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

Slow light in dye-doped chiral liquid crystals

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

Published Mar 6, 2013• 0 citations
Publisher

Abstract

We demonstrate slow light in dye-doped chiral liquid crystals. The medium combines a nematic liquid crystal with methyl-red and chiral agents. When a circularly polarized pump and a pulse beam interact in the cell, the output pulse is delayed thanks to the dispersive properties of the beam coupling occurring in the medium. The origin of the slow-light effect is explained on the basis of photo-isomerization induced transparency of the dye molecules hosted in the chiral liquid crystal structure.

DOI
10.1117/12.2011339

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We demonstrate slow light in dye-doped chiral liquid crystals. The medium combines a nematic liquid crystal with methyl-red and chiral agents. When a circularly polarized pump and a pulse beam interact in the cell, the output pulse is delayed thanks to the dispersive properties of the beam coupling occurring in the medium. The origin of the slow-light effect is explained on the basis of photo-isomerization induced transparency of the dye molecules hosted in the chiral liquid crystal structure.
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.
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.
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.
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.
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.
Two-wave mixing experiments performed in a liquid crystal light-valve leads to optical amplification in the Raman-Nath regime of diffraction. By exploiting the dispersion properties associated with the two-wave mixing gain, we demonstrate slow and fast light effects, with the output signal pulse either delayed or anticipated with respect to the input light field. Group velocity as slow as a 0.2 mm/s are obtained, and a large tunability of the group delay is achieved thanks to the ability of controlling the liquid crystal nonlinear response through the main experimental parameters. A theoretical model based on the Kerr-like response of the liquid crystal light-valve accounts for the experimental results and allows us to calculate the group delays for the different output orders. We show that the large group delay obtained in the slow light regime can be exploited to enhance the spectral sensitivity of a Mach-Zehnder interferometer and, on the other side, the narrow frequency bandwidth of the two-wave mixing gain can be used to realize an adaptive holographic system that achieves picometer detection.