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Optics Express

Transition between superluminal and subluminal light propagation in photorefractive Bi12SiO20 crystals

Fang Bo, Guoquan Zhang, Jingjun Xu

Published Jan 1, 2005• 20 citations
Publisher

Abstract

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.

DOI
10.1364/opex.13.008198

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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 show that the group velocities of light pulses can be decelerated dramatically by the use of a dispersive phase-coupling effect through a wave mixing process. We have observed experimentally such a phase-coupling-induced ultraslow light propagation with a group velocity as low as 0.05  m/s0.05\text{ }\text{ }\mathrm{m}/\mathrm{s}0.05  m/s in a photorefractive Bi12SiO20{\mathrm{B}\mathrm{i}}_{12}{\mathrm{S}\mathrm{i}\mathrm{O}}_{20}Bi12​SiO20​ crystal at room temperature. Moreover, the ultraslow light is amplified in the Bi12SiO20{\mathrm{B}\mathrm{i}}_{12}{\mathrm{S}\mathrm{i}\mathrm{O}}_{20}Bi12​SiO20​ crystal because of the unidirectional energy transfer from a coupling beam to the ultraslow light. This technique to produce ultraslow light propagation is valid for all nonlinear wave mixing processes with a dispersive phase-coupling effect.
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 show theoretically that the group velocity of light pulses can be reduced significantly by use of the steep dispersion properties of the phase coupling effect in the photorefractive two-wave mixing process. The group velocity of light pulses of the order of 0.1 m/s can be achieved in typical photorefractive BSO crystals with an appropriate externally applied electric field and moving gratings of appropriate speeds. It is also shown that the slowly propagating light pulses can be set to be amplified after passing through the photorefractive material.
We demonstrate slow and fast light at room temperature by dispersive phase-coupling in a photorefractive SBN:60 crystal. Non-degenerated wave-mixing is used to obtain delayed and amplified pulses with reduced or increased group velocity of vg = 0. 2c m s −1 and vg =− 1. 3c m s −1 , respectively. The gain spectrum is modulated by using multiple frequency shifted pumps simultaneously. The complete dispersion of the spectrum is determined via the phase modulation technique. We compare the experimental results to numerical simulations based on coupled-wave equations.
We theoretically propose a strategy to use a travelling monochromatic wave in a simple two level atomic system to tune the group velocity over a wide range. Both subliminal and superluminal group velocity are demonstrated from the interaction of a light wave with atoms in the case where the cell length is much less than the light beam diameter. The tunable control of the light group velocity for the negative (superluminal light) to positive (subluminal light) are achieved in different physical conditions. In fact, we control the group velocity by tuning the relaxation parameters such as, the collision between atoms, the incident intensity and the beam diameter. This allows us to switch from subluminality to superluminality and back in a sub-Doppler system.