Luminescent Solar Laser - The Next Step in LSC Research

The the luminescent solar laser was presented as being the next step beyond the luminescent solar concentrator (LSC) at the Nov. 2010 meeting of the Optical Society of America's Frontiers in Optics.

Carmen Rotschild, a researcher at the Massachusetts Institute of Technology (MIT; Cambridge, MA) outlined the work groups at MIT and the University of Michigan have done so far to make such a luminescent solar laser practical.

In a conventional incoherent LSC, a slab of clear plastic is doped with a dye that absorbs sunlight and reradiates it in a longer-wavelength band; the reradiated light (or part of it, at least) makes its way via total internal reflection to the edge of the slab, where it is collected by a long, thin photovoltaic (PV) cell.

Incoherent LSCs have some remarkable qualities--for example, they reach reasonably high concentrations without having to track the sun, and can even concentrate light on a completely cloudy day. However, because the dye in the slab reradiates light in all directions, part of the light escapes the slab, lowering efficiency.

The idea behind the luminescent solar laser is to create a setup where the light would lase rather than simply re-emit incoherently, then the radiation would be emitted within a very narrow angle and would thus all be channeled to the PV cell.

Micro ring Lasers

Rotschild and colleagues are creating microring lasers made of three materials, with one layer's output-wavelength band matching the next layer's absorption-wavelength band.

First, a very thin outer coating absorbs and re-emits very efficiently, but also does not transmit the light very well--which is not a problem, because the coating is so thin.

Next, a second material absorbs the first layer's output and re-emits it (at a longer wavelength), and has a longer transmission length, which allows the light to get into the laser cavity, which has a high Q factor.

Finally, the laser cavity itself, which has a very low absorbance but is compensated for this by its high Q, absorbs the light from the second material and produces laser light.

In Summary

Early experimental results are encouraging, but much more research on device materials and geometry is needed before practical solar-energy-collecting lasers can be created.

However, the potential of cheap, highly efficient solar concentrators using the luminescent solar laser technology, which will require very little PV area and can concentrate light on cloudy days, makes this project one of the most interesting variations on the LSC theme.

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