Plasmonic metamaterials are artificial structures whose optical response can be tailored to achieve several effects by playing with the geometrical parameters of the components. In this talk, we discuss how to apply the metamaterial design rules to develop band-stop linear filters and nonlinear filters, operating as intensity limiters. In both regimes, the filters share some common qualities: their optical response does not change for a broad range of incidence angles, at least up to 30 degrees, and is only weakly dependent on the polarisation of the incident light. These properties make these ultrathin filters useful in open field applications. The metamaterial is based on an array of gold nanotubes (i.e., a cylindrical gold shell with a dielectric core) embedded in a dielectric matrix. In the linear regime, the metamaterial displays an absorption resonance independent of the polarisation and the angle of incidence of the light, which can be tuned throughout the visible spectral range by changing the geometrical parameters of the array. In the nonlinear regime - based on free-electron Au nonlinearity and tested with ns-long pulses at 532 nm - the metamaterial limits the output peak fluence, keeping it constant across several order of magnitudes of the incoming fluence. The proposed metamaterial approach can be useful for designing optical spectral filters and intensity limiters over broad range of wavelengths.
In this work, we present a detailed experimental Raman investigation of nanostructured silicon films prepared by metalassisted chemical etching with different nanocrystal sizes and structures. Interpretation of observed one and two-phonon Raman peaks are presented. First-order Raman peak has a small redshift and broadening. This phenomenon is analyzed in the framework of the phonon confinement model. Second-order Raman peaks were found to be shifted and broadened in comparison to those in the bulk silicon. The peak shift and broadening of two-phonon Raman scattering relates to phonon confinement and disorder. A broad Raman peak between 900-1100 cm-1 corresponds to superposition of three transverse optical phonons ~2TO (X), 2TO (W) and 2TO (L). Influence of excitation wavelength on intensity redistribution of two-phonon Raman scattering components (2TO) is demonstrated and preliminary theoretical explanation of this observation is presented.