The adjoint method is an efficient technique for the topology optimization of complex nanophotonic systems, including nanostructures, metasurfaces and integrated optical circuits. While such method has been traditionally used in the frequency domain, its extension to the time domain opens new opportunities for wideband optimization of dispersive materials for applications ranging from broadband absorbers to enhanced quantum emitters in dispersive environments. We propose a topology optimization technique for the inverse design of linear optical materials with arbitrary dispersion and anisotropy. We introduce a general adjoint scheme in the time-domain based on the complex-conjugate pole-residue pair (CCPR) model. This approach has the advantage of treating dispersive media and broadband response naturally in a single simulation run. We implement this framework within the finite-difference time-domain (FDTD) method and investigate the method for optimizing metallic and dielectric nanoantennas over the optical spectral range of 350 to 1000nm. The combination of the method with parallel computing enables the large-scale inverse design of nanostructures in 3D with extreme field confinement. Nanostructures found via inverse design and featuring the intriguing anapole effect are also discussed. This effect enables nanostructures that show field enhancement, negligible scattering, and low losses. The possibility of reducing losses in plasmonic nanostructures via inverse design is an interesting possibility offered by the method and may open new avenues towards the realization of transparent plasmonic metamaterials for applications in linear and nonlinear nanophotonics.
Gradient-based topology optimization via the adjoint method has been successfully used in nanophotonics to uncover shapes with superior performances compared to what would be possible with traditional design methods. Here, we have used this technique to optimize a dielectric object to engineer its induced multipole moments. As an example, we show the method's application to realize the first Kerker effect in a silicon nanoparticle. The final result shows a rather complex shape with highly suppressed backscattering due to the excitation of in-phase electric and magnetic dipoles with the same amplitude. This promising approach can pave the way for the inverse design of photonic structures based on a set of desired multipole moments, which can exhibit a variety of complex photonic phenomena.
Hyperpolarizability is a measure of the nonlinear optical characteristics of natural or meta-atoms describing how the atoms become nonlinearly polarized by the induced local-field. However, determining hyperpolarizability in the case of structured plasmonic meta-atoms is not straightforward due to their relatively larger sizes, unique shapes, and the index of refraction of the surrounding dielectric medium. Also, the order-of-magnitude of hyperpolarizability may vary with the frequency of light especially when inter-band transitions in metals become dominant. Here, we experimentally and theoretically estimated the order-of-magnitude of the 1st-order hyperpolarizability of gold meta-atoms that can be used in designing nonlinear metasurfaces.
Surface Plasmon polaritons have long been utilized to enhance and confine optical fields at the nanoscale. They have been proven effective in the control and enhancement of optical processes at metal-dielectric boundaries with a variety of applications including nonlinear optics. In this paper we review the application of plasmonic metasurfaces to enhance non-linear processes on semiconductors, crystals, 2-D materials (graphene) and in the metal itself forming the metasurface. We consider applications such as harmonic generation, the generation of vortex beams, and the enhancement of nonlinear processes in 2D materials (graphene).
The use of metal nanostructures to produce colour has recently attracted a great deal of interest. This interest is motivated by colours that can last a long time and that can be rendered down to the diffraction limit, and by processes that avoid the use of inks, paints or pigments for environmental, health or other reasons. The central idea consists of forming metal nanostructures which exhibit plasmon resonances in the visible such that the spectrum of reflected light renders a desired colour. We describe a single-step laser-writing process that produces a full palette of colours on bulk metal objects. The colours are rendered through spectral subtraction of incident white light. Surface plasmons on networks of metal nanoparticles created by laser ablation play a central role in the colour rendition. The plasmonic nature of the colours are studied via large-scale finite-difference time-domain simulations based on the statistical analysis of the nanoparticle distribution. The process is demonstrated on Ag, Au, Cu and Al surfaces, and on minted Ag coins targeting the collectibles market. We also discuss the use of these coloured surfaces in plasmonic assisted photochemistry and their passivation for day-to-day use. Reactions on silver that are normally driven by UV light exposure are demonstrated to occur in the visible spectrum.
In this work, we validate the behavior of 3D Photonic Crystals for Structural Health Monitoring applications. A Finite Difference Time Domain (FDTD) analysis has been performed and compared to experimental data. We demonstrate that the photonic properties of a crystal (comprised of sub-micrometric polystyrene colloidal spheres embedded in a PDMS matrix) change as a function of the axial strain applied to a rubber substrate. The change in the reflected wavelength, detected through our laboratory experiments and equivalent to a visible change in crystal color, is assumed to be caused by changes in the interplanar spacing of the polystyrene beads. This behavior is captured by our full wave 3D FDTD model. This contains different wavelengths in the visible spectrum and the wave amplitudes of the reflected and transmitted secondary beams are then computed. A change in the reflectance or transmittance is observed at every programmed step in which we vary the distance between the spheres. These investigations are an important tool to predict, study and validate our understanding of the behavior of this highly complex physical system. In this context, we have developed a versatile and robust parallelized code, able to numerically model the interaction of light with matter, by directly solving Maxwell's equations in their strong form. The ability to describe the physical behavior of such systems is an important and fundamental capability which will aid the design and validation of innovative photonic sensors.
We show the angle-independent coloring of metals in air arising from nanoparticle distributions on metal surfaces created via picosecond laser processing. Each of the colors is linked to a unique total accumulated fluence, rendering the process compatible with industry. We report the coating of the colored metal surfaces using atomic layer deposition which is shown to preserve colors and provide mechanical and chemical protection Laser bursts are composed of closely time-spaced pulses separated by 12.8 ns. The coloring of silver using burst versus non-burst is shown to increase the Chroma, or color saturation, by 50% and broaden the color Lightness range by up to 60%. The increase in Chroma and Lightness are accompanied by the creation of 3 kinds of different laser-induced periodic surface structures (LIPSS). One of these structures is measured to be 10 times the wavelength of light and are not yet explained by conventional theories. Two temperature model simulations of laser bursts interacting with the metal surface show a significant increase in the electron-phonon coupling responsible for the well-defined LIPSS observed on the surface of silver. Finite-difference time-domain simulations of nanoparticles distributed on the high-spatial frequency LIPSS (HSFL) explain the increase in color saturation (i.e. Chroma of the colors) by the enhanced absorption and enriched plasmon resonances.
This work describes a computational approach for the optical characterization of an opal photonic crystal (PC). We intend, in particular, to validate our approach by comparing the transmittance of a crystal model, as obtained by numerical simulation, with the transmittance of the same crystal, as measured over 400- to 700-nm wavelength range. We consider an opal PC with a face-centered cubic lattice structure of spherical particles made of polystyrene (a nonabsorptive material with constant relative dielectric permittivity). Light-crystal interaction is simulated by numerically solving Maxwell’s equations via the finite-difference time-domain method and by using the Kirchhoff formula to calculate the far field. A method to study the propagating Bloch modes inside the crystal bulk is also sketched.
This paper describes the parallel computational approach for the analysis of the multiple scattering of light from a three dimensional ensemble of many spherical particles having an ordered face-centered cubic lattice structure. The solution is obtained by numerically solving the Maxwell's equations using the FDTD (Finite Difference Time Domain) method with an impinging electromagnetic plane. The aim is to simulate the reflectance and transmittance of the system in the 300÷700 nm wavelength range, calculating also the angular power distribution of the scattered light. This study is suitable for the optical characterization of opal photonic crystals.
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