Lasing in induced cholesteric liquid crystal (CLC) containing highly photosensitive azo chiral dopant (ChD) and pyrromethene laser dye was investigated. Due to the absence of excitation energy transfer from the dye molecules to the molecules of ChD in such a system, a low lasing threshold was achieved. When using violet and green lightemitting diodes, reversible frequency tuning was obtained in the range of about 30 nm with a tuning time 1-2 orders of magnitude smaller than in conventional CLC laser systems based on azo and azoxy photosensitive compounds. Further increase of the intensity of the excitation light by using a green laser pointer allowed us to achieve a record phototuning speed (∼21 nm in 148 ms).Distributed feedback (DFB) lasers based on dye-doped cholesteric liquid crystals (CLCs) were first established in 1980 [1]. Based on the characteristics reported to date, such as threshold excitation intensity, linewidth, and energy efficiency, they can actually be used for development of high-brightness laser displays [2]. Perfection of components for these devices, e.g., development of dye activators with minimal triplet-triplet absorption, CLCs with a high melting point and temperature-independent pitch length make it possible to produce lasing under continuous excitation [3].Since the creation of the DFB laser, a practical problem is to develop a method for tuning of its frequency, which would provide high speed and reversibility of this process. In practice, the tuning is achieved by changing the pitch of the CLC's helix, impacting on it by mechanical, thermal, electrical, or light factors [4]. The last process, photocontrolling of helical pitch, is of particular interest due to the remote, spatial, and temporal activation advantages of light stimulus. The method of tuning the lasing frequency based on this effect, which more generally is seen as a method of controlling light by means of light, is considered the most promising in terms of the above requirements of the process of tuning [4].Variation of the helical pitch under the influence of light is due to a change in the helical twisting power of chiral molecular switches in CLCs as a result of the photoconversion of molecules of the chiral dopant (ChD) [5-9], or liquid crystal host [10][11][12][13]. Often, these phototransformations (photo-Fries reaction, photolysis reaction, etc.) cause irreversible changes in the helical pitch [5,6]. However, some phototransformations (such as a trans-cis isomerization of azo and azoxy compounds) are generally reversible; the photoexcited cis isomers of such molecules relax to initial trans form when heated or exposed to light of a different wavelength. The case of cis-trans photoisomerization is the most interesting, because it allows one to change the frequency of lasing in two opposite directions by means of the same, quite a technological factor, i.e., light.Note that the efficiency of trans-cis and cis-trans photoisomerization and, consequently, the efficiency of lasing frequency tuning, depends on a number o...
