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Slow and Fast Light (SL)

Variable Slow Light Using Coherent Population Oscillation in Quantum Dot Electro-absorption Modulator

Piotr Konrad Kondratko, Shu‐Wei Chang, Hui Su, Shun Lien Chuang

Published Jan 1, 2006• 2 citations
Publisher

Abstract

Room temperature quantum-dot semiconductor electro-absorption modulator is utilized as an optical group delay. Electrical (reverse and forward bias below transparency current level) and optical tunable buffer is realized by means of counter-propagating coherent population oscillation.
DOI
10.1364/sl.2006.mc3

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•Jan 1, 2007•2 citations
Room temperature p-doped and intrinsic quantum-dot semiconductor electroabsorption modulator is utilized as an optical group delay. Electrical (reverse voltage and forward current, below transparency) and optical controllable delay is realized by means of counterpropagating coherent population oscillation. Because of carrier dynamics, the experimental observations show bandwidth increase with reverse voltage and dependence of group delay on background absorption. Moreover, experimental observations show maximum group delay at a particular pump intensity, which is attributed to inhomogeneous contributions and saturation effects in quantum dots.
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
We demonstrate room-temperature slow light that is electrically and optically controllable by using a quantum-dot (QD) semiconductor optical amplifier (SOA) at zero and low bias below the transparency current. The absorption spectrum of the QD SOA exhibits a spectral dip with a corresponding group-index dispersion and group delay owing to coherent population oscillation caused by the interaction of pump and probe laser light near resonance of the first heavy-hole-conduction-state transition. At an optical pump power of approximately 0.3 mW inside the single-mode waveguide without current injection, a group-index change of 3.0 with a bandwidth of 2 GHz was measured. This group-index change can be controlled by injection of electrical current and by changing the optical pump power.
Room temperature quantum-well semiconductor optical amplifier with large input power is utilized in both the absorption and gain regime as an optical group delay and advance (slow and fast light), respectively. Material resonance created by coherent population oscillation and four wave mixing is tuned by electrical injection current, which in turn controls the speed of light. The four-wave mixing and population oscillation model explains the slow-to-fast light switching. Experimentally, the scheme achieves 200 degrees phase shift at 1 GHz, which corresponds to 0.56 delay-bandwidth product. The device presents a feasible building block of a multi-bit optical buffer system.
Controlling the group velocity of light has great potential applications in future optical communication, photonic computing, and microwave signal processing. These applications have recently triggered immense experimental and theoretical interest in the research community. Numerous studies report large optical delays while sacrificing many important aspects of engineering feasibility: bandwidth, device compactness, room temperature operation, ease of integration, and minimized signal insertion loss. Slow and fast light in the quantum-dot and quantum-well semiconductor optical amplifiers (SOAs) address many of these characteristics. First, this dissertation presents variable slow light in quantum-dot SOAs using population oscillation in an absorptive medium. Bandwidth tuning of slow light is observed with forward and reverse bias of quantum-dots. Second, the study explores four-wave mixing and population oscillation in a quantum-well gain medium. Control of optical advance and bandwidth is achieved by the medium's electrical bias or the optical pump power. Moreover, the fast light in the cascade of N number of quantum-well SOAs is experimentally and theoretically investigated. The cascaded scheme uses SOA-to-SOA attenuators to achieve active delay-bandwidth control. Finally, a large delay-bandwidth scheme is presented in which slow-to-fast light is observed by means of absorption to gain switching. More than a half cycle of tunable delay at 1 GHz bandwidth is achieved at optical frequencies.