Photonic Crystals Made by Holographic Lithography
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Photonic Crystals Made by Holographic Lithography A.J.Turberfield Introduction A photonic crystal is a periodically structured composite material, with a unit cell whose dimensions are of the order of an optical wavelength, made from constituents whose refractive indices differ greatly (n is of the order of 2). Three-dimensional (3D) photonic crystals typically consist of interpenetrating networks of dielectric material and air. Holographic lithography1 is a technology for the fabrication of photonic crystals in which the initial step is to define the 3D microstructure by interference of coherent light in a photosensitive precursor. The defining characteristic of a photonic crystal is a photonic bandgap, a range of frequencies within which no propagating electromagnetic modes exist.2 If the bandgap is complete (omnidirectional), then the material acts as an optical insulator. Structural defects in a photonic crystal may give rise to spatially localized electromagnetic modes at energies within the gap: 3 these defect modes are analogous to microcavity-confined modes.4 Waveguides are formed by coupling defects together.5 A waveguide operating at a frequency within a photonic bandgap cannot leak— there are no propagating electromagnetic modes in the surrounding photonic crystal capable of carrying energy away. In principle, this allows the fabrication of waveguides that turn corners in a distance of the order of the optical wavelength,6 requiring 2 orders of magnitude less space than a typical semiconductor ridge waveguide (currently used in integrated optics), which has a minimum bend radius 100 m. The development of holographic lithography is at least partly motivated by a view of the future of photonic-crystal
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engineering which sees waveguides operating at frequencies within a photonic bandgap as the basis of miniaturized integrated optical circuits with length scales comparable with those of integrated electronics. Such engineering applications of photonic crystals will require fabrication technologies for the cheap and rapid production of periodic photonic microstructures that have the potential to incorporate engineered structural defects to create microcavities and waveguides. The relative technological importance of 2D and 3D photonic crystals is still unclear—it seems likely that there will be applications for both. Two-dimensional structures7 can be created by patterning planar waveguides using well-developed lithographic techniques (including the use of 1D and 2D optical interference patterns to define gratings8,9). Two-dimensional photonic crystals allow many aspects of photonic-crystal physics to be demonstrated and exploited. Three-dimensional structures avoid problems created by the out-of-plane diffractive losses suffered by 2D structures; in particular, they will allow the creation of 3D microcavities with quality factors limited only by the intrinsic loss of the dielectric. Three-dimensional photonic crystals are a natural host for 3D opticaldevice and interconnect architectures. Some import
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