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Silicon Structures Offer Tunable Bandgap

Photonics Spectra
Sep 2001
Richard Gaughans

UNIVERSITY PARK, Pa. -- Tunable photonic bandgap materials show promise as light-modulation devices for telecommunications, display and data storage applications, but existing solutions display either a high tunability but slow response speeds or a quick response but limited tunability. Now researchers at Pennsylvania State University have designed a photonic crystal that offers both high tunability and fast response.

Assistant professor of materials science and engineering Venkatraman Gopalan and his colleague Sungwon Kim have designed an array structure consisting of two dielectric materials. They embedded columns of one material in a hexagonal pattern in the second material. Depending on the materials' dielectric constants and the diameter of the embedded columns relative to their spacing, the researchers' calculations indicate that the structure would exhibit an absolute bandgap that prohibits the propagation of both TE- and TM-polarized light.

For the case of air holes in a silicon film -- columns of air in a regular hexagonal pattern in a silicon substrate -- Gopalan and Kim have determined that the absolute bandgap would be roughly 6 percent of the energy associated with an electromagnetic wave of a wavelength equal to the lattice spacing.

Tunability

If two lateral edges of the crystal were sheared with respect to each other, the matrix would deform and thus shift the bandgap. For air-filled holes with a dielectric constant of 1 and a silicon substrate with a dielectric constant of 11.56, the absolute bandgap should vary by 73 percent of its initial value under a 3 percent shear strain. With a piezoelectric structure attached to the film, modulation frequencies in the megahertz range should be possible.

Because silicon is transparent at 1.3 and 1.55 µm, the arrays could find a place as modulators, optical switches or active filters in telecommunications applications. Gopalan is constructing a real device with a silicon matrix on an electrostrictive polymer.

The researchers reported their work in the May 14 issue of Applied Physics Letters.


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