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Optics Letters

Experimental observation of the slowdown of optical beams by a volume-index grating in a photorefractive LiNbO_3 crystal

Shiuan Huei Lin, Ken Y. Hsu, Pochi Yeh

Published Nov 1, 2000• 43 citations
Publisher

Abstract

We investigate the group velocity of light in a one-dimensional volume-index grating inside a photorefractive LiNbO(3) crystal. The slowdown of the group electromagnetic propagation is observed experimentally by tuning of the wave number of the optical beam close to the outside edge of the forbidden bandgap. We obtain a large group index of up to 7.5 in a 3.5-cm crystal sample. The group index is compared with the result of a theoretical derivation. The results are presented and discussed.

DOI
10.1364/ol.25.001582

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We investigate the group velocity of light in a one-dimensional volume-index grating inside a photorefractive LiNbO(3) crystal. The slowdown of the group electromagnetic propagation is observed experimentally by tuning of the wave number of the optical beam close to the outside edge of the forbidden bandgap. We obtain a large group index of up to 7.5 in a 3.5-cm crystal sample. The group index is compared with the result of a theoretical derivation. The results are presented and discussed.
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We show theoretically that the group velocity of light can be slowed down by means of phase coupling in the photorefractive two-wave mixing process. It is shown that the group velocity of light propagating in a photorefractive material could be significantly reduced because of a steep variation of the phase-coupling coefficient with respect to the angular frequency difference between two coupling beams in a two-wave mixing process. The results for the case of a photorefractive Bi12SiO20 crystal are presented and discussed in detail.
We demonstrated superluminal light propagation with a negative group velocity of -5.7 m/s in a photorefractive Bi 12SiO20 crystal by using the dispersive phase coupling effect in a nondegenerate two-wave mixing process. To the best of our knowledge, this is the first experimental demonstration of superluminal light propagation at room temperature in solids by using a classical wave mixing technique. In addition, we showed the tunability of the group velocity of light between the negative (superluminal light) and the positive (subluminal light) by simply tuning the experimental conditions such as the frequency of the coupling beam, the incident intensity, and the externally applied electric fields.
We investigate all-optical tunable index structures, induced in photorefractive materials for the observation of room-temperature slow light. The potential of this technique lies in the possibility of reconfiguration in different spectral windows at low pump intensities and transmission properties comparable to fiber-based resonant delay-lines [1]. Volume-holographic gratings offer low insertion loss and narrow bandwidth selectivity but are nearly insensitive to photo-induced damage in the NIR, thus making them suitable for telecom applications, e.g. WDM filters [2]. The narrow spectral features of the thick volume gratings produce a steep slope in the dispersion which can be considered as the basis of all slow-light experiments. Investigations of increased group index of single band-stop configurations in photorefractive materials without gain have proven this concept [3,4].