Ultrafast All-optical Switches Based on Intersubband Transitions in GaN/AIN Multiple Quantum Wells for Tb/s Operation
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Ultrafast All-optical Switches Based on Intersubband Transitions in GaN/AlN Multiple Quantum Wells for Tb/s Operation Jahan M. Dawlaty, Farhan Rana and William J. Schaff Department of Electrical and Computer Engineering, Philips Hall, Cornell University, Ithaca, NY 14853, U.S.A ABSTRACT Theoretical and experimental results on ultra-fast all-optical switches based on intersubband transitions for Tb/s operation are presented. Designs for engineering intersubband transitions (ISBT) in GaN/AlN quantum wells near communication wavelengths (~1.55 µm) and for realizing all-optical switches requiring small pulse energies are discussed. Optimized designs show all-optical switching at Tb/s data rates with pulse energies as small as 200 fJ. Experimental realization of narrow line-width ISBT in GaN/AlN superlattices is also demonstrated. INTRODUCTION Ultrafast all-optical switches are expected to play an important role in high capacity optical time division multiplexed (TDM) networks. As the demand for bandwidth grows, optical networks are expected to operate at data rates approaching 1 Tb/s. Also, each channel in wavelength division multiplexed (WDM) optical network is expected to operate at data rates exceeding 100 Gb/s. At such high data rates it is desirable to perform switching entirely in the optical domain. In order to be useful in optical networks, all-optical switching devices must also require sufficiently low optical powers. All-optical switching devices based on non-resonant optical non-linearities are fast, but require large pulse energies [1] (greater than 10 pJ.) Resonant non-linearities, such as those associated with interband transitions in semiconductors, require smaller pulse energies for all-optical switching (typically less than 500 fJ) but are generally slow. For example, all-optical switches based on interband saturable absorbers and using cross-loss modulation (XLM) scheme are limited by slow carrier relaxation times to speeds not much greater than few tens of Gb/s. All-optical switching devices for operation at data rates close to 1 Tb/s need to be able to restore their state in time periods smaller than 1 ps. It is well known that electron intersubband relaxation times in semiconductor quantum wells are around 1 ps or less, and are three orders of magnitude smaller than interband relaxation times (~1 ns.) [2,3]. These fast relaxation times can be used to realize ultrafast all-optical switches. Intersubband devices such as quantum cascade lasers (QCL), and intersubband quantum well infrared photodetectors (QWIP) have been developed by many groups [2-5]. However, these devices do not utilize the fast intersubband relaxation times for ultra-fast applications and mostly operate at wavelengths much longer than the communication wavelength (larger than 3 µm). Semiconductor heterostructures with large conduction band offsets, such as GaN/AlGaN/AlN, are required for realizing intersubband transitions at communication wavelengths (~1.55 µm). In this paper, results on all-optical waveguide switches based
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