Quantum cascade lasers are unlike other semiconductor lasers in that their emission wavelength is determined by the thickness of the semiconductor layers in the device rather than their energy band-gap. Now physicists have demonstrated another novel facet of these lasers - the wavelength at which they lase can be changed by reversing the voltage applied to the device. This unipolar two-wavelength device has been demonstrated by Claire Gmachl and colleagues at Bell Laboratories in the US (Science 286 749).
Light is emitted from a quantum cascade laser when an electron falls from an excited energy level in a quantum well to a lower level. The quantum wells are defined by alternating layers of semiconductors with different energy gaps. When an appropriate voltage is applied to the device the electron then tunnels from this lower level to the higher level of the next quantum well. The electron therefore “cascades” through the device, emitting a photon from every quantum well. Unlike previous cascade lasers, the new device switches to different set of energy gaps when the voltage is reversed. This allows it to emit photons at a wavelength of 6.3 microns for a positive voltage and 6.5 microns for a negative voltage.
Quantum cascade lasers are about 1000 times more powerful than conventional semiconductor lasers in the mid-infrared part of the spectrum, which makes them suitable for applications such as atmospheric pollution monitoring.
