Reverse the voltage on an ordinary diode laser and it will stop emitting light—sometimes permanently. In contrast, another form of semiconductor laser, the quantum cascade (QC) laser, is unipolar and thus offers the possibility of use at both positive and negative voltage bias. A group at Lucent Technologies' Bell Labs (Murray Hill, NJ) has for the first time built such a bidirectional QC laser and has done so in such a way that it emits one wavelength for a positive bias and a different wavelength for a negative bias. Potential uses include differential spectroscopy, says Clair Gmachl, a Bell Labs researcher.
Made up of many layers that form a multiple quantum-well structure, a QC laser emits at a wavelength determined by its layer thicknesses rather than by the gain material. The bidirectional laser consists of 17 layers that alternate in composition between aluminum indium arsenide and gallium indium arsenide. Contained in the structure are two active regions separated by 35 nm and sandwiching an injector region. Under a positive bias, the injector causes one active region to emit; under a negative bias, the other active region emits. If the two active regions are made so that they are not identical in layer structure, then light at two differing wavelengths is the result, emitted independently of each other and separated in time. Because the prototype lasers operate best at temperatures below 150 K, they are mounted on the cold finger of a helium flow cryostat. The lasers produce 50-ns pulses at up to a 100-kHz repetition rate, with peak power reaching 300 mW.
