In this paper we demonstrate laser emission from emulsion-based polymer dispersed liquid crystals. Such lasers can be
easily formed on single substrates with no alignment layers. Remarkably, it is shown that there can exist two radically
different laser emission profiles, namely, photonic band-edge lasing and non-resonant random lasing. The emission is
controlled by simple changes in the emulsification procedure. Low mixing speeds generate larger droplets that favor
photonic band edge lasing with the requisite helical alignment produced by film shrinkage. Higher mixing speeds
generate small droplets, which facilitate random lasing by a non-resonant scattering feedback process. Lasing thresholds
and linewidth data are presented showing the potential of controllable linewidth lasing sources. Sequential and stacked
layers demonstrate the possibility of achieving complex, simultaneous multi-wavelength and "white-light" laser output
from a wide variety of substrates including glass, metallic, paper and flexible plastic.
The study of band-edge lasing from dye-doped chiral nematic liquid crystals has thus far been largely restricted to visible
wavelengths. In this paper, a wide range of commercially available laser dyes are examined for their suitability as
infrared emitters within a chiral nematic host. Problems such as poor solubility and reduced quantum efficiencies are
overcome, and successful band-edge lasing is demonstrated within the range of 735-850 nm, using the dyes LD800,
HITC-P and DOTC-P.
This paper also reports on progress towards widely tuneable liquid crystal lasers, capable of emission in the region 460-
850 nm. Key to this is the use of common pump source, capable of simultaneously exciting all of the dyes (both infrared
and visible) that are present within the system. Towards this aim, we successfully demonstrate near-infrared lasing (800
nm) facilitated by Förster energy transfer between the visible dye DCM, and the infra-red dye LD800, enabling pump
wavelengths anywhere between 420 and 532 nm to be used.
These results demonstrate that small and low-cost tuneable visible to near-infrared laser sources are achievable, using a
single common pump source. Such devices are envisaged to have wide-ranging applications including medical imaging
(including optical coherence tomography), point-of-care optical medical diagnostics (such as flow cytometry),
telecommunications, and optical signatures for security coatings.
In this paper we investigate a dye guest-host liquid crystal device based upon the bistable electro-optic
effect in the smectic A phase. The dye used is a pentamethyldisiloxane-grafted disperse red 1 moiety;
the liquid crystal host is an organosiloxane liquid crystal based upon cyanobiphenyl. It is shown that
extremely high dye solubilities can be achieved e.g. greater than 38 % w/w, without dye separation or
adverse effect on device electro-optic properties. The typical solubility limit in liquid crystals is of the
order of several percent. It is proposed that the beneficial degree of dye accommodation is the result of
microsegregation of constituent moieties into siloxane, alkyl chain and cyanobiphenyl core/chromophore regions.
In this work, we examine the phenomenon of random lasing from the smectic A liquid crystal phase.
We summarise our results to date on random lasing from the smectic A phase including the ability to
control the output from the sample using applied electric fields. In addition, diffuse random lasing is
demonstrated from the electrohydrodynamic instabilities of a smectic A liquid crystal phase that has
been doped with a low concentration of ionic impurities. Using a siloxane-based liquid crystal doped
with ionic impurities and a laser dye, nonresonant random laser emission is observed from the highly
scattering texture of the smectic A phase which is stable in zero-field. With the application of a low
frequency alternating current electric field, turbulence is induced due to motion of the ions. This is
accompanied by a decrease in the emission linewidth and an increase in the intensity of the laser
emission. The benefit in this case is that a field is not required to maintain the texture as the scattering
and homeotropic states are both stable in zero field. This offers a lower power consumption alternative
to the electric-field induced static scattering sample.
Organosiloxane liquid crystals have previously been shown to have much potential in bistable smectic-A devices; in this paper we aim to optimise the device performance by reducing threshold voltages and response times. Our results show that mixtures of novel organosiloxanes with enhanced dielectric coupling can significantly these key parameters. The molecules used were of the A/B and A/B/A type, where B refers to the number of siloxane units, and A to the mesogenic unit(s) attached. The molecule 5/2, which has a pentamethyldisiloxane (PMDS) group laterally attached to a pentyl-oxycyanobiphenyl (5OCB) mesogenic unit, was chosen as host for the mixtures. Of the A/B type, two napthylene-core molecules were chosen, which are designated Si2-4-ONEBN and Si3(iso)-4-ONEBN. These molecules have identical cores and alkyl chain lengths but differ in the number and conformation of the siloxane moiety. Of the A/B/A type, 5/2/5 was selected. This molecule consists of two 5OCB units joined via a PMDS group. The concentrations used were 0.3 mol (A/B type) and 0.15 mol (A/B/A type). Threshold voltages of the mixtures were measured as a function of shifted temperature; the response times were measured at fixed temperature as a function of applied voltage. It was found that all the mixtures gave favourable results, with the 0.3 mole fraction Si2-4-ONEBN response times of 20 ms were achieved - an order of magnitude faster than pure 5/2. Threshold voltages were shown to be reduced by approximately 25% for all mixtures with no degradation in mesogenic behaviour.