Engineers at the University of Michigan and Intel have succeeded in creating the first practical, room-temperature polariton laser. The polariton laser is of extreme interest because it requires just 0.4% of the current required by normal lasers, making it a prime candidate for use with on-chip optical interconnects. It is also believed that the polariton laser is the first new practical method of producing coherent laser light since the laser diode debuted more than 50 years ago in 1962.
In a normal laser (which is actually an acronym for “light amplification by stimulated emission of radiation”), a large amount of electrical current is applied to a lasing material. In a standard laser diode, the material is usually consists of a sandwich of semiconductor pairs (gallium arsenide and aluminium gallium arsenide are a common pairing). Electricity is pumped into this sandwich until they cross a certain threshold, at which point it emits photons (that’s the stimulated emission part). The problem is, a large amount of current is required to cross that threshold, and up until that point there’s no lasing at all.
Polariton laser diagram. A layer of gallium nitride (the lasing material) sits atop a layer of indium aluminium nitride, which prevents photons from leaking out the bottom.
A polariton laser, however, produces coherent light by stimulating polaritons — and polaritons start producing photons as soon as you pump some electrons into them, rather than being forced to cross a certain bandgap. This means that the lasing threshold is much, much lower — just 169 amperes per square centimeter, or about 250 times less than an ordinary laser. The different method of operation also means that a polariton laser can be turned on and off much faster than a conventional laser. [DOI: http://dx.doi.org/10.1103/PhysRevLett.112.236802 – "Room Temperature Electrically Injected Polariton Laser"]. In case you’re wondering, a polariton is a quasiparticle formed by an exciton (an electron and an electron hole) and a photon. The polariton entry on Wikipedia has a lot more info if you’re interested, but be warned that you’ll need a solid grasp of quantum physics to get past the first three words.
“For the past 50 years, we have relied on lasers to make coherent light and now we have something else based on a totally new principle,” says Pallab Bhattacharya, the University of Michigan engineer who led the research.
University of Michigan’s polariton laser (the triangular bit), seen under the microscope
While it’s nice to have another method of producing coherent light, by far most significant part of this discovery is the dramatically reduced power requirement. Computer designers have known for some time now that copper wires, with their high resistance and power consumption, are a significant bottleneck. There is a reason that most high-speed interconnects now use fiber-optics (laser), rather than copper wires. By far the biggest speed gains would be realized by on-chip and inter-chip optical interconnects — but so far, there hasn’t been a laser diode that’s small enough or low-power enough to enable the dream of silicon photonics. [Read: HP bets it all on The Machine, a new computer architecture based on memristors and silicon photonics.]
This polariton laser could change all that — and it’s definitely interesting that one of the paper’s authors is from Intel. Obviously we’re a long way away from integrating polariton lasers with advanced VLSI CMOS chips — IBM is much further along with that — but still, this is a very exciting breakthrough.
Top image: An IBM silicon nanophotonic chip
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