A new laser-based source of terahertz radiation that is more efficient and less prone to damage than similar systems could be useful for detecting trace gases or imaging weapons in security screening. Researchers at JILA, a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado, have developed a technology that offers a twist on common terahertz sources. The JILA instrument for generating terahertz radiation. Ultrafast pulses of near-IR laser light enter the device through the lens at the left, where the light strikes a semiconductor wafer studded with electrodes bathed in an oscillating electric field. The light dislodges electrons, which acceleratein the electric field, emitting waves of terahertz radiation. Courtesy of H. Zhang, JILA. The team used a semiconductor surface patterned with metal electrodes and excited by ultrafast laser pulses. The scientists applied an electric field across the semiconductor, while near-IR pulses lasting about 70 fs and produced 89 million times per second dislodged electrons from the semiconductor. The electrons accelerated in the electric field, emitting waves of terahertz radiation. To eliminate a common problem associated with the device, they added a layer of silicon oxide insulation between the gallium arsenide semiconductor and the gold electrodes, preventing any electrons from becoming trapped in semiconductor crystal defects, which would produce spikes in the electric field. Their innovation resulted in a uniform electric field over a large area, which has enabled them to use a large laser beam spot size to enhance system efficiency. Another advantage is that the technique does not require a microscopically patterned sample or high-voltage electronics. To generate terahertz radiation, systems using ultrafast lasers and semiconductors are commercially important because they offer a combination of broad frequency range, high frequencies and high-intensity output. The system currently uses a large laser based on a titanium-doped sapphire crystal, but it could be made more compact by using a different semiconductor and a smaller fiber laser. NIST has applied for a provisional patent on the technique. The research was described in Optics Letters, Vol. 36, Issue 2, pp. 223-225 (2011).