5. PHOTONIC CRYSTALS

Recent advances in optical communications have shifted the frontier in high-speed, wide-bandwidth information processing from electrons to photons. This, in turn, requires new kinds of materials to control light. Recent research has suggested a way to tailor the propagation of light via the creation of periodic optical media dubbed photonic crystals (PhCs).1,2 These crystals consist of a periodic array of macroscopic (wavelength-scale) dielectric and/or metallic "atoms" through which light passes, as described in the pioneering work of Yablonovitch and John.3,4

PhCs are structures that exhibit periodic variation in dielectric constant in one, two or three dimensions. The periodicity is onthe order of the wavelength of light and they exhibit a bandgap that forbids the propagation of a certain range of light frequencies. The propagation of light in these periodic materials is analogous to the well-known wavelike propagation of electrons in a crystalline structure. The periodic structure gives rise to Bragg diffraction, which is associated with stop-gaps for propagation in certain directions. In the direction of a stop-gap, light is excluded from the material. If light is very strongly coupled to a PhC, a full photonic band gap is expected, i.e., a frequency range for which no light can propagate in any direction.1,2

The ability to control the propagation of light by introducing a defect inside the periodic structure makes the photonic bandgap materials very attractive for the fabrication of active and passive elements like lasers, waveguides, beam splitters, switches, etc.

There are several fabrication methods of PhCs. One of more widely used is a multibeam holographic lithography. This method has been known for several years, the first 3D photonic crystals made this way were demonstrated in 2000 and they had the fcc-like and bcc translational symmetry.5 Later it has been established that this technique could also deliver structures with different morphologies such as gyroid-like, diamond-like, etc. Diamond-based structures have received the most attention because theoretical calculations indicated that this morphology provides the largest gaps and needs the smallest refractive index contrast between constituents of the photonic crystal to develop a full band gap (about 1.97).

The staff of Hybrid Technologies (HT) has used the Multibeam Holographic Lithography (MHL) method and demonstrated that it can be used for fabrication of photonic crystal structures of different symmetries, and that it is size scalable as well. Specifically, this fabrication method utilizing two different photopolymer systems, the SU8 and the ORMOCERŪ, is extremely useful and lends itself to the design of practical photonic devices such as wavelength-selective filters, retardation plates, polarizers, multiplexers, etc., for use in communication, sensing, imaging and spectroscopy systems for wavelength detection, monitoring and filtering.

HT has created 2D and 3D PhC structures utilizing the MHL method. The Figure below shows SEM images of 2D structures fabricated in SU-8 resin by recording consecutively two orthogonal gratings using an

exposure of 2.5 Jcm-2 of 532 nm cw laser line for each grating.


For 3D structures, an "umbrella" configuration with two different polarizations of the recording beams is used to fabricate photonic crystal structures in a SU-8 based resin.6 In this umbrella geometry, three side beams, indicated by wavevectors k1, k2 and k3, form a tetrahedron whose base is parallel to x'y' plane in local x'y'z' Cartesian coordinates. The fourth, central beam ko, is the tetrahedron's height and intersects with all three of the other beams (1, 2 and 3) at the tetrahedron's apex point. Each side beam forms an angle of 39 degrees (in air) with the central beam.
 

In conclusion, HT's team has advanced a convenient method for fabricating periodic structures of different types in commercial photoresists. The inclusion of nanoparticles is used to enhance refractive index contrast and to confer other properties. Continued work is focused on the incorporation of intentional "defects" and additional nanoparticle types and to transition this technology into practical devices.

[1] Joannopoulos JD, Meade RD, Winn JN. "Photonic crystals: molding the flow of light." Princeton University Press, 1995.
[2] Johnson SG, Joannopoulos JD. "Photonic crystals: the road from theory to practice." Boston: Kluwer, 2002.
[3] Yablonovitch E. Inhibited spontaneous emission in solid-state physics and electronics. Phys Rev Lett 1987;58:2059-62.
[4] John S. Strong localization of photons in certain dis-ordered dielectric superlattices. Phys Rev Lett 1987;58:2486-9.
[5] M. Campbell, D. N. Sharp, M. T. Harrison, R. G. Denning, and A. J. Turberfield, Nature (London) 2000, 404, 53.
[6] S. Yang, M. Megens, J. Aizenberg, P. Wiltzius, P. M. Chaikin, and W. B. Russel, Chem. Mater. 2002, 14, 2831-2833.
 

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