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.