Monday, February 6, 2012

High-contrast gratings bring new meaning to "integrated" optics

Gail Overton
Senior Editor
Laser Focus World

Photonics West is such a condensed week of awesome photonics technology that it is next to impossible to describe even a fraction of the best presentations and papers. But I was particularly impressed by the Tuesday, 24 January OPTO Plenary presentation by Connie J. Chang-Hasnain, UC Berkeley research professor (and incidentally, an editorial advisory board member for Laser Focus World), entitled "High-Contrast Metastructures for Integrated Optics."

Chang-Hasnain presents her work with such enthusiasm! I found myself, along with the audience of several hundred, paying close attention as she described the incredible science behind how a periodic structure of lines can act as a highly reflective mirror, a high-Q resonator, a focusing element or lens, a vertical in-plane coupler, and even be used to fabricate a slow-light waveguide--no kidding.

The website for Chang-Hasnain's CCH Optoelectronics Group at UC Berkeley at explains the functionality of high-contrast subwavelength gratings in much detail. But in brief, high-contrast gratings (HCGs) have alternating stripes of semiconductor materials and air (or silicon) with subwavelength periodicity. And unlike distributed Bragg reflectors with narrowband operation, HCGs can be as much as two orders of magnitude thinner and are both broadband and sensitive to the incident polarization state of the input.

IMAGE: Unlike conventional diffraction gratings, the high-contrast gratings (HCGs) developed by Connie Chang-Hasnain's group at UC Berkeley have a grating period that is nearly one wavelength; that is, between the incident wavelength in air and that divided by the high refractive index of the grating material. (Courtesy Weimin Zhou, U.S. Army Research Laboratory)

She describes how mathematical simulations can be used to analyze the behavior of light as it passes through the HCG device; rigorous coupled wave analysis (RCWA) and other modeling software can determine the physical parameters of an HCG (length and width of the line structure and line separation or period) and how those parameters can translate into a user-desired light-guiding function. For example, the versatility of HCGs is proving critical in the development of lower-cost, better-performing vertical cavity surface-emitting lasers (VCSELs) in a single epitaxial step, which is a major area of study for Chang-Hasnain's group.

There is even an ABC News report on the HCG work as shown in the YouTube video below:

Imagine replacing bulky optoelectronic devices with thin, smart layers that are engineered to perform particular functions. As the number of research papers and "integrated", silicon-photonics-compatible components continues to grow based on this architecture, HCGs--"a new platform for integrated optics" in the words of Chang-Hasnain--will become commonplace in (and critical for) the photonics engineering community.

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