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Crystal gives tunable terahertz source

23 Aug 2002

The properties of a GaSe crystal allow researchers to build a compact terahertz source with a tuning range in excess of 5 THz.

Researchers in the US claim to have built a compact source of widely tunable terahertz radiation by exploiting difference-frequency generation (DFG) in a GaSe crystal. Their prototype device, which is 6 inches wide and high and 30 long, can tune between 56.8 and 1618 micron (0.18 to 5.27 THz).(Optics Letters 27 1454)

Convenient sources of tunable terahertz waves are currently required for applications in medical imaging and spectroscopy and the team is now looking for companies that wish to commercialize the work. The source was built by researchers from Lehigh University, the Air Force Research Laboratory in Ohio and Picarro (an optical component start-up in California formerly known as Blue Leaf Networks).

"At the moment the only competing techniques for generating tunable and coherent terahertz radiation are free-electron lasers and optical parametric oscillators, " explained Yujie Ding, from Lehigh University. "Key advantages of DFG are an extremely-wide tuning range, high stability, compact size and high peak power."

Although DFG has been tested before, it has never been tried with GaSe, which turns out to have a very large nonlinear coefficient and a small absorption coefficient in the terahertz spectrum. "We used GaSe for the first time to achieve an extremely-wide tuning range," commented Ding. ""No other teraherz source can compete with ours in terms of tuning range."

The source works by illuminating a 15 mm long crystal of GaSe with nanosecond pulses from a Nd:YAG laser and the idler beam from a oscillator power oscillator. The emission wavelength is tuned by adjusting the wavelength of the idler beam and the angle of the crystal. The source's peak power output reached 69.4 W at a wavelength of 196 micron.

Ding says that the research team is now working on a new configuration that will make the system more stable and compact.

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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