TERAHERTZ TECHNOLOGY: Tunable compact terahertz source is powerful, efficient

Practical sources will be needed if terahertz radiation is to be used for proposed applications in biomedical imaging, homeland security, astronomy, and industrial inspection.

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Practical sources will be needed if terahertz radiation is to be used for proposed applications in biomedical imaging, homeland security, astronomy, and industrial inspection. Roughly defined as sources with wavelengths between 30 μm and 1 mm, terahertz radiation can penetrate deep into organic ­tissue without the harmful effects of x-ray sources and are capable of distinguishing between organic materials with varying water content.

While progress has been made in developing terahertz sources using several methods, a team of researchers from the School of Physics and Astronomy at the University of St. Andrews (St. Andrews, Scotland) has demonstrated a compact, room-temperature terahertz source-widely tunable from 1.2 to 3.05 THz (corresponding to 100 to 250 μm)-that shows a greater than 10× reduction in required pump energy and a 25× increase in terahertz pulse energy compared to prior art.1 The terahertz source, which generates 1-W-peak-power and 5-nJ-energy pulses, is based on the technique of noncollinear phase-matched parametric generation.

The terahertz-generation technique uses a novel intersecting-cavity geometry that allows the nonlinear medium to be placed within the cavity of the pump laser, where it is subjected to the circulating intracavity field. Because this field is more than an order of magnitude greater than the external field of the pump laser under optimal output-coupling conditions, a high-­energy pump laser is not required. In fact, the 1‑W-peak-power terahertz radiation generated by this technique used a pump laser with an energy of only 1.3 mJ, compared to typical pump values of more than 20 mJ required for other parametric generation techniques.

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To generate widely tunable, efficient terahertz ­radiation, a laser diode operating near 808 nm pumps a cavity created by two mirrors, M1 and M2, separated by 37 cm. Mirror M1 has a reflectivity of at least 98% at 1064 nm and high transmission at 808 nm, and mirror M2 has a reflectivity of 90% at 1064 nm. The laser gain medium (LG), quarter-wave plate (QW), electro-optic Q switch (QS), and a polarizer (POL) ensure Q-switched operation of the pump laser. Mirrors M3 and M4 enclose the nonlinear crystal cavity (NL) and form the optical parametric oscillator (OPO) cavity from which the terahertz radiation is generated.
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In the experimental setup, a quasi-continuous-wave laser diode (QCW LD) operating near the neodymium (Nd) ­excitation wavelength of 808 nm pumps a laser cavity consisting of two mirrors separated by 37 cm (see figure). Additional optical elements within the pump cavity ensure that the pump laser is Q switched to achieve the desired peak power. The pump-laser gain medium is Nd:YAG. The physical length of the idler-wave or optical-parametric-oscillator (OPO) cavity, bounded by mirrors M3 and M4 and containing the nonlinear crystal, is 13 cm. The OPO cavity is configured to rotate about the pump-laser cavity axis to allow angular adjustment of the idler-wave cavity axis relative to the pump wave, effectively enabling the generation of tunable terahertz radiation within the intersecting nonlinear crystal.

Using this noncollinear phase-matched geometry, the generated terahertz radiation is incident to the 5 × 50-mm side faces of the 5-mm-square and 50-mm-long nonlinear crystal at an angle of 30. Because this angle exceeds the total-internal-reflection (TIR) angle of 11° for the crystal, a prismatic output coupler fabricated from silicon is used to increase the TIR angle at the ­crystal-silicon interface to 38° to enable the terahertz radiation to exit the crystal.

Though the spatial profile of the terahertz beam has not yet been measured, it is anticipated to be Gaussian in the far field and close to diffraction-limited.

“We believe that our recent developments have led to a versatile, compact source of tunable terahertz radiation based on parametric generation techniques,” notes research group leader Malcolm Dunn. “We are very keen to collaborate with others concerning applications. To this end we are already interacting with some potential users in homeland security and medicine-further approaches are very much welcomed.”

Gail Overton

REFERENCE

1. T.J. Edwards et al., Optics Exp.14(4) 1582 (Feb. 20, 2006).

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