Reversible Tuning of Lasing in Cholesteric Liquid Crystals Controlled by Light‐Emitting Diodes

Abstract

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 ...

Self-assembled organic and polymer photonic crystals for laser applications

“…However, the other reports have been successfully made on the photoreversible switching of laser emission peaks by combining photochromic compounds such as azobenzene derivatives in the CLC media. [70][71][72] It is greatly advantageous to realize the reversible photocontrol of the laser emission peak for the next-generation photonics.…”

Color‐ and Reflectance‐Tunable Multiple Reflectors Assembled from Three Polymer Films

“…Here, after breaking the sample to measure the SEM images, the ductility of PPFVB polymer causes the PPFVB film in Figure 4a to intrude into the lower-lying layers. Figure 4b and 4c show the experimental (solid lines) and theoretical (dashed lines) reflection and transmission spectra for M 3 2 system with 14 layers, respectively. All these spectra clearly indicate the presence and reflectance-tuning of multiple PBGs from the multireflector, where the reflection peaks in Figure 4b correspond to the transmission dips in Figure 4c Second, we prepared the multireflector (Fig.…”

“…[1] Tuning of such PBGs in the visible region is attractive for active photonic applications in display, sensor, and lasing devices. [2][3][4][5][6][7][8][9] Here, we design and fabricate a tunable multiple PBGs system consisting of an anisotropic polymeric nematic liquid crystal (PNLC) film and two kinds of isotropic polymer film, and demonstrate tuning of reflectance and colors of the multiple PBGs in the whole visible region by changing the polarization direction of linearly polarized incident light. The only difference between the two tunable systems is the stacking sequence.…”

“…The spectral characteristics of the PBGs can be modulated by changing the refractive indices, thicknesses, and periodicities in given photonic crystals, which can be performed by external light, [2,3] electric [4,5] and magnetic [6] fields, and chemicals. [7][8][9] However, to obtain broad tunability covering the complete visible spectral range, exceptionally large changes such as enormous birefringence or swelling/de-swelling length are required.…”

Different Approaches of Employing Cholesteric Liquid Crystals in Dye Lasers

“…The reversible tuning of the lasing wavelength in CLCs containing nematicazo-or azoxy-photo-sensitive components was accomplished by varying the time of irradiation of samples with the use of two LEDs: at 405 nm wavelength for blue shift and at 466 nm wavelength for red shift [38]. The second harmonic signal from a Nd:YAG laser was used as light source for pumping the cholesteric resonators.…”