Ultraviolet light with a wavelength of about 300 nanometers is invisible to the eye, but it can destroy the DNA of microbes, make certain substances glow, and carry information. Until now, lasers in this range were bulky, expensive, or short-lived. Now, a team of researchers has assembled a compact laser diode that operates from an electric current, like the ordinary laser in a DVD player, but shines in the mid-ultraviolet.
The heart of the device is a crystal of aluminum gallium nitride, grown layer by layer on an aluminum nitride substrate. When current is passed through the crystal, electrons and holes meet in the active zone and emit photons. To amplify the light, a narrow ridge is etched on the surface — a waveguide that directs the waves like a trough directs water. When the current exceeds the threshold, the laser starts to work: the power rises sharply, and the spectrum compresses into a narrow line only 0.2 nanometers wide.
Such a laser could find use in portable spectrometers for analyzing the composition of substances, in sensors for dangerous gases, and in devices for disinfecting water and surfaces. And in quantum photonics, ultraviolet photons can help control the states of individual atoms and ions.
For now, this is a laboratory prototype, but it proves that the mid-ultraviolet is no longer exotic. A laser that once took up an entire table now fits in the palm of your hand.
The wavelength of 298.5 nm is almost 40 times shorter than the thickness of a human hair.