Laser-induced linear dichroism in planar self-organized silver nanostructures

A method for obtaining metallic plasmonic nanostructures with linear dichroism, based on the method of burning out constant spectral dips, is proposed. Isotropic granular silver films obtained by physical vapor deposition in vacuum were irradiated with linearly polarized laser radiation in the spectral region of the plasmon resonance of their constituent nanoparticles. As a result of irradiation, the silver nanostructures change their sizes and shapes, and the films acquire a pronounced linear dichroism. Both the magnitude and the spectrum of linear dichroism depend on the state of the isotropic film before irradiation, which can be changed by heat treatment. For the unannealed films the dichroism does not change the sign over the entire spectral range studied and corresponds to the expected increase in the depth of the spectral dip for the light polarized parallel to the laser radiation polarization plane. For the annealed films, which consist of more distinctly formed and better separated nanoparticles, the dichroism value is greater, and the spectrum turns out to be sign-changing. The appearance of linear dichroism after laser irradiation is due to the differences in the change in the shape and size of the initially anisotropic nanoparticles that make up an isotropic film as a whole, depending on their orientation relative to the polarization plane of the laser beam.