Metasurface Engineering with Tantalum Pentoxide-Coated Microspheres: Tailoring Optical Resonances and Enhancing Local Density of States

Abstract

Hexagonally-packed polystyrene (PS) microsphere lattices coated with tantalum pentoxide (Ta$_2$O$_5$) form scalable dielectric metasurfaces supporting tunable photonic resonances and enhanced local density of optical states (LDOS). Here we combine fabrication, optical and fluorescence spectroscopy, and multi-scale electromagnetic simulations to quantify how the thickness of Ta$_2$O$_5$ shells control far-field resonances and Rhodamine 6G (Rh6G) emission. Experimentally, Ta$_2$O$_5$ shells of 10 - 70 nm deposited on microsphere lattices generate resonances that shift red with the thickness of the shell and systematically enhance the Rh6G fluorescence relative to flat Ta$_2$O$_5$ films. The largest enhancement is obtained for 30 - 50 nm shells, when lattice resonances overlap the Rh6G excitation and emission bands. Finite-cluster finite-difference time-domain simulations reproduce the measured transmittance and reflectance spectra, confirming the assumed geometry of the Ta$_2$O$_5$ shells covering the sphere lattice. Periodic-cell simulations of single electric dipoles yield wavelength-dependent Purcell factors $Fp(λ)$ and directional $β$-factors $β_{top}(λ)$, from which we construct emission-weighted figures of merit that link LDOS modulation to the experimentally accessible top-side fluorescence enhancement. As a complementary test of our emitter-environment model, we compare simulated and measured Purcell factors for PS/Ta$_2$O$_5$ microsphere lattices. A physically motivated averaging that accounts for emitter position, orientation and ensemble spectral smoothing yields very good agreement across all shells. Overall, our results establish Ta$_2$O$_5$-coated microsphere lattices as robust dielectric substrates for surface-enhanced fluorescence and clarify how shell thickness and emitter placement jointly control photonic resonances, LDOS and fluorescence response.

Figures (5)

• Figure 1: (a) SEM image of the two-dimensional PS microsphere monolayer. (b and c) Experimental transmission and reflectance of Ta$_2$O$_5$-coated PS microsphere lattices. (b) Comparison between a PS monolayer, the same monolayer coated with a 50nm Ta$_2$O$_5$ shell, and a flat 50nm Ta$_2$O$_5$ film on glass. (c) Metasurfaces with Ta$_2$O$_5$ shell thicknesses of 10, 30, 50 and 70nm, showing a systematic red-shift of the resonance with increasing thickness.
• Figure 2: Fluorescence of Rh6G/PVP on Ta$_2$O$_5$-coated PS microsphere lattices. (a) Emission spectra for metasurfaces with Ta$_2$O$_5$ shells of 10, 30, 50 and 70nm and for the glass reference (black line). (b) Normalized enhancement spectra (sample/reference), highlighting spectral regions of maximum enhancement and their correlation with the lattice resonances. Inset: integrated fluorescence enhancement factor $E_{\mathrm{F}}$ as a function of resonance wavelength for the different shell thicknesses.
• Figure 3: (a) Time-correlated single photon counting (TCSPC) data showing the three-dimensional data carpet obtained from a 50 nm shell thickness Ta$_2$O$_5$ metasurface when pumped at 545 nm. (b) Normalized kinetic traces recorded at the 590 nm emission maximum. The dotted lines represent the corresponding fitted curves. The inset shows the calculated lifetimes characteristic to the Rhodamine 6G control sample and to each metasurface with Ta$_2$O$_5$ shells of 10, 30, 50 and 70 nm.
• Figure 4: Finite-cluster FDTD simulations of Ta$_2$O$_5$-coated PS lattices. (a) Schematic of the simulated structure: a hexagonal cluster of PS spheres on a glass substrate coated by a Ta$_2$O$_5$ shell and residual film. (b) Simulated transmittance and reflectance spectra for a flat Ta$_2$O$_5$ film, a PS microsphere monolayer, and a Ta$_2$O$_5$-coated microsphere monolayer. The hybrid lattice exhibits a deeper and red-shifted resonance compared to the bare PS monolayer, in agreement with experiment.
• Figure 5: Comparison between the modeled Purcell factor (solid lines) and experimental data (circles) for PS/Ta$_2$O$_5$ microsphere lattices with Ta$_2$O$_5$ shell thicknesses of 10, 30, 50 and 70nm (bottom to top). Squares indicate the model evaluated at the experimental wavelengths. Global curvature parameters are shared across all thicknesses; per-thickness smoothing factors $s$ and wavelength shifts $\Delta\lambda$ are indicated in the inset.