Engineers at the University of California, Berkeley, have created a 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 current high-grade mirrors, called 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. All these characteristics present critical advantages for today’s ever smaller integrated optical devices.

      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 the journal Nature Photonics.

      “Today’s semiconductor lasers demand mirrors that can deliver high reflectivity, but without the extra thickness,” said Chang-Hasnain, who is also a UC Berkeley professor of electrical engineering and computer science. “When you reduce the thickness of a mirror, you are significantly reducing the mass of the device, which also translates into lower power consumption. The mirror we’ve developed overcomes the hurdles that have stalled the advancement of certain lasers.”

      To get the coherent, single wavelength light of a laser beam requires a pair of mirrors at opposite ends of a photon-generating gain medium. 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 said to be saturated, and the light energy is transferred into a laser beam.

            Early versions of semiconductor lasers used crystal for the mirrors, which yielded 30 percent 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...

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