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Submitted on 1 Jan 1988
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POLARISATION SWITCHING AND
SUBMILLIWATT LASER DIODE INDUCED BISTABILITY IN NEMATIC LIQUID CRYSTALS
A. Lloyd, B. Wherrett
To cite this version:
A. Lloyd, B. Wherrett. POLARISATION SWITCHING AND SUBMILLIWATT LASER DIODE
INDUCED BISTABILITY IN NEMATIC LIQUID CRYSTALS. Journal de Physique Colloques, 1988,
49 (C2), pp.C2-141-C2-144. �10.1051/jphyscol:1988233�. �jpa-00227650�
GB-Edinburgh EH14 4 A S , Scotland, Great-Britain
Abstract
-
Polarisation bistability and optical switches at power levels as low as 14 pW, allowing operation with compact disc laser systems, are reported inoptimised cavities containing planar-aligned nematic liquid crystals.
I - INTRODUCTION
Optimisation studies of nonlinear narrow pass-band interference filters /1,2/ have concluded that the independent tailoring of finesse and cavity length made possible by removal of absorption from within the cavity can lead to a significant reduction in operating power levels.
A metallic partial mirror at the rear of the cavity is thus used no both a reflector and an absorber, with thermal conduction into the spacer 1ead.ing tc optothermal bistability.
This design allowed the construction of rigid cavities which have been used to demonstrate optical bistability with a number of liquid phase themo-optic materials /5,4/.
For a fixed cavity design of this type the critical switching poaer is inverse?y proportional to the magrltade of cheimo-optic coefficent dr./dT / 3 , 5 / . Tnis factor was instrumental in the choice of nematic phase liquid crystals as sparer materials. Their high thermo-optic coefficients at room temperature lad t:, a rcducticn ir. oyeret~ng power levels to 140 pW, allowing submilliwatt laser diode bpera+ior, of thsse dsviyes.
For the application gf these thermal devices t~ aptical 2D information processing, it is necessary however to operate within the 1-100 pW reginie in order to realise the
parallelism that optics offers in princig7.e, p.t moderate total power levels.
2
-
LOW POWER SWITCKINGFigure 1 shows the variation of refractive indices x l t h temperature for the nematic liquid crystal 4-cyano-4'-pentylbiphenyl (K15, 5CB or PCB). In priyciple it is clear that a significant reduction in operating powers is eqectad as t!ie nezaric-isotropic phase transition is approached, due to the increase in magnitude cf thsm.c-optic ccefficient.
Experimental confirmation of this tharmo-optk enl~aozement was obtained by enclosing the entire -611 within a specifically desigrled, controlle? teqsreture environment.
This allowed choice of ambient temperature to li100 of a dezzee and stability approaching 1/1000 of a degree /6/.
Figure 2 shows the observed reduction in operating power, fcr ';o.-!: .i'r~e brdinary and extraordinary refractive indices, as the phase transition is app.:osched. The critical power is observed to reduce by approximately one order of magnitute over an ambient range of e0C, and at 34.72OC has a value of just 13.3 pW.
This low power level allows bistable operation well below 100 pW, and when operated in reflection displays an adequate switch contrast of % 2 : l (Fig. 3 ) .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988233
C2-I42 JOURNAL DE PHYSIQUE
Fig. 1. Refractive indices of 4-cyano- 4'-pentylbiphenyl, measured on a heated stage Abbe Refractometer over the full nematic range. o = nl (0-ray), = rill
(e-ray).+ = nl (isotropic phase).
. . 0 ..a
0 " . " " " ' I
24 26 28 30 32 34 36
TEMPERATURE ('C)
Fig. 2. Critical switch power as a function of temperature, measured for a dielectric/gold 26.5 pm cell containing K15 (Rf = .85. Rb = .go).
0 = nl (0-ray)
.
= nil (e-ray).
+
= nl (isotropic phase).Operation of these devices with laser diodes simply requires construction of a cell of the same configuration, but with the dielectric stack reflectivity centred to match that of the gold at the wavelength used. Thus we were able to demonstrate laser diode driven optical bistability using the AlGaAs laser output from a demounted compact disc player read arm (Fig. 4). The higher critical switch power of 62.5 pW is due to the larger spot size of 117 p (l/e2 diameter) used.
Though the switching powers in Figure 2 show a reduction of an order of magnitude, the temporal response of the device remains virtually unchanged for the same temperature range. Elevating the temperature from 26.00°C to 34.70°C, for the same detuning in each case, represents a reduction in the 100% overdrive power from 963 pW to 87 )1W with no penalty in response time. The characteristic switching time in fact showing a reduction
from 2.4 ms to 1.75 ms (Fig. 5).
nnrr m w r ~ (pw) INPUT POWER (pw)
Fig. 3. Bistable operation of a dielectric/ Fig. 4. Bistable operation of a dielectric/
gold K15 cell observed in transmission and gold K15 cell using a CD laser diode reflection. Spot size = 25 pa. package. Spot size = 25 pm.
As Figure 1 shows, we have a system in which both refractive index and birefringence vary in step with the temperature rise produced by absorption of the cavity transmitted power;
which is essentially /a/:
where Po,. Peo, are the respective incident powers, aT/aPa is the temperature rise per unit power absorbed, and A,, A, are the absorptances for each polarisation state. This temperature rise causes phase changes in each of the orthogonal polarisations, related by the expression /a/:
Hence by appropriate choice of initial detuning, it is possible to switch optically from a low to high transmission state for one polarisation and simultaneously to switch from a high to low transmission for the orthogonal polarisation state.
To verify the above, a dielectric/gold cell was constructed with a ?. 25 p planar aligned.
nematic liquid crystal spacer (K15). Light of wavelength 633 nm from a He-Ne laser was focussed to a spot size of fl/e2 diameter) 25 onto the sample. The liquid crystal director axis was at 45' to the laser polarisation. Resulting transmission
characteristics (Fig. 6) clearly display polarisation switching and compare well with the theoretical expectation modelled on the basis of equations (1) and (2).
4 - DISCUSSION
To take advantage of the inherent parallelism of optics with conventional cw laser sources requires devices with high sensitivity. The pursuit of techniques such as pixellation and cavity/substrate optimisation will produce significant increases in the sensitivity of all thermal dispersive optically bistable systems. The limits will therefore be represented by sophistication of construction techniques and the magnitude of refractive index change with absorbed power.
C2-144 JOURNAL DE PHYSIQUE
0 10 20
INPlJT POWER (mW)
I I I
0 10 20
* I W T POWER (mW)
Fig. 6. Comparison of experimental results and theoretical modelling for polarisation switching in a dieletric/gold K15 cell. Spot size = 25 pm.
Liquid crystals have presently demonstrated the greatest exploitable thermo-optic
nonlinearity and have produced switching levels nearly two orders of magnitude lower than that of typical ZnSe interference filter devices /9/. Planar-aligned nematic liquid crystals also display a nonlinear birefringence which has been used to produce switches in both transmitted power and polarisation state. This allied with their dielectric
anisotropy and thus sensitivity to applied electric fields permits their application to a wide range of optical/optoelectronic devices.
5 - ACKNOWLEDGEMENTS
A.D. Lloyd is grateful for an SERC CASE award in cooperation with STL Ltd., and partial funding through the SERC/DTI JOERS p r o g r m e is acknowledged. We would like to thank Linkam Scientific Instruments for assistance in the design of the temperature stage, and M. Brett (Mullard Ltd.) for the supply of the CD laser diode assembly. The K15 liquid crystal sample was obtained from BDH Chemicals Ltd., Poole. Dorset.
REFERENCES
/1/ Wherrett, B.S., Hutchings, D and Russell, D., J. Opt. Soc. Am.,
3
(1986) 351./2/ Walker, A.C., Opt. Commun.,
59
(1986) 145./3/ Lloyd, A.D., Janossy, I., MacKenzie, H.A. and Wherrett, B.S., Opt. Commun.,
61
(1987) 339./4/ Somerton, C. and Tunnicliffe. D.L., Opt. Conmnun..
65
(1988) 143./5/ Hutchings, D.C., Lloyd, A.D., Janossy, I. and Wherrett, B.S., Opt. Commun.,
61
(1987) 345./6/ Model THW 200, Linkam Scientific Instruments Ltd.. Tadworth, Surrey KT20 5HT. U.K.
/7/ Campbell, R.J., Mathew, J.G.H., Smith, S.D. and Walker, A.C., submitted to Appl.
Opt.
/8/ Lloyd, A.D.. Opt. Commun..
64
(1987) 302./9/ Chow, Y.T., Wherrett, B.S., Van Stryland. E.W.. McGuckin. B.T.. Hutchings. D., Mathew, J.G.B., Miller, A. and Lewis, K., J. Opt. Soc. Am. B,
