May 13, 2010 Terahertz (THz) radiation is one of the hottest
areas of modern physics research. This is because THz light
waves, or T-rays as they are sometimes called, have great
potential for spectroscopy and for the scanning of objects in a
homeland security setting that are opaque to infrared and
visible light. The trouble is that THz light waves -- which fall
in the range of 0.3 to 10 trillion cycles per second or,
equivalently, wavelengths of about 30 to 1000 microns -- are
difficult to make with traditional means. Now scientists at MIT
have combined several technologies to obtain a versatile source
of THz light. They start with a quantum cascade laser (QCL)
device, which differs fundamentally from a traditional
semiconductor laser. In most traditional lasers, light comes
from the recombination of an electron with a hole (a vacancy in
the surrounding semiconducting material). But in a QCL device,
light comes from the transition of an electron to a succession
of ever lower energy levels in a series of lay