Nanoparticle-Based 3D Optical Nanostructure for Resonance Manipulation

Photonic crystals have been used for various applications with their distinctive optical properties. They feature synergetic optical properties which combine the controllability of selective wavelength in transmittance and reflectance, light confinement, and light scattering effect inside the periodic structure.¹ The spectacular colors of natural photonic crystals are produced by complex periodic structures, which have proven to be challenging to synthesize artificially. A variety of tools for the fabrication of artificial photonic crystals have been demonstrated, for instance, template-assisted fabrication for inverse opals, silicon double inversion, and laser lithography. However, the inherent constraints of material limitation, complexity, and time-consuming fabrication have hindered the development of high-performance photonic crystals for practical applications.<br/><br/>In this research, we utilized a transfer-printing technique, derived from immersion transfer-printing (iTP)², to fabricate 3D photonic crystals solely consisting of quantum dots (QDs). Following template-guided formation of QD films using spin-casting, simple contacting of polydimethylsiloxane (PDMS) mold selectively removed QDs on undesired areas. Hence, the transferred 2D QD patterns can provide distinct periodicity with a low defect density for the desired functionality of photonic crystals. In order to fabricate the 3D structures with preserving the high crystal quality by capping materials between each layer, the individual 2D layers of QDs were stacked with freely controlled angle variations. The stacked QD patterns for 3D photonic crystals can exhibit useful optical characteristics, in mainly two aspects, combining enhanced light absorption and extraction efficiency. Both contribute to the overall photoluminescence (PL) enhancement. Periodic empty space in photonic crystals can confine traveling light and longer interaction between the incident light and QDs induces increased light absorption. As light scattering on periodic nanostructure lowers total internal reflection, the QD photonic crystals can achieve much enhanced light extraction efficiency as well. We successively fabricated 3D QD-based photonic crystals with a thickness of up to a few hundred nanometers, and it showed a PL enhancement compared to conventional QD films. We characterized the absorptance and extraction efficiency of photonic crystals by means of experiment and calculation, validating the origin of PL enhancement in the structure. Our developed fabrication tool for the bottom-up nanoparticle-based photonic crystals can potentially be exploited for metamaterials³ with unconventional functions or properties such as negative index, delay of carrier lifetime, optical waveguides, and so on.<br/><br/>Reference<br/>1. Eileen Armstrong and Colm O'Dwyer, <i>Journal of Materials Chemistry C</i> <b>3</b>, 6109 (2015).<br/>2. Tae Won Nam <i>et al</i>., <i>Nature communications</i> <b>11</b>, 3040 (2020).<br/>3. Nikolay I. Zheludev <i>Science</i> <b>328</b>, 5978 582-583 (2010).