The last few decades have been marked by the rapid opening of the high-frequency electromagnetic spectrum for communications, detection and imaging. Remarkably quiet, however, has been the terahertz region, those frequencies between microwaves and the infrared, but that, too, is changing. A new laser diode source with terahertz repetition frequencies may offer an optical path to the generation of terahertz radiation. These terahertz rays have a wavelength sufficiently short to provide resolution better than 1 mm but long enough to minimize Rayleigh scattering. They are safe and noninvasive, but they can penetrate skin and packaging materials. In addition, a variety of measurement techniques can be employed to distinguish different organic materials, to image tracks on semiconductors or to measure water content in fuel. Applications in medical diagnostics are clear, and terahertz imaging has already been used to observe tooth decay at a very early stage. The problem has been finding reliable sources of T-rays. Dan A. Yanson of Glasgow University and his colleagues there and at York University may have found a way to access the terahertz region using GaAs/AlGaAs compound-cavity laser diodes. The devices were designed, fabricated and tested at Glasgow, and the Semiconductor Growth Facility at Sheffield University supplied the GaAs material. The 860-nm lasers are mode-locked to a harmonic of their fundamental round-trip frequency. The compound-cavity devices consist of two gain sections, a saturable absorber and an intracavity slot reflector. It is possible to choose the harmonic by controlling the relative lengths on either side of the reflector, and the researchers were able to produce 2.2 mW of optical power sinusoidally modulated at 2.1 THz by using a wet-etch technique to control the length of the cavities to within a few microns. Challenge and promise The laser, which the researchers described in the June 4 issue of Applied Physics Letters, may find application as a local oscillator source in detection circuits for atmospheric sensing and astronomy. In telecommunications, it could be employed as an optical clock in time division multiplexing or as a wavelength comb generator in wavelength division multiplexing schemes. To produce pure T-rays, the optical signal must be down-converted with a terahertz photomixer. Yanson believes that, although the efficiency of such conversion is less than 1 percent, the mode-locked lasers have the potential to generate useful power levels for many applications, including terahertz imaging. "Whilst direct generation of terahertz radiation presents a challenge," he said, "the down-conversion of optical power into the microwave region is a promising way forward."