Broadband Mirrors Built of Porous Silicon
Daniel S. Burgess
A research group at the University of New South Wales in Sydney, Australia, has reported the fabrication of broadband mirrors made from porous silicon for CW and mode-locked Ti:sapphire lasers and for a tunable dye laser. The performance of the high reflectors and output couplers is similar to that of commercial mirrors, demonstrating the potential of the material for the economical and flexible design and manufacture of such passive optical components.
Researchers have fabricated broadband mirrors out of porous silicon for Ti:sapphire and dye lasers. Materials of various porosities -- and thus refractive indices -- are stacked to create distributed Bragg reflectors with the desired optical properties. Courtesy of Michael Gal.
Discovered in the late 1950s, porous silicon is produced by electrochemically etching a silicon wafer in a hydrofluoric acid bath, which yields layers with a porous structure that alters the optical properties of the material. Michael Gal, a professor of physics at the university, explained that by varying the etching parameters, it is possible to control the size of the pores, quantified by a figure of merit known as porosity. In doing so, one can vary the refractive index of the porous silicon as desired between approximately 1.3 and 2.8.
This ability to tune the porosity -- and hence the refractive index -- enables the fabrication of distributed Bragg reflectors by stacking alternating quarter-wavelength-thick layers of higher- and lower-porosity material. In the experiments, the temperature of the 35 percent acid solution was fixed at 222.5 °C, and the porosity and thickness of the etched layers were determined by the magnitude of the applied current and by the duration of the etching. The researchers produced a mirror with a reflectivity of more than 98 percent between 730 and 1000 nm and an output coupler with a transmission of approximately 3 percent between 700 and 1000 nm and a peak reflectivity of higher than 99.4 percent.
To test the performance of the optical elements, they replaced the standard end mirror and output coupler in a commercial Ti:sapphire laser with the porous-silicon components. In CW mode, the laser demonstrated stable, tunable emission between 740 and 960 nm, although its output powers were slightly lower than with the stock mirrors because of the lack of optimization in the porous-silicon output coupler. With an expanded laser cavity and intracavity dispersion-compensating prisms, the setup produced 80-fs pulses with a repetition rate of 85 MHz. Over three months of testing, the laser showed no fluctuation in output powers with the porous-silicon components.
The technique also yielded a high reflector with reflectivity of more than 95 percent between 550 and 635 nm and an output coupler with a transmission of approximately 2 percent between 530 and 620 nm. Using the components, the researchers created an optically pumped rhodamine dye laser, which suggests the possibility of constructing microscale lasers from dye-filled cavities in porous silicon.
The work, Gal said, illustrates that inexpensive, conventional silicon technology can be used to fabricate large, high-quality laser mirrors. The team is looking for partners to develop porous silicon for commercial applications, he said. It also is investigating the construction of mirrors for the 1.3- to 1.6-µm spectral range, the integration of porous silicon mirrors with other optoelectronic components, the fabrication of porous-silicon semiconductor saturable absorber mirrors, and the doping of the mirrors with optically active materials to enable switchable transmission.
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