Invited Paper
Red-Green-Blue 2D Tuneable Liquid Crystal
Laser 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 allorganic 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
Liquid Crystals XIII, edited by Iam Choon Khoo, Proc. of SPIE Vol. 7414
741402 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.831230
Proc. of SPIE Vol. 7414 741402-1
by the distance along the helix for which the director would rotate by S (although the pitch of the helix is the distance
for a 2S 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).
c
a
b
400
450
500
550
600
Wavelength (nm)
650
700
750
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. Defectmode 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. Defectmode 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
Proc. of SPIE Vol. 7414 741402-2
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 ('O) is related
to the pitch (p) of the helix and the birefringence ('n) of the anisotropic ‘pseudo’ layers through the relationship 'O =
'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 Pm 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.
Proc. of SPIE Vol. 7414 741402-3
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 GO ~ 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 (GO = 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 'Or = 0.09 nm. Using Or2/'Or, 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.
Proc. of SPIE Vol. 7414 741402-4
(b)
(a)
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.
(b)
(a)
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 Ȝ = 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
Proc. of SPIE Vol. 7414 741402-5
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.
Emission energy (PJ/pulse)
0.5
90 Pm
100 Pm
131 Pm
181 Pm
203 Pm
231 Pm
0.4
0.3
0.2
0.1
0.0
0
10
20
30
(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 OII = nIIp whereby OII 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
Proc. of SPIE Vol. 7414 741402-6
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 Pm 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
Wavelength (nm)
600
Figure 8. The change in the laser wavelength with an applied inplane electric field.
595
590
585
-1
0
1
2
3
4
-1
Electric field (VPm )
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 VPm-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
Proc. of SPIE Vol. 7414 741402-7
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 VPm-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, K, the coupling coefficient, N, 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,
Q
Or
vS
'Or
Coupling coefficient,
N v 'n
Emission efficiency, K
Gain coefficient,
v ST
g v nII
(19)
Emission energy (nJ/pulse)
150
100
50
0
0.16
0.18
0.20
0.22
0.24
0.26
Figure of merit, f
Figure 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
Proc. of SPIE Vol. 7414 741402-8
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 PJ/pulse, slope efficiency = 1%) whilst the laser which has relatively large values for the physical
parameters has the highest output (excitation threshold = 0.4 PJ/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 PJ/pulse at d = 5 Pm to Eth = 2.5 PJ/pulse between thicknesses of d = 10 – 15 Pm. For
each cell thickness the laser line remained at the gain maximum of DCM. Above d = 15 Pm, an increase in the thickness
results in an increase in the threshold energy. In terms of the slope efficiency, Ks, this is found to increase from Ks =
1.5% at d = 5 Pm to Ks ~ 6.5% for thicknesses ranging for an optimal value from d = 10 Pm to d = 15 Pm.
Emission energy (PJ)
4
3
2
1
0
0
10
20
30
40
Excitation energy (PJ)
Figure 10. Pump energy dependencies for four different photonic band edge lasers. The plot includes three commercially available
nematogen 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).
Proc. of SPIE Vol. 7414 741402-9
Slope efficiency, Ks (%)
Energy threshold, Eth (PJ/pulse)
10
20
15
40
10
5
20
0
0
0
10
20
30
40
5
0
0
20
40
0
5
10
15
20
25
30
35
40
Cell thickness, d (Pm)
Cell thickness, d (Pm)
(b)
(a)
Figure 11. (a) The excitation threshold energy, and (b) the slope efficiency as a function of cell thickness for a chiral nematic bandedge laser.
A previous report has considered the threshold energy in terms of the threshold gain, Jth 22. In this case, it was assumed
that the threshold energy could be expressed in the form Eth = A(Jth)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
Eth
§
E
A¨¨ D Ud
©
·
¸¸d ,
¹
(1)
where, D is a coefficient relating to absorption losses, E is a fitting parameter and U is the density of photon states. It has
been suggested 22 that U = 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 v 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(Jth)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
Eth
AD d AO2
,
'n 2 d 2
(2)
where O 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.
Proc. of SPIE Vol. 7414 741402-10
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
Șs v ((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 O = 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 O = 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
RB-E49*
R6G-E49*
PM580-E49*
DCM-E49*
PM597-E49*
O (nm)
582
572
570
609
582
Table 1
Laser sample
RB-E49*
R6G-E49*
DCM-E49*
PM580-E49*
PM597-E49*
Table 2
Slope efficiency, Șs (%)
0.1
0.8
9
21
29
Proc. of SPIE Vol. 7414 741402-11
Emission energy (PJ/pulse)
6
PM597-E49*
PM580-E49*
DCM-E49*
R6G-E49*
RB-E49*
5
4
3
2
1
0
5
10
15
20
Excitation energy (PJ/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.
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 Pm because it was found that
RB-E49* did not generate laser emission in d = 10 Pm. The slope efficiency of R6G-E49* was found to be Ks = 0.8% as
opposed to Ks = 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 10Pm 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 R6GE49* 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 Ks for PM597-E49* is a factor of three greater than that of DCM-E49*
when excited at O = 532 nm and a repetition rate of 1 Hz. Of course, the laser wavelength of PM597-E49* is at a shorter
wavelength (O = 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 PJ/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 Ks = 29% compared to Ks = 9%
for DCM-E49*. The threshold energies for the three lasers were found to be in the range of Eth ~ 100 – 200 nJ/pulse.
Proc. of SPIE Vol. 7414 741402-12
2.3
3
PM580
PM597
DCM
0.8
0.6
0.4
0.2
0.0
0.0
0.5 wt %
1 wt %
1.5 wt %
2 wt %
2.2
2
Lifetime (ns)
Quantum efficiency
1.0
2.1
1
2.0
0
1.9
9
1.8
8
0.5
1.00
1.5
2.0
2.5
1.7
7
580
Concentrat
ation of dye (wt %)
585
5
590
5
595
600
W
Wavelength
(nm
m)
(a)
(b
b)
Figure 13. (a) Quantum Effi
ficiencies for thhe PM 580, PM
M 597 and DC
CM dyes and (b)
( Fluorescencce Lifetimes as a function of
concentration for
f DCM.
Table 3. Excittation Thresholdds (nJ/pulse) ass a function of dye
d
concentration. Excitation pulsse width 5ns annd Ȝout =610nm
Table 4. Slope Efficieencies as a funcction of dye
ntration. Excitattion pulse widthh 5ns and Ȝout =610nm
concen
From Tables 3 and 4 it is clear that thee pyromethenee dyes have lo
ower thresholdds and higherr slope efficieencies than thee
DCM dye under
u
the sam
me illuminatioon conditionss and opticall path lengthh. It is notew
worthy, generrally, that thee
pyromethene dyes also perrform better at
a lower dye concentrations
c
s 30. This has to
t be taken innto account when
w
designingg
p
(
(c.f.
4.2.2).
cavity lengthss, or sample thhickness, wheen optimising the LC laser performance
5. BLU
UE PHASE LIQUID CR
RYSTAL LASERS
In this sectionn we will conssider, briefly a further lasinng mode in hig
ghly chiral nem
matic liquid crrystals, namelly Blue
31
Phases. Such phases have been
b
shown, recently
r
, to give rise to simultaneous laasing in three orthogonal directions. Bluee
phases are essentially compposed of doubble twist cylinnder that lead to
t a net of discclination liness that have cub
bic symmetry.
Bragg scatterring from these disclination lines then sett up a 3D LC laser
l
cavity orr photonic bannd structure. Until
U
recently
32
, however, these
t
phases were
w only stabble over a tempperature rangee of a few Kellvin.
5.1.1 Wide temperature Range Blu
ue Phase Lasers
It has been shhown32 that, for
f certain muulti-componennt mixtures co
ontaining bimeesogenic com
mpounds and a high twistingg
power chiral additive, Bluee Phase I (BPI) was found to
t occur naturrally over a teemperature rannge of 30oC. The
T BPI phasee
by the spacingg of the cubic units is of th
he order of thee
is representedd by a body-ccentered cubicc lattice of deefects, whereb
wavelength of
o light. Consequently, thiss results in a photonic
p
band
d gap in threee-dimensions which meanss that the bluee
phase could be
b used as a thhree-dimensional band-edgee laser by simp
ply adding a gain
g medium.
Proc. of SPIE Vol. 7414 741402-13
Figure 14 (a) Laser emissioon from a widde temperaturee range
BPI as a funcction of excitattion and waveelength.
Figu
ure 14 (b) Thhreshold curvve for lasing in BPI. Thee
inseet shows localiized fluoresceence from a BP
P platelet.
In Figure 14, band-edge lasing
l
from a dye-doped BPI
B laser is demonstrated.
d
The host mixxture was thee same as thaat
described in the literature 32 and 1.5 wt%
w of DCM was dissolveed into the hoost. The figuree shows the laser
l
emissionn
spectrum for an excitation fluence of 10 mJ/cm2, using 5ns pulses at
a an operatingg temperaturee of T = 30oC along
a
with thee
optical texturre that is obseerved when viewed
v
betweeen the crossed
d polarisers of
o an optical ppolarizing miccroscope. Thee
platelet from which laser emission
e
occuurs is highlighhted in the figu
ure and corressponds to the (200) orientaation. It can bee
seen that lasiing occurs at O = 571 nm, correspondingg to the band-edge of the photonic
p
bandd gap for the (200) platelett.
The band-gapp of this domaain is shifted with
w respect too the gain max
ximum of DC
CM and this iss evident from
m the amplifiedd
spontaneous emission (AS
SE) peak that occurs simulttaneously at O ~ 610 nm. A number of domains are excited at thee
same time duue to the fact that the sizes of
o the individuual domains are
a much less than
t
the pumpp-spot size. Due to the widee
temperature range
r
of this blue
b
phase, laaser emission was
w observed
d over a 10oC temperature rrange with no change in thee
emission specctrum. The results here shoow that the lasser emission iss broader thann that of the chhiral nematic laser althoughh
in this case thhe spectrum was
w recorded using
u
a 1.3 nm
m-resolution spectrometer.
s
I has been shhown 33 that line widths lesss
It
than 0.1 nm are
a achievablee indicating a high
h
quality factor
fa
of the laaser mode in a single domaiin of Blue Phaase I.
Achieving lasser emission in
i either BPI or
o BPII is exttremely encou
uraging as thesse structures eexhibit a phottonic band gapp
in three dimeensions unlikee the chiral nem
matic or chiraal smectic phaases. Consequuently, both bllue phases exh
hibit a numberr
of important features that are not observved for 1D phhotonic band structures.
s
Onne such featurre is that laserr emission cann
t
being thee technologicaal obstacles th
hat need to bee
occur in multtiple directionns simultaneouusly (31). Ignooring for the time
overcome, thiis could be exxploited to prooduce a single element red-g
green-blue ligght emitter by doping in a nu
umber of dyess
so as to correespond with the
t reflection spectrum of the different platelet orienntations, e.g. rred-(110), greeen-(200), andd
blue-(211).
Proc. of SPIE Vol. 7414 741402-14
4000
580
560
2000
O, nm
Intensity, a.u.
3000
540
520
1000
symmetric bimesogen (low 'H)
non-symmetric bimesogen (higher 'H)
500
0
400
500
O, nm
600
700
0
5
10
15
20
Electric Field, V/Pm
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.
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 quasicontinuous 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.
1.0
0.5
0.0
3
600 Hz
750 Hz
1000 Hz
2
1
0
0
5
10
15
5
10
15
20
p u m p e n e rg y / P J
Repetition Rate (H z)
1000 H z
2 .0
750 H z
600 H z
500 H z
1 .5
400 H z
300 H z
1 .0
0 .5
0 .0
0
20
2 .5
emission power / mW
Emission
emissionEnergy
energy / PJ (μJ)
Emission Energy (PJ)
1.5
3 .0
Emission Power (mW)
4
2.0
Pump energy (μJ)
25
0
5
10
15
20
25
pum p pow er / m W
Pump power (mW)
(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.
Proc. of SPIE Vol. 7414 741402-16
(a)
(b)
Figure 17 (a) Schematic
S
of thhe lenslet array illuminating thhe chiral nematiic liquid crystall laser to give aan output array of
o lasing dots
and (b) an imagge taken of an output
o
laser arraay using a conccentration gradiient induced pittch across the L
LC laser cell.
(e)
Figure 18. Moonochromatic LC
L laser array em
missions (a, b & c), the corresp
ponding RGB emission
e
spectra (taken by reco
ombining the
array in the farr field) (d), and the CIE1931 chhromaticity spaace diagram com
mparing the collour gamut from
m a CRT display
y (inner
triangle) as com
mpared with thee liquid crystal laser array(outeer triangle).
Proc. of SPIE Vol. 7414 741402-17
Current studies of LC laserrs are typicallyy conducted using
u
Q-switch
hed solid-statee lasers, such as an Nd:YAG laser, as thee
pump sourcee. The short pulses
p
(ps or ns) ensure reeduced speck
kle, low threshhold energy aand, due to the
t high slopee
efficiencies, large
l
peak poowers can be obtained.
o
In addition
a
it hass been shownn38, 39 that a raange of wavellengths from a
single chiral nematic (N*L
LC) sample can
c be obtaineed by using more
m
than onee laser dye annd forming a pitch gradiennt
across the cell. A broad baand of lasing wavelengths are then obtaainable by trannslating the sample relative to the pumpp
beam. As a reesult, LC laseers are an attraactive alternattive as the fulll color emitteer in a laser prrojection display. Howeverr,
in order to realize the full potential
p
of thhese lasers sim
multaneous R--G-B lasing frrom a single ppump source is required. Byy
combining grradient pitch LC
L samples with
w array-bassed pumping techniques, itt is possible tto achieve a multi-coloured
m
d
laser array coonsisting of reed, green, andd blue emissioons when photto-pumped at a single fixedd pump wavellength. This iss
achieved withhout using coomplex fabrication proceduures. Indeed, R-G-B
R
lasing from a liquidd crystal defeect-mode laserr
has recently been demonstrated40, 41 usiing the innovaative approach of stackingg single-pitcheed polymer films to form a
Fibonacci seqquence. Usingg an alternativve approach42, we have show
wn that by coombining graddient pitch LC
C samples withh
array-based pumping
p
technniques, it is possible
p
to achhieve a multi--colored laserr array consistting of red, grreen, and bluee
emissions whhen photo-pum
mped at a singgle fixed pum
mp wavelength
h, Fig. 17(b). This is achievved without using
u
complexx
fabrication prrocedures.
In Figure 18,, the laser em
mission is show
wn from indivvidual 10 μm thick laser cells arranged in RGB strip
pes, in one LC
C
laser cell bassed geometry. For these ceells, 10 microon path length
h cells were used, photo-ppumped at 43
30 nm to givee
emission at Blue (470 nm
m), Green (5334), Red (617 nm) laser waavelengths. The
T array conssisted of threee well-definedd
columns of red-green-bluue output chhannels. This approach offfers a facilee alternative to laser arraays based onn
semiconductoors and , potenntially, proviides a simple route to fabriication of liquuid crystal laseers and projecction displayss.
In the case of
o a monochroomatic LC lasser, one couldd, of course, scan an arrayy of monochroomatic outputt and sum thee
outputs in thhe far field too give high avverage outputt powers. Thee average outtput power off > 2.5mW, quoted
q
abovee,
referred to sinngle spot outpputs. Thus witth the 2-D arraays there is po
otential to prodduce much higgher average output powerss
in the far fieeld using conndensing optiics. Based onn the conceptts, described above, it hass been possib
ble to build a
microscopic projection
p
laser prototype, Figure 19(a) with a footpriint of less thaan A5. This deevice has been
n designed forr
projection thrrough phase based
b
computeer generated holograms, Fig
gure 19(b).
(a)
Figure 19(a) Foootprint of the A5
A sized LC laser projection system,
s
with (b)) and (c) hologrrams projected at wavelengths of 532 and
632.8 nm.
Proc. of SPIE Vol. 7414 741402-18
6.1.3 LED pumped
p
stim
mulated emiission througgh liquid cry
ystal photon
nic band gap
p cavities.
2
500
1
500
600
700
0
800
15
LC
emissionn
Blue LED
D
10
500
5
0
400
Wavelengthh (nm)
500
600
700
0
800
Output Intensity (arb. units)
LC Emission
Input intensity (arb. units)
7. CONCLUSIO
C
ONS
Output Intensity (arb. units)
Input Intensity (arb. units)
Green LED
0
400
1000
3
1000
Waveleength (nm)
Figure 20 LED
L
pumped exxcitation of chirral nematic liquuid crystal cavitties.
With optimizzation of the materials parameters, caviity design etc, as discussedd above, it m
might be expeccted that laserr
emission migght be achieveed by replacinng the relativvely low energ
gy pulsed laser supply by high intensity
y pulsed LED
D
sources. In Figure
F
20 we show the eviddence for stim
mulated emisssion using optimized LC laaser cavities and materialss.
Figure 21, shows photograaphs of the outtput emission.
Figure 21. Phootographs of thee LED stimulateed emission from
m optimized ch
hiral nematic liqquid crystal sam
mples. (unpublisshed).
7. SUMMAR
RY
The aim of thhis review was to present a summary of recent
r
experim
mental results regarding thee emission chaaracteristics of
chiral nematic band-edge lasers
l
and alsoo to consider briefly recentt advancements with regardds to band-edg
ge lasing from
m
o
we have
h
reported
d how the perrformance of a chiral nemaatic band-edgee
wide-temperaature blue phaases. In this overview
laser varies as
a parameterss such as the pump-spot siize, pump rep
petition rate, and cell thickkness are varried. For largee
pump-spot siizes the threshhold energy is found to bee higher than that for smalll spot sizes, although the fluence at thee
threshold apppears to be low
wer for largerr spot sizes. It was found that
t
for a com
mbination of hhigh excitation
n energies andd
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 v d + fn
(1/d2) in accord with previous observations. In terms of the dependence of the slope efficiency (Ks) on cell thickness, this
was found to be best described by considering that Ks v 1/Eth. In addition, a number of different laser dyes were
considered and the results show that, for pumping at O = 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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