Mikhail N. Pivnenko

Liquid crystal `blue phases' with a wide temperature range

Liquid crystal ‘blue phases’ are highly fluid self-assembled three-dimensional cubic defect structures that exist over narrow temperature ranges in highly chiral liquid crystals. The characteristic period of these defects is of the order of the wavelength of visible light, and they give rise to vivid specular reflections that are controllable with external fields. Blue phases may be considered as examples of tuneable photonic crystals with many potential applications. The disadvantage of these materials, as predicted theoretically and proved experimentally, is that they have limited thermal stability: they exist over a small temperature range (0.5–2 °C) between isotropic and chiral nematic (N*) thermotropic phases, which limits their practical applicability. Here we report a generic family of liquid crystals that demonstrate an unusually broad body-centred cubic phase (BP I*) from 60 °C down to 16 °C. We prove this with optical texture analysis, selective reflection spectroscopy, Kössel diagrams and differential scanning calorimetry, and show, using a simple polarizer-free electro-optic cell, that the reflected colour is switched reversibly in applied electric fields over a wide colour range in typically 10 ms. We propose that the unusual behaviour of these blue phase materials is due to their dimeric molecular structure and their very high flexoelectric coefficients. This in turn sets out new theoretical challenges and potentially opens up new photonic applications.

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.

Optical and electro-optical properties of bimesogenic organosiloxane antiferroelectric liquid crystals with molecular tilt approaching 45 degrees

We have studied the optical and electro‐optical properties of three different bimesogenic siloxane materials. The dimers are symmetrical and the two mesogenic groups are connected by a siloxane unit containing three Si atoms and methylene spacers. The mesogens contain F, Cl and Br substituents, respectively, laterally attached to the phenyl ring lying closest to the chiral centre of the mesogen. These materials each exhibited a broad temperature range antiferroelectric phase and large molecular tilt, which is almost temperature‐independent. We have shown that, at low temperatures, the molecular tilt in the antiferroelectric phase is the same as the apparent molecular tilt in the field‐induced ferroelectric state and is close to 45°. It was found that the antiferroelectric phase of these compounds, aligned in such way that the dimeric molecules were lying in a plane parallel to the confining substrates, exhibited very low in‐plane birefringence. For this reason, samples of the compounds inserted between crossed polarizers in the antiferroelectric state exhibited an extremely dark state, with contrast ratios as high as 1000:1, which did not change noticeably on rotating the sample between the crossed polarizers. It was shown that the optically isotropic siloxane moiety did not contribute to the optical properties, viz. birefringence or tilt angle, but served to enhance the promotion of the antiferroelectric phase and the temperature independent properties. We also showed the existence of a linear electro‐optic response in the pretransitional region of the antiferroelectric phase of these compounds over a moderate range of fields.

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.

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.

The influence of fluorosurfactants on the flexoelectro-optic properties of a chiral nematic liquid crystal

In this paper, we demonstrate an increase in the flexoelectro-optic response of a hypertwisted chiral nematic liquid crystal with the addition of a low concentration (0.7wt%) of a fluorosurfactant so as to reduce the anchoring energy at the liquid crystal∕substrate boundary. Specifically, the results show that the tilt angle per unit of applied electric field is increased by approximately 20% whereas the response time is decreased by approximately 30%. An increase in the concentration of the fluorosurfactant agent does not appear to greatly affect the magnitude of the tilt angle for a given field strength but does result in a further reduction of the response time.

Electro-optic Telecommunication Devices at 1550 nm Employing Electroclinic and Ferroelectric Switching of an Organosiloxane Liquid Crystal

ABSTRACT We demonstrate three electro-optic telecommunication devices operating at a wavelength λ = 1550 nm: an optical switch or modulator, a variable attenuator, and a rotatable waveplate. These devices make use of the electroclinic and ferroelectric properties of a chiral smectic organosiloxane liquid crystal. Under moderate electric fields of ≤23 V/μm we observed an optical power modulation of up to 38 dB and a switching time of ∼100 μs. The waveplate could be continuously rotated over 38°. We also present birefringence data of this liquid crystal at λ = 1550 nm as a function of temperature and the implications on the development of liquid crystal telecommunication devices.