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Optics Express

Slow light in a semiconductor waveguide at gigahertz frequencies

Jesper Mørk, Rasmus Kjær, Mike van der Poel, Kresten Yvind

Published Jan 1, 2005• 182 citations
Publisher

Abstract

We experimentally demonstrate slow-down of light by a factor of three in a 100 microm long semiconductor waveguide at room temperature and at a record-high frequency of 16.7 GHz. It is shown that the group velocity can be controlled all-optically as well as through an applied bias voltage. A semi-analytical model based on the effect of coherent population oscillations and taking into account propagation effects is derived and is shown to well account for the experimental results. It is shown that the carrier lifetime limits the maximum achievable delay. Based on the general model we analyze fundamental limitations in the application of light slowdown due to coherent population oscillations.

DOI
10.1364/opex.13.008136

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Optics Express•Jan 1, 2005•108 citations
We experimentally demonstrate slow-down of light by a factor of three in a 100 microm long semiconductor waveguide at room temperature and at a record-high frequency of 16.7 GHz. It is shown that the group velocity can be controlled all-optically as well as through an applied bias voltage. A semi-analytical model based on the effect of coherent population oscillations and taking into account propagation effects is derived and is shown to well account for the experimental results. It is shown that the carrier lifetime limits the maximum achievable delay. Based on the general model we analyze fundamental limitations in the application of light slowdown due to coherent population oscillations.
Experimental demonstration and theoretical analysis of slow light in a semiconductor waveguide at GHz frequencies slow-down of light by a factor of two in a semiconductor waveguide at room temperature with a bandwidth of 16.7 GHz using the effect of coherent pulsations of the carrier density. The achievable delay is shown to be limited by the short lifetime. The maximum time delay observed reflects an approximately two-fold increase of the group refractive index, corresponding to a time delay of approximately 20 % of the carrier (population) lifetime. The experimental observations are well-explained by a model accounting for the absorption saturation in the waveguide, when using a lifetime that depends on the reverse bias.
We demonstrate the use of coherent population oscillations (CPO) to realize a monolithically integrated semiconductor device which allows voltage controlled tuning of the group velocity corresponding to a phase shift of up to 55 degrees at a frequency of 10 GHz. By combining sections of slow and fast light, corresponding to absorption and gain, we demonstrate control of both the slow-down factor and the signal amplitude, which is important for applications as true-time delay in microwave photonics. The physics of CPO is discussed in relation to electromagnetically induced transparency (EIT). In particular, we demonstrate and explain the possibility of achieving transparency when using the effect of CPO despite the fact that it relies on only a partial saturation of an absorption line.
We develop a model for slow light based on the coherent population oscillation of quantum dots at room temperature. With the absorption dip and corresponding variation of the refractive index due to the coherent population beating induced by the pump and probe signal, quantum dots can be used as a slow-light medium. Our theoretical model matches the experimental results very well. We also experimentally and theoretically demonstrate that both the forward-bias injection current and reverse-bias voltage can change the group index in a semiconductor quantum-dot waveguide. Our results indicate that quantum-dot devices can be potentially used as electrically and optically controllable slow light devices
Investigations of slow and fast light effects in semiconductor waveguides entail interesting physics and point to a number of promising applications. In this review we give an overview of recent progress in the field, in particular focusing on the physical mechanisms of electromagnetically induced transparency and coherent population oscillations. While electromagnetically induced transparency has been the most important effect in realizing slowdown effects in atomic gasses, progress has been comparatively slow in semiconductors due to inherent problems of fast dephasing times and inhomogeneous broadening in quantum dots. The physics of electromagnetically induced transparency in semiconductors is discussed, emphasizing these limitations and recent suggestions for overcoming them. On the other hand, the mechanism of coherent population oscillations relies on wave mixing effects and is well suited for semiconductor waveguides. Recent experimental progress is reviewed, emphasizing new ideas that have significantly enhanced the degree of control that can be exercised and the frequency range that can be achieved. Thus, applications within microwave photonics appear to be within reach.
We demonstrate room temperature optically and electrically controllable group delay using population oscillation in a quantum-dot (QD) semiconductor optical amplifier (SOA). A reduction of the group index up to 10% with a bandwidth of 13 GHz is achieved under different configurations of injection current and optical pump intensity. Our theoretical results based on population pulsation agree well with experimental data. We extract the linewidth enhancement factor and effective carrier diffusion coefficient of the QD SOA. We also observe slow light when the injection current is increased.