Conventional lasers emit radiation at a single wavelength, so applications that require many wavelengths often need several lasers. Existing broadband lasers – which are usually based on titanium-sapphire – are not always suitable because they only emit pulses of radiation. Some of these devices are also bulky because their spectra are artificially broadened by external gadgets.

The ‘quantum cascade’ devised by Gmachl’s group consists of 36 semiconductor layers. Each layer emits a different range of wavelengths, and these add up to produce a smooth spectrum of wavelengths from 6 to 8 micrometres. The light is intense enough for the device to act as a laser, and each layer is transparent to the light emitted by other layers. The layers consist of indium gallium arsenide ‘quantum wells’ separated by aluminium indium arsenide barriers.

A quantum well is a layer of semiconducting material embedded in a semiconductor with a larger bandgap. Charge carriers such as electrons can then be trapped in the well, where they can only occupy certain quantized energy states, similar to the electrons in an atom. When an electron that has been excited into a higher energy state returns to its ground state, it emits light with a frequency corresponding to the energy difference between these states. Unlike atoms, quantum wells can be engineered to emit different wavelengths.

“After we calculated the layer structure, we used molecular beam epitaxy to create the device,” Gmachl told PhysicsWeb, “and it worked on the first attempt!”

Gmachl and colleagues believe that their compact laser technique could lead to single light sources for many applications across the spectrum. The micrometre-sized device is mounted on a chip just millimetres across, and they plan to adapt it to make broadband lasers that operate at fibre-optic wavelengths – around 1.3 micrometres – and in the visible spectrum.