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Multi-color laser created by UCSB scientists

Excitons with extra zing could make networks sing

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A group of University of California Santa Barbara researchers is touting a new technique to create multi-coloured lasers.

The coherent light of lasers is created by pumping a suitable material with energy. The energy is absorbed by electrons in the material, which move briefly to a higher-energy state; when they shed that energy, it is given off as a photon – with the wavelength of the light determined by the resonant characteristics of the material (a simplified description).

Lasers with multiple wavelengths are usually created by mixing the outputs of different devices, but the UCSB approach is different: they’re persuading the material they’re pumping to emit multiple wavelengths, by pumping it with both near-infrared and terahertz-frequency beams.

It works like this: the material – in this case, nanostructures of gallium arsenide – is pumped with lasers of different wavelengths. The near-infrared beam creates excitons (electron-hole) pairs in the material; in other words, pulling the electron completely out of its orbit instead of lifting it into a higher-energy orbit.

The electron, however, retains its attraction to the hole it came from – and this is where the university has added its “secret sauce”, a second, more powerful terahertz beam. As explained by UCSB physics professor Mark Sherwin in the university’s announcement:

“The very strong, low-frequency free electron laser beam rips the electron away from the hole and accelerates it. As the low-frequency field oscillates, it causes the electron to come careening back to the hole."

The electron has excess energy because it has been accelerated, the statement notes, and when it slams back into the hole, the recombined electron-hole pair emits photons at new frequencies.

Even better, from a practical standpoint: in their paper for Nature (of which Sherwin is a co-author), the researchers say they can optimize the spacing of the multiple wavelengths emitted by their multi-color laser.

If miniaturized, that would give the multi-color laser a home in optical communications, where wavelength-division multiplexing (WDM) expands the carrying capacity of networks by using different wavelengths to carry different data streams.

All such an application would need is to replace the building-sized free-electron laser in the UCSB’s Broida Hall (so large because it’s tunable as well as very powerful) with a transistor laser operating in the terahertz range, Sherwin said. “Now that we’ve seen this phenomenon, we can start doing the hard work of putting the pieces together on a chip.” ®

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