A new method of lasing frequency phototuning was studied for dye-doped cholesteric liquid crystal (CLC) mixtures of azoxy-nematic ZhK-440 and cholesterol derivatives by variation of their helical pitch under irradiation by light of different wavelengths. For most lasing dyes introduced into CLC systems the fluorescence quantum yield becomes substantially lower at weight concentrations 0.3-0.5%, which hinders realization of lasing. A dye of pyrromethene class has been found, showing fluorescence quantum yield of more than 50% at the above-indicated concentrations. Lasing of distributed feedback (DFB) laser in CLC on the basis of azoxy nematic ZhK-440 has been obtained, and conditions were studied for minimization of its threshold and broadening of the reversible phototuning frequency range
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The first observation of lasing in liquid crystals was probably reported in , with nematics activated by dichroic polymethine dyes placed in a standard mirror resonator. These experiments have shown that the ordered structure of the dye-doped nematic is stable in the field of short nanosecond laser pumping pulses, which allows control of the threshold excitation intensity and emission polarization of such a laser under reorientation of the nematic director with respect of the pumping polarization direction.
The use of cholesteric liquid crystals (CLC) in lasers started by their application as resonator mirrors of tunable lasers involving isotropic solutions of dyes . By variation of CLC temperature, tuning was obtained of a wide lasing spectrum, which was synchronous with changes in CLC helical pitch, supporting the assumption of stability of CLC helical ordering in the field of nanosecond pumping pulses.
Combining CLC and organic dyes in one and the same medium allows creation of microlasers with distributed feedback (DFB) [3,4], which is realized due to Bragg scattering and amplification of the dye emission on helical periodic structure of the liquid crystal. This feature makes DFB-lasers based on dye-doped CLC very promising for display systems of high brightness, because the absence of a mirror resonator allows fabrication of the active medium of such lasers in the form of a surface with arbitrary area and curvature.
In studies of polarization characteristics of dye fluorescence in CLC in the direction along helical axis, a new phenomenon was recorded  – a gap in the dye fluorescence spectrum for diffracting circular polarization at frequencies of selective reflection (SR), being an evidence of “locking” of photons in the periodical structure. It should be noted that a possibility of such deformation of the fluorescence spectrum in the periodical structure was not considered even theoretically , and fluorescence studies of dye-doped CLC using a standard procedure at 45° to the helical axis could not reveal such deformation in principle, because under such conditions the SR band did not fall into the fluorescence band.
At first, lasing in dye-doped CLC was obtained  and further studies of its characteristics were carried out [6–9] using three-component mixtures of cholesterol derivatives, characterized by low birefringence (∆n≈0.05) and weak temperature dependence of helical pitch (< 3 nm/K). The first experiments suggested that spectral and energy characteristics of lasing in such CLC could be explained within the coupled wave model [10,11]. This model was supported by location of the lasing spectrum at the SR band center, strong selection of longitudinal modes, as well as low energy yield due to strong wave coupling in the CLC layer. Recent studies of lasing thresholds and lasing spectra in steroid CLC as function of the planar texture perfection have also confirmed that lasing in these systems can be explained by the coupled wave model .
The main difference between the coupled wave model  and photonic model  is that in the first case the optical band gap, which is relatively narrow due to low birefringence, disappears in the presence of gain and gain modulation coefficient, while in the photonic model the gain is maximal at the band gap edge and minimal in its center. Within the band gap, the wave exponentially decays when propagating into the CLC layer; correspondingly, the density of states in the band gap is strongly decreased. Since the intensity of spontaneous emission is proportional to the number of photon states, it is suppressed within the gap, i.e., within the selective reflection (SR) band; accordingly, it increases at the band edge. Since in dye-doped steroid CLC there is gain within the SR band, as well as gain modulation due to partial ordering of the dye in CLC (orientational order parameter S = 0.25 ), location of the lasing spectrum at the SR band center, which is observed experimentally in such materials, suggests that lasing mechanism in this case corresponds to the coupled wave model. Respectively, in induced cholesterics (including those based on azoxy nematics) location of the lasing spectrum at the SR band edge shows that lasing in such CLC should be explained by the photonic crystals model.
It should be noted that studies of eventual damaging of CLC helical structure by powerful nanosecond pulses  have revealed substantial changes in optical characteristics after ~100 ns, which sets a limit for pumping pulse duration in CLC-based lasers. With CLC planar textures of high perfection, diffraction rings were observed in the spatial emission pattern of such DFB lasers . An increase in lasing energy yield with higher excitation intensity, presumably due to weakening of the wave coupling in steroid CLC as an effect of the induced thermal lattice, was reported in .
Prospects of practical application of CLC-based DFB lasers for information display stimulated their active studies by many researchers [13–18]. In most cases, instead of highly viscous cholesterol derivatives, CLC of other types were used – so-called “induced cholesterics”, in which helical twisting is induced in nematic matrices by optically active (chiral) dopants. Characteristics of lasing in such materials show certain peculiar features that require the use of other models for their explanation. Location of the lasing spectrum at the edge of SR band, higher energy yield, lower selectivity (generation of more than three longitudinal modes , which was never observed in steroid CLC ) cannot be described by the coupled waves model. These features, first of all, the lasing spectrum location with respect to SR band, were explained by the model of lasing at the non-transmittance band edge of photonic crystals . Such description of lasing in induced CLC was first proposed in . It is the photon crystals model [20,21] that is the base of modern theory of lasing in induced CLC [22,23]. Results of experimental studies of lasing in induced CLC are presented in several review papers [24,25].
The main method used for tuning of lasing frequency in CLC-based DFB lasers is still temperature variation of the helical pitch [3,8]. Low tuning speed and the need for thermostabilization of CLC hinder broad application of such lasers. Therefore, search for new and more efficient methods for frequency tuning of CLC lasing is an important problem.
It is known that in some types of induced CLC based on nematic liquid crystals (NLC) with chiral dopants (CD) molecular conformation of these dopants can change under UV irradiation, with corresponding changes in their helical twisting power. CLC of this type, including those with non-mesogenic CD, because of the presence of different photoisomer forms of molecules and easiness of their phototransformation  can show variation of their helical pitch under excitation of certain molecular stereoisomeric forms.
Earlier the effect of changes in the dopant helical twisting power due to transition from trans- to cis-conformation, which is accompanied by variation of the helical pitch, was used for holographic recording , as well as for frequency control of CLC-based DFB-laser [27–29]. Since the absorption bands of trans- and cis-conformations of the CD used were very close , this method of phototuning did not allow selective irradiation of specified molecular conformations and control of their composition in the irradiation zone. Therefore, the frequency variation (tuning) process could be only unidirectional, with increased concentration of cis-conformation under irradiation, decreased helical twisting power of the dopant, and continuous shift of the lasing line towards longer wavelengths. The reverse process, i.e., restoration of the initial stereoisomer composition, occurs spontaneously during 17-20 hours and is practically independent of the external factors (temperature, electric field, etc.) .
Recently, new ways have been proposed for realization of controlled reversible phototuning of helical pitch in CLC. To achieve this, it was proposed [30,31] to use nematics on the basis of azoxy compounds (azoxy nematics) with absorption bands of stereoisomers located in such a manner that irradiation with light of wavelength above ~410 nm results in selective irradiation of only one cis-conformation. Such location of the absorption bands allows their selective excitation and reversal of the direction of frequency tuning of lasing.
However, it appeared that at concentrations of lasing dyes in azoxy nematics even at the level of several tenths of one percent strong fluorescence quenching is observed, and the obtained quantum yield is not higher than several percent, which is not sufficient to reach the lasing threshold . By the beginning of our work, there have been no information in literature on lasing in such materials; however, recently a possibility of lasing in such materials was reported .
Therefore, our work was aimed at studies of spectroscopic characteristics of dyes of different classes in CLC based on the available azoxy-nematic ZhK-440 and selection of dyes with sufficiently high fluorescence quantum yield, which should allow us to obtain sufficiently low lasing threshold, to realize reversible frequency tuning under irradiation by light with different wavelengths and to broaden the frequency range of tunable lasing.
2. Idea and experimental details
Studies of frequency tuning of DFB lasers on the basis of induced CLC realized by UV irradiation leading to trans-cis isomerisation of the chiral dopant in the nematic liquid crystal have shown that the lasing frequency shift is correlated with the UV radiation dose and is linear at small doses . Such linear shift can be used for controlled tuning of lasing spectrum of such DFB lasers, provided a method is found for efficient reverse cis-trans transformation of molecules in induced CLC. One of the ways could be using dopants with greater difference between absorption peaks of both photoisomeric forms and controlling the ratio of photoisomers in the active substance of DFB laser by their selective excitation.
For realization of reversible phototuning, we used CLC mixtures composed of ~75% of nematic ZhK-440 and ~25% of mesogenic chiral dopant M5 (a mixture of cholesterol esters: 30% cholesteryl formate, 5% cholesteryl butyrate and 65% cholesteryl nonanoate). CLC were activated by dyes of different types (benzanthrone, phenolene, pyrromethene) at weight concentration 0.2-0.3%. The selective reflection (SR) band maximum was located close to the fluorescence maximum of the dye (in the spectral region close to 600 nm). The basic mixture in our temperature, spectral and lasing studies was the mixture of ZhK-440 and M5 of the above-noted composition. To vary the SR band location inside the fluorescence band of the dye, the composition of M5 could be changed within 1-3%. Absorption (transmission) spectra of the CLC planar texture were measured using SF-20 and Hitachi-330 spectrophotometers, fluorescence spectra and their quantum yield were determined using a Hitachi MPF-4 spectrometer.
Planar textures of CLC were prepared by rubbing the surfaces of the cell substrates forming an ordered liquid crystal layer. For transmission and lasing measurements in the visible range, we used quartz substrates covered with a layer of polyimide lacquer rubbed unidirectionally. In studies of absorption spectra in UV range, sapphire substrates were used. The planar texture was formed by application of a polyvinyl alcohol (PVA) layer by dipping the substrate into 0.1% PVA water solution, drying and rubbing by a soft fabric in one direction.
Layer thickness of CLC (from 3 to 30 microns) was set by Teflon spacers. For UV irradiation, we used a low pressure Hg lamp of DRK-120 type with air cooling (radiation power 120 W, stabilized discharge current). Optical pumping of the DFB laser on the basis of CLC was carried out by the second harmonic (λ = 530 nm) of a Q-switched Nd3+ laser operating in a slow pulse repetition rate mode with the pulse duration τ і ≈18 ns. The lasing spectra of the dye-doped CLC corresponding to each pumping pulse were optically imaged in a focal plane of a spectrograph with an inverse dispersion 0.6 nm/mm and then displayed by the web camera on a PC monitor.
3. Results and discussion
The effects of UV-irradiation on CLC based on azoxy nematics were first studied by optical absorption spectra. Figure 1 shows absorption spectra of a thin (3 microns) CLC layer in the liquid crystalline phase at 28°С. After UV irradiation (20 mW/cm2, 10 minutes), the absorption band of ZhK-440 with maximum at ~360 nm (Fig. 1,curve 2) becomes less intense as compared with the same band before irradiation (Fig. 1, curve 1), indicating the transition from trans- to cis-isomer. After subsequent 25 min irradiation with ZhS-10 filter (which cuts off light with λ < 410 nm), absorption of the 360 nm band is restored (Fig. 1, curve 3) as a result of partial reverse transition from cis- to trans-form of the azoxy compound.
In the isotropic phase of the same CLC, as it can be seen from (Fig. 2 ), a similar trans- to cis- transition is observed (curves 1 and 2), but the reverse transition after irradiation with ZhS-10 filter is not noted (Fig. 2, curve 3). In absorption spectra of the same CLC mixture dissolved in standard organic solutions, the reverse cis- to trans- transition was also notobserved. This behavior is a peculiar feature of the photoisomerization process in azoxy nematics.
It should be noted that at temperatures corresponding to the isotropic phase the measured values of optical density D for one and the same sample were noticeably higher than in the cholesteric phase. This can be related to absorption dichroism of azoxy-compounds – in the cholesteric planar texture the measured D value, in fact, is close to , while above the isotropic transition temperature Ti, when orientational ordering vanishes, a contribution appears from the much larger value of D║  ( and D║ are optical density values for directions respectively perpendicular and parallel to the director of the quasinematic layer). Similar behavior – substantial increase in the measured optical density at the nematic to isotropic transition – was noted in an earlier paper .
Alongside with changes in the SR band location, changes in other optical characteristics are also observed in the process of trans-cis isomerisation of azoxy-nematics.
Figure 3 shows SR bands of the basic mixture of ZhK-440 and M5 before (curve 1) and after (curve 2) UV irradiation, as well as reversing of the SR band in the process of cis-trans isomerisation upon irradiation with ZhS-10 filter (curve 3). It can be seen that in the cis-form the birefringence is substantially lower, which is reflected in narrowing of the SR band. This, in turn, will lead to an increase in the lasing threshold of the DFB laser, since the Q-factor of such structure is related to the modulation depth of the refractivity index, i.e., the birefringence value. This fact, alongside with amplification selectivity of the dye, should be accounted for in optimization of characteristics of such DFB lasers and broadening of the reversible frequency phototuning.
In our studies of spectroscopic characteristics of dyes in CLC based on azoxy nematics, quenching of dye fluorescence appeared to be a major problem. For most of efficient fluorescent dyes, quantum yield in CLC was 5-10 times lower as compared with standard solutions. Since normal operation of CLC-based DFB lasers requires fluorescence quantum yield of the doping dye at the level of 50%  and more, our search led to two classes of dyes apparently suitable for this purpose – phenolenone dyes (NIOPIK, Russia) and pyrrhomethene dyes produced by Exiton. The latter are distinguished by high fluorescence quantum yield and high lasing efficiency in isotropic solutions and polymers . Among them, the highest quantum yield (60.9%) in CLC based on mixture 27.6% М5 + 72.4% ZhK-440 was found for pyrrhomethene dye No 597, which was used for our lasing experiments.
Figure 4 shows spectral characteristics of this dye in CLC based on mixture of ZhK-440 and M5. The absorption maximum (curve 1) in the region of 410 nm is the absorption band edge of nematic ZhK-440, the peak close to 540 nm is the absorption maximum of the dopant dye, and the shoulder in the region of 590-570 nm corresponds to the SR band of CLC. To broaden the frequency tuning range and obtain lasing at the largest possible distance from the dye flourescence maximum, an optimized mixture of composition 26,9% М5 + 73.1% ZhK-440. In this mixture, lasing was obtained at 613 nm, which corresponds to the long-wave edge of the SR band of the CLC matrix.
The lasing spectra obtained were composed of one broad line or two lines, the distance between them corresponding to the distance between longitudinal modes for the layer thickness of 15 μm. Such shape of the spectrum is mainly due to low quality of the planar texture formed on substrates without SnO2 layer, higher scattering in CLC based on azoxy nematics and rather high threshold excitation intensity.
The values of threshold intensity of lasing excitation, at the layer thickness of 15 μm, were in the range of 600- 800 kW/cm2, which is noticeably higher than in DFB-lasers based on steroid CLC, as well as induced cholesterics of other types . From the standpoint of lasing threshold, the method of formation of CLC planar texture used in these experiments was probably not optimal. Our experiments on lasing threshold optimization have shown that the use of SnO2 layer covered by polyimide lacquer makes it possible to decrease the threshold excitation intensity by 12-15 times with the same CLC material at the same layer thickness. However, absorption of UV light by the SnO2 layer can affect the speed of CLC pitch variation and the range of its possible changes.
Figure 5 shows the location of the medium line in the lasing spectrum of pyrrhomethene dye in the CLC used as function of irradiation time by Hg lamp (curve 1). As it can be seen, the achieved tuning range in the short-wave direction is about 43 nm and is limited by dye amplification selectivity and high threshold excitation intensity. The reverse lasing frequency tuning (on irradiation by Hg lamp with ZhS-10 filter) requires larger time and, as in the CRC material without dye, the obtained shift is by nearly 8 nm smaller. Our studies of the dynamics of lasing frequency tuning have shown no noticeable spontaneous frequency reversal during the measurement of lasing spectrum, which, together with sample irradiation time and recording of the transmission spectrum, lasted from 2 to 6 min. Check-up of the transmission spectrum after UV irradiation shows that lasing spectrum is invariably located at the long-wave edge of SR band.
Figure 6 shows tunable lasing spectra for another CLC mixture, where the tuning range of 35 nm was obtained. It can be seen that, due to rather high lasing threshold, the main Bragg mode is involved, which is broadened over spectrum due to generation of non-collinear beams at high excitation levels. When the amplification maximum at 575 nm is approached, one of the side modes also begins to contribute because of lowering of the threshold in the 564-570 nm range (insets a and b).
Thus, in our studies we have shown that the use of pyrrhomethene dyes as activators of azoxy nematics is promising for DFB lasers based on induced cholesterics. Sufficiently high fluorescence quantum yield of these dyes in azoxy-nematics, together with the use of more perfect planar textures, will allow substantial lowering of threshold excitation intensities in such DFB lasers, thus widening the range of lasing frequency tuning.
Shortening of the tuning time is possible by optimization of illumination power and its spectral range. It should be noted that, because the absorption band edge of the nematic is located in the blue spectral region, the best suited for application in such DFB lasers, from the viewpoint of broadening of the tuning range, will be dyes with fluorescence bands in orange and red spectral ranges.
The main task in this method of frequency tuning is creation of high-quality planar texture, together with minimization of radiation losses of UV illumination at sufficiently high fluorescence quantum yield of the doping dye.
Authors are grateful to Prof. E. A. Tikhonov for providing the pyrromethene dye used in our experiments and to M. I. Serbina for providing the optimized CLC mixtures and measurements of their absorption spectra.
The work was partially supported by the State Fund for Fundamental Researches (project F 40.2/066) and Goal-Oriented Program of Presidium of NAS of Ukraine (Project VTs-138).
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