CC BY
1*
ALT'23
The 30th International Conference on Advanced Laser Technologies
LM-I-15
Optical anisotropy of 2D and 3D metal nanoparticle ensembles induced by optical and mechanical treatments
I.A. Gladskikh, D.R. Dadadzhanov, D.A. Kafeeva, T.A. Vartanyan
ITMO University, Kronverksky Pr. 49, bldg. A, St. Petersburg, 197101, Russia
Main author email address: 138020@mail.ru
In this work two alternative approaches for development of polarization-dependent systems based on anisotropic ensembles of plasmonic nanoparticles were proposed.
The first approach involves the phenomenon of longitudinal plasmon resonance of uniaxially oriented nanorod assemblies in polymer matrices. Polyvinyl alcohol is often chosen as a polymer matrix, since films are fairly easy to obtain with conventional drying, they are transparent in the visible range, have a low glass transition temperature, and the amount of possible stretching depends on the length of the molecules. The alignment of the nanorods in this case is carried out by heating the film to a softening temperature while simultaneously subjecting it to tensile stress. In this process, the nanorods in contact with the polymer chains interlock and reach the final orientational position parallel to the stretching direction. It turns out that the degree of orientation of the nanorods is directly proportional to the degree of stretching of the film. The second approach is proposed to obtain anisotropic metasurface by modifying thin metal layers using powerful linearly or circular polarized laser radiation. The method consists in spectral hole burning in the inhomogeneously broadened spectra of silver nanostructures. Under the action of laser radiation, resonant nanoparticles are heated and, depending on the intensity of heating, change their shape and/or size, and also lose mass due to evaporation.
Both methods lead to corresponding changes in the absorption spectra (Fig. 1). The measured extinction spectra (Fig. 1a) confirm that unstretched composite films have isotropic optical properties, since both longitudinal and transverse modes can be excited regardless of the polarization state of the incident light. This is due to the random orientation of the rods, which averages their individual anisotropy. Stretched films, on the other hand, are optically anisotropic at the macroscale due to the uniform orientation of metal nanorods caused by stretching so the longitudinal and transverse surface plasmon resonance modes can only be excited by light polarized parallel and perpendicular to the stretching axis of the film, respectively.
a b
Fig. 1. Extinction spectra of silver nanorods in stretched polymer film (a) and irradiated granular silver film by 532 nm pulsed laser (b) in polarized light.
Laser irradiation of granular silver films also led to formation of metasurfaces with strong linear (Fig. 1b) or circular dichroism at the wavelengths close to laser wavelength depending on laser polarization. The action of polarized radiation on anisotropic particles depends not only on their size and shape, but also on their orientation. Therefore, the spectral dips after laser irradiation are not the same for different probe polarisation. The greatest changes are observed when the polarization of probe beam coincides with laser polarization. The study was supported by the Russian Science Foundation grant No. 21-72-10098, https://rscf.ru/en/project/21-72-10098/.
