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In this chapter, we present our work on the development of coherent THz sources based on intersubband transition in quantum-well structures. The main focus is on electrically pumped or quantum-cascade structures, which have been quite successful in generating coherent radiation at mid-infrared frequencies. Relevant issues, such as various depopulation intersubband scattering rates, the role of complex phonon spectra, and coherent vs. incoherent tunneling are discussed in details. Optically pumped sources, including optical parametric amplifiers, and both intersubband and interband pumped THz emitters, are also investigated for their feasibility in generating coherent THz radiations.

The prospects and advantages of silicon germanium quantum cascade lasers are discussed, from both physical and technological perspectives. A range of Si/SiGe intersubband laser configurations are discussed, for both edge and surface emission. Recent experimental activity on mid- and far-infrared devices is reviewed, and the value of detailed theoretical tools for heterostructure design is highlighted. Steps towards silicon optoelectronic integration are also considered.

Semiconductor solid-state lasers based on conduction-valence band recombination are now commonplace for low-power red emission, and are available commercially at nearly continuous wavelengths throughout the near-UV and near-IR communications bands. Furthermore, new lasers and broadband spontaneous emission sources are available through a wide wavelength range, including 3-8μm based on both conduction-valence band and intersubband transitions, and up to 70μm using cascaded intersubband transitions. Competing processes make the design of these semiconductor lasers extremely difficult when extended to the very long, 300μm wavelength regime corresponding to low terahertz frequencies. We discuss material and device-design considerations for extending semiconductor lasers to this regime. We suggest a new set of device structures based on a semiconductor quantum dot (QD) gain medium, where the lasing occurs through discrete conduction states. In one implementation, two QDs are coupled to make a coupled-asymmetric quantum dot (CAD) laser. In another implementation, an ensemble of non-coupled QDs is selectively placed in a high quality cavity, called a microdisk, which is resonant with an intersublevel QD transition. We demonstrate the initial fabrication of these structures.

Nonlinear processes in quantum well infrared photodetectors (QWIP) are reviewed. Being an intersubband dipole transition based photoconductor, the nonlinear behaviors in QWIPs are caused by both the (extrinsic) photoconductive transport mechanism and (intrinsic) nonlinear optical processes. Extrinsic nonlinearity leads to a degradation of QWIP performance at high incident power or low operating temperatures. Some intrinsic nonlinear QWIP properties are useful in applications, such as in autocorrelation of short pulses by two-photon absorption. The general area of QWIP nonlinear properties has not been extensively investigated. We point out some directions for further studies and hope to stimulate more research activities.

In this chapter, we present our work on the development of coherent THz sources based on intersubband transition in quantum-well structures. The main focus is on electrically pumped or quantum-cascade structures, which have been quite successful in generating coherent radiation at mid-infrared frequencies. Relevant issues, such as various depopulation intersubband scattering rates, the role of complex phonon spectra, and coherent vs. incoherent tunneling are discussed in details. Optically pumped sources, including optical parametric amplifiers, and both intersubband and interband pumped THz emitters, are also investigated for their feasibility in generating coherent THz radiations.

Semiconductor solid-state lasers based on conduction-valence band recombination are now commonplace for low-power red emission, and are available commercially at nearly continuous wavelengths throughout the near-UV and near-IR communications bands. Furthermore, new lasers and broadband spontaneous emission sources are available through a wide wavelength range, including 3-8μm based on both conduction-valence band and intersubband transitions, and up to 70μm using cascaded intersubband transitions. Competing processes make the design of these semiconductor lasers extremely difficult when extended to the very long, 300um wavelength regime corresponding to low terahertz frequencies. We discuss material and device-design considerations for extending semiconductor lasers to this regime. We suggest a new set of device structures based on a semiconductor quantum dot (QD) gain medium, where the lasing occurs through discrete conduction states. In one implementation, two QDs are coupled to make a coupled-asymmetric quantum dot (CAD) laser. In another implementation, an ensemble of non-coupled QDs is selectively placed in a high quality cavity, called a microdisk, which is resonant with an intersublevel QD transition. We demonstrate the initial fabrication of these structures.

The prospects and advantages of silicon germanium quantum cascade lasers are discussed, from both physical and technological perspectives. A range of Si/SiGe intersubband laser configurations are discussed, for both edge and surface emission. Recent experimental activity on mid- and far-infrared devices is reviewed, and the value of detailed theoretical tools for heterostructure design is highlighted. Steps towards silicon optoelectronic integration are also considered.