Nanometer‐Scale Precision Tuning of 3D Photonic Crystals Made Possible Using Polyelectrolytes with Controlled Short Chain Length and Narrow Polydispersity

Abstract

Herein we report the development of an effective method which enables nanometer-scale precision tuning of prefabricated 3D colloidal photonic crystals (PCs). In our approach which involves the layer-by-layer (LbL) electrostatic assembly of polyelectrolytes in the photonic crystals, a key conceptual novelty lies in the development and utilization of polyelectrolytes with controlled short chain lengths and narrow polydispersity as the infi ltrating materials. This allows uniform deposition of the polyelectrolytes in the PCs while circumventing the leading problem in the LbL approach:blockage of the PC interstitial openings by large polyelectrolytes during the early stages of infi ltration. As a result, nanometer-scale precision tuning of photonic stopband of 10 nm or less has been achieved, which few other methods can attain. The method is facile and reproducible, and can be applied to photonic crystal lattices ranging from small to large dimensions. Photonic crystals are an interesting class of photon-control materials characterized by periodic array of dielectric lattices at the scale of light wave lengths. [ 1 ] These materials possess photo nic band gaps or stop bands where light within a range of frequencies is prohibited from propagating along specifi c lattice directions in the structure. As a result, PCs have attracted signifi cant recent attention as powerful tools for the manipulation of photons with potential applications ranging from telecommunications to optical computing. [ 2 ] Three-dimensional photonic crystals can be created by top-down methods such as holographic lithography and direct laser writing or bottom-up methods such as self-assembly of monodisperse colloidal silica or polymer spheres. [ 3 ] In both fabrication methods, the PC structures generated are of a passive nature with fi xed optical properties, and any slight change in the requirement of the photonic band gaps or stop bands, such as in solar cell, light emitting diode or lasing applications, will generally need the fabrication of a new photonic crystal. In comparison, nanometer-scale fi ne tuning of existing photo nic structures through controlled infi ltration represents an attractive and cost-effective alternative. However, since many of these photonic structures possess an interconnected pore network, control at the nanometer regime of the infi ltrated materials is a challenge for designed optical response. [ 4 ] In our efforts to develop post-fabrication strategies to tailor the optical properties of PCs, LbL assembly [ 5 ] of polyelectrolytes (PEs) [ 6 ] in preformed photonic crystal lattices [ 7 ] emerged as a promising approach. Importantly, we found that PEs which possessed controlled short chain lengths and narrow polydispersity were suitable as infi ltrating materials for the fi ne tuning of photonic stop bands. In this Communication, we report the development of the method and the precision tuning of photonic properties on the nanometer scale.