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

Abstract

Photonic crystals (PCs) offer far greater control over the generation and propagation of light than any other material structures. [1,2] 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. [3,4] 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. [5][6][7][8][9] 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. [10][11][12][13] 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. [12] 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.[14] 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 TiO 2 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 TiO 2 structures are of high interest. In this...

Reinforcing nanocolloidal crystals by tuning interparticle bonding via atomic layer deposition

“…Unlike CVD, ALD is self-limiting by keeping the precursors separate, enabling ultrahigh-precision control over the deposition and providing superior film conformity and uniformity even for complex structures with ultrahigh aspect ratios [15][16][17][18][19]. Thus, ALD has been used as a way of infiltrating NCC templates to generate inverse opals [19][20][21][22][23][24][25][26]. Our previous studies [27,28] demonstrated that ALD can be used to significantly reinforce films composed of randomly packed polydisperse nanoparticles.…”

Sacrificial‐Layer Atomic Layer Deposition for Fabrication of Non‐Close‐Packed Inverse‐Opal Photonic Crystals

“…Recently, Míguez et al and King et al reported techniques to partially overcome this limitation by using extended heavy presintering of the opal template to increase the necking (or overlap) between neighboring spheres, thus increasing the inverse-pore diameter and allowing a higher degree of backfilling. [18,19] Although this technique can lead to enhanced properties, heavy presintering is unable to provide the nanometer-scale precision and control required to produce high-quality inverse opals. Additionally, it is incompatible with many template materials and structures.…”

Templating multiple length scales by combining phase separation, self-assembly and photopatterning in porous films

“…The lack of features is not surprising in any case as a 'dense' silica layer is generally observed at the film surface when using the vapor pressure of TEOS to deliver the silica to the preformed template [38,49]. Previously, King et al used ion beam milling to remove a capping TiO 2 layer to expose colloid templated pores [52]. In attempts to expose the pore structure of these materials, similar ion beam milling was used.…”