Degenerated four-wave mixing in chiral nematic liquid crystal exhibiting Bragg-like reflection

Abstract

Summary form only given. We investigate the optical phase conjugation phenomenon in chiral nematic liquid crystal. Nematic chiral liquid crystals exhibit Bragg like reflection - or light stop band - for left or right circular polarization. We use the mixture of chiral liquid crystal cholesteryloleate and nematic E7 mixture in volume proportion 2:1. The photonic band gap of such a chiral mixture can be tuned by changes of temperature and shifts over the whole visible light spectrum (cf. Fig.1a). We enhanced the nonlinear optical properties of liquid crystal mixture by addition of photochromic azo-dye Disperse Red 1.In proposed novel optical configuration shown in Fig. 1b for degenerated four-wave mixing we used only two incident beams: a pump beam and a signal one. The pump light beam propagates parallel to the long axis of a chiral structure. The counter propagating, second pump beam results from a Bragg reflection of the pump beam on the chiral structure with a suitable pitch. The signal beam illuminates the sample at small angle with respect to the pump beam. Both incident beams came from the same cw Nd:YAG laser doubled in frequency (λ = 532 nm) and are coherent. Due to Bragg condition the phase conjugate to the signal beam is observed only in the small temperature interval. The intensity of phase conjugate beam strongly depends on pump and signal beams polarization states, being maximal for left circular polarization and vanishing for right circular polarization states of both beams. The signals of the optical self-pumped phase conjugation and self-diffraction are observed at low laser light intensities (P <; 1 mW) due to resonant enhancement. This enhancement is due to the matching of the light wavelength and natural material photonic band gap which 'slow downs' the light and thus can amplify light - matter interaction. The role of azo-dye is crucial for creating of suitable diffraction gratings which arise in a material. We estimated a value of third order optical susceptibility χ(3) =2.8 x 10-17 m2/V2 and we attribute it to enhancement of azo-dye induced molecular reorientation process of liquid crystal molecules.