ანდრო ჭანიშვილი

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.

Tuning of a dye-doped cholesteric liquid crystal mirrorless laser in the violet– ultraviolet wavelength range is achieved using two different strategies for controlling the periodicity of the cholesteric helical structures. Both methods are shown produce a shift of the lasing wavelength by 30–40 nm, and smooth variation of the helical pitch can be achieved.

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.

LCDs based on a luminescent dichroic-dye-doped non-absorbing cholesteric LC with positive dielectric anisotropy is proposed. In the initial state, the orientation of the dye molecules provides effective light absorption and irradiation. By applying an electric field to the cell, the absorption and thus the luminescence is absent. A two-color luminescence could be achieved by sandwiching two cells: the upper cell consists of a cholesteric LC with two dyes (sensitizer and emitter) and is used with an applied voltage (active cell); the lower cell consists of a cholesteric LC doped with one dye and works without applying a voltage (passive cell). The performance characteristics of luminescent dye-doped cholesteric-LCDs were investigated.

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.

It has been shown that UV radiation can produce observable color changes in induced cholesteric systems (nematic + optically active dopant) additionally doped with ergosterol (provitamin D2). The effect is based on the photoinduced conversion of ergosterol into vitamin D2, which has the opposite sign of its helical twisting power. The observed shifts of the selective reflection peaks (more then 100 nm after 10 min of UV irradiation) allow both instrumental and visual monitoring of biologically active UV radiation.

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 Förster 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.

A new class of photoactive chiral liquid crystals based on a photoactive nematic host material and a photoinactive chiral dopant was utilized to investigate the behavior of the blues phases when trans-cis isomerization is induced. While the general behavior follows what has been observed in the cholesteric phase, the sensitivity of the blue phases to external parameters causes different behavior when these systems are exposed to UV radiation. The results for four different mixtures are reported and include (1) modulation of the blue phase selective reflection wavelength with low levels of UV and visible light, (2) conversion of one blue phase to another upon exposure to UV light, and (3) induction of blue phases due to UV irradiation when no blue phases are stable beforehand. It is also noted that the supercooled blue phase behaves differently from the other blue phases. All of these results can be understood qualitatively from the ratio of non-nematogenic cis isomers to nematogenic trans isomers and chiral molecules. 

Shifts of selective reflection peaks under UV irradiation (which can be observed as color changes) are reported for induced cholesteric systems (nematic + optically active dopant) additionally doped with ergosterol (provitamin D2) or 7-dehydrocholesterol (provitamin D3). the effect is based on the photoinduced conversion of ergosterol into vitamin D2, and 7-dehydrocholesterol into vitamin D3, with the vitamins and provitamins having opposite signs of their helical twisting power. The observed shifts of the selective reflection peaks (more than 190 nm after 45 min of UV irradiation) allow both instrumental and visual monitoring of biologically active UV radiation.

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.

This work describes the observation of lasing in an intermediate chiral phase of a dye-doped cholesteric liquid crystal mixture. This intermediate phase exists between cholesteric and smectic A phase and it presents anomalous selective reflection properties. The lasing was observed at the long-wavelength edge of the photonic band gap.

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.

Stability of dye doped cholesteric liquid crystal laser emission from several minutes up to two hours and more was achieved by rotating the liquid crystal cell. Significant dependence of stability on surface treatment was observed.

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.

We have demonstrated the LED-controlled tuning of a dyedoped CLC laser. The laser can be tuned in both directions, from longer to shorter wavelengths and vice versa by exploiting the reversible photoisomerization of the azo- and azoxynematic components of the cholesteric mixture. This photoisomerization is controlled by the exposure time and wavelength of incident light. Tuning over a 70 nm range, from 575 to 645 nm, was possible for the sample containing azoxycompounds.

Moreover, the use of LEDs to induce phototransformations and laser tuning represents a very simple but relevant innovation over similar light-controlled tunable CLC laser systems that use UV lamps, primarily because of the possibility of incorporating such small devices inside compact laser systems.

We have investigated defect mode lasing when a cell containing a dye solution is sandwiched between two cholesteric liquid crystal cells. When the cholesteric pitch of both cells is equal, the dye emission generates the typical multi-mode lasing peaks inside the photonic stop band. We prepared two cholesteric mixtures whose pitches were shifted one respect to the other in such a way that only the edges of band gaps of the cholesteric liquid crystals layers overlapped. Using these mixtures we have observed single mode lasing. Moreover the divergence of the laser beam spot is considerably lower than the one observed in dye doped cholesteric liquid crystal lasers.

We have investigated defect mode lasing in a multilayer system consisting of a dye doped isotropic solvent sandwiched between two cholesteric liquid crystal (CLC) cells. In this case we can use dyes not soluble in liquid crystals and avoid the degradation of the CLC structure caused by the absorption of the pumping energy. When the CLC pitches of both cells are equal, the dye emission generates the typical multi-mode lasing peaks inside the photonic stop band. In CLC mixtures whose pitches were shifted one respect to the other in such a way that only the edges of band gaps of the CLC layers overlapped, we have observed single mode lasing. The divergence of the laser beam spot is considerably lower than the one observed in dye doped CLC.

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.

The Erythemal Response Spectrum is a scientific expression that describes the skin sensitivity to the ultraviolet radiation. A long exposure to UV radiation causes skin erythema once a threshold dose has been exceeded.

We present a sensor based on a liquid crystalline mixture, containing dye, that absorbs photons at UV wavelength and emits them in the visible range of the electromagnetic spectrum. In particular the absorption spectrum of the mixture can be modulated to be similar to the Erythemal Response Spectrum. This system presents several innovative features: the absorption range of the mixture can be varied to be sensitive to different wavelengths, the luminescence intensity can be tuned, the system can be implemented on flexible devices, providing the possibility to create a dosimeter for ultraviolet radiation.

Novel combinations of liquid crystalline materials for liquid crystals lasers tuning are presented. The cell is based on a three layer system, with two cholesteric films sandwiching a third layer containing an isotropic mixture of a photoluminescent dye. One of the cholesteric films has a wide band gap and the other one presents a narrow band gap, allowing the laser emission tuning over a wide spectral range, from UV to near IR. Further, using mixtures containing cholesteryl esters, a temperature control can be achieved.

Two ways of employing cholesteric liquid crystals in tunable dye lasers are considered: the cholesterics as distributed feedback medium and the cholesterics as resonator mirrors. In the dye doped distributed feedback cholesteric liquid crystal lasers the frequency tuning is achieved exploiting light induced effects or using a specially designed cell assem-bling a chiral dopant concentration gradient in combination with suitable distribution of different dyes. Another ap-proach represents the lasing in a multilayer system consisting of a dye doped isotropic solvent sandwiched between two CLC layers.

In this paper we propose a new method of information recording based on the photo controllable shift of the selective reflection wavelength in photo sensitive cholesteric liquid crystals.

We have investigated penetration of NIR through the prostate tissue. It was shown that NIR of 850 nm has highest penetration. Infrared image of noncancerous prostate represents picture with approximately even brightness. However obtained near-infrared images of the cancerous outgrowths of the prostate revealed dense and dark foci, which proved to be malignancy on pathology (adenocarcinoma, Gleason score 6[3+3] in 7 cases and adenocarcinoma, Gleason score 7[3+4] in 3 cases). At brighter areas the pathology report did not reveal the presence of malignant cells. Obtained IR images give opportunity to distinguish prostate cancerous outgrowth with the dimensions of several millimeter. 

A new type of lasing was obtained on the basis of a dye-doped cholesteric liquid crystal laser with transversally distributed excitation. Two coherent beams of the pumping laser formed an interference pattern in the planar dye-doped cholesteric layer and provided laser generation in the separate narrow strips of the pumped area. Each of the strips demonstrated lasing along the cholesteric axis. But, due to the mutual coherence of the separate strips emission, the total picture of lasing represented an interference pattern. Thus, in contrast to the conventional lasers, the intensity distribution in the laser emission pattern had a periodical character similar to diffraction from an elementary dynamic hologram.

Spatial modulation of laser emission controlled by the structure of excitation light field was demonstrated. A dye doped polymer film as an active medium was sandwiched between two laser mirrors forming a laser cell. The pumping was performed by an interference pattern formed with two mutually coherent beams of the second harmonic of a Q-switched Nd:YAG laser (532 nm) and located in the plane of the laser cell. The laser emission was observed normally on the plane of the cell. The cross section of the obtained laser emission was modulated in intensity with an interval between maximums depending on the period of the pumping interference pattern. Thus, the emitted light field qualitatively looks like diffraction from an elementary dynamic hologram, that is, a holographic diffraction grating.

With the rapid development of modern information technology, massive amounts of data are stored that require an information security. Here, we introduce a spiropyran doped polymer dispersed cholesteric liquid crystal film based rewritable medium that can produce high quality visual two-dimensional barcodes for anti-counterfeiting, authentication and security purposes. The prepared film exhibits excellent photochromic properties, improved photosensitivity, high spatial resolution, and numerous rapid write/erase capabilities. Such rewritable material can be efficiently printed over 70 cycles without compromising its readability. Furthermore, the fabricated film is mechanically resistant to small stresses and, hence, it can be directly handled.

A few microns thick layer of an optically activecholesteric liquid crystal is realized by using a photoiso-merizable nematic component and a chiral dopant. It isshown that such a photosensitive optically active mediumcan be used as a holographic material for optical informa-tion and dynamic grating recording. The photo-inducedgratings are written by exploiting the light-induced pho-toisomerization phase transition from an optically activechiral liquid crystal to an isotropic liquid, which resultsin the rotation of the light polarization plane from 90 to0 degrees and corresponding to maximum, respectively, zero transmittance. The results highlight applications in the eld of optical storage by the recording of static gratings, as well as in the eld of nonlinear beam-coupling viathe holographic writing of dynamic gratings

The real-time thermal imaging systems, which allow the rapid conversion, acquisition, and manipulation of obtained optical information, are the emerging technologies that offer a variety of imaging applications. Here, we present a novel type of thermal imaging device, based on the thermo-optical properties of liquid crystal blue phases. Herewith, the novelty lies in the use of a weak first-order phase transition between the blue phases controlled by external thermal fields. In turn, the stimulated interconversions of the selective reflections between the blue phases enable the visualization of the two-dimensional spatial distribution of the thermal fields. The real-time room temperature operation capabilities of the proposed thermal imaging device may enable applications in areas such as medicine, astronomy, security, surveillance, people tracking, aerospace monitoring, and artworks inspection.

A thin layer of the photosensitive cholesteric liquid crystal possessing a high value of the optical activity is studied. Reversible change in the optical activity controlled by two light emission diodes (LEDs) with different emission wavelengths is applied for information recording. The behavior of this structure under exposure to UV and blue light is investigated. A smooth decrease in optical activity caused by the UV light resulting in a change in the color of the structure between crossed polarizers is obtained. The reverse process occurs under exposure to the blue light. On the basis of the obtained results, two methods of optical information recording in such a layer have been demonstrated.

In this work, we demonstrate micro resonators made of liquid crystal blue phase (BP) microspheres, embedded in a polymer/water matrix. The omnidirectional 3D lasing from BPII and BPI microspheres and the temperature-controlled laser tuning within the range of 55 nm from the BPI microspheres were observed for the first time. The potential applications of BPs microlasers range from temperature-controllable, omnidirectional, coherent light micro sources to informational displays and micro sensing devices.

We have prepared the liquid crystal blue phase (BP) microdroplets emulsified in the glycerol environment and investigated their electro-optical properties upon the applied electric field. We showed that the lasing from the supercooled luminescent dye-doped BPI microdroplets can be controlled electrically. When the electric field intensity is increased, the lasing intensity is increased too along with the linewidth that decreases from 6 to 2.5 nm. Besides, the laser lines were shifted toward the shorter wavelength by 11 nm. All effects were achieved via electric field-stimulated structural and symmetrical modification of BPI microdroplets, resulting in their optical anisotropy and birefringence. Luminescent dye-doped liquid crystal blue phase-based microdroplets can find applications as electrically controlled microlaser sources for microphotonics and environmental sensing.

We have developed a cholesteric liquid crystal (CLC) mirror-based innovative model for detection and visualization of images in turbid media. Due to its unique optical-polarization properties, the CLC mirror is suggested as the basic element of the imaging Stokes polarimeter. The particular design of the proposed polarimeter, coupled with its distinguished operational simplicity, reliability, and real-time operational facilities, promises to fabricate a new generation of the imaging Stokes polarimeter, which can find applications in areas such as diagnostics, biology, astronomy, and remote sensing.