Angular Dispersion of Photonic Pseudogap in Opal vs Inverse Opal
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1014-AA07-06
Angular Dispersion of Photonic Pseudogap in Opal vs Inverse Opal Lay Kuan Teh, and Chee Cheong Wong School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Singapore, 639798, Singapore
ABSTRACT We have recently achieved a non-dispersive, low-order pseudogap in the blue region near the L-point in electrodeposited ZnO inverse opal. This behavior is a consequence of the change in the interconnecting network of interstices from a low dielectric (air) in the opal to a high dielectric one in the inverse opal. To verify this, we present the calculated photonic bands along the LW direction, which corresponds to the angular range explored experimentally by means of angle-resolved spectroscopy. We also studied the dispersion as a function of refractive index contrast (RIC). Further increase of the RIC above the threshold necessary to open up a complete photonic bandgap between the 8th and 9th bands does not have significant effects on improving the non-dispersive characteristic in the pseudogap. The results could be extended to make other inverse photonic structures of different symmetry with non-dispersive bands suitable for the study of optical processes involving low group velocity. INTRODUCTION In an artificial opal of dielectric spheres, the low refractive index contrast (RIC) opens a photonic pseudogap between the 2nd and 3rd bands in the photonic band diagram. [1] The inverse opal, on the other hand, exhibits a complete photonic band gap (PBG) between the 8th and 9th bands for sufficiently high RIC (>2.8). [2,3] This can be realized by infiltrating the opal with high dielectric materials followed by removal of the opal, leaving behind air spheres within a high refractive index material backbone. Theoretical simulations have shown that the opening of the PBG is a function of the infiltration degree while the gap width is dependent on the RIC. [4] The band gap is extremely sensitive to geometrical non-uniformities such as size and site randomness. [5] One approach to experimentally probe the existence of a PBG in the opal is the collection of transmission/reflection spectra along high symmetry directions of the first Brillouin zone (BZ) by varying the angle of incidence. This reveals the angular dispersion of the stop-band, which is an important factor towards a complete PBG. The overlapping of the stop-bands for all directions corresponds to an omni-directional (complete) PBG. Due to the complexity in correlating the high energy spectral features to the high-order bands,[6] most work focus on investigating the evolution of the low-energy pseudogap with the incident angles. In this work, we report an improvement in the stop-band characteristics, i.e., a nondispersive pseudogap in the blue region near the L-point in the Brillouin zone, in an electrodeposited ZnO inverse opal. Although the refractive index of ZnO (~2.0) is insufficient for the opening of a complete PBG, the RIC is high enough to produce pseudogaps. While the opening condition of a complete PBG in an inv
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