Plasmonic phenomena have greatly contributed to nanooptics and nanophotonics owing to their features such as light localization and high sensitivity to the surrounding environment. The nanoparticles of poor metals (e.g. Al) exhibit plasmonic properties in the UV range (240-350 nm) where many organic molecules and semiconductors absorb light, which was recently confirmed and utilized in enhanced Raman spectroscopy and UV photocatalysis. The present study demonstrates the efficient TiO2 photocatalysis with indium nanostructures resonant in the near-UV range. Indium (In) nanograins were densely distributed on a TiO2 thin film, where methylene blue (MB) was applied to test the photocatalytic activity. The photocatalytic reaction was initiated by irradiating the samples with UV light, and the time-dependent decay of the MB absorbance was observed. A reaction rate was found to increase by factors as high as 7 while the enhancement of photocatalysis shows particle size dependence. The increase and downward trend in the enhancement shows a good agreement with that in the field intensity simulated by the discrete dipole approximation (DDA). Simulation results also suggest that the largest enhancement of photocatalysis be obtained with In nanograins whose resonance is close to the bandgap of TiO2. It is expected that the light at the absorption edge wavelength confined at plasmonic nanostructures effectively for the photocatalytic reaction.
Deep-UV (DUV) plasmonics can expand the possibilities of DUV-based techniques (i.e. UV lithography, UV spectroscopy, UV imaging, UV disinfection). Here we present that indium is useful for research of DUV plasmonics. According to dielectric function, indium and aluminum are low-loss, DUV plasmonic metals, of which the imaginary parts are far smaller than those of other metals (i.e. rhodium, platinum) in the DUV range. Additionally, the real parts in the whole DUV range are close to but smaller than -2, allowing efficient generation of surface plasmon polaritons on an indium or aluminum nanosphere. In comparison to aluminum, indium provides a distinctive feature for fabricating DUV-resonant substrates. It is highly apt to form a grainy deposition film on a standard, optically transparent substrate (i.e. fused silica). The surface plasmon resonance wavelength becomes promptly tailored by simply varying the deposition thickness of the films, resulting in different grain sizes. Thus, we fabricated indium-coated substrates having different plasmon resonance wavelengths by varying the deposition thicknesses from 10 to 50 nm. DUV resonance Raman scattering of adenine molecules was best enhanced using the 25 nm deposition thickness substrates by the factor of 2. Furthermore, the FDTD calculation simulated the electromagnetic field enhancement over a grainy, indium-coated fused silica substrate. Both results indicate how indium plays an indispensable role in study of DUV plasmonics.