Terahertz (1–10 THz, ℏω = 4-40 meV, and λ = 30–300 µm) frequencies are among the most underdeveloped electromagnetic spectra, even though their potential applications are promising in biochemical sensing, imaging for medical and security applications, astrophysics and remote atmospheric monitoring, and high-bandwidth communications. This underdevelopment is primarily due to the lack of coherent solid- state THz sources. This so called "THz gap" falls between two other frequency ranges in which conventional semiconductor devices have been well developed. One is the microwave and millimeter-wave frequency range, and the other is the near infrared and optical frequency range. Semiconductor electronic devices (such as transistors) are limited to below ~1 THz by the transit time and parasitic RC time constants. Semiconductor photonic devices based on interband transitions (such as bipolar laser diodes), however, are limited to frequencies higher than those corresponding to the semiconductor energy gap (>10 THz). Thus, the frequency range of ~1–10 THz is inaccessible for conventional semiconductor devices. Semiconductor quantum wells are human-made quantum-mechanical systems in which the energy levels can be designed and engineered to be any value. Our group at MIT has developed THz quantum cascade lasers based on two novel features, resonant phonon-assisted depopulation of the lower lasing level and metal-metal waveguides for mode confinement. Based on these two features, we have achieved several records in the performance of THz QCLs. These include but not limited to: the highest pulsed operating temperature of ~200 K, the highest pulsed operating temperature relative to photon energy (kTmax ≈ 1.9 ℏω), the first CW THz QCL operating above the important liquid nitrogen temperature of 77 K (Tmax ≈ 117 K), and the highest output power level of ~250 mW. Using these lasers, we are now able to perform real-time THz imaging using focal-plane cameras at video rate, that is, making movies in "Trays". Our collaborators have demonstrated heterodyne spectroscopy using the THz QCLs as tunable local oscillators. These rapid developments demonstrate great potentials for THz QCLs in various applications.
© 2012 Optical Society of AmericaPDF Article