ABSTRACT
The major problem in silicon optoelectronics is the lack of a laser or efficient electroluminescent device.
There have been many attempts to realize silicon-based lasers including porous silicon, erbium-doped
silicon, and SiGe along with silicon nanocrystals [1, 2]. The indirect bandgap of silicon precludes the
efficient recombination of electrons and holes, which to date has prevented the realization of an
interband laser. The quantum cascade laser (QCL) [3-5] is a unipolar laser utilizing intersubband
transitions, and therefore, can be applied both to direct and indirect materials systems such as silicon.
QCLs were originally proposed in 1971 [6] but the first experimental realization did not happen until
1994 using GaInAs and AlInAs heterostructures [7]. In particular for far-infrared or terahertz applica-
tions where no practical semiconductor materials exist with appropriate bandgaps, the potential for use
in applications is high [8]. Potential terahertz applications include medical and dental imaging (for
instance skin cancer detection) [8], security imaging [9], molecular spectroscopy, and bioweapons
detection [10]. QCLs were first demonstrated at mid-infrared wavelengths [7] and more recently
there have been a number of far-infrared demonstrations [11].
