Toward a solar-powered laser

To most people, the phrase “solar energy” conjures up images of photovoltaic panels or solar thermal collectors. But one long-explored application is the solar-pumped laser, which offers the promise of greater efficiencies both in generating coherent radiation and in converting the Sun’s energy into usable forms. Research on such devices dates back 50 years. Solar irradiation can average several hundred watts per square meter at Earth’s surface, yet early prototypes never produced even 0.1 watt per square meter of sunlight-collecting mirror. One path toward higher efficiencies lies in appropriating a wider swath of the solar spectrum. Doping a common laser material, neodymium-doped yttrium aluminum garnet (Nd:YAG), with chromium ions has shown promise. Shermakhamat Payziyev and Khikmat Makhmudov of the Uzbekistan Academy of Sciences now report on a refinement to further improve performance: shifting the spectral-harnessing responsibilities to a separate region that couples to the laser’s active region. In the pair’s model, a mirror focuses sunlight into the Nd:YAG active region, which has been further doped with cerium ions and which primarily absorbs in the yellow and red. Photons of other colors travel down to an end cap containing Cr-doped gadolinium scandium gallium garnet (Cr:GSGG), which primarily absorbs in the blue-violet and orange regions of the solar spectrum and reemits in the red; a mirror reflects the emitted red photons back into the Nd:YAG. The Cr:GSGG therefore effectively serves as a frequency converter that channels much more of the solar energy into the laser. In their simulation the researchers found up to 32% of the incident sunlight going into the laser light. The pair notes that even higher outputs should be possible by separately optimizing the regions’ operating temperatures. (S. Payziyev, K. Makhmudov, J. Renew. Sust. Ener. 8, 015902, 2016, doi:10.1063/1.4939505 .)