Anisotropic light absorption by localized surface plasmon resonance in a thin film of gold nanoparticles studied by visible multiple-angle incidence resolution spectrometry.

Anisotropic light absorptions via localized surface plasmon resonance in a gold-evaporated film parallel (in-plane; IP) and perpendicular (out-of-plane; OP) to the film surface are studied using visible multiple-angle incidence resolution spectrometry (Vis-MAIRS). When the thin film was aged for eighteen days, the time-dependent Vis-MAIRS IP spectra exhibited significantly different variation from that of the OP spectra: the IP spectra exhibited a large shift to the shorter wavelength side, whereas the OP spectra were explained by a linear combination of three-constituent spectra. The surface topographical analysis of the film revealed that a continuous film coalesced to form aggregates of metal particles. The intrinsic difference between the IP and OP spectra was readily elucidated by considering the surface-parallel and -perpendicular dipoles interaction depending on the topographical changes, which was confirmed by performing spectral simulation using metal particle array models.

Analytical understanding of multiple-angle incidence resolution spectrometry based on a classical electromagnetic theory.

Infrared multiple-angle incidence resolution spectrometry (IR-MAIRS) is a unique spectroscopic technique to retrieve both surface-parallel (in-plane; IP) and -perpendicular (out-of-plane; OP) molecular vibration spectra simultaneously from an identical thin-film deposited on a high refractive index substrate, and the measurement theory was constructed by the use of a theoretical framework of regression equation. The core part of the MAIRS theory is found in the weighting factor matrix, R, used for a linear combination, which was constructed in an unusual manner. Because a regression equation does not strictly correlate the left- and right-hand sides of the equation, R matrix cannot directly be deduced from Maxwell's equation. Although the conventional studies using IR-MAIRS gave excellent empirical results, a strict physical understanding of MAIRS is necessary; otherwise, we cannot rely on it at least quantitatively. In the present study, the MAIRS theory has first been analyzed by the use of Maxwell's equations inductively. As a result, both MAIRS-IP and -OP spectra have readily been expressed as a linear combination of the Im(epsilon(x)) and Im(-1/epsilon(z)) functions that correspond to the conventional transmission and reflection-absorption spectra. Through the analysis of coefficients of the linear combination, MAIRS has proved to be reliably useful for analysis of thin film on a high refractive index substrate.

Development of UV-visible multiple-angle incidence resolution spectrometry and application study of anisotropic surface plasmon excitation in a silver thin film on a glass substrate.

Multiple-angle incidence resolution spectrometry (MAIRS) that was originally developed in an infrared region for analysis of molecular orientation in a thin film on a substrate has been extended to develop a novel UV-visible spectrometry. The new technique, named Vis-MAIRS, has been employed for analysis of anisotropic surface plasmon excitation in a silver thin film with a thickness of 5 nm deposited on a glass slide. The Vis-MAIRS spectra yield two spectra at a time, which correspond to absorption spectra whose transition moments are parallel and perpendicular to the film surface. The two spectra of the silver thin film were largely different from each other in shape, which strongly suggested that the silver nanoparticles in the thin film were in an ellipsoidal shape. In addition, absorption due to long-range surface plasmon propagation across the nanoparticles aligned parallel to the film surface, which is a result of the dipole or quadruple couplings of plasmon in each particle, was clearly monitored for the first time in the Vis-MAIRS spectra. In this manner, Vis-MAIRS is expected to be a useful tool to study aggregates of metal nanoparticles in a film.

Experimental optimization of p-polarized MAIR spectrometry performed on a fourier transform infrared spectrometer.

Optimized experimental conditions of infrared p-polarized multiple-angle incidence resolution spectrometry (p-MAIRS) for the analysis of ultrathin films on glass have been explored. When the original MAIRS technique is employed for thin-film analysis on a substrate of germanium or silicon, which exhibits a high refractive index, an established experimental condition without optimization can be adapted for the measurements. On the other hand, the p-MAIRS technique that has been developed for analysis on a low-refractive-index material requires, however, optimization of the experimental parameters for a 'quantitative' molecular orientation analysis. The optimization cannot be performed by considering only for optics in the spectrometer, but for optics concerning the substrate should also be considered. In the present study, an optimized condition for infrared p-MAIRS analysis on glass has been revealed, which can be used for quantitative molecular orientation analysis in ultrathin films on glass.