Optical Characterization of Plasmonic Indium Lattices Fabricated via Electrochemical Deposition

The optical properties of periodic metallic nanoparticle lattices have found many exciting applications. Indium is an emerging plasmonic material that offers to extend the plasmonic applications given by gold and silver from the visible to the ultraviolet spectral range, with applications in imaging, sensing, and lasing. Due to the high vapor pressure/low melting temperature of indium, nanofabrication of ordered metallic nanoparticles is nontrivial. In this work, we show the potential of selective area electrochemical deposition to generate large-area lattices of In pillars for plasmonic applications. We study the optical response of the In lattices by means of angle-dependent extinction measurements demonstrating strong plasmonic surface lattice resonances and a good agreement with numerical simulations. The results open avenues toward high-quality lattices of plasmonic indium nanoparticles and can be extended to other promising plasmonic materials that can be electrochemically grown.


Rayleigh anomaly dispersion curves
The RA dispersion curve calculations are based on the geometry of the array which has a nearest neighbour separation Λ of 534 nm as shown in the main text. The direct and reciprocal lattice vectors as shown in the main text are described as: (1) (2) The sample was rotated around the y-axis from θ 0 to 46 • in steps of 2 • . The RA energies are solutions to where m 1 and m 2 are the diffraction orders, k // = E c · sin θk x and n = 1.5 is the refractive index of the surrounding medium.

Soft conformal imprint lithography
The In plasmonic lattices were fabricated by Substrate Conformal Imprinting Lithography (SCIL) as follows: ITO on glass substrates was cleaned with soap and subsequent sonication in acetone and IPA. A layer of silica sol-gel (SCIL Nanoimprint solutions T1100) was spincoated (30-40 nm) onto the ITO substrate. This was followed by curing at 150 • C for 1 min.
Next, a layer of PMMA (PMMA 950k A8 in anisole, 250 nm) was spin-coated and baked

Electrochemical deposition
The imprinted substrate is used as a working electrode for In electrodeposition in a 2 mL three-electrode cell with a Pt wire counter electrode and an Ag/AgCl (0.21 V vs SHE) reference electrode. The deposition area was 0.  Figure 1 shows the electrochemical current during the growth time. During the growth, the solution was not stirred. After nucleation and coalescence, a single particle per hole emerges and periodicity is established. The pillars grow vertically following the trench. In the exposed area, some regions presented defects, indicated by the lack of pillar growth ( Figure 2). Defects are used as reference for the height measurements with the AFM.

FDTD simulations
To simulate the reflection and transmission spectra, Maxwell's Equations were solved numerically, using an FDTD-code (Ansys Lumerical -FDTD: 3D Electromagnetic Simulator). 1 The optical properties for In was taken from 2 and approximated by fitting curves to the datasets. 1 The shape of the In pillars was approximated by using a rounded cylinder object, with only the upper dome of 50 nm in radius. The pillar radius and total height were 170 nm and 225 nm, respectively. The hexagonal array was achieved by considering a rectangular simulation area, as indicated in Figure 4.
Periodic boundary conditions were used for the in-plane boundaries, that were overwritten by the BFAST plane wave source under off-normal incidence conditions. In all cases, antisymmetric boundary conditions were kept for the boundaries parallel to the magnetic field (y max and y min ). Perfectly matched layers (PMLs) are used at the boundaries along the source injection plane. Two frequency domain field and power-monitors were used to obtain reflected and transmitted spectra. The mesh refinement was set to conformal variant 1 to account for the metals in the simulations, and the mesh size was determined by mesh convergence testing with a minimum mesh step of 1 nm.

Optical characterisation
To experimentally determine the plasmonic lattice modes dispersion, angle-resolved transmission was measured. The data was collected with a Spectra Pro 2300i spectrometer equipped with a Pixis 400 CCD. The sample was mounted on a rotating stage and illuminated with polarized collimated white light from a SuperK EXTREME/FIANIUM supercontinuum laser.
The transmitted light was collected by an integrating sphere and sent to the spectrometer through a multimode fiber. The zeroth-order transmission was selected using an iris at the entrance of the integrating sphere.
To ensure a symmetric dielectric environment, the sample was immersed in a Nikon immersion oil (n=1.518). Figure 6 shows the effect on transmission of using the oil compared