Terahertz laser tuning comes down to the wire

Marie Freebody, marie.freebody@photonics.com

The progress of terahertz technology continues its steady march with news of a tunable terahertz laser developed by a group at MIT.

Tunable terahertz lasers are particularly useful for sensing and spectroscopy applications because many biochemical species have strong spectral fingerprints at terahertz frequencies. Despite this, the terahertz range is among the most underdeveloped in the electromagnetic spectrum. This is largely a result of the “terahertz gap” between solid-state electronic devices and photonic devices.

Qing Hu and colleagues have managed to overcome some of the technological hurdles facing terahertz research to develop a terahertz quantum cascade laser with a frequency tuning of ~0.14 THz.

An enlarged view of the laser (right) shows the plunger (transparent blue) lying on top of guide rails, ready to be actuated by the shaft of the linear bearing (left). Courtesy of professor Qing Hu.

Conventionally, the frequency of a laser is tuned in a manner similar to a stringed musical instrument, such as a violin. The pitch of the instrument is varied by changing the length – the longitudinal component of the wave vector – and the tension – the refractive index – of a string. However, this method is difficult to implement at terahertz frequencies because of the relatively long wavelength of a semiconductor laser compared with its cross section.

Instead of fighting the battle with brute force, the group developed an approach to tuning that actually takes advantage of the laser’s tiny cross section. The new line of attack is based on manipulating the evanescent propagating mode of a device known as a “wire laser”; i.e., any laser with a cross section that is much smaller than the wavelength it produces.

“In a typical wire laser, a large fraction of the mode propagates outside of the solid core,” Hu said. “By placing a movable object close to the wire laser, we can manipulate the laser’s transverse mode profile, thereby tuning its resonant frequency.”

In the researchers’ experiments, which were described in the November 2009 issue of Nature Photonics, a movable metallic or dielectric object is placed at a distance of ~1 to 15 µm from a wire laser with a 13-µm-wide ridge. Using a gold object next to the wire laser resulted in a blueshift in frequency, and, conversely, a silicon object produced a redshift.

Hu now hopes to develop broadly tunable terahertz lasers based on microelectromechanical systems technology.

“The aim is to integrate the tunable wire lasers with power amplifiers to create high-power frequency-tunable sources for sensing, spectroscopy and imaging applications,” he said. “Furthermore, the concept of tuning a wire laser by manipulating the transverse mode profile applies to other frequency ranges. For wire lasers at visible frequencies, one can envision using a scanning probe to tune its frequency for sensing and spectroscopy at nanometer scales.”