Gia Petriashvili

Phototunable lasing in dye-doped cholesteric liquid crystals

In this letter, we report the results of phototunable lasing in dye-doped cholesteric liquid crystals (DD–CLC). Photoexcitation of DD–CLC films gives rise to laser emission in the violet-UV range. Control of the structure of the chiral dopant driven by UV phototransformation is exploited in order to obtain a permanent variation of the cholesteric pitch. Laser emission wavelength tuning, by means of photoinduced shifting of the selective reflection band of the cholesteric liquid crystals is established. A tuning interval of about 35 nm, in the wavelength range of 385–415 nm, is observed.

Widely tunable ultraviolet-visible liquid crystal laser

Quasicontinuous tuning of a dye doped cholesteric liquid crystal (CLC) mirrorless laser in the ultraviolet-visible wavelength range is demonstrated using a single original device based on CLC as resonator and several resonant dyes. The thought is to combine the CLC pitch gradient and the distribution of different dyes. In the same cell, six dyes are combined in order to nearly cover the whole wavelength range from ultraviolet (370 nm) to red (680 nm). Some of the used dyes work as emitter, while others work in the Forster regime to decouple the excitation and emission processes. The relevant aspect of the device is that a simple translation of the cell respect to the same pump beam enables fine tuning of the laser wavelength in almost all the visible range, up to the ultraviolet.

Light Induced Effects In Cholesteric Mixtures With A Photosensitive Nematic Host

For the first time the photooptic effect in induced cholesteric mixtures using photosensitive nematic host has been observed. The changes of the pitch (color) and optical anisotropy (Δn) and also cholesteric-isotropic phase transition have been studied. Potential applications of these systems are color filters, UV image recording, UV sensors, and optical data processing systems.

Trans-Cis Isomerization Of An Azoxybenzene Liquid Crystal

Trans–cis isomerization was investigated in a room temperature liquid crystal mixture of two azoxybenzene compounds. Experiments were performed on isolated molecules in dilute solutions and on the liquid crystal phase composed of the pure compounds. The absorption spectra of the trans and cis isomers were found to be similar to those of azobenzene compounds, as were the birefringence and order parameter of the nematic liquid crystal phase. The photo‐optic properties were also similar in that irradiation by ultraviolet light caused the conversion from trans to cis isomers, while short wavelength visible light incident on these compounds resulted in the conversion from cis to trans isomers. The activation energy for thermal relaxation from the cis to trans isomer in the liquid crystal phase was determined to be (66±7) kJ/mole, which is less than for azobenzene in solution. While a photostationary state in a dilute solution with approximately equal numbers of trans and cis isomers was achieved, the nematic–isotropic transition of the mixture of the pure compounds decreased from 70°C to room temperature with a cis concentration of only about 12%. One unusual finding was that the photostationary concentration of trans and cis isomers due to irradiation with light of a specific visible wavelength depended on the starting concentrations of the two isomers, indicating that there may be a molecular conformation that is not photo‐responsive and relaxes only thermally.

Laser emission from a dye-doped cholesteric liquid crystal pumped by another cholesteric liquid crystal laser

We report the observation of laser emission from a luminescent dye-doped cholesteric liquid crystal (CLC) excited by another dye-doped CLC laser. The idea is based on the cascade of two CLC cells containing two different dyes where the emission band of the first overlaps the absorption band of the second. This system of low threshold mirrorless lasers emphasizes the main advantages of these organic materials for lasing applications and identifies a simple laser device. Preliminary characterizations of these CLC laser systems produce evidence of remarkable features that suggest fascinating developments. The main aspects are related to the wavelength tunability of the laser emission and to the miniaturization of the device.

Light Control of Cholesteric Liquid Crystals Using Azoxy-Based Host Materials

The characteristics of cholesteric liquid crystals can be controlled by light irradiation if conformationally photo-active molecules are present. Recently, control of the selective reflection band (spiral pitch) in nemato-chiral mixtures was demonstrated when photosensitive molecules, namely nematic azoxy-based compounds, were used as the host material. In this report, the investigation of light induced effects in cholesterics with azoxy-based host materials is continued to highlight the mechanisms of the response. Different non-photosensitive chiral materials were added to different azoxy-nematic liquid crystals and the pitch change caused by UV irradiation was investigated. A change in the pitch of 50–210 nm was observed depending on the exposure time and the intensity of the light. This effect is reversible: under illumination at wavelengths greater than 410 nm, the pitch shifts in the opposite direction. The dependence of the selective reflection band and the full-width-at-half-maximum of the band on the exposure time and the temperature dependence of the selective reflection band were investigated. The lowering of the phase transition temperature and narrowing of the width of the selective reflection band can be explained by a decrease in the orientational order parameter. The blue shift of the selective reflection band is due to a decrease in both the order parameter and the concentration of linear nematogenic molecules.

Wide band gap materials as a new tuning strategy for dye doped cholesteric liquid crystals laser

A new tuning strategy for mirror-less liquid crystals laser is presented. A three layer cell is prepared with two cholesteric layers sandwiching a layer containing an isotropic mixture of a photoluminescent dye. One of the chiral layers contains a wide band gap material while the second layer consists of a series of small band gap materials. Through the combination of these two layers, a set of mirrors that can selectively reflect different wavelengths is obtained. A different laser wavelength is emitted from different regions of the cell under the pumping beam irradiation.

Cholesteric Liquid Crystal Mixtures Sensitive to Different Ranges of Solar UV Irradiation

ABSTRACT Three photosensitive cholesteric liquid crystal mixtures have been investigated as UV sensors. Each of them is sensitive to a well definite part of the solar UV radiation spectrum, the UV A or the UV B or the UV C range, with suitable absorption spectra. The photosensitive elements in the mixtures are either photoisomerizable nematic hosts or photoisomerizable optically active dopants. The selective reflection peaks of these cholesteric mixtures are in the visible part of the light spectrum. The UV exposure changes the cholesteric pitch and hence the position of the selective reflection peak. The consequence is that the colour reflected by each mixture varies under influence of the UV and it can be used as an indicator of the UV exposure itself. We present the mechanisms of the observed effects and possible application features.

Chiral Azobenzene Nematics Phototunable with a Green Laser Beam

Phototuning of Bragg reflection bandgap from green to blue and from red to green wavelengths was induced with a green (λ = 532 nm) laser beam in a cholesteric liquid crystal based on highly sensitive azobenzene nematic BEAM 1005 (BEAM Co.) and UV transparent non-photoisomerizable optically active dopant ZLI-6248 (R-2011) (Merck Ltd.). Optical switching between reflected and transmitted beams, as well as nonlinear transmission and reflection due to Bragg reflection band shift are studied in detail.

Thermal and electrical laser tuning in liquid crystal blue phase I

Thermal and electrical tuning of laser emission from optically pumped blue phase I of dye-doped short pitch cholesteric mixtures have been achieved. Temperature changes or applied electric field to the liquid crystal cells induce structural changes in the blue phase configuration, producing a shift of the photonic band gap. The emission tunability in a structure that in addition allows multidirectional emission may herald a new age of multipurpose laser sources. Furthermore, the reversibility of the effect points out the potential applications of these soft photonic self-assembled materials.