Record power in the terahertz gap

The universe teems with terahertz radiation, though here on Earth you’d never know—partly because most of it is absorbed by atmospheric molecules, partly because it’s difficult to detect. Heterodyne receivers can do the trick, but they must be driven by a narrowband source at the frequency of interest. Optical devices such as lasers make suitable sources at frequencies above 4 THz or so, and electronic devices work well at frequencies below about 0.8 THz. But nestled in between, just longer than the IR but shorter than microwaves, is an elusive region known as the terahertz gap. Now Jerome Moloney (University of Arizona, Tucson ) and an international team of colleagues have designed a room-temperature source that delivers narrowband, milliwatt beams at terahertz-gap frequencies. They exploited a nonlinear optics effect whereby two frequencies of light are mixed in a nonlinear crystal to emit a beam at the difference frequency, so that appropriately spaced frequencies of IR laser light beget a terahertz beam. Though not alone in having adopted such an approach, the team achieved record power by positioning all the necessary photomixing elements—the nonlinear crystal; a Brewster window, which polarizes light; and an etalon, which establishes two IR lasing frequencies—inside the laser cavity, where the beam is most intense (see illustration). The device, dubbed a terahertz external-cavity surface-emitting laser, or TECSEL, could help to reveal previously obscured cosmic structures and events. (M. Scheller et al., Opt. Express 18, 27112, 2010 .)—Ashley G. Smart