Laser THz Source Demo’d

A room-temperature electrically pumped semiconductor laser source of terahertz (THz) radiation, reportedly the first of its kind, has the potential to become a standard source and could support applications such as security screening and chemical sensing.

The device was demonstrated at the Harvard University School of Engineering and Applied Science by research associate Mikhail Belkin, physics professor Federico Capasso, and Vinton Hayes, a senior research fellow in electrical engineering.

“Terahertz imaging and sensing is a very promising but relatively new technology that requires compact, portable and tunable sources to achieve widespread penetration," Capasso said. "Our devices are an important first step in this direction. We believe our laser has great development potential, because the nanoscale material used was grown by molecular beam epitaxy, a commercial and widely used thin-film growth technique which ‘spray paints’ atoms on a surface one layer at a time.”

A photograph of a bar with 10 terahertz laser sources developed by Harvard University engineers. One of the lasers is connected to the contact pad (seen on the left) by two thin gold wires. A 2-mm-diameter silicon hyper-hemispherical lens is attached to the facet of the device to collimate the terahertz output. The emission frequency is 5 THz, corresponding to a wavelength of 60 µms.

The ability of terahertz rays, or T-rays, to penetrate efficiently through paper, clothing, cardboard, plastic and many other materials makes them ideal for use in many applications, Harvard said in a statement. For example, a device emitting T-rays could be used to image concealed weapons, detect chemical and biological agents through sealed packages, see tumors without causing any harmful side effects and spot defects in materials, such as cracks in the Space Shuttle’s foam insulation.

Using lasers in the terahertz spectral range, which covers wavelengths from 30 to 300 µm, has long presented a major hurdle to engineers. In particular, making electrically pumped room-temperature and thermoelectrically cooled terahertz semiconductor lasers has been a major challenge. These devices require cryogenic cooling, greatly limiting their use in everyday applications.

“By contrast, our device emits T-rays with several hundreds of nanowatts of power at room temperature and microwatts of power at temperatures easily achievable with commercially available thermoelectric coolers,” said Belkin. “Further, there is the potential of increasing the terahertz output power to milliwatt levels by optimizing the semiconductor nanostructure of the active region and by improving the extraction efficiency of the terahertz radiation.”

To achieve the breakthrough and overcome the temperature limitations of current laser designs, the researchers engineered a room temperature infrared quantum cascade laser (QCL) that emits light at two frequencies simultaneously. The generation of T-rays occurs at room temperature inside the laser material via the process of difference-frequency generation. The frequency of the emitted radiation is 5 THz (equal to the difference of the two infrared QCL frequencies).

QCLs were invented and demonstrated by Capasso and his team at Bell Labs in 1994. The compact, millimeter-length semiconductor lasers operate routinely at room temperature with high optical powers and are increasingly used in the commercial sector for wide range of applications in chemical sensing and trace gas analysis. The devices, made by stacking ultrathin atomic layers of semiconductor materials on top of each other, are variable and tunable, allowing an engineer to adjust the energy levels in the structure to create artificial laser medium. (See also: Laser Study Shines at CLEO, THz Laser Temp. Tweaked)

Belkin and Capasso’s co-authors are Feng Xie and Alexey Belyanin, department of physics at Texas A&M University, College Station; and Milan Fischer, Andreas Wittmann and Jérôme Faist, Institute of Quantum Electronics at the Swiss Federal Institute of Technology (ETH), Zürich, Switzerland. Their findings are published in the current Applied Physics Letters, and they have applied for US patents covering the technology.

The research was supported by the Air Force Office of Scientific Research and the National Science Foundation.

For more information, visit: www.seas.harvard.edu