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Porous silicon mirrors show promise

11 May 2004

Researchers produce broadband high-quality laser mirrors made from layers of porous silicon.

Researchers in Australia have fabricated broadband laser mirrors from porous silicon (PSi). The team says that a tunable (740 – 960 nm) Ti:Sapphire laser fitted with its PSi mirrors showed stable operation over a three month testing period. (Applied Physics Letters 84 3519)

Mike Gal and his colleagues from the University of New South Wales start with a highly boron-doped p+-type (100) silicon wafer. They produce mirrors by electrochemically etching an alternating stack of high and low porosity silicon layers into the wafer.

“As the refractive index is a function of the porosity, it is possible to etch a “dielectric stack” into the silicon with a predetermined refractive index profile,” explained Gal. “Due to the large refractive index ratio that can be achieved with PSi, high quality (reflectivity > 99.8%), very broadband mirrors can be made. Fabrication costs are also very low.”

Another possible advantage is that very large area mirrors can be made, although to date the largest mirror the researchers have produced has a diameter of 15 mm.

Gal’s team has made PSi equivalents of the high-reflector end mirror and the output coupler found in commercial Ti:Sapphire lasers. The team exchanged the standard components with their PSi versions in a 5W argon-pumped Ti: Sapphire tuning between 740 and 960 nm. The laser was then operated CW and modelocked where it produced 80 fs pulses at a repetition rate of 85 MHz.

According to the authors, there was negligible difference between the CW output powers when using its PSi mirrors. “The dispersion of the PSi mirrors in the given spectral range did not measurably increase the temporal width of the pulses,” they report. “Nor did the high-intensity pulses damage the mirrors.”

By placing a curvette filled with Rhodamine 6G dye between two PSi mirrors, the researchers have also made a PSi/dye laser. “The combination of laser dyes with PSi cavities may open the possibilities of producing dye-filled PSi microlasers embedded within the Si wafer,” say the authors. “Such a laser could prove to be useful in microelectronic and MEMS applications.”

The team is now making mirrors for the 1.3 to 1.6 micron spectral range. “We are also working on integrating PSi laser mirrors with other optoelectronic devices fabricated directly onto silicon and on ways to dope the mirrors with optically active components so we can switch the mirrors on and off,” Gal told Optics.org.

Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.

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