Conformally Back-Filled, Non-close-packed Inverse-Opal Photonic Crystals

Photonic crystals (PCs) offer far greater control over the generation and propagation of light than any other material structures. Their defining characteristics, omnidirectional and directional photonic bandgaps (PBGs), can be manipulated to enable effects such as low losses in optical circuits and control of spontaneous emission. PCs are also creating much excitement because of their ability to form flat bands, yielding phenomena such as slow light and negative refraction. The infiltration and inversion of synthetic-opal templates has been established as a promising method for obtaining the periodic structure and refractive-index contrast required in PCs. In addition, topological tuning by shifting the distribution and filling fraction of a high-dielectric material within PCs offers a way to significantly change their photonic-band properties. For example, increased tunability, functionality, and bandgap properties can be accomplished through advanced architectures such as multilayered inverse opals and non-close-packed (NCP) inverse opals. NCP geometries differ significantly from inverse opals by the formation of extended “air cylinders” between neighboring air spheres, as shown in Figure 1a, whereas, in the more-limited inverse opal, the air spheres are connected only by narrow “sinter necks.” As reported by Doosje et al., silicon NCP architectures are predicted to yield a 100 % increase in the width of the omnidirectional PBG between the eighth and ninth photonic bands. In this paper, we show that modification of PC-template topology by both heat treatment and multiple conformal infiltrations facilitates precise control and optimization of photonic-band properties. Static tuning is possible by precisely controlled backfilling of inverse opals to increase the filling fraction of dielectric material. However, inverse opals have inherently limited filling-fraction tunability because the narrow sinter necks quickly close with backfilling. To solve this limitation, we report the implementation of a new, two-step atomic layer deposition (ALD) infiltration process to form a TiO2 NCP inverse opal. Significantly, this new process allowed a static tunability of ∼ 400 nm in the position of the directional bandgap. In the first step, the void space available for infiltration was reduced to ∼ 25 % of the original volume by controllably collapsing the opal template by sintering followed by ALD infiltration, a method that is capable of high-finesse deposition of dense films within a nanoporous template. This facilitated the formation of an ultralow-filling-fraction inverse opal (5.8 %) with 193 nm diameter sinter necks. In the second step, backfilling of the low-volume-fraction inverse opal resulted in large tuning of the photonic-band properties and, ultimately, the formation of an NCP structure. Because of its many applications in biosensing, solar cells, catalysis, and environmental cleanup, optically active and tunable TiO2 structures are of high interest. In this study, silica opal templates were first grown on silicon substrates in a manner similar to that of Park and coworkers, as described elsewhere. The steps that were next taken to form an NCP inverse opal are illustrated in Figure 1b. First, a 10 lm thick opal template with 460 nm sphere diameter was sintered at 1000 °C for 3 h (Fig. 1b, step i), which is considerably longer and at a higher temperature than the “normal” sintering condition of 800 °C for 2 h, in order to partially collapse the template and increase the sinter-neck diameter. TiO2 was then conformally deposited in the very small air voids (< 10 nm) using ALD, a technique that has reC O M M U N IC A IO N S