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 layers in a sandwich-style structure of thin semiconducting layers.
This type of laser has a unique property: one electron (as it moves through the layers) triggers the release of many photons. The emitted light energy of the device can be changed by altering the thickness of the layers.
Population inversion is provided over a range of energies provided by the cascaded energy levels described above with the fine energy or wavelength selection provided by the laser cavity. In the MIT approach, tuning is achieved by changing the width of the laser light beam (and hence cavity) by precisely controlling the distance between a specially designed block material and the laser. This technique is analogous to changing the pitch of a guitar string by changing its diameter. In this case, the laser waveguide is much narrower than the wavelength of the light, hence the description of this setup as a "wire" laser.
Qi Qin of MIT says their cascade laser can be tuned continuously and controllably to produce terahertz radiation over a broad range. "At present, this is the only viable mechanism to achieve broad continuous tuning in terahertz quantum-cascade lasers," says Qin.
The work is being reported at the 2010 Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference (CLEO/QELS) May 16-21 at the San Jose McEnery Convention Center in San Jose, Calif., where researchers from around the world are presenting the latest breakthroughs in electro-optics, innovative developments in laser science, and commercial applications in photonics.
Presentation: "Development of Tunable Terahertz Wire Lasers" by Qi Qin et al. is at 3 p.m. on Thursday, May 20.
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