations. Left: the Poincare representation of the output polarisation. The output variesfrom S3 = +1 → S2 = −1 → S3 = −1 → S2 = +1 → S3 = +1, for increasing wavelength. Right:the Stokes parameters S1, S2, S3 in function of the wavelength.

Chapter 3

Modelling and optical measurements

3.1 The computer model

The three models (model Linear, model Circular and model QWP) make use of different materials:reflective polarisers, glass plates, liquid crystal and a QWP. The impact and interaction of theseplates on the polarisation of light can be simulated in order to predict and/or confirm lab resultsof the experiments. The simulation model used in this work, was implemented using the Matlabsoftware, which is extremely useful for matrix manipulations.

The simulation is based on the scattering matrix method of Ko [16]. For this method, themodel is sliced in thin layers that are defined by a certain thickness, director orientation andbirefringence. A plane wave of a certain polarisation and wavelength is incident on this stack ofthin layers. The program calculates the polarisation of light in transmission and reflection. Thecalculations used in this work use Matlab functions created by Lieven Penninck.

First of all, the model is divided in the existing layers: a reflective polariser, a glass plate,a retardation plate and a liquid crystal layer. Each plate in the model is characterized by thethickness, the birefringence and the director orientation through the plate. Using enough layersfor each plate, the stack is then put together.

The input polarisation is defined using the Jones-formalism. A vector is defined that containsall the wavelengths that need to be evaluated. For each wavelength, the transmission throughall the layers is calculated and stored. At the end of the simulation, the graphs can be createdfor all the wavelengths. The simulation duration is strongly dependent on the number of layersthat is used for defining the stack. It is useful to define as much layers as possible to have betterresults, but there is clearly an optimum in the trade-off extra layers or longer calculation time.The simulation duration depends also on the number of wavelengths that need to be evaluated,so for testing it is better to evaluate only roughly (i.e. few wavelengths).

An example of the definition of CLC stack and resulting reflection band is shown in figure 3.1.The typical transmission spectrum can be recognized in this figure, meaning that the simulationreturns a very good result. After thoroughly testing of different kind of plates, models, . . . , it canbe said that the simulation model is correct.

3.2 Lab results 19

0 2 4 6

x 10−6

−1

−0.5

0

0.5

1

Distance (µm)

c

x

cy

−10

1

−10

1

0

2

4

6

x 10−6

xy

z

550 600 650 700 750 800 850 900 9500

0.5

1

Tra

nsm

issi

on

Wavelength (nm)

Figure 3.1: Top: left figure represents the director structure (c x, cy) through the stack, right figurerepresents the helical structure of the cholesteric liquid crystal. Bottom: the transmission of RCPdecreases where Bragg reflection occurs.

For better understanding of the simulation results, the models are being sandwiched betweenindex matching plates, resulting in a possibility of 100% transmission. Doing so, it is now easierto focus on the more important effect of the different layers on the polarisation of the light.

3.2 Lab results

Three devices based on the three models were also fabricated and measured in the lab. In thissection, more information on the different layers and measurement setup is presented. The fabri-cation of the liquid crystal cell can be found in the appendix B.

3.2.1 The reflective polarisers

The models are created using different reflective polarisers. For model Circular and model QWP,circular reflective polarisers are used. Two different cholesteric liquid crystals (CLC) were used.The CLC constructed by D.J. Broer [12] has a reflection band over the visible spectrum. An-other type of circular reflective polariser was available in the NIR spectrum, and used layers ofthree different pitches to obtain broadband RCP reflection. Model Linear uses linear reflectivepolarisers. These polarisers were only available in the visible spectrum and not in the NIR.

3.2 Lab results 20

3.2.2 The measurements

To measure transmission spectra of an object, a spectrophotometer is used. To measure trans-mission spectra for different input polarisation, the polarisation of the spectrophotometer shouldbe adjusted. After the output of the spectrophotometer, one places a linear polariser so linearlight is obtained. For all experimental transmission spectra in the following chapter, the baselineis the transmission of air. Placing the object in two orthogonal directions results in the trans-mission spectra for the two linear polarisations. To measure the transmission for RCP and LCPinput, a quarter-wave plate (QWP) can be used. Turning the QWP to −π/4 or +π/4 with respectto the transmission axis of the polariser, results in circular polarised light. To determine whichpolarisation is RCP/LCP, one can use a cholesteric liquid crystal that reflects RCP light. If thetransmission is low, one has created RCP light.

The linear polariser and QWP of the spectrophotometer are designed to work in a certainwavelength range. The available polariser and QWP work only in the VIS spectrum. The QWPexhibits dΔn = 140nm, so it is a zeroth order QWP for λ = 560nm. These spectra resulted in moreinsight in the models and the individual layers. The insights in the visible spectrum, enabled topredict the results in the NIR. For the NIR, no polarisation dependent transmission was necessary,and therefore a depolarisator was used.

The measurements in the VIS spectrum were executed at the laboratory of Eindhoven Techni-cal University. The measurements using a depolarisator were done at Lumilab at the Universityof Ghent.

Chapter 4

Experimental and simulation results

In this chapter, the results of the experiments and simulations are presented for the three models.First, the performance of the TNLC alone will be investigated. This layer is used for two differentpurposes: in model Circular, the TNLC is used for its retardation effect. While for model Linearand model QWP, the polarisation rotating property of the TNLC is used. Then the performanceof model Circular, model Linear and model QWP is presented.

For the experiments, the results of the transmission spectra are always given and never thereflection. The transmission (T) spectra are more easy to measure and reproduce. The reflection(R) spectra are measured with much more deviations and were almost impossible to reproduceexactly. As the absorption (A) of the used layers is known to be small, and T = 1 − R − A, it isenough to measure the transmission. Where transmission is low, the reflection is high and viceversa. For the simulations, no absorption was taken into account. The results of the simulationsare presented in transmission spectra in order to better compare with the experimental results.

In this section, two models will not only be investigated for perpendicular angle of incidence,but also for different angles of incidence. The angles that are mentioned, are the angles measuredin air.

4.1 Numerical analysis of the performance of TNLC layer 22

4.1 Numerical analysis of the performance of TNLC layer

In this section, the performance of the twisted nematic liquid crystal layer is investigated. First,we will look into the effects that occur when the LC is used as a retardation plate by veryfing theinfluence of thickness variations and the influence of a twist. Then the polarisation rotating effectis investigated.

4.1.1 TNLC as a retardation plate

Thickness variations

With the right LC and a perfectly tunable thickness, it is possible to obtain a λ/2 effect for the SIRapplication. However, the thickness of the LC layer is quite difficult to tune due to the wavinessin the used glass plates. The effect is severe, as can be seen in figure 4.1. The HWP effect shiftsfrom λ = 610nm to λ = 616nm, 632nm and 658nm for the variation of respectively 1%, 5% and10% in the thickness d = 15μm. It is already clear that this model will be difficult to fabricate forthe desired wavelength range.

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1515.1515.7516.5

Figure 4.1: Thickness variation of 1%, 5%, 10% around the desired thickness of d = 15μm has asevere impact on the wavelengthrange where the HWP effect is desired.

Twisted Liquid Crystal as λ/2 retardation plate

The LC layer that will be used as an HWP to obtain the SIR effect, is bound to the configurationof the SEG. It is already stated that a minimum thickness is necessary, and on top of that, the LClayer needs to be twisted in order to obtain higher absorption with the dyes. If the planar stateis not twisted, all the dyes are oriented in the same direction, and they all absorb light that ispolarised along this direction. In the case of perfect alignment, maximum 50% of the light can beabsorbed, as random polarised light can be splitted in 50% pol1 and 50% pol2, where pol1 andpol2 are two orthogonal linear directions. To improve the absorption of the LC guest host effectof the SEG in the visible spectrum, a twist is introduced of at least π/2.


The introduction of a twist in a retardation plate, results in two important deviations. Infigure 4.2, the Stokes S3 parameter is plotted for a retardation plate of thickness d = 15μm andtwist values φ = 0, π/2, π, 3π/2. The input polarisation is RCP, so the λ/2 retardation is obtainedwhen the Stokes parameter S3 = −1. When the LC has no twist φ = 0, the HWP effect is perfectlyobtained (S3 = −1) at 610nm. Increasing the twist introduces two deviations: firstly, the HWPeffect is not perfectly obtained as S3 �= −1. Secondly, the retardation spectrum has redshifted.The full-wave plate effect, or a retardation of ±2Mπ is always perfectly obtained (S3 = 1), but thewavelength is not predicted with the simple formula 2.6. This can be verified in table 4.1, wherethese parameters are summarized.

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phi = 0phi = pi/2phi = piphi = 3pi/2

Figure 4.2: S3 parameter for RCP input light on a LC layer d = 15μm and varying twist.

Simulation results indicated that three factors have an influence on the decreased and red-shifted retardation effect: the thickness of the cell d, the birefringence of the LC Δn and the twistφ. Model Circular can only work perfect if the λ/2 effect is perfectly obtained. As the decreasedretardation effect is highly undesired for this model, it should be minimized.

The effect of the twist is clearly visible in 4.2: the smaller the twist, the smaller the deviation.As a minimal twist is needed for the absorption effect for the SEG, a twist of φ = π/2 is chosenfor model Circular. Simulations indicate that increasing the optical path difference dΔn, decreasesthe undesired effect. The birefringence cannot be changed as the SIR is bound to the LC usedin the SEG. The thickness can be increased. However, increasing dΔn means a smallband HWPeffect. From table 4.1 and figure 4.2, we notice that a twist φ = π/2 still results in a highly leftcircular polarised wave or that the HWP is still well obtained using this configuration, so we willcontinue to work with the π/2 twist for model Circular.

φ 0 π/2 π π/4

λhwp 610nm 615nm 630nm 660nmS3 −1 −0.961 −0.849 −0.6573

Table 4.1: Value of Stokes parameter S3 and wavelength for the HWP effect.

4.1.2 TNLC as a polarisation rotator

In this section, the polarisation rotating effect of a TNLC is investigated. From section 2.4.3 it isclear that a better rotation of the light, or a better extinction of Tparallelpolarisers, is obtained for high


optical path differences: high optical anisotropy and a thick layer. There is however a trade-off:thicker cells have slower switching times, require higher voltages and more material.

In figure 4.3, the Stokes S1 parameter is plotted for TNLC cells with a thickness d = 15μm, 25μmand 100μm and y polarised input. After the TNLC the linear y polarised light should becomex polarised light. From this figure, it is clear that thicker cells exhibit smaller variations aroundthe perfect x polarisation (S1 = +1). For the d = 15μ cell, the S1 Stokes parameter is minimumS1,minimum = 0.91 indicating a good quality of linear x-polarised light. As the intention of theswitchable reflector is to work in the infrared spectrum, it is best to work with cells of d > 15μm.

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Figure 4.3: Stokes S1 parameter for y polarised light incident on a TNLC with thickness d =15μm, 25μm and 100μm. The output is mainly S1 ≈ 1 indicating x polarised light.

4.2 Model Circular 25

4.2 Model Circular

In this section, the performance of model Circular will be presented. First, the simulation resultsin the visible spectrum are explained. Then the experimental results are presented and comparedto the simulations. Model Circular consists of the following stack: CLC + LC + CLC, where theLC layer is used as a retardation plate.

4.2.1 Simulations

The simulation of model Circular will use a circular reflector similar to the one constructed in[12], i.e. a broad reflection band in the visible spectrum. The parameters of this cholesteric liquidcrystal can be found in section 2.3.2. The two circular reflectors sandwich a LC layer that canbe switched between homeotropic (low reflection - high transmission) and twisted nematic state(high reflection - low transmission). The LC layer has a thickness of d = 15.8μm and a twist ofφ = π/2. As this model uses circular polarisation properties, the two circular polarisations oflight are simulated. In figure 4.4, the results are presented.

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Figure 4.4: Simulated transmission spectra for model Circular. Left: LC is in TN configuration.Right: random polarised light results in high (homeotropic) and low transmission (TN). Averagetransmissions are THomeotropic = 50% and TTN = 29%.

At the left, the LC layer is in TN configuration. For RCP input, there is no transmission asthe first CLC reflects all RCP light. For LCP input, a low transmission is desired, but instead thetransmission fluctuates. This is due to the phenomenon explained in section 2.4: the HWP effectis strongly λ-dependent. Only a small part of the LCP light will be transformed to RCP light andwill be totally reflected by the bottom circular reflector. It is impossible to obtain a reflection of100% over the desired wavelength range due to these fluctuations. In the right figure (randompolarised light), the transmission is visualized for the two configurations of the LC layer: twistednematic (high reflection) and homeotropic (low reflection). The reflection band of the CLC rangesfrom λ = 430nm − 730nm. The average transmission in the wavelength range of the reflection


band of the CLC are: THomeotopric = 50% and TTN = 29%. It is important to note that thesesimulations are done in index matching materials, so that perfect transmission (T = 100%) ispossible.

4.2.2 Experimental results

In order to interpret the results of model Circular, the transmission spectra of the circular re-flective polarisers are measured for different input polarisations, in order to put later results inperspective. The circular reflective polariser is a cholesteric liquid crystal (CLC). The results ofthis measurement are presented in figure 4.5. The legend is explained below:

• clc rcp/lcp = transmission of a cholesteric liquid crystal for RCP / LCP input polarisation.• clc clc rcp/lcp = transmission of two cholesteric liquid crystal for RCP / LCP input polarisa-

tion.

• clc clc tn rcp/lcp = transmission of two cholesteric liquid crystal and a TNLC, with the TNLCbehind the two reflectors, for RCP / LCP input polarisation.

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clc lcpclc rcpclc clc lcpclc clc rcpclc clc tn lcpclc clc tn rcp

Figure 4.5: Experimental transmission spectra of the reflectors and LC layer with different inputpolarisations.

The circular polarisations are generated using a linear polarisator and a zeroth order QWPwith dΔn = 140nm. For this explanation, we must assume that the circular polarisations areexactly RCP or LCP polarised. This is true for 550nm − 700nm, where the transmission spectraare almost flat. The baseline for these measurements is air, with Tair = 100%. In the followingexplanation, the transmission values are at λ ≈ 600nm, unless the wavelength is clearly specified.


The reflective polarisers consist of two glass plates with a cholesteric liquid crystal in between.For LCP light, the transmission decreases to about 91% due to the two glass-air interface reflec-tions. For RCP the transmission decreases to 14.8%. This is due to two factors: the air-glassinterfaces and the Bragg reflection. As calculated in 4.1, RBragg ≈ 83.9% for RCP.

Tclc,RCP ≈ 14.8%≈ 100%(1 − Rglass−air)(1 − RBragg)(1 − Rglass−air)≈ 100%(1 − 0.04)(1 − RBragg)(1 − 0.04)

RBragg ≈ 83.9% (4.1)

LCP transmission of two clc’s (clc clc lcp) is about 84.4% due to four glass-air interfaces. ForRCP (clc clc rcp), the transmission is 7.46%. This transmission is higher than expected. One wouldexpect Tclc,clc,rcp = Tclc,rcpTclc,rcp = 2.2%, assuming that the transmission of the first reflector isthe same as the input polarisation (RCP). We can conclude that the circular reflector used in theexperiment, transforms the RCP input in two ways: 83.9% is reflected due to Bragg reflection,while 16.1% that is transmitted, will not be perfectly RCP anymore. The extra reflector decreasesthe transmission of RCP input, by only 50.4%.

Adding the LC layer (clc clc tn lcp/rcp), results in a transmission of 74.8% for LCP and 6.20%for RCP. This decrease is due to extra reflections and small absorption of the ITO layer.

In figure 4.6 the experimental results of model Circular are presented for different input po-larisations. In the top figure, the transmission spectra for different input polarisations is given.In the bottom figure, the transmissions are given for randomly polarised light and the LC layerin the two configurations. The simulated result of the TN configuration is also presented in thesame graph, to show the agreement between simulation and experiment.

The list below, explains the most important information observed in the transmission spectraof figures 4.5 and 4.6.

• LCP in - TN: We clearly see the retardation effect in action when LCP light is incident on theTN cell. Where this transmission peaks, the TN acts as a full wave plate which is similarto no retardation: LCP stays LCP light. The maximum transmission equals LCP in - Homeo,which confirms the full wave plate effect: in the homeotropic state, the LCP light will notchange due to the LC layer.

• We notice that the minimal transmission of LPC in - TN from λ = 500 − 700nm is not aslow as one may expect. One would expect that the minimal transmission should be equalto RCP in - Homeo for example. This is due to many different reasons. First of all, the twistin the LC layer prohibits the full HWP effect, as explained in section 4.1.1. Secondly, as thereflective circular polarisers are far from perfect, not all the RCP light will be reflected bythe second reflector at the bottom. So, one should compare the minimal transmission ofLCP in - TN to clc rcp from figure 4.5: they are the same. Therefore, if one wants a lowertransmission, better reflectors are needed.

• We notice that the minimal transmission of LCP in - TN is lower than expected for λ =400 − 500nm, as this transmission is lower than RCP in - Homeo. This paradox is because of


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Figure 4.6: Experimental results of model Circular in the two states: twisted nematic andhomeotropic. TOP: model Circular for different input polarisations, bottom: model Circularfor random polarised input.

comparing the wrong numbers. In this wavelength range, 100% LCP light is the input. Thefirst reflector transmits ≈ 78% (see clc lcp) for λ = 455nm. After the TNLC layer, this LCPpolarisation is transformed to RCP polarisation which is incident on the second reflector.One reflector will transmit only 14.8%, so the transmission should decrease to 11.5%. Thetransmission is TLCPin−TN,455nm = 13.0%, which is only a small deviation from 11.5%.

• From the bottom figure, we notice that the simulation and experimental result do agree.The retardation effect is well predicted. The minima and maxima do not agree becausethe simulated reflectors are perfect in contrast to the reflectors used for the experiment andbecause the glass-air reflections are not incorporated in the simulation.

• The averaged experimental transmissions calculated in wavelength range λ = 400nm −700nm are TTN = 23.3% and THomeotropic = 37.2%. The experimental results are about25% lower than calculated, because the calculations do not take into account the glass-air interfaces, absorption due to polymide, ITO, . . . As can be seen by clc clc tn lcp, theexperimental transmission of LCP through two CLCs and one TNLC already decreases to≈ 75% whereas the simulations would predict a 100% transmission.

As model Circular has a reflection that is strongly wavelength dependent and which is difficultto tune (see the effect of thickness variation, figure 4.1), and the characteristics of the LC-layerare bounded to the SEG-functionality, the investigation of this model ends here. The angulardependence of the transmission and reflection spectra will not be presented here, the resultswould be beyond the scope of this text. It is more interesting to investigate the other models that(hopefully) do have more interesting results, with respect to the SIR.

4.3 Model Linear 29

4.3 Model Linear

Model Linear consists of the following stack: reflector + LC + reflector, the reflectors reflectthe same linear polarisation. Model Linear uses linear reflective polarisers with transmissionaxis parallel to the first director of the LC. The two reflective polarisers are parallel aligned sothey reflect the same linear polarisation. This model cannot be simulated as the structure of thereflective polariser is not exactly known. We do know that the linear reflector is based on Braggreflection, it consists of multiple thin films with each a different period of refractive index change,to obtain broadband reflection. The reflection spectrum of the linear polariser ranges from 400nmto about 950nm. The reflection band is flat in the visible spectrum of light as its main use is indisplays. In the NIR spectrum, the transmission starts to increase. It was not possible to obtain alinear reflective polariser that reflects only a wavelength band in the NIR spectrum.

Before explaining the transmission spectrum of model Linear under varying angle of inci-dence, the transmission spectrum of the linear reflector under varying angle of incidence is pre-sented and can be found in figure 4.7. In the visible spectrum the transmission of randomlypolarised light is close to 45% which indicates that the reflectors are very good, but not perfect.Increasing the angle of incidence results in a blueshift of the reflection band and a decrease intransmission. These graphs can be used as reference values for later discussion. The angle vari-ation is only in the plane defined by the transmission axis of the linear reflective polarisers andthe direction perpendicular to the model.

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0°30°45°60°

Figure 4.7: Measured transmission spectrum of the linear reflector under varying angle of inci-dence.

In figure 4.8, the transmission spectrum of model Linear is given for perpendicular inci-dent random polarised light. The LC layer is tuned in a twisted nematic (high reflection) andhomeotropic state (low reflection). From λ = 400nm − 800nm the average value of the transmis-sion of the twisted nematic and homeotropic state are respectively TTN = 9.8% and Thomeotropic =

39.7% for random polarised light. The switching ratio (ThighTlow

) is very good and as the reflection isvery broadband, the design looks like a metallic mirror reflection.

In figure 4.8, model Linear is also subjected to random polarised light that is incident under

4.4 Model QWP 30

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Figure 4.8: Transmission measurement of model Linear in the high (dotted) and low (full line)transmission state for angles of incidence 0◦, 30◦, 45◦ and 60◦ respectively drawn in color green,black, blue and magenta.

varying angles. It is clear that the overall transmission decreases for higher angles, as well asthe switching ratios. For increasing angles of incidence, the average transmission in the twostates and the switching ratios are given in table 4.2. The decrease in overall transmission isdue to decreasing transmission of the reflector under increasing angle of incidence as a result ofincreasing Fresnel reflection.

α 0◦ 30◦ 45◦ 60◦

TN 9.8 8.8 10.1 9.8Homeotropic 39.7 31.0 23.2 16.6Switching ratio 4.1 3.5 2.3 1.7

Table 4.2: Average transmission (λ = 400 − 800nm) of model Linear in two states, for varyingangle of incidence.

4.4 Model QWP

In this section, the simulation and experimental results are presented of model QWP. First, themodel is investigated using simulations in the visible spectrum. As these results are promising,the simulations are also performed for the near infrared spectrum. Finally, the experimentalresults of model QWP for the NIR spectrum are presented.

Model QWP consists of the following stack:

CLC + QWPβ=−π/4 + LC + QWPβ=+π/4 + CLC.

4.4.1 Simulation and experiments - visible spectrum

The simulations of model QWP will use circular reflectors similar to [12], they reflect a broadbandwavelength range in the VIS spectrum. The LC cell has a thickness of d = 15.8μm and a twist

4.4 Model QWP 31

φ = π/2. At both sides of the LC layer, a zeroth order QWP film is applied with dΔn = 140nm.In figure 4.9 the results are presented.

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Figure 4.9: Simulated transmission spectra for model QWP. Left: LC is in TN configuration.Right: random polarised light results in high (homeotropic) and low transmission (TN). Averagetransmissions (λ = 450 − 700nm) are THomeotropic = 50.7% and TTN = 3.9%.

In figure 4.9 at the left, the LC layer is in a TN configuration. When RCP light is incident, allthe light is reflected and there is no transmission in the desired wavelength range. The LCP inputis important: it is transmitted by the first reflector, but will be transformed by the QWP-TNLC-QWP layers to RCP and eventually be reflected by the bottom reflector. The transmission is indeedvery low, however, it does not reach zero. The QWP-effect is only perfect for a single wavelength(λ = 560nm) which explains the imperfect behavior. At λ ≈ 550nm, the transmission is indeedlower then elsewhere. The imperfect polarisation rotation of the TNLC (section 2.4.3) also playsan important role as the thickness of the LC is small. The desired effect, low transmission forLCP input, is obtained for the simulations.

In figure 4.10 the experimental transmission results are shown of model QWP for differentinput polarisations. The bottom figure presents the transmission spectra for the LC layer in TNand homeotropic state for random polarised light. The spectra for random polarised are equal tothe transmission of LCPin+RCPin2 as random polarised light is 50% LCP and 50% RCP.

The transmission of LCP in - TN is low, meaning that the intention of transforming LCPinto RCP by the QWP-TNLC-QWP layers works, as predicted by the simulations. However, thisexperimental transmission is higher (TLCP,TN = 14.6%) than the simulated one. This is mainlydue to the imperfections of the reflectors which only reflect about 84.4% of RCP light, insteadof 100% as in the simulations. From the simulations, we also concluded that this model doesnot work perfect: the transmission remained 3.9%. These are the main factors for the highertransmission of LCP in - TN. Not only the simulations, but also the experimtal results indicate ahigh switching ratio. This model appears to be a good candidate for the SIR, as it works well inthe visible spectrum.

4.4 Model QWP 32

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Figure 4.10: Experimental transmission spectra for model QWP. The top figure represents thetransmission for different states of the LC (homeotropic / TN) for different input polarisation(LCP/RCP). The bottom figure represents the transmission of random polarised light for the LCin two configurations. Average transmissions (λ = 450 − 700nm) are THomeotropic = 36.1% andTTN = 11.2%.

4.4.2 Simulations in the near infrared spectrum

Perpendicular angle of incidence

The simulation and experiments in the previous paragraph uses the cholesteric reflective polaris-ers of [12], reflecting a

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