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Journal of Lightwave Technology

Slow and Fast Light in Semiconductor Quantum-Well and Quantum-Dot Devices

Connie J. Chang-Hasnain, Shun Lien Chuang

Published Dec 1, 2006• 131 citations
Publisher

Abstract

The ability to manipulate the speed of light has recently become one of the most exciting emergent topics in optics. There are several experimental demonstrations showing the capability to slow down light more than six orders of magnitude in a variety of media, ranging from atomic vapor, solid state crystal, to semiconductors. These results have led to intensive research into new materials, devices, and system studies that examine their impact to new applications. It is believed that we are on the verge of a dramatic change in the way we envision and construct communication, processing and control systems. One direct application of slow and fast light devices is in the area of communications. One grand challenge remaining in information technology today is to store and buffer optical signals directly in optical format. As such, optical signals must be converted to electronic signals to route, switch, or be processed. This resulted in significant latencies and traffic congestions in current networks. In addition, keeping the data in optical domain during the routing process can greatly reduce the power, complexity and size of the routers. To this end, a controllable optical delay line can effectively function as an optical buffer, and the storage is proportional to the variability of the group velocity. In addition to optical buffers, slow and fast light devices can be used as tunable true-time delay elements in microwave photonics, which are important for remotely controlling phased array antenna. Other novel applications include nonlinear optics, optical signal processing, and quantum information processing. There are various approaches that can be used to vary the optical group velocity. Ultraslow or fast group velocity may result from a large material dispersion, waveguide dispersion, or both. In this paper, the authors provide a review of recent progress of slow and fast light using semiconductor devices. Specifically, they will discuss results obtained using semiconductor quantum-well/quantum-dot absorber and optical amplifiers. Slow and fast light are controllable electrically by changing the bias current or voltage as well as optically by changing the pump laser intensity and wavelength. Delay-bandwidth tradeoff and other figures of merits are analyzed

DOI
10.1109/jlt.2006.885767

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The ability to manipulate the speed of light has recently become one of the most exciting emergent topics in optics. There are several experimental demonstrations showing the capability to slow down light more than six orders of magnitude in a variety of media, ranging from atomic vapor, solid state crystal, to semiconductors. These results have led to intensive research into new materials, devices, and system studies that examine their impact to new applications. It is believed that we are on the verge of a dramatic change in the way we envision and construct communication, processing and control systems. One direct application of slow and fast light devices is in the area of communications. One grand challenge remaining in information technology today is to store and buffer optical signals directly in optical format. As such, optical signals must be converted to electronic signals to route, switch, or be processed. This resulted in significant latencies and traffic congestions in current networks. In addition, keeping the data in optical domain during the routing process can greatly reduce the power, complexity and size of the routers. To this end, a controllable optical delay line can effectively function as an optical buffer, and the storage is proportional to the variability of the group velocity. In addition to optical buffers, slow and fast light devices can be used as tunable true-time delay elements in microwave photonics, which are important for remotely controlling phased array antenna. Other novel applications include nonlinear optics, optical signal processing, and quantum information processing. There are various approaches that can be used to vary the optical group velocity. Ultraslow or fast group velocity may result from a large material dispersion, waveguide dispersion, or both. In this paper, the authors provide a review of recent progress of slow and fast light using semiconductor devices. Specifically, they will discuss results obtained using semiconductor quantum-well/quantum-dot absorber and optical amplifiers. Slow and fast light are controllable electrically by changing the bias current or voltage as well as optically by changing the pump laser intensity and wavelength. Delay-bandwidth tradeoff and other figures of merits are analyzed
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.
Abstract : Slow and fast light schemes are attractive for many applications including optical signal processing and RF phased array antennas. Semiconductor based schemes offer electrical control of velocity at very high bandwidths in an extremely compact device. Further they operate at room temperature and can be easily integrated into various optical systems. In this program, we used ultrafast nonlinearities in semiconductor optical amplifiers (SOAs) to achieve tunable time shifts at THz frequencies. In particular, we leveraged the spectral hole burning phenomena and demonstrated fast light in quantum well (QW) and quantum dot (QD) SOAs. The research included both theoretical and experimental efforts. Theoretical work was developed to understand and simulate experimental results and to optimize device designs. We introduced a novel pulse compression technique to reduce the pulse width to 100 fs to efficiently use ultra-fast nonlinearities in SOAs to achieve a large and cascadable delay. We implemented a novel chirped pulse scheme and obtained the largest advance-pulse-product of 10.7 with THz bandwidth. We demonstrated slow and fast light with GHz RF signal in QW/QD SOAs using coherent population oscillation and four-wave mixing. We also demonstrated cascaded SOAs in a loop to scale up the delay/advance.
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.
Slow and fast light in quantum-well (QW) and quantum-dot (QD) semiconductor optical amplifiers (SOAs) using nonlinear quantum optical effects are presented.We demonstrate electrical and optical controls of fast light using the coherent population oscillation (CPO) and four wave mixing (FWM) in the gain regime of QW SOAs.We then consider the dependence on the wavelength and modal gain of the pump in QW SOAs.To enhance the tunable photonic delay of a single QW SOA, we explore a serial cascade of multiple amplifiers.A model for the number of QW SOAs in series with variable optical attenuation is developed and matched to the experimental data.We demonstrate the scaling law and the bandwidth control by using the serial cascade of multiple QW SOAs.Experimentally, we achieve a phase change of 160 • and a scaling factor of four at 1 GHz using the cascade of four QW SOAs.Finally, we investigate CPO and FWM slow and fast light of QD SOAs.The experiment shows that the bandwidth of the time delay as a function of the modulation frequency changes in the absorption and gain regimes due to the carrier-lifetime variation.The tunable phase shift in QD SOA is compared between the ground-and first excited-state transitions with different modal gains.