Spectropolarimetric Properties of a Gallium Nanoparticle Layer on a Sapphire Substrate

Gallium nanoparticles (Ga NPs) are currently the subject of vigorous research as possible substrates in surface-enhanced Raman scattering (SERS) experiments in the ultraviolet spectral domain. Verification of any comprehensive model of the interaction of electromagnetic radiation with Ga NPs requires that complete polarimetric measurements be made. These spectropolarimetric properties can be obtained using a Mueller matrix spectropolarimeter (MMSP). The position of localized surface plasmon resonances (LSPRs) and spectral depolarization data of Ga NPs in the 300 to 1100？nm spectral region are presented. Spectral depolarization data may be of value in creating a better understanding of how light couples to individual nanoparticles, as well as the role played by interparticle coupling and the connection to phenomena such as SERS. 1. Introduction The optical properties of metal nanoparticles have caused them to be considered in recent years for applications ranging from sensing to modifying the emission and absorption spectra of a variety of materials. Typical metals considered for such nanoparticle studies have traditionally been Au and Ag, both of which have LSPR peaks in the near UV, visible, or IR region. Ga NPs, on the other hand, can be tuned into the deep UV as a result of the high plasma frequency of Ga. Additionally, the LSPR for Ga NPs have broad tunability, stability across a wide temperature range, excellent plasmon resiliency when oxidized, and simplicity of deposition even at room temperature [1–3]. Ellipsometric measurements determine the amplitude ratio and phase shift between the parallel and perpendicular components of the polarization state. The complex refractive index and thickness of the thin film can be calculated from the spectral dependence of and [4, 5]. The peaks in the imaginary component of the refractive index (corresponding to absorption) identify the positions and strengths of the LSPRs. Ellipsometric measurements can be performed over a range of wavelengths and incident angles in order to achieve additional insight and a better estimate of the optical constants. However, conventional ellipsometry cannot measure polarization parameters such as circular retardance, circular diattenuation, and depolarization. Recently, a Mueller matrix spectropolarimeter (MMSP) was developed at the University of Alabama in Huntsville (UAH) with the aim of studying the polarization properties of various optical/electrooptical elements, materials, microparticle deposits, and so forth. The MMSP configured in the reflection mode can parallel the