Ultrashort pulses of optical superradiance. A group of excited atoms normally emits light randomly. But when the atoms interact with an appropriate light field, they can act collectively to spontaneously emit coherent, polarized, and intense radiation. Distinct from stimulated laser emission, that so-called superradiance has been explored in systems such as a Bose–Einstein condensate (see PHYSICS TODAY, July 2010, page 16) and a semiconductor at near-IR wavelengths. In a new twist, researchers at the University of Cambridge’s Centre for Photonic Systems generated superradiance in a commercial 405-nm diode laser like the one shown here. The team etched a gap into the diode, producing a gain section and an absorber section. As the gain section is driven by 9-ns current pulses and the absorber is reverse-biased, a dense array of electron–hole pairs is created in the gain section. Those pairs eventually act in concert to spontaneously produce coherent pulses of superradiance as short as 1.4 ps, much shorter than the 22-ps pulses they could obtain from the unmodified diode laser. By adjusting the bias voltage of their room-temperature device, the researchers can get a single pulse per 9-ns interval, although the timing jitter is significant. Assuming the jitter is corrected, the new method of generating ultrashort pulses at visible wavelengths may find applications in high-density optical storage and in biological imaging. (V. F. Olle et al., Opt. Express 20, 7035, 2012.)
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