Materials with periodicity on the order of the wavelength of light, photonic crystals (PCs), that possess alternating regions with high dielectric contrast have been shown to exhibit photonic band gaps (PBGs), or forbidden ranges of electromagnetic frequencies (see Figure 1). In fact, for materials with sufficient dielectric contrast and three-dimensional periodicity, complete omnidirectional PBGs have been demonstrated, essentially rendering these materials perfect mirrors throughout their bulk for those frequencies within the band gap. Since midgap states (where the otherwise forbidden frequencies can exist) occur in regions where the periodic alternation in the dielectric contrast is interrupted, the controlled incorporation of line and point defects in a photonic crystal would result in the ability to direct the flow of light through intricate pathways. In fact, light has been successfully guided without loss through ninety degree bends with radii of the wavelength of light in PBG materials (which would be impossible in the current total internal reflection-based waveguides), potentially enabling the development and on-chip integration of photonic components and devices.
Figure 1. Frequency v. wave vector plot in which the complete omnidirectional PBG is highlighted in yellow. (Joannopoulos, J., Meade, R.D., Winn, J.N., Photonic Crystals: Molding the Flow of Light. Princeton University Press, 1995, p. 82)
The inexpensive and reliable fabrication of a material with a three-dimensional PBG at the communications wavelength of 1.5 microns remains a challenge that is currently being addressed. The two-dimensional techniques commonly used in microelectronics have been extended in layer-by-layer fabrication approaches toward PBG materials however these approaches are rather tedious and expensive to be scaled to production levels. While self-assembly approaches do not have these problems, they do present their own challenges due to their inherent lack of defect control and insufficient dielectric contrast. Using template-directed colloidal sedimentation, researchers have demonstrated single crystalline
Figure 2. Reconstruction of confocal micrographs of a high-quality colloidal crystal grown on a patterned template (sphere diameter ~1.6 microns). (van Blaaderen, A; Ruel, R; and Witzius, P. Nature. Vol. 385, 1997, p. 321-324
Since the colloids do not have sufficient dielectric contrast and it is more desirable to have the high index material in the inverse FCC structure, these PCs are typically used as templates for infiltration with high refractive index materials. A variety of infiltration techniques have been explored including: CVD, nanoparticle imbibing, electrodeposition, and melt imbibing. Removal of the colloids is very straightforward (a simple etch in hydrofluoric acid for
with sufficiently high dielectric constants is still a challenge being addressed by researchers. While they have made substantial progress in these areas, a perfect high index replica of a perfect colloidal crystal would still not be appropriate for most photonic band gap applications. Their operation typically relies on structures within the photonic crystal that are specifically designed to guide and otherwise interact with light so as to function as desired. This necessitates an appropriate method for the controllable incorporation of pre-defined structures in the interior of colloidal crystals. We have recently demonstrated controlled fabrication within photonic crystals via multi-photon polymerization and laser scanning confocal microscopy. This will extend the utility of self-assembled photonic band gap materials and may enable the inexpensive development and on-chip integration of PBG-based devices.
There has been recent work by our group on various aspects of self-assembled photonic band gap materials.
Yun-Ju (Alex) Lee: Conducting Polymer Based Photonic Crystals
Wendy Chan: Novel Routes to Photonic Crystals
Sandeep Mariserla: Photonic Biosensors
Seokwoo Jeon: Hydrothermal Synthesis of Er-doped Titania Particles for Photonic Band Gap Applications