Ultrafast all-optical switch based on intersubband transitions in InGaAs/AlAs/AlAsSb quantum well is described.
Because of very fast intra-band relaxation in conduction band, we can obtain very fast response of around 1ps. The
operation principles and characteristics as an absorption saturation type device are described. Also described is the
operation as an all-optical phase modulator. With Mach-Zehnder interferometer configuration, error free all-optical
demultiplexing operation from 160-Gb/s to 40-Gb/s was achieved.
We have dramatically improved the optical properties of extremely thin QWs required for ISBT devices operating at optical communication wavelengths using novel InGaAs/AlAs/AlAsSb QW structures with 4-7 monolayers (MLs) of AlAs. The intersubband saturation intensity (Is) was reduced to 3fj/μm2. This represented an Is reduction of nearly 3 orders of magnitude relative to the previous samples whether or not such sample featured 1 ML of AlAs interface layer. This paper reviews the recent results of novel InGaAs/AlAs/AlAsSb quantum well properties grown by molecular beam epitaxy, and discusses the linear and nonlinear optical responses of ISBT.
Based on our line shape analysis of temperature dependent absorption spectra on InGaAs/AlAsSb single quantum wells, we expect a fast carrier redistribution with in the broad inhomogeneous intersubband absorption spectrum from a wavelength as short as 1.72 micrometers . In addition, due to large resonant 3rd order susceptibility but weak absorption, we expect small saturation intensity (Is) at this short wavelength. We present wavelength dependent saturation measurements to show that the Is is, indeed, lower by more than an order of magnitude compared to that at the main peak (1.88 micrometers ). We also show from the figure of merit estimates that the carrier relaxation time at 1.72 micrometers is expected to be faster at 1.72 micron, consistent with the line shape analysis predictions.
We report the near-infrared intersubband absorption characteristics in In (formula available in paper) heterostructures lattice matched in InP substrate. We have investigated for the first time the excitation power dependence of the intersubband transitions in Sb based quantum wells using a femtosecond optical parametric amplifier tuned over wavelength ranging from 1.8 micrometers - 2.4 micrometers . The bandgap of the InGaAs material system in the regime of 1.0 eV facilitates nonlinear interband optical absorption effects in the presence of strong near-infrared intersubband resonant optical excitation. We have observed a novel nonlinear optical phenomenon,- an intersubband transition induced interband absorption due to the two- photon interband excitation after the onset of the intersubband absorption saturation. The excitation wavelength-dependence of the absorption saturation characteristics has also been studied. The absorption saturation measurements have been performed in quantum wells with various well widths.
Modulators based on interband (IB) light absorption by intersubband (ISB) excitations in undoped quantum well structures (QWs) has the inherent advantage of ultrafast response without thermal dissipation at high bit rates. In this report we present an efficient scheme to achieve ultrafast modulation in the femtosecond regime using IB and ISB light pulses in a step-like type II semiconductor QW. The threshold control-light intensity for 100% modulation in the proposed structure is less than 1 pJ, which is at least an order of magnitude lower than in any excitonic optical switch proposed until now. The peak modulation efficiency in asymmetric QW's at 1 MW/cm2 is 40% which is twice than that estimated in symmetric QW's and can be enhanced to 100% at 10 MW/cm2. A modulation speed of 500 fs can be achieved without any serious degradation of the IB signal due to thermal dissipation. This is an important step towards the development of novel ultrafast optoelectronic devices based on the pulse shaping techniques.