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IEEE Journal of Quantum Electronics

Effect of Pump Intensity on Slowing Down of Light in ${\rm Ce{:}BaTiO}_{3}$ Crystal via Degenerate Two-Wave Mixing Using Chopped Pulses

Satchi Kumari, Alika K. Khare

Published Jun 10, 2011• 9 citations
Publisher

Abstract

The slowing down of light in Ce:BaTiO3at room temperature via degenerate two-wave mixing for chopped pulses is reported. Delay of 0.9 ms is observed for chopped pulses of duration 8 ms, corresponding to a group velocity of 5.5 m/s. Effect of pulse width of signal beam and pump intensity onto the slowing down action of light in the crystal is studied. The building up and decay time of photo grating is also recorded as a function of pump beam intensity.

DOI
10.1109/jqe.2011.2147280

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Effect of Pump Intensity on Slowing Down of Light in Ce:BaTiO3{\rm Ce{:}BaTiO}_{3}Ce:BaTiO3​ Crystal via Degenerate Two-Wave Mixing Using Chopped Pulses
The slowing down of light in Ce:BaTiO3at room temperature via degenerate two-wave mixing for chopped pulses is reported. Delay of 0.9 ms is observed for chopped pulses of duration 8 ms, corresponding to a group velocity of 5.5 m/s. Effect of pulse width of signal beam and pump intensity onto the slowing down action of light in the crystal is studied. The building up and decay time of photo grating is also recorded as a function of pump beam intensity.
Slow-down of the light velocity has potential applications in information processing [1] and interferometers. Recently, several works have experimentally [2] demonstrated the deceleration of light in photorefractive crystals using a two wave mixing (TWM). A low velocity value of the order of 0.02 cm/s has been reported in BaTiO3crystal for Gaussian pulse profiles of long duration of 1.3 s [3]. It was also reported that the time delay depends mainly on the coupling photorefractive gain. However, the photorefractive grating build-up time in that crystal does not allow to slow down pulses shorter than ms, which limits its interest for telecoms applications.
The ability to delay short light pulses is a promising solution for all-optical telecommunications, but suffers from a large distortion of the delayed pulse as a consequence of the high material dispersion. In this Letter, we demonstrate the possibility to all-optically control the group delay in a photorefractive (PR) crystal by the use of the two-wave mixing (TWM) effect in the pulse regime at room temperature. Most importantly, we show that a proper choice of the pump pulse width in the TWM process enables us to slow down shorter or longer signal pulses without distortion. The technique is demonstrated both at visible (638 nm) and infrared (1064 nm) wavelengths and for slowed-down pulses with durations ranging from 10 ns up to 30 ms, hence confirming its broad applicability.
A slowing down of light pulses using backward-wave four-wave mixing is achieved in photorefractive crystals with different coupling strength. The delay and width of the output pulse are studied as a function of the input pulse width and pump intensity ratio for the amplified transmitted beam and for the phase-conjugated beam. The delay characteristics are compared with those of the two-beam coupling. It is demonstrated that the four-wave mixing process ensures a larger slowing down of short pulses (pulses with width shorter than the photorefractive response time) as compared to the photorefractive two-beam coupling scheme and guarantees the elimination of forerunners. The delay of long pulses in the four-wave mixing configuration is almost the same as the one that can be reached with two-beam coupling. The effect of absorption on the nonlinear shape transformation of the output pulses is discussed qualitatively.
Recently, several works have shown that the light group velocity can be reduced using the recording of dynamic refractive index gratings in photorefractive (PR) crystals [1] , [2] , [3] . The slowdown of light is performed using optical beams wave mixing: it yields the output pulse amplification and increases the light dispersion inside the PR crystal. For example, the TWM method demonstrated that the delay time ∆ τ can be varied as a function of the TWM gain values Γ and the input pulse widths T 0 . However, several works have shown that the results achieved by this method are limited in terms of pulse duration. Indeed, only long pulses with time duration of ms−s can be slowdown in a PR crystal both at the visible and the infrared wavelengths [4] .
Lately, active research has been conducted in slowing down light pulses to the order of tens of meters per second or below. An interesting application would be an optical buffer for all optical routers, where solid state devices operating under room temperature are desired. In this paper, we investigate the possibility of such a device with a Ce:BaTiO3 crystal. Group velocity as slow as 2 mm/s is obtained in the two-wave coupling experiment. By modulating the angular-multiplexed pump beams in the experiment, we also demonstrate the modulation of the output waveform, which could be used to address the problem of pulse broadening that so far limits the application of room temperature slow light.
In the demonstrations of slow light in photorefractive (PR) materials, the delays are only reported for long pulse durations (ms to s) that are in discrepancy with typical ns-pulse length in data processing applications. Here we demonstrate that the slowdown of light in a PR crystal can be also achieved in the nanosecond regime at room temperature. We experimentally show that the use of a pulsed laser with a high peak intensity may indeed reduce the Sn2P2S6{\mathrm{Sn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}Sn2​P2​S6​ crystal response time to a scale of ns and achieve a two-wave mixing (TWM) gain of $7.9\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}1}$. We show the slowdown of light pulses whose duration ranges from 10 to 100 ns with a fractional delay up to 0.3. These results are similar to those obtained in optical fibers, except that the slow light with PR TWM is achieved through smaller propagation lengths and at wide wavelength ranges. This could reveal potential applications of PR slow light systems and have a significant impact for future optical communications systems.