The stady investigates the optical response of metal - dielectric nanostructures of the nanocup type to external excitation. Analytical relations are derived for the diagonal components of the polarizability tensor, the electric field enhancement tensor, and the absorption cross section. It is shown that variations in the overall radius of the nanoparticle have a more pronounced effect on the spectral positions and amplitudes of the maxima of the imaginary parts of the transverse and longitudinal components of the polarizability tensor than variations in the radius of the dielectric core. An original method is proposed for obtaining size-dependent characteristics of the transverse and longitudinal components of the effective relaxation rate. Within the plasmon hybridization approach in the lossless approximation, size dependences of the transverse and longitudinal surface plasmon resonance frequencies are derived. The calculated results make it possible to establish a correspondence between the resonances of a core-shell particle and two low-amplitude resonances of the nano cup. It is demonstrated that, for the composite nanostructure under study, the surface plasmon resonance is excited at a higher frequency than the transverse resonance, which distinguishes the nano cup from other axially symmetric nanoparticles of different shapes. It is established that the contribution to plasmon resonance splitting due to the reduced symmetry of the nano cup significantly exceeds the contribution associated with the presence of two metal-dielectric interfaces. It is shown that, of the four absorption cross-section maxima predicted by theory, only one or two may be observable, depending on the ratios between the geometric parameters of the nano cup, while the remaining maxima may not manifest themselves due to their low amplitudes. The absence of longitudinal field enhancement is demonstrated, whereas the transverse field enhancement is shown to be substantial. The influence of the core and shell materials on the spectral positions of the maxima of field enhancement, absorption cross section, and the imaginary parts of the transverse and longitudinal polarizabilities is explained.
Emission-line spectra of star-forming dwarf galaxies provide key diagnostics of their chemical composition and physical conditions. In this work, photoionization modeling with detailed treatment of diffuse ionizing radiation is applied to the chemo dynamical simulation results of a star-forming dwarf galaxy at 100 Myr. A thin dense shell is included to reproduce the emission lines of low-ionization-stage ions. The synthetic spectra obtained in this way are within the observed range. The model spectra diagnostics using Te-and R23-methods provide oxygen abundances close to the mass-weighted values from the chemo dynamical simulation results, while the outward-only approximation reproduces luminosity-weighted ones. Also, the modeling results allowed us to test the Kennicutt H alpha-based estimator of the star formation rate. We find that this calibration substantially overestimates the true star formation rate adopted in the chemo dynamical simulation. The modeling escape fraction of ionizing photons is larger in the model with detailed treatment of diffuse radiation than in the one based on the outward-only approximation. This means that accurate treatment of diffuse ionizing radiation is necessary for the correct interpretation of emission-line data in dwarf galaxies.
