Picosecond laser pulses have been used as a surface colouring technique for noble metals, where the colours result from plasmonic resonances in the metallic nanoparticles created and redeposited on the surface by ablation and deposition processes. This technology provides two datasets which we use to train artificial neural networks, data from the experiment itself (laser parameters vs. colours) and data from the corresponding numerical simulations (geometric parameters vs. colours). We apply deep learning to predict the colour in both cases. We also propose a method for the solution of the inverse problem – wherein the geometric parameters and the laser parameters are predicted from colour – using an iterative multivariable inverse design method.

The tuning of 3D topographical features on silver for the production of plasmonic colors is reported. The topography is produced by applying closely time‐spaced laser bursts. Using laser bursts increases the Chroma of the colors produced by up to 100% compared to the nonburst coloring method. By adjusting the energy distribution of the laser pulses in a burst, while maintaining the total burst energy constant, significantly different color palettes and topographical structures are produced. Scanning electron microscope analysis of the surfaces produced reveals the creation of three distinct sets of laser‐induced periodic‐like surface structures (LIPSS): low spatial frequency LIPSS (LSFL), high spatial frequency LIPSS (HSFL), and large LIPSS that have a period about 7× that of the laser wavelength. Two‐temperature model simulations of silver irradiated by a laser burst show a significant increase in the electron–phonon coupling which is mainly responsible for the creation of LIPSS. Finite‐difference time‐domain simulations of a model of the surface, consisting of nanoparticles arranged on a sinusoidal‐modulated surface of varying amplitude (0 to 150 nm) and period (200 and 1000 nm), elucidate the importance of the HSFL and LSFL structures for color formation, including the increase in Chroma (saturation) observed experimentally.

The promise of active nanophotonics technology relies on the confinement and control of light at the nanoscale. Confinement via plasmonics, dielectric resonators, and waveguides can be complemented with materials whose optical properties can be controlled using nonlinear effects. Transparent conducting oxides (TCOs) exhibit strong optical nonlinearities in their near zero permittivity spectral region, on the femtosecond

Nanostructured surfaces, or metasurfaces, allow exquisite control of linear and nonlinear optical processes by reshaping the amplitude, phase, and polarization of electric and magnetic fields near wavelength-scale heterogeneities. Recently, metasurfaces have broken new ground in high-field attosecond science where they have been utilized to amplify the emission of high-order harmonics of femtosecond infrared laser pulses, a notoriously inefficient process, by enhancing the incident field, and to shape the emitted high harmonics in space. Here we show control of the polarization and phase of high harmonics with a plasmonic metasurface. We design and fabricate perpendicularly aligned rectangular gold antennas on a silicon crystal that generate circularly polarized deep-ultraviolet high harmonics, from a circularly polarized infrared driver, providing a simple path for achieving circular emission from patterned crystals. Our metasurface enhances the circularly polarized harmonics up to ∼ 43 times when compared to the unpatterned surface, where harmonics are quenched. Looking forward, circularly polarized high harmonics will be useful tools for sensing chiral laser–matter interactions and magnetic materials. Our approach paves the way for polarization control at even shorter, extreme ultraviolet, wavelengths.

The promise of dynamic nanophotonic technologies relies on the confinement and spatiotemporal control of light at the nanoscale. Confinement via plasmonics, dielectric resonators, and waveguides can be complemented with materials whose optical properties can be controlled using nonlinear effects. Transparent conducting oxides (TCOs) exhibit strong optical nonlinearities in their near‐zero permittivity spectral region, on the femtosecond timescale. Harnessing full spatiotemporal control over the nonlinear response requires a deeper understanding of the process. To achieve this, a self‐consistent multiphysics time‐domain model for the nonlinear optical response of TCOs is developed and implemented into a 3D finite‐difference time‐domain code. Simulations are compared and tuned against recently published experimental results for intense laser irradiation of thin indium tin oxide (ITO) films, achieving good quantitative agreement; the time‐domain dynamics of the nonlinear response and the phenomenon of time‐refraction are also explored. Finally, by simulating intense laser irradiation of a plasmonic particle on an ITO film, the applicability of the approach to time‐varying metasurfaces is demonstrated. As expected, significant enhancement of the nonlinear response of an ITO‐based metasurface over bare ITO thin films is found. This work thus enables quantitative nanophotonics design with conductive oxides in their epsilon‐near‐zero region.

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Plasmonic metasurfaces are promising as enablers of nanoscale nonlinear optics and flat nonlinear optical components. Nonlinear optical responses of such metasurfaces are determined by the nonlinear optical properties of individual plasmonic meta-atoms. Unfortunately, no simple methods exist to determine the nonlinear optical properties (hyperpolarizabilities) of the meta-atoms hindering the design of nonlinear metasurfaces. Here, we develop the equivalent RLC circuit (resistor, inductor, capacitor) model of such metaatoms to estimate their second-order nonlinear optical properties, that is, the first-order hyperpolarizability in the optical spectral range. In parallel, we extract from second-harmonic generation experiments the first-order hyperpolarizabilities of individual meta-atoms consisting of asymmetrically shaped (elongated) plasmonic nanoprisms, verified with detailed calculations using both nonlinear hydrodynamic-FDTD and nonlinear scattering theory. All three approaches, analytical, experimental, and computational, yield results that agree very well. Our empirical RLC model can thus be used as a simple tool to enable an efficient design of nonlinear plasmonic metasurfaces.

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