We present theoretical and experimental studies that explain the observed strong enhancement of the magneto-optical (MO) Faraday rotation in all-metal core-shell Co-Ag nanoparticles (NPs) attributed to localized surface plasmon resonance (LSPR). We also explain why the optical absorption and MO spectra peaks appear blue-shifted with increased Co core size while keeping the NP size constant. Further, we demonstrate direct correlation between the strong LSPR induced electromagnetic fields and the enhanced MO activity of the NPs.

Metallic nanoparticles ͑NPs͒ are suitable platforms for miniaturized biosensing based on their optical and magneto-optical properties. It is possible to enhance the sensitivity of specific kinds of NPs by exploiting their optical and magneto-optical properties under suitable external magnetic field modulation. Here, the magneto-optical properties of Fe-Ag core-shell ferromagnet-noble metal NPs have been investigated as a function of the incident light frequency. For Fe-Ag NPs with a concentration ratio around 25:75, an optical absorption band centered at 3 eV due to localized surface plasmon resonance ͑LSPR͒ excitation is observed. A strong enhancement of the Faraday rotation is also observed, greatly exceeding the value estimated for pure Fe NPs, also associated with the LSPR excitation. Our findings open up the possibility of highly sensitive miniaturized magneto-optically modulated biosensing.

Using both Raman spectroscopy and direct laser reflectivity measurements, we investigate the optical properties of vanadium dioxide (VO2) thin films deposited on different substrates as they undergo the thermally induced insulator to metal phase transition. Comparing similarly prepared VO2 films grown on quartz, sapphire, and rutile substrates, we observed a significant difference in the transition temperatures without hysteresis loop broadening after heating and cooling the samples. We attribute these different transition temperatures to differences in the VO2 microstructure, mainly the difference in average grain sizes. We also observed variations in the contrast of the detected Raman resonances using different wavelengths for the excitation laser, and found that in all cases a longer wavelength (in our case 785 nm) yielded the clearest VO2 Raman spectra.