Terahertz Emitters Based on Intersubband Transitions

摘要

Terahertz (1-10 THz, or 4-40 meV, or 30-300 1.1m) frequencies are among themost underdeveloped electromagnetic spectra, even though their potentialapplications are promising for spectroscopy in chemistry and biology,astrophysics, plasma diagnostics, remote atmospheric sensing and imaging,noninvasive inspection of semiconductor wafers, and communications. Thisunderdevelopment is due primarily to the lack of coherent solid-state THz sourcesthat can provide high radiation intensities (greater than a milliwatt). The THzfrequency falls between two other frequency ranges in which conventionalsemiconductor devices have been well developed. One is the microwave andmillimeter-wave frequency range, and the other is the near-infrared and opticalfrequency range. Semiconductor electronic devices that utilize the transport offree charge carriers (such as transistors, Gunn oscillators, Schottky-diodefrequency multipliers, and photomixers) are limited by the transit time andparasitic RC time constants. Consequently, the power level of these classicaldevices decreases as 1/f 4, or even faster, as the frequency f increases above 1 THz.Semiconductor photonic devices based on quantum-mechanical interbandtransitions, however, are limited to frequencies higher than those corresponding tothe semiconductor energy gap, which is higher than 10 THz even for narrow-gaplead-salt materials. Thus, the frequency range of 1-10 THz is inaccessible forconventional semiconductor devices.