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Rocky Mountain Instruments - Infrared Optics LB

Terahertz laser tuning comes down to the wire

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Marie Freebody, [email protected]

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.

Meadowlark Optics - Building system MR 7/23

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.”

Published: February 2010
Glossary
quantum cascade laser
A quantum cascade laser (QCL) is a type of semiconductor laser that operates based on the principles of quantum mechanics. It is a versatile and powerful device used for emitting coherent light in the mid-infrared to terahertz range of the electromagnetic spectrum. Quantum cascade lasers were first proposed by Federico Capasso, Jerome Faist, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, and Alfred Cho in 1994. Key features and principles of quantum cascade lasers: Quantum cascade...
redshift
The displacement of spectrum lines, as determined by the increasing distance between, and the relative velocity of, the observer and a light source, causing the lines to move toward the red portion of the spectrum. It is used in astrophysics to determine the rate of recession or expansion of celestial bodies. Also known as the Hubble effect.
terahertz
Terahertz (THz) refers to a unit of frequency in the electromagnetic spectrum, denoting waves with frequencies between 0.1 and 10 terahertz. One terahertz is equivalent to one trillion hertz, or cycles per second. The terahertz frequency range falls between the microwave and infrared regions of the electromagnetic spectrum. Key points about terahertz include: Frequency range: The terahertz range spans from approximately 0.1 terahertz (100 gigahertz) to 10 terahertz. This corresponds to...
blueshiftFreebodyHuImagingMarie FreebodyMEMSMITNature PhotonicsOpticsQing Huquantum cascade laserredshiftResearch & Technologyscanning probesemiconductor laserssensingspectroscopyTech Pulseterahertzwire laserLasers

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