Indium Plasmonic Thin Films as Substrates for Kretschmann Configuration-Based SPR Sensing and FDTD Analysis of Oxide-Enhanced UV-SERS

Fundamental studies of uncommon plasmonic materials expand their potential applications in nanoplasmonics. In this work, nanometers thin films of indium, a plasmonic yet rarely explored metal, were characterized for its surface plasmon polariton (SPP) properties and the performance in surface plasmon resonance (SPR) sensing and surface-enhanced Raman spectroscopy (SERS). Direct deposition of indium films on glass slides exhibited coalescence of discrete particles and generated no SPR response due to gaps disrupting SPP propagation. Addition of a 2 nm Cr base layer reduced the particle size and produced a resonance dip under the Kretschmann configuration. SPR sensing with a 2/40 nm Cr/In film was demonstrated for the first time and compared to the standard 2/50 nm Cr/Au film, revealing a sensitivity approximately 77% that of the gold film. The optical properties of the passivating In2O3 layer are investigated through analogous RF magnetron sputtered In2O3 films to allow for improved computational modeling and optimization of the substrates for plasmonic applications. Low extinction coefficient (k) at typical SPR wavelengths (∼650 nm) found for the oxide layer indicates minimal effects on SPR response. However, finite-difference time-domain (FDTD) simulations revealed that the presence of the oxide layer resulted in a > 50% increase in the SERS enhancement in the UV range, while exhibiting little effect with incident light in the visible range. This work provides insights into the plasmonic properties of thin indium films for optical sensing applications and highlights the critical role of the oxide layer in wavelength-specific performance.