Realizing ultrafast optical control of materials is imperative for advancing the field of optical information processing, nonlinear optics, and time-varying materials. Noble metal-based plasmonics has provided many platforms for achieving optical switching, using strong local field enhancement offered by plasmonic resonances and free-electron plasmonic nonlinearity. However, the switching times in such systems are traditionally constrained by the relaxation of photoexcited hot electrons. In this study, we investigate an interplay between electron relaxation lattice vibrations of the nanostructure. This is achieved by harnessing a temporal Fano-type interference between the rapid relaxation of hot electrons and vibrational dynamics within the plasmonic nanostructure. The effect provides high spectral selectivity and sensitivity to the polarisation of light and geometric parameters of the nanostructure. The results are important for development of nonlinear nanostructure with the tailored transient response.
Metamaterials provide unique opportunities for manipulation of dispersion of light waves and, therefore, polarisation and phase, as well as amplitude of transmitted and reflected waves. Here we report on using linear and nonlinear properties of nanorod and nanotube based metamaterials for shaping ultrashort optical pulses. Intensity limiters, temporal pulse shape control, as well as polarisation switching will be presented. The role on nonlocal effects in pulse propagation in metamaterials will be discussed. Using nanotube based metamaterials allows to introduce additional degree of freedom for passive and active tunability of the optical response.
Optical vector field topologies will be discussed on the example of plasmonic systems and anisotropic metamaterials. An anisotropic metamaterial platform for local control of polarisation in complex vector beams, including radial and azimuthal beams, will be presented. The approach enables a flexible platform for tailoring complex vector beams and achieving required polarisation patterns on demand for harvesting functionalities and applications of complex light beams with complex polarisation and phase information in numerous photonic and quantum technologies, imaging and metrology.
Acoustic vibrations of nanoparticles have raised a significant interest due to the potential to study mechanical phenomena at nanometer length scales. We investigated the transient optical response of multilayer spherical plasmonic nanoparticles formed by alternating layers of gold and silica. The nanoparticles provide a platform for a broadband nonlinear optical response from visible to near-IR spectral ranges as well as a tunable optomechanical system in which different acoustic modes can be selectively switched on/off tuning the excitation wavelength. The nonlinear response monitored through the transient absorption spectra, follows the linear optical properties of the nanoparticles with the transient signal decay relatively independent of the pump wavelength in the visible (514 nm) and infrared (1028 nm). At both pump wavelengths, more than one vibration mode is excited on the nanoparticles. The numerical modelling allows differentiating the core and shell vibrations excited at different pump wavelengths. In addition to two vibration modes which can be excited at both pump wavelengths, and related to the shell vibrations, the acoustic vibration existed by the 514 nm pump illumination can be assigned to the nanoparticle core. The observed behaviour shows that different vibrational modes of the nanoparticle can be independently optically excited by tuning the wavelength of the excitation. This observation not only expands the knowledge about the internal structure of composite plasmonic nanoparticles but also allows for the additional degree of freedom to control their optical and mechanical properties.
Second harmonic generation is one of the fundamental nonlinear optical processes that is at the heart of communication and sensing applications. Due to the underlying crystal symmetry, second harmonic generation in noble metal-nanostructures is dominated by metal/dielectric interfaces with only weak (magneto-dipole and quadrupolar) contributions coming from the bulk of the metal inclusions. Here we demonstrate that, in metamaterials, nonlinear contributions from individual plasmonic inclusions can add up together, resulting in the bulk nonlinear polarization. The resulting nonlinear response can be described in terms of volumetric second harmonic polarizability that relates unit-cell averaged nonlinear polarization to a product of unit-cell averaged fundamental fields. The amplitude of this effective nonlinear polarizability is comparable to that of common nonlinear crystals.
In order to analyze nonlinear response of the plasmonic nanowire arrays we compare experimental results to numerical solutions of Maxwell equations where second harmonic response is calculated using nonlinear hydrodynamic model. Numerical solutions of Maxwell equations are also used to analyze the spatial and spectral distributions of fundamental and nonlinear fields across the composites and, in the end, to guide and validate the development of analytical description of effective second harmonic polarizability. The developed analytical description of the second harmonic generation in plasmonic composites opens new avenues for engineering of nonlinear response.
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