High index contrast subwavelength grating produces super thin mirror

Feb. 15, 2007
February 15, 2007, Berkeley, CA--Engineers at the University of California, Berkeley, have created a new high-performance mirror that could dramatically improve the design and efficiency of the next generation of devices relying upon laser optics, including high-definition DVD players, computer circuits and laser printers.

February 15, 2007, Berkeley, CA--Engineers at the University of California, Berkeley, have created a new high-performance mirror that could dramatically improve the design and efficiency of the next generation of devices relying upon laser optics, including high-definition DVD players, computer circuits and laser printers.

The new mirror packs the same 99.9 percent reflective punch as distributed Bragg reflectors (DBRs), but it does so in a package that is at least 20 times thinner, functional in a considerably wider spectrum of light frequencies, and easier to manufacture.

Connie J. Chang-Hasnain, director of UC Berkeley's Center for Optoelectronic Nanostructured Semiconductor Technologies, developed the super-thin mirror, or "high-index contrast sub-wavelength grating (HCG)," with her graduate students, Michael Huang and Ye Zhou. Their work is described in this month's issue of the journal Nature Photonics.

"When you reduce the thickness of a mirror, you are significantly reducing the mass of the device, which also translates into lower power consumption," said Chang-Hasnain, who is also a UC Berkeley professor of electrical engineering and computer science. The mirror we've developed overcomes the hurdles that have stalled the advancement of certain lasers."

"DBRs can reflect 99.9 percent of light, but it can take up to 80 layers of material to achieve this high reflectivity," said Huang, lead author of the paper. "The DBR ends up being a relatively thick 5 microns wide. The precision necessary for the layers also requires a complicated manufacturing process. Our mirror is thinner and will be easier to manufacture, which keeps the cost low."

Instead of multiple levels of alternating refractive-index layers, the HCG mirror developed by the UC Berkeley engineers contains only one pair. In this study, the engineers used aluminum gallium arsenide for the high refractive index layer, coupled with a layer of air. In addition, the high refractive index layer contained grooves spaced by a distance that is less than a wavelength of light.

In this configuration, light hitting the mirror surface was directed over the grooves. As the light waves passed each semiconductor-air interface, they were strongly reflected back in the opposite direction. The researchers noted that other materials could replace air as the low refractive index material. Silicon dioxide, for instance, has a refractive index of 1.5.

To demonstrate the reflectivity of the HCG, the researchers replaced one of the two DBRs in a vertical-cavity surface-emitting laser with the new mirror. They confirmed that the HCG is capable of providing reflectivity greater than 99.9 percent, equivalent to the DBR.

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

Potential applications include high density data storage as well as providing the mobile mirror in micro-electromechanical systems (MEMS), such as wavelength tunable lasers.

"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.

The researchers added that it may be possible to print this mirror on various surfaces, and that it could one day be used to create organic, plastic displays that can be rolled up for easy transport.

"There is a wide range of products based upon laser optics that could benefit with this thinner mirror," said Huang. "It includes light emitting diodes, photovoltaic devices, sensors, computer chips and telecommunications equipment."

For more information, contact Connie Chang-Hasnain.

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