Scientists from the University of Michigan and Intel Corporation in the US have demonstrated what appears to be the first electrically powered, room-temperature polariton laser. The device, based on a GaN-based microcavity diode, could advance efforts to replace on-chip wire connections with lasers, leading to smaller and more powerful electronics, say the researchers.
A polariton is a quasiparticle that results from a coupling between a photon and an electron-hole pair (an "exciton") in a semiconductor material. In 1996, researchers realised that-under certain conditions-polaritons will condense into a single quantum state, from which they will spontaneously emit coherent, monochromatic light. In contrast to stimulated lasing, the polariton emitters do not need to be constantly pumped up into excited states (so-called population inversion). As a consequence, polariton lasers begin lasing at a relatively low threshold power.
Experimental realisations of polariton lasers have so far required either low temperatures or a pump laser to create the initial polaritons. Described in the journal Physical Review Letters, the room temperature polariton laser produced a beam of UV laser light at a threshold current density of 169 A/cm2, which is almost a factor of 100 less than for conventional GaN-based lasers.
The device consists of a thin strip of gallium nitride, sandwiched between stacks of metal oxide mirrors. (In the diagram above, the mirrors are represented by the grey bars. The yellow is the electrode through which the researchers stimulate the laser. The purple is the gallium nitride semiconductor). When electric current enters such a microcavity, it can generate polaritons. But unlike previous designs in which electricity passed through or around the high-resistance mirrors, the team injects current orthogonally to the microcavity's emitting direction, thus avoiding overheating the device and destroying the lasing.
‘Room Temperature Electrically Injected Polariton Laser’ by P Bhattacharya et al, Phys. Rev. Lett. 112, 236802 (2014)