Solar-Pumped Laser

September 20, 2011

The archetypal aluminum garnet is yttrium aluminum garnet (YAG, Y₃Al₅O₁₂), which is used as a laser host. Neodymium-doped YAG (Nd:YAG) is an important commercial solid state laser that emits at 1064 nm. The neodymium atoms that substitute for about one percent of the yttrium atoms in these solid state lasers absorb energy in two broad bands between 730–760 nm and 790–820 nm (see figure).

Relative optical transmission of neodymium-doped YAG. Data from published sources. Graph via (Gnumeric).

It's these absorption bands that allow efficient conversion of the pumped light energy into useful infrared laser light that makes Nd:YAG such a commercial success. These absorption wavelengths overlap nicely with the spectral output of many flash lamps and light emitting diodes.

Many years ago, I did research on the epitaxial crystal growth of layers of aluminum garnet for optical waveguide applications.[1-2] I also prepared epitaxial layers of chromium-doped YAG, which is interesting as a tunable laser material. In that case, we wanted the chromium to exist in the crystal as the Cr⁴⁺ cation, so it was necessary to co-dope with Ca²⁺ for charge compensation.

As is often the fate of the crystal grower, my very nice wafers of this material were never characterized by the optical physicist for whom I had prepared them. As far as the scientific literature is concerned, these crystals were never made, so it's important to choose your collaborators wisely.

A small wafer of neodymium gallium garnet (Nd3Ga5O12) in a two-inch wafer carrier. Neodymium-doped oxides have this pleasant color, although the color is not as intense as for this crystal. (Photo by author).

It's also possible to prepare (Nd,Cr):YAG with Cr³⁺, and this leads to enhanced optical absorption at visible wavelengths. Although not as bad as its hexavalent cousins, chromium (III) oxide is still a nasty chemical; but it has a green color, and green is right in the middle of the visible spectrum. Cr³⁺, when added to Nd:YAG, captures visible light and transfers its energy to excite the lasing neodymium atoms.

The absorption spectra of Cr³⁺in YAG extends from 400-700 nm.[3] This enhanced optical absorption of visible light can be put to use in a solar pumped laser. Furthermore, ceramic processing techniques have advanced to the point at which you can prepare an optically adequate laser without the expense of preparing a single crystal.[3,4]

Recently, a team of scientists at the arcanely named Uzbekistan Scientific and Production Association (Akadempribor) has been researching solar pumped (Nd,Cr):YAG ceramic lasers formed as thin slabs. These slab lasers are placed in optical devices that resemble Newtonian reflector telescopes (see figure). The sunlight is concentrated by a one meter diameter parabolic mirror into a focused spot of intense radiation about an inch in diameter that pumps the ceramic laser slab.

A Newtonian reflecting telescope. The solar pumped laser presumably has the same arrangement of optical elements with the slab laser replacing the secondary mirror (M2). (Drawing by H. Hahn, via Wikimedia Commons).

Heat, of course, is the enemy of most good things, so it's necessary to extract heat from the ceramic laser with a water-cooled heat sink. The Uzbekistan research team claims that their solar pumped laser should convert thirty-five percent of solar energy into laser light. Other solar pumped lasers have shown just one, or two, percent efficiency.

More detailed information can't be found just yet, since the paper that describes this research is still in press. The paper, "Transformation of concentrated sunlight into laser radiation on small parabolic mirrors," by Shermakhamat Payziyev, Sagdilla Bakhramov and Abdugappar Kasimov will appear in the Journal of Renewable and Sustainable Energy.

References:

1. D.M. Gualtieri and R.C. Morris, "Epitaxial Waveguides of Aluminum Garnet," J. Appl. Phys. vol. 74, no. 1 (July 1, 1993), pp. 20-23.
2. Devlin M. Gualtieri and Robert C. Morris, "Optical Waveguides of Aluminum Garnet," U.S. Pat. No. 5,113,472, May 12, 1992.
3. M. Yamaga, Y. Oda, N. Kashiwagura, K. Hasegawa, H. Ito and S. Mizuno, "Energy transfer in YAG ceramics codoped with Nd³⁺ and Cr³⁺," European Conference on Lasers and Electro-Optics 2009 and the European Quantum Electronics Conference (CLEO Europe - EQEC 2009), Munich, June 14-16, 2009.
4. Nanograined Y₂O₃ ceramic transmits as well as single crystal, Laser Focus World, vol. 45, no. 6 (June 2009), p. 14.
5. Charles E. Blue, "Parabolic mirrors concentrate sunlight to power lasers," American Institute of Phyics Press Release, September 12, 2011.