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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Interferometric measurements by using slow light in liquid crystal media

U. Bortolozzo, Jean‐Pierre Huignard, S. Residori

Published Feb 21, 2014• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.2047569

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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.
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.
Abstract Steep dispersion is obtained by exploiting the large optically induced refractive index change in liquid crystal media. This effect is observed either in devices made by the association of liquid crystals with a photoconductive substrate or by employing the photo‐isomerization of azo‐dye molecules hosted in a chiral liquid crystal structure. When interacting in these media, a pulse of light that is relatively long becomes delayed, hence, slow light phenomena are obtained with steep dispersion, a feature that can be exploited for improving the performances of optical sensors.
We present the nonlinear optical properties of liquid crystal light-valves and wave-mixing experiments from which slow-light effects are obtained. Group velocities as low as fractions of mm/s are achieved, the corresponding group index becoming very large. We show how this property that can be exploited to realize interferometric systems, for instance, an enhanced sensitivity Mach-Zehnder interferometer containing the light-valve as a slowlight medium is shown. Then, we present a common-path polarization interferometer based on the anisotropic character of the slow-light process occurring in the light-valve. Finally, we present a nonlinear optical cavity where a Doppler shift is introduced by the controlled displacement of one of the mirrors. Self-pulsing is obtained in the cavity due to the strongly dispersive response of the light-valve.
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.
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.