A new approach based on the use of cholesteric liquid crystals (CLCs) and dye-doped light-sensitive chiral dopants was employed to create lasing materials with reversible tuning and switching. The lasing wavelength of optically-pumped dye-doped cholesteric liquid crystals (CLCs) is shifted by irradiation with UV light. The shift depends on the UV light exposure. Lasing is switched off at high levels of UV light irradiation. A qualitative model describing different lasing regimes is proposed.

Chiral hydrogen-bonded polymer films that respond to the presence of some amino acids (arginine, lysine, histidine) in water by changing the color and shifting the wavelengths of the selective reflection band (SRB) were synthesized and studied. The kinetics of the film's response depends on the concentration of donor/acceptor groups in the polymer matrix. A higher concentration of hydrogen-bonded groups results in a faster shift of the SRB and color changes. This effect is explained in terms of structural changes and the breakage of hydrogen bonds that occurs between the components of a cholesteric polymer, immersed in various aqueous solutions of amino acids. Optical pumping of cholesteric films doped with laser dyes leads to lasing. The changes in the selective reflection induced by amino acids in water solutions result in a shift of the lasing wavelength.

Cholesteric liquid crystals (CLC), polymer stabilized cholesteric liquid crystals (PSCLC), and polymer dispersed cholesteric liquid crystals (PDCLC) are promising dye-doped optically pumped lasing materials with an inherent photonic band gap structure. At low polymer concentrations, lasing from PSCLC occurs in modes near the edge of the selective reflection band. Electric field applied to the samples sandwiched between conducting glasses enhances PSCLC lasing emission. Lasing behavior changes dramatically at higher polymer concentrations in PDCLCs:  numerous lasing peaks appear on the top of the emission band. New effects are explained in terms of lasing in photonic defect modes and photonic band gap modes.

The twisting ability of a novel series of bridged binaphthol derivatives with substituents of various lengths and chemical nature in the 6,6'-positions, recently synthesized and used as dopants in nematic solvents, is investigated with the help of the model based upon surface-helicity tensors. Structures of the low energy conformers of these compounds have been generated by molecular mechanics calculations. Their orientational behaviour and the coupling between anisotropy in the alignment and molecular chirality, which are at the origin of the helical twisting power, are analysed on the basis of the anisometry and the chirality of the shape

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The light-controlled tuning of cholesteric liquid crystal (CLC) lasers has attracted considerable interest over the last few years because of the possibility of developing tunable chiral mirrorless lasers emitting over a wide range of wavelengths. It is expected that these laser sources will be easily tunable because of the high sensitivity of the helical pitch of CLCs to external fields. Several different approaches for changing the helical pitch have been proposed, such as changing the chiral dopant concentration, varying the temperature, and applying an external voltage. However, particular attention has been paid to controlling the helical pitch using light in order to build an optically tunable system. [1][2][3][4][5][6][7] After the first report on phototunable lasing in dye-doped CLCs, [1] where this possibility was demonstrated by exploiting the phototransformation of the chiral dopant based on a photo-Fries mechanism, [2] several other reports have also been published. [3][4][5][6][7] Using photo-Fries processes, these novel approaches offer the possibility of extending the irreversible tuning range to ca. 40 nm, [1] based on huge pitch variations that can be larger than 400 nm. [1,8] However, in the original case, as in the method reported by Fuh et al., [4] the irreversibility of the photoinduced processes significantly narrows the possible applications of these systems. In order to achieve fully controllable and reversible laser tuning, novel effective compounds and mechanisms for controlling the pitch are required. One possibility is to use chiral and achiral photochromic azobenzene derivatives. These molecules can be dissolved in the cholesteric mixture, where they act as molecular switches by inducing changes in the chirality of the whole system upon irradiation with UV light. [6][7][8] In recent reports, reversible light-controlled lasing has been achieved by the trans-cis photoisomerization of chiral azodopants. [6,7] In these publications, the azobenzene is a structural part of the chiral molecules. Under UV (350 nm) irradiation, rod-shaped trans molecules are transformed into bent cis molecules. This transformation alters the twisting power of the chiral dopants, inducing helical changes in the cholesteric matrix, and therefore shifting the selective reflection band (SRB). Shibaev et al. [6] have reported a rapid shift of the lasing wavelength (by ca. 40 nm) and position of the SRB after 2 min of low-power UV irradiation; relaxation back to the initial lasing wavelength and SRB position takes about 20 min. A larger change of pitch (up to 110 nm) has been reported by Lin et al. [7] after 20 min of UV irradiation; relaxation to the initial state takes a much longer time in this case (about 20 h). Azobenzene materials also appear to be very attractive for other applications: several reports not related to lasing have been published pertaining to investigations of a wide variety of novel mixtures of chiral and achiral azobenzene derivatives. Several different experimental efforts in the literature ...

Lasing is reported at the edge and in the middle of the selective reflection band produced in the visible region of the spectrum for the cholesteric phase of a lyotropic liquid crystal formed from an optically active polyisocyanate in toluene. This is the first observation of lasing from a stiff polymer based lyotropic liquid crystal. Lasing was observed at a relatively low threshold when the selective reflection band overlaps with the emission peak of an incorporated dye molecule. The absence of fine structure of the selective reflection band at wavelengths near the edge of this band indicates that the cholesteric liquid crystal is not perfectly periodic. Lasing in these multidomain films may therefore be associated with defects in the periodic structure.

Polymer materials able to change reflective properties due to mechanical deformation fundamentally challenge the theory of soft materials and are important for a number of emerging applications. The most promising of those are chiral lasers. In this communication, we report novel cholesteric materials that display large color change from far red to blue and a shift of the position of the selective reflection band under uniaxial strain from near infrared to ultraviolet. Optical pumping of these materials which are doped with laser dyes, leads to lasing at the wavelengths controlled by strain within the emission interval of laser dyes of 80 nm.