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Crystal tunes terahertz emission

04 Apr 2003

Researchers build a terahertz radiation source based on a GaP crystal that has a tuning range of 2.5 THz.

Scientists in Japan have used difference-frequency generation (DFG) to produce tunable, high power terahertz radiation from a GaP crystal. The team says the crystal's emission ranged from 0.5 to 3 THz, with a peak pulsed power of 480 mW at 1.3 THz. (Journal of Applied Physics 93 4610).

The technique works by firing two beams, a "pump" and a "signal", at the crystal. The researchers from Tohoku University and the Semiconductor Research Institute in Kawauchi, Japan, uses a Nd:YAG laser emitting at 1064 nm as the signal source and a BBO-based optical parametric oscillator (OPO) as the pump.

The frequency of terahertz emission is tuned by varying both the output wavelength of the OPO and the small angle between the pump and signal beams as they enter the crystal. The team varies the OPO's wavelength between 1050 and 1063 nm, while also changing the angle between the two beams by less than 35 minutes of a degree.

"The mirror mounts are adjusted using actuators [to change the angle between the beams]," explains researcher Tadao Tanabe from Tohoku Univeristy. "The angle is so small that we can change it without serious decrease of the beam overlap."

Tanabe and colleagues saw continuously-tunable emission for three lengths of GaP crystal: 2.6 mm, 5 mm and 20 mm. They say that the frequency increases nearly linearly with the angle between the beams, irrespective of the crystal length. They add that the THz power increases steeply as the crystal length increases from 2.6 to 5 mm.

The team now hopes to broaden the tuning range. "We have extended the frequency region up to 7 THz," Tanabe told Optics.org. "We are also seeking various materials that have a wide frequency range and at the same time high power. A narrow linewidth is also important."

Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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