Machine Design

Ultra-thin mirror for high-performance optics

Engineers at the University of California, Berkeley, have created mirrors that pack the same 99.9% reflectivity as current distributed Bragg reflector (DBR) mirrors, but do so in packages 20 times thinner.

The high-index-contrast sub-wavelength grating reflects 99.9 percent of light, the same reflectivity as the much thicker distributed Bragg reflectors. (Images courtesy Michael Huang/UC Berkeley)

The high-index contrast sub-wavelength grating (HCG) mirrors were developed by Connie J. Chang-Hasnain, director of UC Berkeley's Center for Optoelectronic Nanostructured Semiconductor Technologies and her graduate students, Michael Huang and Ye Zhou.

HCG mirrors also function in a considerably wider light frequency spectrum and will be easier to manufacture, says Chang-Hasnain who is also a UC Berkeley professor of electrical engineering and computer sciences. Super-thin mirrors could dramatically improve the design and efficiency of next-generation laser-optic-based devices.

Inside high-definition DVD players, computer circuits, and laser printers, for example, a pair of mirrors at opposite ends of a photon-generating gain medium produces the coherent, single wavelength light of a laser beam. Light photons of a specific frequency bounce back and forth between the mirrors, building up energy with each pass. As this effect levels off, the gain is saturated, and the light energy is transferred into a laser beam.

"Today's semiconductor lasers demand mirrors that can deliver high reflectivity, but without the extra thickness," explains Chang-Hasnain. "When mirror thickness is reduced, devices get significantly lighter, which in turn translates into less power consumption."

Early versions of semiconductor lasers used crystal for the mirrors, which yielded a mere 30% reflection. Such a low reflectivity is too inefficient for vertical-cavity surface-emitting lasers (VCSEL) - used in short-range optical communications, optical mice for computers, and other applications requiring low power consumption. VCSELs have a particularly short gain medium, so a highly reflective mirror is needed.

High reflectivity is possible with DBRs, in which light passes through alternating layers of aluminum gallium arsenide (AlGaAs), which has a refractive index of 3.0, and gallium arsenide (GaAs), which has a higher refractive index of 3.6. The difference in refractive indices lets a small amount of light reflect from each pair of alternating layers. The light from the multiple layers adds up to form a strongly reflected coherent beam.

"DBRs can reflect 99.9% of light, but need up to 80 layers of material," said Huang. DBRs end up being a relatively thick 5 micrometers wide. The precision necessary for the layers also requires a complicated manufacturing process. In contrast, the UC Berkeley HCG mirrors contain only one pair - AlGaAs for the high refractive index layers, coupled with a layer of air, which has a very low refractive index of 1. In addition, the AlGaAs layer contains grooves spaced by a distance that is less than a wavelength of light.

In this configuration, light hitting mirror surfaces is directed over the grooves. As the light waves pass each semiconductor-air interface, they strongly reflect back in the opposite direction. The researchers report that other materials could replace air as the low refractive index material. Silicon dioxide, for example, has a refractive index of 1.5.

Researchers replaced one of the two DBRs in a vertical-cavity surface-emitting laser with a HCG mirror. Test showed that the HCG mirror provides reflectivity greater than 99.9%, equivalent to the DBR.

"The HCG mirror overcomes many of the hurdles that has slowed the advance of VCSEL research," says Zhou. "In addition to being thinner, it has the advantage of working in a broader range of light frequencies."

The latter attribute is particularly important as optical disc technologies increasingly employ blue-violet lasers, which operate on a shorter wavelength than red lasers. Shorter wavelengths make it possible to focus on smaller units, enabling significantly higher-density data storage. In addition, the researchers say it may be possible to print HCG mirrors on various surfaces to create organic, plastic displays that can be rolled up.

Future efforts by the researchers will include applications in microelectromechanical systems (MEMS), such as wavelength tunable lasers, which are used in broadband communications. "Reducing the size of the laser's mirror also means a dramatic reduction in weight, which is particularly important for high-speed MEMS devices," said Chang-Hasnain.

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