Lattice Kerker Effect and Out-Of-Plane Anisotropic Radiation of Photoluminescence from Nanoparticle Array of Titanium Oxide with Tuned Packing Volume

Surface lattice resonance (SLR) is a collective resonance induced by radiative coupling between localized resonances in periodically arranged nanoparticles. When the local scattering elements are Mie-resonators such as dielectric nanoparticles, Mie-SLRs are formed by coupling between local Mie resonances mediated via in-plane diffraction [1]. Because of the orthogonality of local magnetic and electric dipoles excited in each Mie resonator, their scattering fields extend to the orthogonal direction and couple to the diffraction modes that match the direction of field expansion. Especially, in rectangular lattice the spectral positions of magnetic and electric SLRs are tunable independently via periodicities along two axes.<br/>The multipolarity of Mie resonances can cause an optical response known as the Kerker effect. In this effect, interference between the far fields deriving from electric and magnetic polarizations suppresses either forward or backward scattering. Rectangular lattices of Mie resonators are very suitable platforms for inducing the Kerker effect, i.e., lattice Kerker effect, because of the independent tunability of spectral position of magnetic and electric SLRs. Zero-backscattering has been reported numerically by carefully tuning the periodicity of one axis [2].<br/>In this study, we focused on Photoluminescence control by utilizing the Lattice-Kerker effect in the square array of rectangle-shaped TiO₂ nanoparticles. We fixed the lattice periodicity while changing the length of one axis of TiO₂ nanoparticles in order to achieve mode crossing of electric and magnetic SLRs. We found asymmetric out-of-plane PL radiation from the light-emitting layer deposited on top of the array at the spectral and angular point where the lattice Kerker effect is achieved.<br/><br/>As a precursor of TiO₂, titanium nanoparticle array was fabricated on SiO₂ substrate by utilizing nanoimprint lithography. Then TiO₂ nanoparticle array was obtained by thermal oxidation of titanium nanoparticle array [3]. Excited resonances were attributed by measured and calculated optical extinction. PL enhancement of TiO₂ nanoparticle array was obtained by measured PL intensity of these arrays embedded in polymethyl methacrylate (PMMA) containing coumarin 153.<br/><br/>The optical extinction of TiO₂ nanoparticle array was measured through optical transmission at various incident angles. By using in-plane diffraction conditions that dominate SLR and calculated extinction through the finite element method, the peaks in the extinction spectra were attributed to electric and magnetic SLRs. In the angle-resolved extinction spectra, we confirmed the intersection of electric and magnetic SLR due to changes in the dispersion relation near <i>θ</i>_in = 0° with increased packing factor of nanoparticles.<br/>The PL enhancement in the front and back of the array as a function of the emission angle was evaluated for each thickness of the emitter layer. Significant PL enhancement was observed at the intersection of electrical and magnetic SLR independent of film thickness. In addition, the PL enhancement at the front of the array was greater than at the back, given a certain film thickness. This result suggests a coupling of Lattice-Kerker effect and PL.<br/><br/>[1] S. Murai et al., <i>Adv. Opt. Mater.</i> <b>8</b>, 10902024 (2020)<br/>[2] V. E. Babicheva et al., <i>Laser Photonics Rev.</i> <b>11</b> 1700132 (2017)<br/>[3] S Murai et al., <i>J. Appl. Phys</i>. <b>129</b>, 163101 (2021)