This is the title page

Contents

I Beginning/Introduction 5

1 Introduction 6

2 Optical Notch Filters 7

3 Setup 10

II EuCl3·6H2O 15

4 EuCl3·6H2O 164.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.2 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184.4 Further Research . . . . . . . . . . . . . . . . . . . . . . . . . 26

III Eu3+:PrCl3·7H2O 28

5 Eu3+:PrCl3·7H2O 295.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.2 Linewidth Measurements . . . . . . . . . . . . . . . . . . . . . 29

5.2.1 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.2.2 results . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

5.3 Optical Depth Measurements . . . . . . . . . . . . . . . . . . 305.3.1 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . 305.3.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

5.4 Further Research . . . . . . . . . . . . . . . . . . . . . . . . . 32

1

IV The End 33

6 End 34

2

List of Figures

2.1 Properties of a notch filter. . . . . . . . . . . . . . . . . . . . . 8

3.1 The Crystal Cell . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.1 Structure of EuCl3·6H2O . . . . . . . . . . . . . . . . . . . . 174.2 Transmission spectrum at 0 deg, perpendicular to the crystal

surface and parallel to the crystal b axis. . . . . . . . . . . . . 204.3 Transmission spectrum at 2.8 deg. . . . . . . . . . . . . . . . . 214.4 Transmission spectrum at 5.6 deg. . . . . . . . . . . . . . . . . 224.5 Transmission spectrum at 7.6 deg. . . . . . . . . . . . . . . . . 234.6 Optical Depth and Polarisation in EuCl3·6H2O . . . . . . . . 24

5.1 Photograph of the flourescence seen from the Eu3+:PrCl3·7H2Ocrystal. This is taken on the “output” side of the crystal; thereis 2.3 mW of laser beam striking the other side. . . . . . . . . 31

3

List of Tables

4.1 Fit coefficients for polarisation dependence. . . . . . . . . . . . 25

4

Part I

Beginning/Introduction

5

Chapter 1

Introduction

This is the introduction: it will give a synopsis of what is to come.

6

Chapter 2

Optical Notch Filters

Optical notch filters, as their name suggests, have “notch”, or bandstoprange, in their optical transmission spectrum. The optical range is commonlyconsidered to be light of wavelengths ca. 300 nm to ca. 650 nm. For ourpurposes this will be extended into the infra-red, up to a wavelength of ca.1200 nm, as these longer wavelengths are often used for spectrographic andother scientific purposes.

Notch Filters The wavelength of light can equivalently measured in threedifferent units: frequency (Hz), wavelength(m) and wavenumber (cm−1).Frequency (f) and wavelength (λ) are familiar units, and have the well-known relation:

c = f · λ (2.1)

Wavenumbers are a measure of particular use in spectroscopy. They aretraditionally measured in units of reciprocal centimetres. This gives therelation of:

w(cm−1) =107

λ(nm)(2.2)

It is also possible to measure the wavelength of light in terms of the energyof the associated photons: in this case, the we have

E = hf (2.3)

Properties of Notch Filters There are three important blocking proper-ties to these filters (four, if the wavelength of the notch is considered.) These

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Figure 2.1: Properties of a notch filter.

are the optical depth, the linewidth and the edgewidth. The relationshipbetween these is shown in figure 2.1.

The optical depth is the property of how much light is blocked, generallyas a ratio of transmitted power over input power. This makes the transmis-

sion a unit-less measure, from 0 to 1. The transmission is also measured asa percentage. A useful descriptor when dealing with highly blocking filters— where the transmission is very low — is the Optical Density, or OD. Thisis defined as the negative common logarithm of the transmission:

OD = − log10 (T ) (2.4)

This is also convenient for use when combining optical filters, as th ODis additive whereas the transmission is multiplicative.

The linewidth, or spectral bandwidth, of an optical filter can be measurein a number of ways. Here, we define the linewidth to be the distance betweenthe OD 0.5 points. This is close to being the T = 0.5 transmission point,and in many cases is taken to be the same.

The edgewidth of an optical filter is the rate at which the transmissiondecreases. There are many ways to define this parameter. There are sourceswhich define this to be the distance between the OD 0.3 and OD 4 points[8].This is a methodology which has a decided flaw, in that for high optical

8

depths it may not be an accurate representation of the edge. However, for ourpurposes this was found to be at least adequate, and so is the measurementused.

Types of Optical Notch Filters There are two broad types of opticalfilters available today: absorption filters, and interference filters.

Absorption filters are generally glass doped with a coloured substance,which absorbs some light while letting other light pass. The substances areoften organic in nature. These are generally used where precision is not overlyimportant. Particularly low quality examples can be made from plastics, suchas acrylic or polycarbonate. Often used by small children.

Interference filters work on the basis of creating an electric field that isequal-but-opposite electric field to that which is exciting it. These fields can-cel, and result in a line is formed in the transmission spectrum. Interferencefilters are generally dichroic, and reflective.

There are a number of methods of making interference filters. Dielectricinterference filters have a number of layers of dielectric materials spaced atspecific 1

4λ and 1

2λ thicknesses, where λ is the desired filter frequency[10].

Current dielectric filters for the 300 nm – 1200 nm range have linewidths of10nm (Semrock) or < 350cm−1 (Kaiser), edgewidths of 3nm (Semrock) or150cm−1 (Kaiser) and maximum optical density of ¿6 in both cases. Semrock:[15]; Kaiser:[8].

A variation on this is the “holographic” filter, where the construction isnot by depositing a number of layers, but by holographically recording aninterference pattern into a gelatinous medium[8]. Similar standards are metby this type.

A relatively recent type of filter is the Guided Mode Resonant filter. Thisworks on the properties of resonance in wave-guides[14]. These have thepotential to be have finer properties than the dielectric and holographic filtersabove, but are not made on a commercial scale.

All of these methods have a number of problems, relating to construc-tion and longevity. Dielectric filters rely on the coating remaining unscathedthroughout usage - this puts constraints on the temperatures and atmo-spheres where these can be used[17][16]. Some of these concerns have beenrelieved with the newer “hard” coating dielectric filters, but there are stillproblems with variations in the methods used in making these types[12].

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Chapter 3

Setup and CommonComponents of Experiment

In this chapter, the common components of the setup used for investigatingboth the EuCl3·6H2O and Eu3+:PrCl3·7H2O crystals will be detailed.

Laser The laser used to excite the crystals is a Coherent 699 Ring dyelaser. This has a frequency stability of 20 MHz.

The construction of this laser has four major adjustments to the frequencyof the light emitted. In order of decreasing magnitude of adjustment, theseare the bi-refringent filter (BRF), the intra-cavity apparatus (ICA) align-ment, the thin etalon offset, and the galvanometer scan. These representdecreasing orders of magnitude from THz to GHz in the order written..

The BRF is adjusted manually, by means of a micrometer. This enablesa coarse adjustment of the cavity length, and is the first adjustment whenchanging the frequency of the laser.

Thee ICA adjustment is also manual, by means of a thumbscrew. Thisprovides the second “level” of adjustment, within a range of ca. 0.4 THz.

The thin etalon offset is the third “level” of adjustment, within a rangeof ca. 80 GHz. This is electrically controlled, by means of a manual poten-tiometer on the control panel of the laser.

The galvanometer scan is the finest control of the set. This has controlover a range of up to 30 GHZ, by means of a potentiometer on the controlpanel. There is also an external control, which enables an outside voltagefrom −5–5 V to be supplied, which enables outside control of the laser scan.On a good day the galvanometer can be scanned across the full range of30 GHz, although in normal use it usual for this range to be closer to 10–12 GHz. The resolution of this depends on the resolution of the supplied

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voltage source (or on the deftness of hand of the operator) and is variable tothe limit of the laser, 20 MHz.

The dyes used were Rhodamine 6G and Sulforhodamine. A mixture ofthese enabled the frequency to be varied across the range of 560–590 nm,and a different mixture, after the dye circulator died and was replaced, gavea range of 580–610 nm.

The Dye laser is possible of putting out up to 300 mW, on a good day.Powers of ca. 150-200 mW are more common. However, the power outputvaries by up to 10%. This problematic, if ones measurements are to consistof power spectra. Therefore two methods were used to account for this.

The first was to take a reference signal, from before the beam reachedthe crystal, and divide the signal from after the crystal by this referencesignal. This method was satisfactory, but proved to be of lower resolutionthan was possible from the second method. The second method was toactively power stabilise the laser output. In order to control the power of thelaser, a PI intensity stabiliser was used in the later parts of the experiment.This used an Acousto-Optic Modulator (AOM) to control the intensity ofthe laser on the sample. The AOM was driven at 80 MHz, which resulted ina first-order sideband being generated at an angle of about 5 deg from thezeroth-order/primary beam. By adjusting the RF power to the AOM, it ispossible to vary the power of the first-order beam, which was used to excitethe crystal. To control the power to the AOM, a PI control system was used,with the same silicon photodiode used as a reference signal. This systemresulted in a power output which was stable to within 0.3%.

Wavemeter In order to determine the frequency of the laser, a HighfinesseGmBH WS7 high-speed fibre coupled wavemeter was employed. This usesinterference to indirectly measure the frequency of the incident light, and hasa frequency resolution of up to 40 MHz, over a range of 320 nm – 1120 nm.This was more than sufficient for all of our purposes, and proved to be ofgreat convenience.

Cryostat and Cell To obtain the narrow linewidths desired from our crys-tals, they had to be cooled to a temperature of a few kelvin to tens of kelvin.To do this, a two stage heluim-condenser cryostat was used. This used a Cry-omech, Inc., cold head and compressor mounted in a commercially availablebody, with the cold head capable of reaching 2.7 K. Furthermore, this par-ticular model was supplied with a Lakeshore temperature controller, whichcombined with a heater element attached to the cold head enabled PID con-trol of the temeprature of the head from 2.7 K to 70 K. Details of this will

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Figure 3.1: The cell which was used to contain the crystals in a heliumatmosphere.

be mentioned later. (See page ??.)The crystals used were very hydroscopic[1], and could not be left under

a vacuum at room temperature for more than ca. five hours before dryingout and becoming useless for our purposes. Because the cryostat relies on avacuum for insulation, which takes at the least 10 or so hours to achieve, thesecrystals were not able to simple be inserted into the chamber of the cryostat.Hence, a cell to contain these crystals in a helium atmosphere was designedand manufactured by the Physics Department Mechanical Workshop. Thisis shown in figure 3.1. In short, it consists of a block of aluminium machinedto have a number of flat surfaces, with a hole of ca. 18 mm drilled through.On each side of this hole, a NKB-7 anti-reflection coated glass window wasmounted, with an indium-wire seal preventing the helium from escaping.

To hold the crystals, a sample holder was made. This is a segment of acylinder, made to fit within the hole in the block. On the flat face of this, agroove was cut, and a pair of holes tapped for screws. Using a bridge, also ofaluminium, held with nylon screws, the sample was secured. Nylon screwswere used in order to reduce the risk of damage to the sample: it is intended

12

that the screws should break before the sample crystals. This is most likelyto happen as the cell is cooled and re-heated.

In order to prevent stray light from going around the crystal, a shieldwas made. This consists of a washer of blackened aluminium with a segmentremoved, cut to fill the remainder of the circle left after the sample holderis in place. In the centre of this a small aperture was made, proportional tothe size of the crystal sample, so that the sample could be illuminated.

To fill the cell with Helium, an improvised glove bag was utilised. Thisconsisted of a large clear plastic bag (a former bread bag, supplied by thekitchens at Knox College, Dunedin), which was taped to the arms of the op-erator (the requisite tools and supplies being already enclosed) and filled withhelium “party gas”, a low grade helium intended for filling party balloons.This low grade was permissible, as at the temperatures we were dealing with,any impurities would liquefy and sit in the bottom of the cell.

Three of these cells were built; the first contains a EuCl3·6H2O crystal,the second a PrCl3·7H2O crystal, and the third is kept as a spare, in thehope that more measurements could be made. The EuCl3·6H2O cell wasassembled first; this was then tested by Mr. Richard Easingwood at theOtago Centre for Electron Microscopy for helium leaks. The cell was cooledto 100 K, under a maximum vacuum of 2.3 × 10−7 mbar, and the rate ofescape of helium measured with a Ultratest UL 100 PLUS detector, attachedto a BALTEC freeze-fracture machine. The freeze-fracture machine was usedfor the sole reason that it had a chamber big enough to fit our cell. The rateof escape was measured to be 2.8×10−8 mbar · ℓ · s−1, and remained constantas the cell was cooled and reheated. Compared to a background reading of9.0×10−10 mbar · ℓ · s−1 it was decided that this represented helium diffusingthrough the glass windows, and showed that there was still helium in the cell.

Light Detectors Two type of detectors were used to measure light here:the silicon photo-diode, and the photomultiplier tube.

Two types of silicon photodiode were used: the first a Thorlabs PD155A,and the second a home-built fast photodiode, which has better response, atthe cost of having a smaller detector area. The vast majority of measurementswere made with these devices, as they have very good resolution and arerather hard to break.

Photomultipliers use the photo-electric effect to give a very low noisesignal, and are excellent for measuring very small signals, such as were foundat the maximum absorption of our crystals. However, being sensitive has thedownside of being rather delicate to overloading, which causes the tetrodesto burn out, destroying the photomultiplier. Hence, this was only used for a

13

very few measurements where the properties of it rendered it desirable.An EMI 9781 RA 11-stage photomultiplier was used, with the cathode

held at −700 V by a stable high-voltage supply. A photomultiplier is a con-stant current source, whereas the vast majority of devices are only able tomeasure voltages. This easily accommodated by appropriate termination ofthe line from the photomultiplier. In our case, termination of 100 MΩ wasused, across which the signal voltage was measured. This enabled accuratemeasurement of the steady-state power of light, which is what we were in-terested in.

A photomultiplier, like a photodiode, only measures the broad-band in-tensity of the light incident. In order to determine if the light measured wasat the laser line, or from other sources, the photomultiplier was mounted to aSpex “Micromate” diffraction-grating monochromator. This served two pur-poses: firstly by providing a light-proof mounting, any stray room light to thephotomultiplier was eliminated, removing some of the risk of overloading thephotomultiplier, and secondly, it enabled the measurement of the frequencyof any fluorescence found.

Lock-In Detection Aiming to measure the quality of high absorption fil-ters means that there will be some very low-level signals on the light detec-tors. These small signals have a sizeable component of noise, which must beminimised. In order to remove as of this noise as possible, a method knownas Lock-In Detection was used. It is known that the majority of the noise ina signal is in the DC component, and falls off as 1

fwith increasing frequency

f . Lock-In detection makes use of this fact, modulating a slowly varyingsignal with a known reference function, which effectively results in movingthe low-frequency (often DC) signal higher in frequency space. This higherfrequency can then be band-passed, which removes most of the noise andleaves a signal at a known frequency that can be shifted back to DC, andmeasured.

We used a Stanford Research Systems optical chopper to square-wavemodulate the signal from the laser before it reached the cell and crystal, anda SRS SR-830 Lock-In amplifier to measure the resultant signal.

In order to further remove electrical noise, a SRS low-noise pre-amp wasutilised, between the detector and the Lock-In. This meant that the resolu-tion of the lock-in was further increased by up to 100-fold, to enable signalsof order nV to be measured where the noise (from room lights and othersources) was of order 10 nV. (VERIFY THIS)

14

Part II

EuCl3·6H2O

15

Chapter 4

EuCl3·6H2O

Initially, we measured an EuCl3·6H2O crystal. This proved to have a verynarrow line-width, but due to a misunderstanding, gave less than desiredabsorption.

4.1 Background

EuCl3·6H2O has a monoclinic crystal system, and has space group P2/n [4].The monoclinic crystal system means that the crystal in optical biaxial[5].A diagram of this structure is given in figure 4.1. This, in short, means thatthere are two colour-dependent axes and one unique optical axis. In the caseof EuCl3·6H2O , this one axis is the C2 or crystal b axis, and is coaxial withthe axis of the dipole moment of the 4F0 →

5D0 transition, which you willrecall is the transition we are using.

According to the literature, the 4F0 →5D0 transition is at around 580 nm,

or 517 THz [2, 11]The linewidth of this specimen is so narrow for a number of reasons;

primary amongst these is because there is very little crystal strain. Tra-ditionally, the rare earths have been used as dopants, often in crystals ofother rare earth compounds — for example, the (relatively) widespread useof Y2SiO5 . The rare earths all have similar atomic sizes, which means thatthe presence of dopants cause a minimum of disturbance to the crystal lattice.However, there are still differences between the different elements, and theaddition of a dopant does cause a disturbance to the structure of the crystals.In EuCl3·6H2O , however, our atom of interest is present in stoichiometricamounts. This means that the crystal lattice is under as undistorted asis possible. This means that each of the centres experience a very similarenvironment, which leads to very little inhomogeneous broadening.

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Figure 4.1: Structure of EuCl3·6H2O , viewed long the b axis. This is thestructure that the beam encounters. The numbers represent the relativeheights of the individual atoms in hundredths of the b parameter. The Hatoms are not shown here; each O is to be read as an H2O, and in thisdiagram the Eu3+ions are represented by Ln . From [3].

17

4.2 Specific Setup for this case

The optical set-up was simple, as we did not progress past making prelim-inary measurements: a laser beam going through the sample onto a siliconphotodiode, which was connected to an oscilloscope. At the start of eachday, and whenever the laser was adjusted, the scan was set to four posi-tions, the frequency and galvanometer scan voltage at each measured — bythe wavemeter and the scope respectively — and a linear fit made of these.Thus, by simultaneous measurement of the scan voltage and the power onthe detector, it was possible to create an absorption spectrum.

4.3 Results

A number of spectra were measured. These showed a very narrow line-width:it is measured to be ca. 0.56 GHz. This is substantially narrower than anyfilter available commercially, which is a promising sign. However, it wasalso immediately apparent that the power of the beam — instead of beingattenuated by a factor of 106 or so,which is OD 6—was only being attenuatedby a factor of 7

8, or an optical density of OD 0.05. This is far from what was

expected. After much consternation and confusion, and consultation withthe suppliers of the crystals, it ws found that due to a poor label and amisunderstanding, the crystal used was no the cut one that was expected,but an uncut one that was also supplied. This uncut crystal was so alignedthat the C2, or crystal b axis, was aligned with the laser. This has the lowestabsorption, and so our results follow.

A number of other measurements were made, in order to characterise asmany of the properties of the crystal as possible, whilst it was at ca. 5 K.This temperature was not directly measured; at this time there were notspare temperature sensing diodes at hand. However, when measuring theEu3+:PrCl3·7H2O crystal, a temperature diode was attached to the cell, andthis read a temperature of 4.3 K as a resting minumum, rising to ca. 5 Kwhen measurements were made. (See page 29 for the details of this.) Sincethese two sells were effectively identical (excepting a slight change in designof a retaining ring for the windows), and temperature sensor on the cold headread 2.7 ± 0.1 K in both cases, it was safely assumed that this was true. Inany case, the temperature of the crystal was below 10 K throughout thesemeasures.

These further measures consisted of adjusting the polarisation and angleat which the beam struck the crystal.

By the nature of its construction, the dye laser emits a strongly vertically

18

polarised beam. Ideally, to rotate this, first the beam was put through avertical polariser, to ensure that it was indeed vertically polarised. It wasthen put through a quarter-wave plate, to make it circularly polarised, andthen through another linear polariser to pick the desired linear polarisation.

In practice, though, the combination of these three optics meant that thesignal was too weak to measure. Hence a much simpler, even if not ideal,method was used: the beam was simply put into a linear polariser, which wasrotated. This caused a significant variation in the power onto the crystal, andso the conclusions drawn here are simple from comparing relative amplitudes.

Initially, the laser struck the crystal approximately perpendicular to theflat surface. To adjust this, the input mirror was mounted onto a horizontaltranslation stage, which was moved to a number of positions in the horizontalplane. These positions were limited by the dimensions of the windows intothe cryostat: a maximum angle of ∼ 7.6 deg was possible.

The light output from the crystal was somewhat scattered. To measurethe light output, a lens was placed close to the appropriate window on thecryostat to capture as much of the light as possible. This could be improvedby the use of an imaging system, as used for the Eu3+:PrCl3·7H2O experi-ment.

To make the measurements, the position of the input mirror was adjusted,the beam was aligned to the hole in the sample washer, and the amplitude ofthe signal was recorded as the laser scanned. This was saved to the PC, theinput polariser rotated to the next desired setting, and the process repeated.The polarisations were measured between angles of 0 deg and 150 deg, inintervals of about 30 deg, and the input angle varied from perpendicular to7.6 deg in four steps.

A number of spectra were recorded, as shown in figures 4.2, 4.3, 4.4 and4.5.

Immediately it is seen that the line here has a definite shape, and seemsto consist of two or perhaps three peaks. This is believed to be the resultof isotope shifts, as the Eu3+has six H2O and two Cl− ions surrounding it.Each of these ligands has multiple natural isotopes, which can cause increasedinhomogeneous broadening. Furthermore, the Eu3+ion itself has two naturalisotopes: Eu-151 and Eu-153. The combination of these factors leads to thecomplicated structure of the line[2].

It is indicative of the low quality of our signal, resulting from the clumsypreliminary setup that the poorly resolved peaks —believed to be causedby the presence of the O-18 isotope — seen at frequencies ∼ −450 MHzto ∼ −1300 MHz by Ahlefeldt et al. [2] are invisible in our measurements.These peaks are small because the natural abundance of 0-18 is low, at ca.0.2% by weight[6].

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Figure 4.2: Transmission spectrum at 0 deg, perpendicular to the crystalsurface and parallel to the crystal b axis.

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Figure 4.3: Transmission spectrum at 2.8 deg.

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Figure 4.4: Transmission spectrum at 5.6 deg.

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Figure 4.5: Transmission spectrum at 7.6 deg.

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Figure 4.6: The relationship between optical depth and polarisation. Thepoints record the measured data points, and the lines are generated fromequation 4.1, using the coefficients given in table 4.1. The vertical dashedlines at 19 deg and 113 deg give the approximate locations of the crystal aand c axes respectively.

To determine the effect of polarisation on the optical depth of the crystal,the transmission at each horizontal angle was plotted against the polarisation.This is shown in figure 4.6.

The measured absorptions were fitted an expression for a sinusoid, of theform:

T = A sin (Bθ − C) +D (4.1)

where A,B, C and D are coefficients, T is the transmission and θ id theangle of polarisation (measured in degrees from vertical). The coefficientswere determined by the method of least squares, using an in-built MatLabroutine. These are given in table 4.1.

The definition of vertical here is more or less arbitrary, and was deter-mined by the way the crystal and the mounting fit within the cell. Thiscan be seen in figure 3.1. The grey dashed lines represent the approximate

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A B C D Norm of Residuals0 1st 0.0515 0.9832 32.5268 0.1329 74.0× 10−4

2nd 0.0550 1.6957 70.7614 0.2918 24.0× 10−4

2.8 1st 0.1586 1.9873 246.6061 0.2280 4.90× 10−4

2nd 0.1958 1.9578 246.8617 0.3321 14.0× 10−4

5.9 1st 0.4807 1.8458 232.7267 0.4653 450.0× 10−4

2nd 0.4521 1.9292 234.5977 0.4653 433.0× 10−4

7.6 1st 0.4155 1.9515 245.9127 0.4412 5.659× 10−6

2nd 0.4444 1.9629 246.0508 0.4795 6.010× 10−6

Table 4.1: Fit coefficients for polarisation dependence.

placements of the a and c crystal axes. These were estimated from a photo-graph of the cell taken soon after it was in the cryostat, and from a diagramsupplied by the ANU[1]. The angle between the a and c axes in EuCl3·6H2Ohas been measured as 94 deg [3]. Thus, from our photograph, the angle ofthe a axis was measured at 19 deg, and so the c axis is at 113 deg. Both ofthese are approximate; these were made somewhat after the fact, and relyon a certain amount of assumption regarding the precision of the cryostatmanufacture.

I suspect,however, that these are approximately 10 deg too low in thecase of both the minima and maxima as it can quite clearly be seen thatthe minima of the fitted curves are at an average of 31 deg, compared to thevisually sighted 19 deg. For the maxima, this discrepancy is worse still: themaxima of the fits is at an average 140 deg, whereas the “sighted” angle isapproximately 113 deg, which is a difference of about 25 deg. This could bethe result of poor perspective in the photograph; it is also possible that theaxes have no bearing on the transmission properties. Further research couldbe done in this, I believe.

As can be seen, from figure for a beam perpendicular to the surface ofthe crystal (that is to say, parallel to the crystal b or C2 axis), the polar-isation is approximately independent of the polarisation. As the “dipoles”of the Eu3+centres are aligned with the beam, this is hardly surprising: thebeam experiences a similar crystal structure regardless of the polarisation itencounters.

As the horizontal angle is increased, however, the dependence on polari-sation increases. The data collected is only of use to show that this is true;it would be fallacious too much into this, as the maximum angle here is only7.6 deg. This does raise the possibility that when aligned along the a or cangles the crystal transmission will be very polarisation dependent. In terms

25

of optical filters, this could be a great inconvenience, or a great convenience.In any case, further work is needed to measure these properties of the crystal.

It was intended to continue measuring this material; however, before thiscould be done we came into possession of a set of Eu3+:PrCl3·7H2O crystals.In terms of optical filters these were thought to be of perhaps greater utility,and so measurements commenced on these. Unfortunately, there was not fur-ther time to return to the EuCl3·6H2O , and so a great deal of measuremetnsthat could be made are at this point uncompleted.

4.4 Further Research

There is a great deal of further research that could be done on this crystal,in order to further characterise the optical properties of it.

The first measurement to make could be to rotate the crystal in the cell.This would be done two-fold, with both the crystal a and then c axes alignedalong the laser beam. Doing this would allow an accurate characterisationof the maximum absorption of EuCl3·6H2O to be made. This is a majorparameter for the utility of EuCl3·6H2O as an optical filter, and unless thecrystals have an optical depth of greater than OD 6 there is little practicalreason to continue — barring of course the gain of knowledge for the sakeof knowledge. This could be achieved using an apparatus similar to the onemade for the

This notwithstanding, for suitably small signals, even the poor attenua-tion we have seen could be of use, when it removes such a narrow line-width.To characterise the filtering properties of this, a side-band signal could beintroduced to the laser beam from at varying frequencies from (say) 100 MHzto 1 GHz. This would allow the usefulness of the crystal for the transmissionof data close to the laser line to be investigated. This could be done with theuse of an Electro-Optic Modulator (EOM): indeed, as an early part of thisexperiment involved the measurement of the modulation power of a 800 MHzEOM in anticipation of doing this.

Another property to measure, which was not considered initially, is theamount (if any) of fluorescence that might occur. Fluorescence is the resultof radiative decay from the excited states to the ground state, or to anotherstate. This can results in the transmission of light at both the same anddifferent frequencies than the exciting laser. If present in our measurements,would be so insignificant as to be unseen. However, at high absorptions,this can be a major limit on the optical depth of the crystal. This can bemeasured and characterised by measuring the spectrum of the transmittedlight; this was done as part of the measurements for Eu3+:PrCl3·7H2O which

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follow.

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Part III

Eu3+:PrCl3·7H2O

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Chapter 5

Experiments and Results withEu3+:PrCl3·7H2O

This details the experiments and measurements made with Eu3+:PrCl3·7H2O.

5.1 Background

Eu3+:PrCl3·7H2O was chosen because it was hoped that there would be verylittle radiative decay to “contaminate” our filter effect, combined with anabsurdly high absorption.

Body of literature: -PrCl3·7H2O has a well know spectrum, and has beenmeasured many times. -Eu3+:PrCl3·7H2O has not been so well characterised.-Eu3+has a set of very well known energy levels, but these have been mea-sured in other hosts. Not cthis one. - Should have a fairly narrow line: bothof the Rare earths here do. - Maybe crystal strain affects this?

5.2 Linewidth Measurements

5.2.1 Setup

In this case, the cell was modified slightly from the initial design, This in-volved mounting a temperature diode onto it, so that an accurate indicationof the temperature of the cell could be made. This temperature diode wasthen wired to the temperature controller, which was configured to read thetemperature at the cell. This enabled a a study of the effects of temperatureon the linewidth to be made.

::ALSO:: - From ANU again; -Intensity Stabiliser - 2.31 mW???CHECK???

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5.2.2 results

5.3 Optical Depth Measurements

5.3.1 Setup

In order to measure the optical depth, it was necessary to alter the experi-mental setup. The cell in the cryostat was returned to a low temperature.In this case, the amount of time available dictated that it be a mere ∼ 10 K,and that no temperature study take place.

Also, in order to improve the accuracy of the measurements, the Si pho-todiode was replaced with a photomultiplier tube and monochromator, as de-scribed above. in section \refsec:lightdetectors. Can’t get this to work.

5.3.2 Results

Initial measurements consisted of comparing the absorption at the centreof the line with a known optical depth. To do this, the laser was movedoff of the line of the crystal, and a number of neutral density (ND) filtersput in the beam. By measuring the voltage across the PMT, it was hopedthat a reference could be generated. Unfortunately, this was less successfulthan expected, as the reference signal changed significantly depending on theprecise alignment of the ND filters. It is believed that this is a result of anetalon effect, resulting from reflections between the discrete ND filters. Thiscould be remedied by altering the design, or using continuous ND filters, buttime prohibited this. The data from this is presented below for completeness.

The laser was tuned well off the line of interest (by 500 GHz), and a stackof ND filters totalling OD 10.5 inserted. The monochromator was adjustedto the wavelength of the laser, and to a slit width of 4 nm. This gave ameasurable signal out of the PMT, but did not overload it By the nature ofdiffraction monochromators, this gave the limit on the frequency resolutionof the signal. This was decreased in OD 0.5 steps until a signal was measured,at OD 6.5. This means the limit on the maximum resolvable optical depthhere was be between OD 6.5 and OD 7.0, for this specific setup.

With the laser returned to the line and the ND filters removed, the signalwas again measured. This gave us an optical density of OD 6.66 ± 0.25, onthe line. This was greater than the measurements made by the photodiode,which suggested that there were other factors to account for.

A visual inspection of the crystal under illumination showed that therewas a green light being emitted from the crystal, which could explain the lowoptical depth. A photograph of this is shown in figure 5.1.

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Filler P

ictu

re:

See photo downloads.

Figure 5.1: Photograph of the flourescence seen from the Eu3+:PrCl3·7H2Ocrystal. This is taken on the “output” side of the crystal; there is 2.3 mW oflaser beam striking the other side.

The monochromator was then scanned from 312 nm to 919 nm to lookfor this fluorescence. There were two wavelengths where this was found: at586± 2 nm, and at 608± 3 nm. Compared to the power transmitted at theline centre, these had relative powers of 127 times and 4.3 times respectively.

The fluorescence at 608 nm is easily attributable to the 5D2 ←4F6 tran-

sition of Eu3+,which has been measured at be 609.8 nm[?] in a LaCl3 host.La and Pr are sumilar here: GET CRYSTAL DATA TO CONFIRM SIMI-

LARITY OF PR AND LA STRUCTURE IN LnCl3.The other transition is of more interest. In general, emissions result from

the decay of the atoms; this by necessity means that the light released isof the same energy or lower than the than the exciting signal. In this case,the emission is of higher energy. This means that there is some from ofup-conversion occuring.

Up- conversion can result from a number of methods: Photon Avalanche,. . .

In this case there is insufficient data to even hazard a guess as to themethod of up-conversion.

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5.4 Further Research

–Use ND filters out of alignment?: Maybe could be a useful reference.–More reliable method of monochromating? Maybe better monochroma-

tor?–Effects of power? Only 2.3 mW in, here.

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Part IV

The End

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Chapter 6

Conclusions and the End-Piece

This is the end piece — this will have a number of conclusions and the like.

34

This is a piece that ought to be referenced[13] as it is not my own. Noris this[9], or this [7].

35

Bibliography

[1] A pair of notes, which came with the crystals supplied by Ahlefeldt,Sellars, and Co. at the ANU.

[2] R. L. Ahlefeldt, A. Smith, and M. J. Sellars. Ligand isotope structureof the optical 7F0 →

5D0 transition in EuCl3 · 6H2O . Phys. Rev. B,80(20):205106, Nov 2009.

[3] V. V. Bakakin, R. F. Klevstova, and L. P. Solov’eva. Crystal structureof LaCl3 · 7H2O and the topotactic relation between hexa- and heptahy-drated chlorides of rare-earth elements. Journal of Structural Chemistry,15:723–732, 1975. 10.1007/BF00747277.

[4] N. K. Bel’skii and Yu. T. Struchkov. The crystal structure and opti-cal properties of europium chloride hexahydrate, EuCl3 · 6H2O. Soviet

Physics — Crystallography, 10(1):15 – 20, 1965. Originally publishedby the Institute of Heteroorganic Compounds, Academy of Sciences ofthe USSR. Translated from Kristallografiya, Vol. 10, No. 1, pp. 21–28,January–Feburary, 1956.

[5] Max Born and Emil Wolf. Principles of Optics. Pergamon Press, sixthedition, 1980.

[6] F. Albert Cotton and Geoffrey Wilkinson. Advanced Inorganic Chem-

istry. John Wiley and Sons, fifth edition, 1988.

[7] R. W. Equall, Y. Sun, R. L. Cone, and R. M. Macfarlane. Ultraslowoptical dephasing in Eu3+ : Y2SiO5. Phys. Rev. Lett., 72:2179–2182, Apr1994.

[8] Page from Kaiser Optical Sytems,Inc., catalogue. Accessed onlinefrom http://www.kosi.com/Holographic_Filters/standard.php onthe 8th of August, MMXI.

[9] Charles Kittel. Introduction to Solid State Physics. John Wiley andSons, eighth edition, 2005.

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[10] P. H. Lissberger and W. L. Wilcock. Properties of all-dielectric inter-ference filters. II. filters in parallel beams of light incident obliquelyand in convergent beams. Journal of the Optical Society of America,49(2):126–130, 1959.

[11] J.P.D Martin, M.J Sellars, P Tuthill, N.B Manson, G Pryde,and T Dyke. Resolved isotopic energy shift and hole burning inEuCl3 · 6H2O. Journal of Luminescence, 78(1):19 – 24, 1998.

[12] Mike May. Getting inside optical filters. BioOptics World, 1(5):38–39,Sep/Oct 2008.

[13] David L. McAuslan. Quantum Computing Hardware Based on Rare-

Earth-Ion Doped Whispering-Gallery Mode Resonators. PhD thesis,University of Otago, 2011. As of September 2011, this has been submit-ted, but is unmarked.

[14] David Rosenblatt, Avner Sharon, and Asher A. Friesem. Resonantgrating waveguide structures. IEEE Journal of Quantum Electronics,33(11):2038–2059, 1997.

[15] Page from Semrock,Inc., catalogue. Accessed online from http://www.

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[16] Ben Standish. High performance with fluorescence optical filters. BioOp-

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[17] Newton NJ 07860-0366 U.S.A Thorlabs Inc. Route 206, P.O. Box 366.Thorlabs catalogue v.20., 2010.

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