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.
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.
In this paper we investigate Photonic Band Edge (PBE) lasing from a homologous series of non-symmetric bimesogen liquid crystals (the mesogenic units are not identical) with varying physical parameters. A homologous series was synthesised, where the number of methylene units in the linking flexible spacer chain ranged from 6 to 12. Our results show a clear odd-even effect within the threshold values and slope efficiencies, of the PBE lasers, when plotted as a function of the number of methylene units in the spacer chain. The even spacer bimesogen PBE lasers performed with an overall higher efficiency (< 2μJ/pulse threshold values and 8% slope efficiency) than the odd spacer bimesogens PBE lasers (3.5μJ/pulse threshold values and 1% slope efficiency). We believe that this odd-even effect is due to the odd-even effect observed within the host physical parameters; a consequence of the molecular shape anisotropy
In this paper, we review our recent experimental work on coherent and incoherent liquid crystal lasers. For the coherent lasers, results are presented on thin-film photonic band edge lasing using dye-doped low molar mass liquid crystals in the self-organised chiral nematic phase. We show that potentially high Q-factor lasers can be fabricated from these materials by demonstrating that a single mode output with a very narrow linewidth is readily achievable in well-aligned monodomain samples. Moreover, from our investigations 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 liquid crystal. Specifically, the slope efficiency was found to vary from 1% to 12% depending upon the liquid crystalline material employed. Using this information, the important parameters of the host liquid crystal are highlighted. As regards to the functionality, 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. Finally, for the incoherent lasers, we summarise our findings on random lasers that are fabricated from liquid crystals which exhibit a smectic A* phase.
Nonresonant random lasing from a dye-doped smectic A* scattering device is demonstrated. The field-induced scattering state of a low molar mass liquid crystal in the smectic A* phase is found to provide sufficient feedback to generate random lasing when a gain material, such as a fluorescent dye, is doped into the liquid crystal host. Furthermore, we found that the emission intensity of the random laser at a given excitation energy can be adjusted by altering the strength of the applied electric field so as to modify the scattering texture and consequently the transport mean free path. This change in the transport mean free path results in a change in the random lasing threshold. Large values for the transport mean free path, which indicate a weak scattering strength, result in large threshold values and vice-versa. Finally, we discuss the benefits of controlling the scattering strength with an applied electric field in terms of potential device applications.
The lasing output characteristics of two different types of photonic band edge liquid crystal lasers have been investigated. The required helical structure or periodic change in the refractive index was realised by using either a chiral nematic or a chiral smectic C as the liquid crystal host. The fluorescent dye doped photonic band edge lasers exhibited very different emission characteristics. A Q-switched Nd:YAG pulsed laser, frequency doubled to 532nm with pulse lengths of 5ns was used to excite the samples. Typical laser parameters such as slope efficiency and input energy threshold values were examined for each laser and results indicated that the chiral smectic C laser was more efficient. We believe that the higher performance of the chiral smectic C laser is attributed to the increase in the degree of order of the host.
In this paper, we measure the emission energies of three different photonic band edge lasers containing nematic hosts which have slightly different physical properties. The low molar mass monomers used as nematic hosts are identical in terms of molecular shape except for the length of the terminal alkyl chain. The photonic band edge laser which contains an even number of methylene units in its alkyl chain is found to have the lowest lasing threshold at both the same absolute temperature and the same shifted temperature. At the same excitation energy the laser output for each sample is found to be proportional to the orientational order parameter of the nematic host. However, when plotted simultaneously the absolute value of the order parameter does not correlate with the same emission energies in each sample. As a result we consider other factors that maybe of equal importance to the operating efficiency of the PBE laser in order to obtain viable candidates that are able to explain the discrepancy in our results. Finally, we introduce a figure of merit parameter which contains the candidates most likely to affect the operating efficiency and obtain much better agreement with our results than with the orientational order parameter alone.