One key defining property of lasers is that they each produce a single, pure color-or are tunable within a limited range of wavelengths. Now researchers at Lucent's Bell Labs have built the first laser capable of emitting light at two distinct wavelengths.

The two-tone laser was built by Bell Labs researcher Claire Gmachl and her colleagues. At its heart is a semiconducting crystal that does double-duty, emitting one wavelength when a positive voltage is applied and a different one when the voltage is negative. Since the polarity can be switched rapidly, this versatile laser can do the work of two for such applications as pollution detection, which requires one color of light that's absorbed by the target gas and a second color for reference. The prototype device emits mid-infrared light, which is most appropriate for such sensing; future devices could produce different wavelengths for medical and communications uses.