Red-Green-Blue 2D Tuneable Liquid CrystalLaser Devices

H. J. Coles*, S.M. Morris, A.D. Ford, P.J.W. Hands & T.D. Wilkinson Centre of Molecular Materials for Photonics and Electronics, Electrical Engineering Division, Cambridge University Engineering Department, 9 JJ Thomson Avenue, Cambridge, CB3 0FA,

United Kingdom

ABSTRACT

In this paper, we review our recent experimental work on coherent and blue phase liquid crystal lasers.We will present results on thin-film photonic band edge lasing devices using dye-doped low molar mass liquid crystals in self-organised chiral nematic and blue phases. We show that high Q-factor lasers can be achieved in these materials and demonstrate that a single mode output with a very narrow line width can be readily achievable in well-aligned mono-domain samples. Further, we have found that the performance of the laser, i.e. the slope efficiency and the excitation threshold, are dependent upon the physical parameters of the low molar mass chiral nematic liquid crystals. Specifically, slope efficiencies greater than 60% could be achieved depending upon the materials used and the device geometry employed. We will discuss the important parameters of the liquid crystal host/dye guest materials and device configuration that are needed to achieve such high slope efficiencies. Further we demonstrate how the wavelength of the laser can be tuned using an in-plane electric field in a direction perpendicular to the helix axis via a flexoelectric mechanism as well as thermally using thermochromic effects. We will then briefly outline data on room temperature blue phase lasers and further show how liquid crystal/lenslet arrays have been used to demonstrate 2D laser emission of any desired wavelength. Finally, we present preliminary data on LED/incoherent pumping of RG liquid crystal lasers leading to a continuous wave output.

1. INTRODUCTION

Lasers, with their coherent and monochromatic output and narrow divergence angle, are widely used in industry, as well as in the field of medicine for numerous techniques ranging from surgical operations to the treatment of dermatological conditions. Other applications include holography, telecommunications, scientific research, displays, consumer electronics, and data processing. There are therefore a range of different lasers in existence ranging from solid-state lasers, such as the Nd:YAG systems, through to gas lasers, such as Argon ion or HeNe to those based on semiconductor materials. Since the development of the first ruby laser in 1960 the potential applications have continued to increase whilst at the same time the typical dimensions of the device have decreased. In spite of the plethora of laser types, in general, high output powers and small device size tend to be mutually exclusive properties. Furthermore, the highly desirable ease of wavelength tuning, is not readily achievable in the majority of currently available laser devices. Due to the significant advances made in recent years in the fields of photonics and molecular materials, the possibility of an all-organic micro-source with the capability of relatively high output peak powers, continuous wave operation and facile wavelength tuning appears to be an achievable goal.

The recent progress to which we refer to is the realisation of photonic band gap materials and the ability to control the propagation of light in one, two, or potentially three dimensions. The most prominent examples of photonic band gap materials are photonic crystals and liquid crystals. Photonic crystals are usually fabricated from semiconductors or organic materials using photolithography, crystal growth and electron beam etching. In contrast, liquid crystalline materials such as chiral nematics, chiral smectics and blue phases form, through self assembly, a supra-molecular periodic structure. This spontaneous formation of a structure with macroscopic periodicity makes liquid crystals highly useful for photonic devices. The formation of a photonic band gap for light with these materials can be considered simply as the combination of two essential features; anisotropy and periodicity. The chiral nematic phase, which forms a one-dimensional photonic band gap, has a geometrical configuration whereby the average pointing direction of the molecules (the director) rotates about a single common axis, the helix axis, to form a helix. The period is then described

Invited Paper

Liquid Crystals XIII, edited by Iam Choon Khoo, Proc. of SPIE Vol. 7414741402 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.831230

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ab

c

by the distance along the helix for which the director would rotate by (although the pitch of the helix is the distance for a 2 rotation, due to the inherent symmetry of the phase the period is only half the pitch length). With the advent of high twisting power chiral additives the band gap can then be tuned throughout the visible spectrum with a high degree of accuracy merely as a function of concentration of the additive.

The first clear demonstration of lasing in chiral nematic liquid crystals was in 1998 by Kopp and co-workers 1 who showed that, with the incorporation of a gain medium (i.e. a fluorescent dye), and under appropriate photo pumping conditions, laser action could occur when one edge of the photonic band gap overlapped the spontaneous emission spectrum of the gain medium (i.e. the dye) and it was shown that low energy threshold lasing could be achieved in such organic self-organising media. This form of lasing is now commonly referred to as photonic band edge (PBE) lasing. Further investigations have since showed that PBE lasing can be observed in a variety of liquid crystal phases so long as they possess a suitable periodic structure 2-6. It has been shown that wavelength tuning can be readily achieved 7 – 11.

For non-defect mode dye-doped chiral nematic liquid crystals, two resonant modes exist for lasing where the density of photon states (DOS) tends to infinity, i.e. one at the short wavelength edge (SWE) and one at the long wavelength edge (LWE), and thus these two modes are separated significantly in k-space by the photonic band gap. A wide separation of the lasing modes is not a feature that is reflected in conventional Fabry-Perot based lasers. The width of this gap, and thus the separation of these two modes, is dependent upon the degree of anisotropy intrinsic to the liquid crystalline material, for a given constant helix pitch. The criterion for LWE lasing is that the average orientation of the transition dipole moment of the dye (ST) must be such that it is aligned along the local chiral nematic director, assuming that the gain is the same at both band edges. For lasing at the SWE the transition dipole moment must point in a direction perpendicular to the local director axis. For most dyes the gain is wavelength dependent with a peak at the fluorescence maximum. Therefore, in order to obtain the maximum efficiency possible, the preferred band edge must be matched in k-space to the gain maximum or fluorescence peak. Indeed the first minimum of the reflection band should match in wavelength space the peak of the dye fluorescence spectrum (cf Figure 1).

Figure 1, shows (a)the absorption and reflection band of the dyed chiral nematic liquid crystal host, (b) the fluorescence output of the DCM dye and (c) the narrow band laser output of a typical liquid crystal laser.

In addition to PBE lasing, defect-mode lasing has also been demonstrated whereby an artefact is incorporated into the system so as to disturb the periodicity. This then results in a resonant lasing mode at the centre of the photonic band gap which is not depleted by spontaneous emission at wavelengths either side due to the presence of the band gap. Defect-mode lasing was first predicted by Yablonovitch in 1987 12) but was not demonstrated in liquid crystal materials until 2003 13 . Although potentially this type of lasing offers a lower threshold condition, in comparison to PBE lasing, it is not clear how the high slope efficiencies of such lasers may be achieved and are not considered further herein. Defect-mode lasers involve complex fabrication procedures but are potentially interesting candidates for low power cw or quasi-cw lasers.

We have found that the performance of the PBE liquid crystal laser depends upon the physical properties inherent to the liquid crystalline material used as the host, the fluorescent dye used as the gain medium, and the device geometry. By optimising these properties we have shown that low threshold and high slope efficiencies (greater than 60%) may be achieved and wavelength tuned in electric or thermal fields. Using these materials we have further shown how lasing

400 450 500 550 600 650 700 750Wavelength (nm)

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may be achieved in Blue Phase I (BPI) and may be used to produce 2D lasing arrays, as well as LED pumped laser devices. These observations are discussed further in this paper

2. SAMPLE PREPARATION

For the study of coherent light sources, liquid crystal samples were prepared as follows. A low molar mass (LMM) liquid crystal, which possessed an enantiotropic nematic phase, was dispersed with a small concentration by weight of a high twisting power (HTP) chiral additive (BDH1281, Merck) to form a chiral nematic phase. We refer to this as an induced chiral nematic (N*) phase. The result of adding the HTP dopant to the LMM liquid crystal is a self-organising periodic structure whereby the anisotropic nematic layers are forced to rotate about a single helix axis, which is oriented everywhere perpendicular to the local nematic director. Macroscopically, the configuration of the liquid crystalline molecules is such that the director traces out a helical structure. The bandwidth of reflected wavelengths ( ) is related to the pitch (p) of the helix and the birefringence ( n) of the anisotropic ‘pseudo’ layers through the relationship =

np. The chiral nematic mixture was then doped with a low concentration of the relevant laser dye, i.e. 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostryl)-4H-pyran (DCM), or Rhodamine 6G Chloride (R6G), Rhodamine B Chloride (RB), Pyrromethene 597 (PM597) or Pyrromethene 580 (PM580), and left in a bake oven for a period of twenty-four hours. Afterwards, mixtures were then capillary filled into cells which were 7.5 m thick and had a rubbed polyimide alignment layers to give planar alignment at the cell surfaces. For the application of an electric field these surfaces also incorporated ITO transparent electrodes over an area of 25 mm2. This resulted in a standing helix geometry (USH), or Grandjean texture, whereby the helix axis is orientated perpendicular to the cell substrates. The emission from the sample is in a direction parallel to axis of the helix and thus perpendicular to the cell walls.

3. EXPERIMENTAL PROCEDURE

The experimental apparatus used to study the coherent lasers has been discussed in detail elsewhere 6. Here we outline the important experimental elements. Firstly, the filled cells described above are positioned on a combined heating and rotation stage which has three-way translation along the X, Y, and Z axes. The temperature of the sample can be controlled to within a 0.1ºC accuracy. The samples are then photo-pumped by a Q-switched frequency doubled Nd:YAG laser which emits 5 ns-long pulses at variable repetition rates up to 10kHz at 532 nm and the spatial profile of the energy distribution is near-Gaussian. For the pumping of the coherent laser sample a quarter wave plate is used to convert the incident wave to the opposite circular polarisation sense to that of the helix. This is in order to maximise the excitation conditions. Otherwise a component of the linearly polarised pump beam would be reflected by the helical structure. The laser light emitted from the sample is then collected over a narrow solid angle by a microscope objective optically coupled into a universal serial bus spectrometer (USB2000 or HR2000, Ocean Optics) with a fibre optic attachment and an energy meter (Laserstar, Ophir). Optical micrographs of the textures were obtained using a digital camera mounted onto a polarising microscope (C4040, Olympus).

4. CHIRAL NEMATIC LIQUID CRYSTAL LASERS

In this section we will initially consider the properties of chiral nematic based lasers that make them interesting for potential applications. We will consider output beam characteristics, illumination conditions, and single mode operation before considering materials properties and experimental conditions that lead to high optical gain or slope efficiencies. We will then consider wavelength tuning of these lasers.

4.1 Laser Properties

Herein we will discuss the importance of the sample texture on the lasing spectrum and show that the spatial distribution of the energy is near-Gaussian. Furthermore, we will report on some of our results 15 – 17 for the pump energy dependencies of the liquid crystal lasers.

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4.1.1 Laser Output beam profile and excitation conditions

In Figure 2, optical micrographs of the sample textures are shown, along with the laser emission spectra, for the same sample but prepared using two different procedures. The first one is a poly-domain texture which was prepared by quench cooling the sample from the isotropic phase at a rate of 50ºC/min. It can be seen that the texture comprises many different domains that are separated by an oily streak network. The corresponding laser spectrum is broad ~ 3.5 nm

Figure 2. Sample textures and corresponding laser spectra (a) poly-domain sample and (b) mono-domain sample.

and contains more than one mode, which indicates that the different domains have slightly different pitch lengths. In contrast, the mono-domain sample, which was prepared by cooling the sample slowly from the isotropic phase at a rate of 5ºC/min and then left to anneal for a period of twenty-four hours. On inspection of the sample texture (c.f. Fig. 2b) it is clear that the texture is without the multiple domains and the oily streak defect as observed for the poly-domain sample. Therefore, as a result the laser spectrum is found to be considerably narrower than that of the poly-domain texture ( = 0.6 nm) and is reminiscent of a single laser mode. The full width at half maximum of the resonant mode is found to be r = 0.09 nm. Using r

2/ r, the coherence length for this laser is found to be ~ 5 mm. For the Q-factor we obtain a high value of Q ~ 6700. Figure 3 shows an image of the laser spot in the far-field (a) and a three-dimensional plot of the energy distribution (b). The intensity is highest at the centre and then falls off as the radial distance from the centre increases. The innermost spot (the brightest region) is surrounded by a series of concentric circles. A previous study 18 has shown that the coherence area for these lasers is large and this increases the lifetime of dye molecules by effectively removing the heat from the system.

The large coherence area is an added advantage of photonic band edge lasers and is the result of the fact that the oblique modes do not contribute to the lasing mode. In Figure 3(b) it can be seen that the emission profile, in terms of energy distribution, is near-Gaussian in form. These photonic band edge lasers thus produce a diffraction-limited beam with a large coherence area, a narrow line width laser mode and a near-Gaussian energy profile. Further these lasers can be excited at any angle to the helix axis and this is shown in Figure 4 where the two extremes of 0º and 90º excitation are demonstrated.

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Figure 3. Photographs of the laser spot, (a) in the far-field and (b) in the near field, with a three-dimensional plot of the spatial distribution of the energy showing a near-Gaussian profile.

Figure 4. (a) Photographs showing laser pumping along the helix axis of the microscopic chiral nematic laser and (b) an image of pumping transversely, i.e. at 90º to the helix axis, where the laser emission is towards the observer.

4.1.2 Experimental optimisation

An investigation was carried out to determine the effect the Laser amplifying volume had on the emission characteristics of a liquid crystal laser 19. We studied the change in the excitation threshold and slope efficiency as the pump-spot area was varied. Figure 5 shows the emission energy as a function of the excitation energy for six different pump spot areas; in each case the laser wavelength is = 610 nm. The emission energy from the liquid crystal sample was collected over a narrow solid angle of 0.12 sr in the forward direction. The figure shows a typical input-output curve that is characteristic of a laser. The solid lines represent the lines of best fit to the data above the excitation threshold. As the spot size increases, the excitation threshold and slope efficiency are seen to increase and decrease, respectively. For a spot size of d = 90 μm (Area = 6.4 x 103 μm2) the excitation threshold was found to be Eth = 3.8 μJ/pulse (fluence ~ 50 mJ/cm2) whereas for d = 231 μm (Area = 42 x 103 μm2) the excitation threshold was Eth = 11.9 μJ/pulse (fluence ~ 17 mJ/cm2). By increasing the spot diameter by a factor of 2.6 (area by a factor of 6.6) the result was a three-fold increase in the excitation threshold energy. It was found that when the threshold energy was plotted as a function of pump-area a linear function best described the relationship. In a space-independent model, the threshold can be shown to be linearly proportional to the cross-sectional pump-beam area, where it is assumed that pump area (mode area) is less than, or equal to, the cross-sectional area of the active medium 20. On the other hand, for the slope efficiency, this appeared to decrease exponentially with pump-spot area. A somewhat intriguing observation is that the threshold

(a) (b)

(a) (b)

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fluence was actually lower for the larger spot size than the smaller spot size. This may be due to the fact that the important factor is actually the cross-sectional area of the laser mode which is different to the pump-area at the sample.

0 10 20 300.0

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0.2

0.3

0.4

0.5 90 m 100 m 131 m 181 m 203 m 231 m

Emis

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(a) (b) (c) Figure 5 – (a) The emission energy as a function of excitation energy of a chiral nematic band-edge laser for different pump-spot diameters, (b) the excitation threshold and (c) the slope efficiency.

4.1.3 Functionality

One feature that is of particular interest with these lasers is the ability to select a single wavelength from a continuous range contained within the large gain bandwidth of the dye. To achieve this, the concentration of chiral additive can be adjusted so as to alter the periodicity and consequently the position of the band edge relative to the spontaneous emission curve. However, an even more remarkable feature is that the emitted wavelength can be tuned in-situ to different levels with the aid of external stimuli such as temperature or an electric field

4.1.4 Temperature tuning of the wavelength

Figure 6. The change in laser wavelength with temperature: (a) laser emission spectra for a range of different temperatures on cooling, (b) the laser wavelength as a function of temperature.

In Figure 6, we show the change in the laser wavelength for a 50ºC change in temperature. Plots of the laser spectra and the laser wavelength as a function of temperature are shown in Figs. 6a and 6b, respectively. It can be seen that as the temperature is reduced from 80oC the laser wavelength increases in small increments and continues in this manner down to 60ºC at which point two laser modes are present. Between 80oC and 60oC the change in laser wavelength is the result of a temperature-induced refractive index change since II = nIIp whereby II is the laser wavelength and nII is the extraordinary refractive index. However, at 60ºC there is a temperature-induced pitch change in a region of the sample which results in the appearance of a second mode at a longer wavelength. Due to the constraints imposed by the surfaces the pitch change is not gradual with temperature and is instead a step-wise function of the temperature. These two modes remain present down to ~45ºC where the laser output returns to a single mode. Further cooling to 30ºC is found to result in a steady increase in the laser wavelength with decreasing temperature. The total variation in the laser

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wavelength shown here is 30 nm. It is possible to remove the pitch jumps altogether by combining chiral twisting agents of opposite thermal dependency 21.

4.1.5 Fine tuning

Figure 7. The change in the laser spectra as the sample is translated in an orthogonal direction relative to the propagation vector of the pump beam. (a) The laser emission spectra for different cell positions, (b) laser wavelength as a function of cell position.

We found that another way to fine tune the laser wavelength was to vary the position of the cell with respect to the pump beam. This then resulted in a change in the area of the sample that was illuminated by the pump beam. To carry out these measurements the cell was translated along the horizontal axis that was orthogonal to the propagation vector of the pump beam. By doing this a gradual change in the pitch length, due to a weak Cano-Wedge effect, is observed resulting in a gradual change in the laser wavelength. The results for the measurements of the laser wavelength as a function of cell position are shown in Figure 7. The laser spectra are shown in Fig. 7a whereas the wavelength dependence on the cell position is shown in Fig. 7b. The total variation recorded was 3.3 nm for a translation of the cell of 1000 m and thus the incremental change was very small, 0.1 nm change in the laser wavelength for a 40 micron translation of the cell. Using this method very fine tuning indeed of the laser wavelength can be achieved.

4.1.6 Electric field tuning

-1 0 1 2 3 4585

590

595

600

Wav

elen

gth

(nm

)

Electric field (V m-1)

In terms of potential device applications, using an electric field to change the laser wavelength, would be a highly desirable feature. We have found that this can be achieved by applying an electric field in a direction perpendicular to the helix axis so as to deform the structure. For such measurements cells with in-plane electrodes were used whereby the electrodes themselves acted as the cell spacers. A more detailed description of the configuration of these cells can be found elsewhere14. In Figure 8 the field dependence of the laser wavelength is shown. It is apparent that the laser wavelength changes from 587 nm to 596 nm for an increase in the field strength of E = 3.5 V m-1. At this stage it is not possible to state definitively whether the change in wavelength is due to flexoelectric coupling or dielectric coupling. As a matter of fact, the low molar mass liquid crystal used for these measurements has a low dielectric anisotropy and thus dielectric coupling is unlikely to dominate at low field strengths. The two different mechanisms, flexoelectric and

Figure 8. The change in the laser wavelength with an applied in-plane electric field.

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dielectric, may be manifested in the same way, i.e. by a change in the laser wavelength. This is easily understood for the case of dielectric coupling which would result in a unwinding of the helical structure and an increase in the pitch. On the other hand the influence of flexoelectric coupling is less clear since it would serve to deform the helix by rotating the director planes about the direction of the applied electric field. How this deformation of the helix changes the lasing conditions is not known exactly although it is possible that the effective refractive index is being altered thus changing the laser wavelength. However, since flexoelectric coupling is linear with the applied field whereas dielectric coupling is quadratic in the applied field, it would appear that up to 3 V m-1 the behaviour is dominated by flexoelectric effects whilst above this, dielectric effects are more prominent.

4.2.1. Excitation thresholds and slope efficiencies

In several recent studies15 – 17 we have been primarily interested in optimising specific laser parameters such as the excitation thresholds and the slope efficiency. We found that both the excitation threshold and the slope efficiency depended upon the low molar mass liquid crystal used as the host material. More importantly the difference in behaviour that was observed was interpreted in the context of the physical parameters of the liquid crystalline material. That is to say that, parameters such as the Q-factor, the emission efficiency, , the coupling coefficient, , and the gain coefficient at the threshold, g, all of which influence the excitation threshold and slope efficiency, can be considered to be dependent upon liquid crystal parameters such as the order parameters (S and ST), the birefringence, n, and the extraordinary refractive index, nII. Simple relationships for each of these parameters are:

Q-factor, SQr

r

Coupling coefficient, n

Emission efficiency, TS

Gain coefficient, IIng (19)

0.16 0.18 0.20 0.22 0.24 0.260

50

100

150

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puls

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Figure of merit, fFigure 9. A plot of the emission energy as a function of the figure of merit for three different photonic band edge lasers based upon mono-mesogenic liquid crystals.

In a previous report 15 we defined a simplistic figure of merit parameter, f, as a gauge for the performance level a liquid crystal photonic band edge laser would have based upon its physical parameters. The figure of merit parameter was defined explicitly as f = n S nII and a plot of the emission energy for three different monomesogens as a function of the figure of merit is shown in Figure 9. These three monomesogens were from the same homologous series and the figure of merit parameter was varied by altering the temperature so as to change each of the physical properties. It is evident that the agreement between them is not that bad, e.g. at a specific value of f, the emission energy is approximately the

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same for the three materials. Since this investigation a wide range of different low molar mass liquid crystal compounds have been examined 16. However, for a wide range of different samples, this figure of merit parameter was not in such good agreement and could not be applied satisfactorily. This was due to the fact that there were many other parameters that influence the performance which were not taken into account by the simple figure of merit parameter.

Nevertheless, we have found that there is a strong correlation between the physical parameters and the performance of the liquid crystal laser. Figure 10 is a plot of the pump energy dependencies for four different photonic band edge liquid crystal lasers. The liquid crystal laser with low values for the physical parameters has the lowest output (excitation threshold = 2.4 J/pulse, slope efficiency = 1%) whilst the laser which has relatively large values for the physical parameters has the highest output (excitation threshold = 0.4 J/pulse, slope efficiency = 12%). The emission energies plotted in Figure 10 take into account the emission in the backward direction as well as the forward direction. Therefore, to obtain a high performance photonic band edge liquid crystal laser both the order parameters and the optical parameters must be high.

4.2.2 Role of Cell thickness

The dependence of the excitation threshold and the slope efficiency on cell thickness is shown in Figure 11. Here the threshold decreases from Eth = 12 J/pulse at d = 5 m to Eth = 2.5 J/pulse between thicknesses of d = 10 – 15 m. For each cell thickness the laser line remained at the gain maximum of DCM. Above d = 15 m, an increase in the thickness results in an increase in the threshold energy. In terms of the slope efficiency, s, this is found to increase from s = 1.5% at d = 5 m to s ~ 6.5% for thicknesses ranging for an optimal value from d = 10 m to d = 15 m.

0 10 20 30 400

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Excitation energy ( J)

Figure 10. Pump energy dependencies for four different photonic band edge lasers. The plot includes three commercially availablenematogen mixtures E7 ( n = 0.21 S = 0.63, ST = 0.35) (closed squares), BLO93 ( n = 0.24 , S = 0.68, ST = 0.28) (closed triangles), E49 ( n = 0.25, S = 0.68, ST = 0.37) (open circles), and a bimesogen FFO8OCB ( n = 0.26, S = 0.73 , ST = 0.48) (closed diamonds).

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0 20 400

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Eth (

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Cell thickness, d ( m)

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10

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e ef

ficie

ncy,

s (%

)

Cell thickness, d ( m)

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Figure 11. (a) The excitation threshold energy, and (b) the slope efficiency as a function of cell thickness for a chiral nematic band-edge laser.

A previous report has considered the threshold energy in terms of the threshold gain, th22. In this case, it was assumed

that the threshold energy could be expressed in the form Eth = A( th)d where A is a constant and d represents the cell thickness. The parameter A would be related to factors involving the pumping conditions. By considering the threshold gain term in detail it was found that the dependence of the excitation threshold energy on the cell thickness could be expressed as

dd

AEth , (1)

where, is a coefficient relating to absorption losses, is a fitting parameter and is the density of photon states. It has been suggested 22 that = Bd2 where is a fitting constant specific to the material. This implies that the relationship between the threshold and the cell thickness is of the form Eth d + fn (1/d2). In Fig 11(a), the solid line represents the fitting curve using this relationship, which is shown to be in good agreement with the experimental data.

Alternatively, it is possible to arrive at a similar result by using the same starting point i.e. (Eth = A( th)d) but in this case using the threshold gain derived by Kogelnik and Shank for a distributed feedback laser 23. The threshold energy can then be written as

22

2

dnAdAEth , (2)

where is the wavelength of the laser line and n is the birefringence. A recent report has found that the density of states increases as a function of the birefringence which suggests that the material constant B is actually related to the birefringence 24. Experimentally, we have found 19 that the excitation threshold does indeed appear to change with the birefringence.

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From both the space-independent and space-dependent rate equations, the slope efficiency can be shown to be proportional to 1/Eth, which in this case would imply that

s ((d + fn (1/d2))-1. (3)

The solid line in Fig 11(b) represents the fitting curve using this expression and the agreement appears to be rather good. The dashed line in the threshold and slope efficiency graphs represent the condition of no absorption losses whereby the threshold continues to decrease with increasing thickness to approach a ‘threshold-less’ laser and the slope efficiency increases quadratically with cell thickness.

4.2.3 Choice of Laser Dye

In addition to varying the parameters of the nematic host, we have also considered the effects of changing the gain medium on the performance of a laser when pumped at = 532 nm, but using the same nematic host E49 and chiral additive BDH1281 (both from Merck). A number of dyes have been examined, including two rhodamine dyes (Lambda Physik) and two pyromethene dyes (Exciton). The emission energy as a function of excitation energy is shown for five different lasers in Figure 12 for pumping at = 532 nm by a Q-switched Nd:YAG laser (NanoT, Litron Lasers) with a repetition rate of 1 Hz. Slightly different concentrations of BDH1281 (of the order of 4 wt%) were required to position the long-wavelength band-edge at the gain maximum of each dye. The dyes examined were: rhodamine 6G chloride (R6G), rhodamine B chloride (RB), pyrromethene 597 (PM597), pyrromethene 580 (PM580), and DCM. The concentrations by weight of each dye were as follows: 0.3 wt% for R6G and RB, and 1 wt% for PM597, PM580, and DCM. The laser emission wavelengths are shown in Table 1.

The rhodamine dyes, which are part of the xanthene family, were chosen for two reasons. Firstly, the xanthenes (particularly R6G) are widely used in conventional dye lasers 25 and should therefore provide a useful reference point for the comparison of desirable dye properties for conventional and chiral nematic laser systems. Secondly, the relatively isotropic molecular shape of the rhodamine dyes should prevent guest-host alignment of the dye within the liquid crystal host (26) allowing laser emission characteristics to be interpreted solely in terms of the behavior of each dye in the polar solvent. However, the rhodamines are not readily soluble in liquid crystal solvents and it was noted that only a very small percentage (~ 0.4 wt%) was required for the dyes to crystallize out of solution. Even though these dyes can be considered to be isotropic in terms of molecular shape, the lasing threshold was much lower at the long-wavelength band-edge than the short-wavelength edge for both dyes, implying a certain degree of alignment of the transition dipole moment with the liquid crystal director 27.

Laser sample (nm) RB-E49* 582 R6G-E49* 572 PM580-E49* 570 DCM-E49* 609 PM597-E49* 582

Table 1

Laser sample Slope efficiency, s (%) RB-E49* 0.1 R6G-E49* 0.8 DCM-E49* 9 PM580-E49* 21 PM597-E49* 29

Table 2

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1

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4

5

6 PM597-E49* PM580-E49* DCM-E49* R6G-E49* RB-E49*

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Excitation energy ( J/pulse)

For a concentration of 0.3 wt% dye, the excitation threshold and slope efficiency were found to be lower and higher, respectively, for R6G-E49* than RB-E49*: in this instance the cell thickness was d = 14 m because it was found that RB-E49* did not generate laser emission in d = 10 m. The slope efficiency of R6G-E49* was found to be s = 0.8% as opposed to s = 0.1% for RB-E49*. Overall, when doped into E49* both dyes had very low slope efficiencies; this is apparent from Figure11, where it can be seen that the outputs are extremely small in comparison to those from the lasers containing 1.0 wt% of the other three, more elongated, dyes (DCM, PM 580 and PM 597) in 10 m cells. This is due to several factors including weak absorption at the pump wavelength (resulting from the low solubility of the dyes) and poor alignment between the transition dipole moment of the dye and the director. The higher slope efficiency of R6G-E49* is believed to be due to a slightly better alignment of the transition dipole moment of R6G with the director arising from hydrogen bonding between the dye and the liquid crystal host (28) these results are discussed in more detail in the literature 29.

In terms of the input-output characteristics of the other three lasers, DCM-E49*, PM580-E49*, and PM597-E49*, Figure 12 shows that the emission energies vary rather significantly. The slope efficiencies for each laser are summarized in Table 2 where it can be seen, for example, that s for PM597-E49* is a factor of three greater than that of DCM-E49* when excited at = 532 nm and a repetition rate of 1 Hz. Of course, the laser wavelength of PM597-E49* is at a shorter wavelength ( = 582 nm) than that of DCM-E49* which means that the energy per photon is greater. Taking this into account, we find that the number of photons emitted by the PM597-E49* laser is still a factor of three times larger (no. of photons ~ 7 x 1012) than that of DCM-E49* (no. of photons ~ 2 x 1012) when excited with an energy of 10 J/pulse. Clearly from Fig 13(a) the Quantum Efficiency of the two pyromethene dyes are significantly higher than for DCM at a given weight concentration. It is noteworthy that the slope efficiency of PM597-E49* is s = 29% compared to s = 9% for DCM-E49*. The threshold energies for the three lasers were found to be in the range of Eth ~ 100 – 200 nJ/pulse.

Figure 12. The input-output characteristics for five chiral nematic band-edge lasers with different laser dyes but the same chiral nematic host (E49*). The operating temperature is T = 31oC.

Proc. of SPIE Vol. 7414 741402-12

(a)

Figure 13. (a) concentration f

From Tables DCM dye upyromethene cavity lengths

In this sectionPhases. Such phases are esBragg scatter32, however, t

5.1.1 Wide

It has been shpower chiral is representedwavelength ophase could b

0.00.0

0.2

0.4

0.6

0.8

1.0

Qua

ntum

eff

icie

ncy

Table 3. Excitconcentration.

Quantum Effifor DCM.

3 and 4 it is under the sam

dyes also pers, or sample th

n we will consphases have bsentially comp

ring from thesthese phases w

temperatur

hown32 that, fadditive, Blued by a body-cof light. Consbe used as a th

0.5 1.0

Concentrat

tation ThresholdExcitation puls

ficiencies for th

clear that theme illuminatiorform better ahickness, whe

5. BLU

sider, briefly abeen shown, rposed of doube disclination

were only stab

e Range Blu

for certain mue Phase I (BPcentered cubicequently, this

hree-dimensio

0 1.5

ation of dye (wt

ds (nJ/pulse) asse width 5ns an

he PM 580, PM

e pyrometheneon conditionsat lower dye cen optimising

UE PHASE

a further lasinrecently 31, to ble twist cylinlines then set

ble over a temp

ue Phase La

ulti-componenI) was found tc lattice of des results in a pnal band-edge

2.0 2.5

PM580 PM597 DCM

%)

s a function of dnd out =610nm

(b

M 597 and DC

e dyes have los and opticalconcentrationsthe LC laser p

LIQUID CR

ng mode in higgive rise to si

nder that lead tt up a 3D LC lperature range

sers

nt mixtures coto occur natur

efects, wherebphotonic bande laser by simp

1.7

1.8

1.9

2.0

2.1

2.2

2.3

Life

time

(ns)

dye Tableconcen

b)

CM dyes and (

ower thresholdl path lengths 30. This has tperformance (

RYSTAL L

ghly chiral nemmultaneous lato a net of disclaser cavity ore of a few Kel

ontaining bimerally over a te

by the spacingd gap in threeply adding a g

580 5857

8

9

0

1

2

3 0.5 wt % 1 wt % 1.5 wt % 2 wt %

W

4. Slope Efficientration. Excitat

(b) Fluorescenc

ds and higherh. It is notewto be taken in

(c.f. 4.2.2).

ASERS

matic liquid crasing in three clination linesr photonic banlvin.

esogenic comemperature rang of the cubic e-dimensions gain medium.

5 590 5

Wavelength (nm

encies as a function pulse width

ce Lifetimes a

r slope efficieworthy, genernto account w

rystals, namelorthogonal di

s that have cubnd structure. U

mpounds and ange of 30oC. Tunits is of thwhich means

595 600

m)

ction of dye h 5ns and out =

s a function o

encies than therally, that the

when designing

ly Blue rections. Bluebic symmetry

Until recently

a high twistingThe BPI phasehe order of thes that the blue

610nm

f

eeg

e.

geee

Proc. of SPIE Vol. 7414 741402-13

Figure 14 (a) BPI as a func

In Figure 14described in spectrum for optical texturplatelet from seen that lasiThe band-gapspontaneous same time dutemperature remission specin this case ththan 0.1 nm a

Achieving lasin three dimeof important occur in multovercome, thiso as to correblue-(211).

Laser emissioction of excitat

, band-edge lthe literature an excitation

re that is obsewhich laser e

ing occurs at p of this domaemission (AS

ue to the fact trange of this bctrum. The rehe spectrum ware achievable

ser emission iensions unlikefeatures that tiple directionis could be exespond with t

on from a widtion and wave

lasing from a 32 and 1.5 wfluence of 10erved when vemission occu

= 571 nm, ain is shifted wSE) peak that hat the sizes oblue phase, lasults here sho

was recorded ue indicating a h

in either BPI oe the chiral nemare not observns simultaneouxploited to prothe reflection

de temperatureelength.

dye-doped Bwt% of DCM

mJ/cm2, usinviewed betweeurs is highlighcorresponding

with respect tooccurs simult

of the individuaser emission wow that the lasusing a 1.3 nmhigh quality fa

or BPII is extmatic or chiraved for 1D phusly (31). Igno

oduce a single spectrum of

e range Figuinse

BPI laser is dwas dissolveg 5ns pulses aen the crossed

hted in the figug to the bando the gain maxtaneously at ual domains awas observed

ser emission ism-resolution sfactor of the la

tremely encoual smectic phahotonic band soring for the t

element red-gthe different

ure 14 (b) Thet shows locali

demonstrated. ed into the hoat an operatingd polarisers oure and corres-edge of the pximum of DC ~ 610 nm. A

are much less td over a 10oCs broader thanspectrometer. Iaser mode in a

uraging as thesases. Consequstructures. Ontime being thegreen-blue ligplatelet orien

hreshold curvized fluoresce

The host mixost. The figureg temperatureof an optical psponds to the photonic band

CM and this isA number of than the pumptemperature r

n that of the chIt has been sh

a single domai

se structures euently, both blne such feature technologicaght emitter by ntations, e.g. r

ve for lasing ence from a BP

xture was thee shows the l

e of T = 30oC apolarizing mic(200) orienta

d gap for the s evident from

domains are p-spot size. Drange with no hiral nematic hown 33 that liin of Blue Pha

exhibit a photlue phases exhre is that laseral obstacles thdoping in a nured-(110), gre

in BPI. TheP platelet.

e same as thalaser emissionalong with thecroscope. The

ation. It can be(200) platelet

m the amplifiedexcited at theue to the widechange in thelaser althoughne widths less

ase I.

tonic band gaphibit a numberr emission canhat need to beumber of dyeseen-(200), and

e

atneeet.deeehs

prnesd

Proc. of SPIE Vol. 7414 741402-14

Figure 15 The upper photo-micrographs demonstrate that large area homogeneous BPI phases can be formed, the reflection spectra of such ‘mono-domains’ and how the peak of the BPI band gap can be varied as a function of applied electric field for mixtures composed of symmetric and non-symmetric bimesogens

Another very important characteristic is that, if the band-gap in two of the three-directions is positioned so as to overlap the laser wavelength in the remaining direction, then spontaneous emission into those directions is prohibited. This should result in a further decrease in the threshold. For chiral nematic and chiral smectic lasers spontaneous emission into angles other than the narrow solid angle subtended by the laser mode is a loss mechanism. Control of the lattice spacing in three directions can be achieved, to some extent, with the application of an electric field across the sample. This results in an extension in the lattice spacing in one direction but a lattice contraction in the other two directions.

Before the advent of the wide-temperature blue phases, either naturally occurring (32) or stabilized with the addition of polymer 34), it would have been very difficult to envisage the usefulness of blue phase lasers beyond the academic arena. However, whilst there is still a great deal to be done, in terms of understanding why these blue phases exist over such a wide temperature range, the ability to form single large-area mono-domain textures opens up interesting technological possibilities.

400 500 600 7000

1000

2000

3000

4000

Inte

nsity

, a.u

.

, nm0 5 10 15 20

500

520

540

560

580

symmetric bimesogen (low ) non-symmetric bimesogen (higher )

, nm

Electric Field, V/ m

Proc. of SPIE Vol. 7414 741402-15

6. LIQUID CRYSTAL LASER DEVICES

In the above we have shown that liquid crystals can be optimised as the basis for coherent photonic band edge lasers. For the photonic band edge lasers we have described three different possible methods for tuning the laser wavelength; the temperature tuning, fine tuning by cell translation, and electric field tuning. The output from these coherent lasers can potentially consist of a single narrow line width mode, a near-Gaussian profile, and a high Q-factor. Moreover, the relationship of the liquid crystal parameters to the performance of the laser has also been reviewed. In short, both the excitation threshold and the slope efficiency are found to correlate strongly with the magnitudes of the optical and order parameters of the low molar mass liquid crystalline host. We will now consider how such lasers may be used as quasi-continuous working sources, 2-D arrays for projection through holograms and potentially LED pumped devices.

6.1.1 Quasi-continuous working laser operation

In our early work35 it was shown that, as the repetition rate of the pump laser increased, the laser output decreased, Figure 16(a). However, optimising the materials parameters, even with a single pass cavity, led to an increased output repetition rate of up to 1 kHz with a slope efficiency of approximately 15%. The sample here was 0.3% PM580 with an optical path length of 20 μm36. Thus we have been able to achieve quasi-continuous working with an average power of > 2mW, Figure 16(b) and output powers were doubled using a reflective cavity.

(a) (b) (c) Figure 16 shows early measurements on emission energy as a function of pulsed repetition rate35 and figures (b) and (c) show the optimised high repetition rate laser output as a function of input energy and pump power, respectively.

6.1.2 2-D Liquid Crystal Laser Arrays

In a typical band-edge LC laser, based on display technology construction, the lateral area of the sample is generally of the order of a few cm2. This is a substantially larger area than that of an LC laser, typically illuminated by the focused pump beam (5 – 100 μm spot diameters). As a result, the majority of the sample is redundant and does not contribute to the laser output. Recently37, we have shown that the active gain region can be substantially increased by using a lenslet array to create an array of 100 plus excited regions within a single LC system, Fig. 17(a). This distributes the pump energy across the entire LC cell, and as a result, a two-dimensional array of LC lasers is generated. Moreover, these individual lasers recombine to form a single output at distances greater than ~5 cm from the sample (or alternatively can be recombined using condensing optics). Such an approach has previously been applied to vertical cavity surface emitting lasers (VCSELs) to increase the output power, although for VCSELs more complicated fabrication procedures are required.

0 5 10 15 200.0

0.5

1.0

1.5

2.0

Em

issi

on E

nerg

y (

J)

Repetition Rate (Hz)0 5 1 0 1 5 2 0 2 5

0

1

2

3

4

6 0 0 H z

7 5 0 H z

emis

sion

ene

rgy

/ J

p u m p e n e rg y / J

1 0 0 0 H z

0 5 1 0 1 5 2 0 2 50 .0

0 .5

1 .0

1 .5

2 .0

2 .5

3 .0

3 0 0 H z

4 0 0 H z

5 0 0 H z6 0 0 H z

7 5 0 H z

emis

sion

pow

er /

mW

p u m p p o w e r / m W

1 0 0 0 H z

Em

issi

on E

nerg

y(μ

J)

Em

issi

on P

ower

(mW

)

Pump energy (μJ) Pump power (mW)

Proc. of SPIE Vol. 7414 741402-16

Figure 17 (a) Sand (b) an imag

Figure 18. Moarray in the fartriangle) as com

Schematic of thge taken of an o

onochromatic Lr field) (d), and mpared with the

(a) he lenslet array output laser arra

LC laser array emthe CIE1931 che liquid crystal

illuminating thay using a conc

missions (a, b &hromaticity spalaser array(oute

he chiral nematicentration gradi

(e) & c), the correspace diagram comer triangle).

ic liquid crystalient induced pit

ponding RGB emparing the col

(b) l laser to give atch across the L

emission spectrlour gamut from

an output array oLC laser cell.

a (taken by recom a CRT display

of lasing dots

ombining the y (inner

Proc. of SPIE Vol. 7414 741402-17

Current studipump sourceefficiencies, lsingle chiral across the cebeam. As a rein order to recombining grlaser array coachieved withhas recently Fibonacci seqarray-based pemissions whfabrication pr

In Figure 18,laser cell basemission at columns of semiconductoIn the case ooutputs in threferred to sinin the far fiemicroscopic pprojection thr

Figure 19(a) Fo632.8 nm.

es of LC lasere. The short plarge peak ponematic (N*Lll. A broad baesult, LC lasealize the full pradient pitch Lonsisting of rehout using cobeen demonsquence. Usingpumping technhen photo-pumrocedures.

, the laser emsed geometryBlue (470 nmred-green-blu

ors and , potenof a monochrohe far field tongle spot outpeld using conprojection lasrough phase b

ootprint of the A

rs are typicallypulses (ps or owers can be oLC) sample cand of lasing

ers are an attrapotential of thLC samples wed, green, andomplex fabrictrated40, 41 usi

g an alternativniques, it is pmped at a sing

mission is show. For these ce

m), Green (53ue output chntially, proviomatic LC laso give high avputs. Thus witndensing optier prototype,

based compute

A5 sized LC la

y conducted uns) ensure re

obtained. In acan be obtaine

wavelengths active alternathese lasers simwith array-basd blue emissioation proceduing the innovave approach42,possible to achgle fixed pum

wn from indivells, 10 micro

34), Red (617hannels. This ides a simple ser, one couldverage outputth the 2-D arraics. Based onFigure 19(a)

er generated h

ser projection s

using Q-switcheduced speckaddition it hased by using mare then obta

tive as the fulmultaneous R-sed pumping

ons when photures. Indeed, Rative approac we have showhieve a multi-

mp wavelength

vidual 10 μm on path length nm) laser waapproach of

route to fabrid, of course, t powers. Theays there is pon the conceptwith a footpriolograms, Fig

system, with (b)

hed solid-statekle, low threshs been shownmore than oneainable by tranll color emitte-G-B lasing frtechniques, itto-pumped at R-G-B lasingh of stackingwn that by co-colored laserh, Fig. 17(b).

thick laser ch cells were avelengths. Tffers a facileication of liquscan an arraye average outotential to prodts, described int of less tha

gure 19(b).

) and (c) hologr

e lasers, such hold energy a

n38, 39 that a rae laser dye annslating the s

er in a laser prrom a single pt is possible ta single fixed from a liquid

g single-pitcheombining gradr array consistThis is achiev

ells arranged used, photo-p

The array conse alternative uid crystal lasey of monochrotput power ofduce much higabove, it has

an A5. This de

(a)

rams projected

as an Nd:YAand, due to t

ange of wavelnd forming a ample relativrojection disppump source ito achieve a md pump waveld crystal defeed polymer fidient pitch LCting of red, grved without u

in RGB strippumped at 43sisted of threeto laser arra

ers and projecomatic outputf > 2.5mW, qgher average s been possibevice has been

at wavelengths

G laser, as thethe high slopelengths from apitch gradiene to the pumplay. Howevers required. By

multi-colouredlength. This isect-mode laserlms to form a

C samples withreen, and blueusing complex

pes, in one LC30 nm to givee well-definedays based onction displayst and sum thequoted aboveoutput powersble to build an designed for

of 532 and

eea

ntpr,ydsrahex

Cedns.ee,sar

Proc. of SPIE Vol. 7414 741402-18

6.1.3 LED p

With optimizemission migsources. In FFigure 21, sh

Figure 21. Pho

The aim of thchiral nematiwide-temperalaser varies apump-spot sithreshold app

Figure 20 L

pumped stim

zation of the ght be achieveFigure 20 we ows photogra

otographs of the

his review wac band-edge lature blue phaas parametersizes the threshpears to be low

LED pumped ex

4000

500

1000

Inpu

t Int

ensi

ty (a

rb. u

nits

)mulated emi

materials pared by replacinshow the evid

aphs of the out

e LED stimulate

s to present a lasers and alsoases. In this os such as the hold energy iwer for larger

xcitation of chir

500 600

Wavelength

Green LED

ission throug

7. C

ameters, caving the relativdence for stimtput emission.

ed emission from

7

summary of ro to consider overview we hpump-spot sis found to ber spot sizes. I

ral nematic liqu

700 8000

1

2

3

h (nm)

LC Emission

gh liquid cry

CONCLUSIO

ity design etcvely low energmulated emiss

m optimized ch

. SUMMAR

recent experimbriefly recenthave reportedize, pump repe higher than t was found t

uid crystal cavit

50

100

Inpu

t int

ensi

ty (a

rb. u

nits

)

Out

put I

nten

sity

(arb

. uni

ts)

ystal photon

ONS

, as discussedgy pulsed lassion using op

hiral nematic liq

RY

mental results t advancemend how the perpetition rate,that for smal

that for a com

ties.

400 5000

0

0

Wavele

Blue LED

nic band gap

d above, it mer supply by timized LC la

quid crystal sam

regarding thets with regard

rformance of aand cell thickll spot sizes,

mbination of h

600 700

ength (nm)

D LCemission

p cavities.

might be expechigh intensityaser cavities

mples. (unpublis

e emission chads to band-edga chiral nemakness are varalthough the

high excitation

8000

5

10

15

Out

put I

nten

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(arb

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ts)

n

cted that lasery pulsed LEDand materials

shed).

aracteristics oge lasing fromatic band-edgeried. For largefluence at the

n energies and

rDs.

fmeeed

Proc. of SPIE Vol. 7414 741402-19

high repetition rates the emission energy is significantly reduced compared to the emission energies obtained for low repetition rates. The dependence of the threshold energy (Eth) on cell thickness (d) follows the relationship Eth d + fn (1/d2) in accord with previous observations. In terms of the dependence of the slope efficiency ( s) on cell thickness, this was found to be best described by considering that s 1/Eth. In addition, a number of different laser dyes were considered and the results show that, for pumping at = 532 nm, pyromethene laser dyes were preferred. We have also reported on band-edge lasing in a naturally occurring wide-temperature Blue Phase I. With such high slope efficiencies, low threshold energies and narrow line width, but with laser outputs tunable throughout the visible spectrum, the recent research in this field is beginning to lead to new miniature light sources suitable for novel projection and display applications. Finally we have discussed such lasers emphasizing that the technology is based on well established liquid crystalline materials and LCD fabrication techniques thus simplifying the route to practical applications.

ACKNOWLEDGEMENTS

We gratefully acknowledge the financial support of the Engineering and Physical Science Research Council, EPSRC (UK), through a Basic Technology Research Grant COSMOS (EP/D04894 X/1), that has enabled this work to be carried out.

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