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A breakthrough for terahertz
semiconductor lasers

A Nanoscale injection technique enables higher operating temperatures and a variety of potential applications.

Light’s versatility is awesome – it inspires painters, it regulates auto traffic and it guides a moonlit stroll on the beach.

Visible light waves occupy just one portion of the electromagnetic spectrum. Radio waves, with their relatively low frequency, let the world communicate wirelessly. Higher-frequency infrared rays enable telecommunications, the Internet and consumer electronics. Farther up the spectrum, X-rays are used for medical imaging and baggage screening.

One largely unexploited region of the electromagnetic spectrum, says Sushil Kumar, is the narrow “gap” between the radio and infrared bands that is occupied by terahertz (THz) waves.

In the presence of THz light, says Kumar, assistant professor of electrical and computer engineering, some large molecules yield specific spectral signatures, like the lines of a barcode.

This sensitivity opens up a variety of potential applications. THz technology can be used to sense trace amounts of drugs or explosives concealed in clothing or packages. It can detect hidden metal objects without harming the body, and it can decode light emanating from the outer universe to reveal clues to the formation of stars and galaxies.

Other applications include the remote sensing of the earth’s atmosphere, medical imaging and disease diagnosis, and quality control in drug manufacturing. And scientists have found that THz radiation can be used to diagnose skin cancer with advantages over X-rays.

One large hurdle stands in the way of these advances, says Kumar – the lack of a commercially viable technique to generate narrowband and high-power THz radiation.

Traditionally high-power THz radiation was produced by bulky, expensive lasers fueled by a molecular gas such as methane. In 2001, scientists in Italy invented the first THz semiconductor quantum-cascade laser (QCL). QCLs are attractive because of their size – they are as tiny as the diode in a laser pointer. But the lasers must be cooled to temperatures as low as that of liquid nitrogen (320 degrees below zero Fahrenheit) before they emit powerful THz radiation.

Kumar has collaborated with researchers at MIT and Sandia National Laboratories to make a QCL that emits THz radiation at higher operating temperatures than ever before. The group, whose work is supported by NASA and NSF, reported their achievement recently in Nature Physics.

Using nanoscale technology, Kumar’s group assembled a super lattice of 1,500 alternating layers, placing atoms of two semiconducting materials – gallium-arsenide and aluminum gallium-arsenide – in periodic arrangements. Because each layer in the sandwich must be of an extremely precise thickness, it takes one day to “grow” a wafer. An electric current applied to the sandwich causes electrons to travel, or cascade, within its structure, according to the rules of quantum mechanics, and generate light along the way.

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Sushil Kumar utilizes thermal vibrations in a semiconductor lattice to obtain the optical amplification necessary to generate a laser.