Nanooptics and photonics

Random lasing in chiral nematic liquid crystals
Iljin A.1, Gao S.2, Li W. 2, Zhang X.2,
1 Department of Physics of Crystals, Institute of Physics, NAS of Ukraine. Prospect Nauki, 46, Kyiv 03028 Ukraine. 
E-mail: lgtc@iop.kiev.ua
2 The MOE Key Laboratory of Weak-Light Nonlinear Photonics, TEDA Institute of Applied Physics and School of Physics, Nankai University, Tianjin 300457 China.

The studies of random lasing (RL) in strongly distorted chiral liquid crystals (CLC) are presented. The model CLC-system is based on mixtures of the nematic E7 LC with different concentrations of a chiral agent CB15. It was doped with efficient laser dye of the boron dipyrromethene type and pumped with the second harmonic of a Nd:YAG laser (λ = 532 nm). Spectral properties of emission are characterized in dependence on the spectral detuning of the pumping wavelength from the position of the selective reflection band; cell parameters and the morphology of the multidomain CLC textures are to be also taken into account. 
Multi-domain CLC samples were fabricated by quenching the CLC from the isotropic state by the fast cooling down in the unaligned cell. The use of CLC having short helical periods has been proved very useful for the fabrication of random strongly scattering media, parameters of which could be easily controlled by the LC composition. These polydomain CLC textures are easy to create, they strongly scatter light in a wide range of wavelengths and exhibit interesting optical properties.
The average size of CLC domains determines the transition of the type of emission from feedback CLC lasing for larger domains to the random one for a strongly disturbed medium. Ratio of the mean path length to the transport mean free path determines the character of the emission band broadening (homogeneous or inhomogeneous) and its half-width energy dependence.