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Results in Optics

Ultraslow light propagation in photorefractive SBN:75

Satchi Kumari, Stuti Joshi

Published Oct 6, 2021• 1 citation
Publisher

Abstract

Slowing down of light pulses to ∼1.9 mm/s has been obtained theoretically in SBN:75 via degenerate two-wave mixing (TWM) configuration. The optical delay has been tuned by varying the input pulse width & beam crossing angle. At the optimum beam crossing angle (2θ ∼ 10◦), very high TWM gain ∼107 has been achieved. Maximum fractional delay (FD) of 1.04 and 0.20 is obtained for 150 ms pulse in case of SBN:75 and SBN:60 respectively. Further, the optical delay, group velocity, FWHM, pulse broadening and amplification in case of SBN:75 is compared with SBN:60. SBN:75 showed minimal pulse broadening, large amplification (∼105), five times more FD and two degrees of higher optical delay than that of SBN:60.

DOI
10.1016/j.rio.2021.100157

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Slowing down of light pulses to ∼1.9 mm/s has been obtained theoretically in SBN:75 via degenerate two-wave mixing (TWM) configuration. The optical delay has been tuned by varying the input pulse width & beam crossing angle. At the optimum beam crossing angle (2θ ∼ 10◦), very high TWM gain ∼107 has been achieved. Maximum fractional delay (FD) of 1.04 and 0.20 is obtained for 150 ms pulse in case of SBN:75 and SBN:60 respectively. Further, the optical delay, group velocity, FWHM, pulse broadening and amplification in case of SBN:75 is compared with SBN:60. SBN:75 showed minimal pulse broadening, large amplification (∼105), five times more FD and two degrees of higher optical delay than that of SBN:60.
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] .
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.
Slow and fast light are achieved in two experiments using SPS photorefractive crystal at room temperature. We report that the photorefractive gain achieved by the two wave mixing (TWM) method can control the group velocity of the transmitted light pulses at visible wavelength. It is shown theoretically and experimentally that the time delay and the shape of the output pulse change as a function of the photorefractive gain. The beam fanning has also been used to control the velocity propagation of a single light pulse in the same crystal at λ = 1064 nm. Depending on the orientation of the polar axis, it is possible to accelerate or decelerate a short pulse with duration of order of μs in the crystal.
We experimentally observe an ultralow effective group velocity of 0.9 cm/s of light pulses using the two-wave mixing process in an Sn2P2S6 (SPS):Te crystal at a visible wavelength. The time delay can be controlled through the nonlinear photorefractive gain and the input pulse duration. By optimizing the nonlinearity strength, we achieve a maximum fractional delay of 0.79 for a pulse duration of 100 ms. Our photorefractive slow light system allows combining low group velocity with large delay-bandwidth product for pulse durations spanning three orders of magnitude.
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.
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.