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Optics Letters

Phase conjugation and slow light in dye-doped chiral nematics

Dong Wei, S. Residori, U. Bortolozzo

Published Nov 9, 2012• 11 citations
Publisher

Abstract

Optical phase conjugation and slow light are obtained in dye-doped chiral nematics via degenerate four-wave mixing. The creation of the light-induced dynamical grating is described and the amplitude of the phase-conjugate wave is shown to follow the changes of the input signal, hence providing efficient distortion correction. Associated to these capabilities, slowing down of light pulses is obtained thanks to the dispersive properties of the wave mixing occurring in the cholesteric liquid crystals, opening the way to applications in interferometry and imaging through highly scattering media.

DOI
10.1364/ol.37.004684

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Optical phase conjugation and slow light are obtained in dye-doped chiral nematics via degenerate four-wave mixing. The creation of the light-induced dynamical grating is described and the amplitude of the phase-conjugate wave is shown to follow the changes of the input signal, hence providing efficient distortion correction. Associated to these capabilities, slowing down of light pulses is obtained thanks to the dispersive properties of the wave mixing occurring in the cholesteric liquid crystals, opening the way to applications in interferometry and imaging through highly scattering media.
Liquid crystal media are characterized by large and tunable dispersive properties and hence allow achievement of large group delays. At the same time, liquid crystals provide large areas and are easily reconfigurable and highly sensitive devices; they are, therefore, well adapted for interferometric applications. Two different ways of achieving slow light in liquid crystals are presented. The first method consists of exploiting photoisomerization-induced transparency in dye-doped chiral liquid crystals, and the second method makes use of two-wave mixing optical resonance in pure nematics. In both mechanisms, two beams are sent to the medium, where they create a grating, either of absorption or of refractive index. Both physical mechanisms are elucidated in the context of slow light, then, as examples of sensing applications, Doppler shift measurements and adaptive holography are presented.
Slow light in liquid crystal media o ers the way of implementing interferometric measurements in large area, easily recon gurable and highly sensitive devices. Here we present two different ways of achieving slow light in liquid crystal media, namely, photoisomerization induced transparency in dye-doped chiral liquid crystals and two-wave mixing optical resonance in pure nematics. The two mechanisms arise from absorption, respectively, refractive index gratings induced in the medium. Doppler shift measurements and adaptive holography are shown as examples of optical sensing applications.
Decelerating and stopping light is fundamental for optical processing, high performance sensor technologies and digital signal treatment, many of these applications relying on the ability of controlling the amplitude and phase of coherent light pulses. In this context, slow-light has been achieved by various methods, as coupling light into resonant media, Brillouin scattering in optical fibers, beam coupling in photorefractive and liquid crystal media or engineered dispersion in photonic crystals. Here, we present a different mechanism for slowing and storing light, which is based on photo-isomerization induced transparency of azo-dye molecules hosted in a chiral liquid crystal structure. Sharp spectral features of the medium absorption/dispersion, and the long population lifetime of the dye metastable state, enable the storage of light pulses with a significant retrieval after times much longer than the medium response time.
Degenerate four-wave mixing (DFWM) is a simple method to achieve phase conjugation. It uses the third-order nonlinearity of the electric susceptibility to produce changes in the optical properties of materials. A dynamic grating is then induced. Such gratings might be used as fast optical memories in optical computing. Our aim is to find media with large nonlinear susceptibilities and fast response time. We report phase conjugation of 532-nm laser radiation by DFWM in dye solutions. The time dependence of the conjugated wave is investigated experimentally and compared to theoretical predictions. We have also studied the influence of the solvents, dye concentration, and pump intensity on the phase conjugated signal. The results are compared with those predicted by three nonlinear mechanisms. Thermally induced refractive-index changes, Kerr effect, and saturable absorption are considered.