Prototyping of three-dimensional photonic crystal structures using electron-beam lithography
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Prototyping of three-dimensional photonic crystal structures using electron-beam lithography G. Subramania, J.M. Rivera Sandia National Laboratories, P.O Box 5800, Albuquerque, New Mexico 87185 Abstract We demonstrate the fabrication of a three-dimensional woodpile photonic crystal in the near-infrared regime using a layer-by-layer approach involving electron-beam lithography and spin-on-glass planarization. Using this approach we have shown that we can make structures with lattice spacings as small as 550 nm with silicon as well as gold thus allowing for fabrication of photonic crystals with omnidirectional gap in the visible and near-IR. As a proof of concept we performed optical reflectivity and transmission measurements on a silicon structure which reveal peaks and valleys expected for a photonic band gap structure. The approach described here can be scaled down to smaller lattice constants (down to ~400 nm) and can also be used with a variety of materials (dielectric and metallic) thus enabling rapid prototyping full three-dimensional photonic bandgap based photonic devices in the visible. Introduction Since the publication of the original paper by Eli Yablanovitch [1]and Sajeev John [2] in 1987 tremendous progress has been made in the area of photonic band gap structures. The unique properties of photonic band gap structures or photonic crystals as they are also called, arise from the creation of forbidden light propagation modes which allow for molding and confinement of light over small spatial regimes. These properties have enormous implications in the field of quantum optics and photonics particularly, for applications in integrated optics, optical computing and energy efficient lighting. Theoretical models predict that a three dimensionally periodic structure with a right geometry and material parameters can give rise to an omnidirectional electromagnetic forbidden gap which provides the best case scenario to harness the full potential of the photonic band gap. A large number of applications such as the ones mentioned above, require photonic crystals that operate in the near infrared (e.g. communications and optical computing) and visible ( e.g. light emission) part of the electromagnetic spectrum. This implies that structures with submicrometer periodicity are required which makes fabrication of three dimensional structures of these kinds extremely challenging. Early successes in this direction were achieved using techniques such as state-of-the-art submicron lithography in the CMOS process [3, 4] and wafer fusion [5]. Technological demands required by these techniques make fabrication of these structures quite difficult hence the slower progress compared to the two dimensional photonic crystals. Alternative approaches have been taken by various groups to fabricate 3D photonic crystals over large area which include self-assembly[6, 13],three dimensional interference lithography [7], pore diameter modulation [8], autocloning [9], glancing angle deposition(GLAD) [10], microtransfer molding [11] a
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