Alison D. Ford

Photonics and lasing in liquid crystals

Lasers were invented some 40 years ago and are now used in a plethora of applications. Stable liquid crystals were discovered at about the same time, and are now the basis of a large display industry. Both technologies involve photonics, the former in the creation and use of light and the latter in the control and manipulation of light. However, it is only recently that these two mature technologies have been combined to form liquid-crystal lasers, heralding a new era for these photonic materials and the potential for novel applications. We summarize the characteristics of liquid crystals that lead to laser devices, the wide diversity of possible laser systems, and the properties of the light produced.

Enhanced emission from liquid-crystal lasers

The performance of a photonic band-edge laser fabricated from a low molar mass dye-doped chiral nematic liquid crystal is found to have a strong thermal dependence. At each temperature the performance of the laser has been characterized by the slope efficiency which was calculated from a plot of the emission energy as a function of excitation energy. This slope efficiency was found to increase by 36% when the dye-doped chiral nematic liquid crystal was cooled from 53to43°C. The increase in slope efficiency is considered to be due to a change in the lasing conditions, in particular, changes in the emission efficiency of the dye and possibly the quality factor of the liquid-crystal resonator, which is dependent upon the linewidth of the resonant mode. The wavelength dependency of the spontaneous emission intensity and the quantum efficiency of the dye were not found to influence the lasing conditions in this case. The order parameters relating to the dye-doped chiral nematic liquid crystal were considered t...

The emission characteristics of liquid-crystal lasers

— Liquid-crystal lasers exhibit narrow linewidth, large coherence area, and low threshold laser emission. Moreover, the wavelength of the laser line can be readily tuned using a variety of different external stimuli, including electric fields. These combined features make them particularly attractive as compact tunable laser light sources. Recent experimental results with regards to the emission characteristics of chiral nematic photonic band-edge lasers are discussed. This type of liquid-crystal laser consists of a self-organizing one-dimensional photonic band structure and a gain medium in the form of a laser dye. Some of the generic features that are observed for these lasers are discussed, including the typical emission linewidth of the laser line, the change in emission energy of the laser for high excitation energies and high pump repetition rates, and the dependence of the excitation threshold and slope efficiency on the cell thickness. In addition, how the performance changes when either the molecular structure of the chiral nematic host or the gain medium is varied is considered. To conclude, results are presented on the laser emission for a wide-temperature-range blue phase I band-edge laser which consists of a self-organizing three-dimensional photonic band structure.

Electronic control of nonresonant random lasing from a dye-doped smectic A* liquid crystal scattering device

The electronic control of the excitation threshold for random lasing in a dye-doped smectic A* liquid crystal is demonstrated. Random lasing is the term given to the nonlinear amplification of light which is the result of feedback due to multiple scattering. With the application of an electric field the smectic A* phase forms a highly scattering texture for which the nonlinear amplification of light occurs at an excitation threshold of 10μJ∕pulse. In comparison, nonlinear amplification is not observed in the field induced homeotropic texture. As a result, a device has been conceived and demonstrated whereby random lasing is switched “on” or “off” with an applied electric field.

The effects of reorientation on the emission properties of a photonic band edge liquid crystal laser

The effect of altering the excitation rate on the emission energy of a photonic band edge liquid crystal laser has been investigated. The results showed that for excitation energies greater than 18 µJ/pulse, an increase in the excitation rate from 1 to 20 Hz caused a reduction in the total emission energy of the photonic band edge liquid crystal laser of up to 90%. In order to establish the cause of this reduction several factors were considered: dye bleaching, thermal/density effects and director-axis reorientation (either by virtue of a light-induced torque or other effects such as a flow-induced orientational decay). Although some dye bleaching was observed, the main factor responsible for the reduction in the overall emission efficiency was found to be nonlinear changes in the director-axis reorientation. Furthermore, our results suggest that the director-axis reorientation is the result of an optical torque, enhanced by the presence of the dye, which for a chiral nematic liquid crystal results in a dilation of the helical pitch.

Removing the discontinuous shifts in emission wavelength of a chiral nematic liquid crystal laser

The laser wavelength of a band-edge liquid crystal laser is known to vary in a stepwise fashion with temperature. This report describes a method for removing the discontinuous shifts in the laser wavelength by doping a nematic sample with two different chiral dopants which exhibit opposing dependences of the natural pitch on temperature. On cooling from the isotropic phase to room temperature the total shift of the laser wavelength is 15 nm with no discontinuities. This is in contrast to that observed for samples consisting of only one chiral dopant whereby the wavelength shift was as large as 75 nm for the same temperature range.

Liquid crystal lasers: coherent and incoherent microsources

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.

Red-Green-Blue 2D Tuneable Liquid Crystal Laser Devices

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.

A comparison of photonic band edge lasing in the chiral nematic N* and smectic C* phases

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.

Photonic band edge lasing in dye-doped chiral nematic bimesogen liquid crystals

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