We demonstrate experimentally a nonlinear plasmonic metasurface that exhibits strongly asymmetric second-harmonic generation (SHG), depending on the illuminating direction: when the metasurface is illuminated from one side, it produces a significant SHG signal, whilst this nonlinear response is strongly reduced (approx. 10 dB), upon illumination from the opposite side. This surprising behavior stems from the bianisotropic response of the system, as confirmed by a homogenization analysis and the extraction of the effective susceptibility tensor. At first sight, it could be tempting to interpret this asymmetric response as a non-reciprocal phenomenon, but we will show that actually it is time-reversal asymmetric.
Among the material families used in nanophotonics, the fundamental mode for metal nanostructures is electric, while that for dielectric nanostructures is magnetic. Here, we consider hybrid nanophotonics, an emerging field of research that mixes both materials into one hybrid structure to benefit from the best of both worlds. It is demonstrated that the magnetic dipole in dielectrics can be entirely suppressed for small interparticle distances by the near-field produced by a nearby metal nanostructure. The explanation of the observed effect is given by considering the formation of a standing wave between the incident field and the light scattered from the metal particle. The analytical coupled electric and magnetic dipole method (CEMD) along with the full wave surface integral equation method (SIE) are used to examine this phenomenon. The conditions required for the observation of the magnetic dipole suppression in the visible range for high refractive index dielectric nanoparticles are described. The influence of the effect on the ability to control the directivity of the radiation in the far-field is considered. We further show that the electric and magnetic responses can be enhanced or suppressed by positioning the dielectric particle in the nodes of the standing wave formed by the metallic particle. This controlled near-field interaction provides a handle on the far-field response of the system, with possible applications as optical switches.
While multipoles expansions have become ubiquitous in the analysis of plasmonic and dielectric nanostructures and metasurfaces, their zoology remains quite complex. Depending on the physical system, it appears that different multipoles families – including somewhat exotic toroidal multipoles – are required to compute the response of the system. In this work, we present a facile derivation of the three multipoles families: the Cartesian primitive multipoles, the Cartesian irreducible multipoles and the spherical multipoles. We illustrate the links between them and demonstrate their utilization in practical situations to guide experiments.
In this paper, we aim at unveiling the underlying physical mechanism for transversal optical forces, appearing due to the simultaneous illumination of a spherical object with two plane waves possessing different polarizations. The appearance of such a transversal force is quite counterintuitive since it seems to contradict the law of momentum conservation. We consider the cases of perfect electric conductor (PEC) and silver spheres illuminated by two orthogonally polarized plane waves propagating obliquely with respect to each other. Interestingly, the Poynting vector in these cases acquires a nonzero component transverse to the plane of propagation. Since the momentum transfer is related to the energy transfer, or equivalently, to non-negligible Poynting vector pointed in a particular direction, an arbitrary object placed in such external field is expected to experience a transversal force. To cast light upon this peculiar effect, we use a surface integral equation method and, along with the Maxwell stress tensor formalism, find the optical force acting on various spheres. We observe this effect for PEC spheres of different sizes and find that they are indeed subject to such transversal force. We find an explanation for this phenomenon via interference effects between selected multipoles excited in the structure. With recently developed methods, we expand the optical force into contributing pairs of selected multipoles and show that, depending on the phase between each multipole pair, the sign and direction of the force can be controlled. We also compare the results for silver and PEC spheres and find that the transversal force magnitude in silver has higher values for more limited range of sphere radii, as compared to PEC.
Hybrid structures that combine dielectric resonators with plasmonic structures hold great promises due to the diversity of optical modes they possess. Here, we explore the physics underlying the scattering response of a hybrid nanoantenna made of a metal disk placed on top of a dielectric cylinder and study the hybridization of the different modes excited in the dielectric and metallic parts. Surprisingly, we note that the signature of an anapole state – usually only seen in high refractive index dielectrics – can be observed in the metallic part of the system. The Cartesian multipoles excited in the dielectric and metal interfere in a complex manner, leading to an unexpected high-order vector spherical multipolar response in the far-field. These effects are thoroughly studied in terms of the near-field and absorption enhancements. We also show that very fine control over the multipoles' resonant positions can be achieved by varying the geometry of the structure. This flexibility renders this system very promising for sensing applications. Based on these developments, we have designed and fabricated such hybrid nanoantennas using silicon and aluminum and measured a preliminary sensitivity of 160 nm/RIU, which is competing with conventional sensors based on localized surface plasmon resonances.
A large amount of experimental and theoretical works deals with the second harmonic generation from different plasmonic geometries. Since they often consider relatively long optical pulses, many of these studies are focused on the investigation of a quasi-monochromatic response of the system and can be understood through the excitation of one, possibly two, optical modes. On the other hand, when the excitation pulse duration is short (say, below several tens of fs), the excitation spectrum becomes broader and a very interesting dynamics emerges from the interplay between several optical modes. In this work, the dynamics of modes at the second harmonic frequency for two silver spheres of different diameters and a nanorod is investigated numerically and shown to be quite different. For the pulsed illumination with length close to the modes lifetime, apart from different relative contributions of dipolar and quadrupolar multipoles in the far-field, we have been able to observe and explain non constant phase difference between multipoles, which is not accessible in continuous wave regime. Short pulse durations also allow us to observe only one mode, while another one has already decayed. For the case of the nanorod we also perform an eigenmode analysis, which allows to understand the modes interplay that explains the observed spectra. In the paper, we also show a method allowing a significant reduction of required computational steps to find the response of a plasmonic nanostructure to a pulsed illumination with arbitrary frequency-domain method.
A novel approach is introduced to determine the time evolution of optical forces and torques on arbitrary shape nanostructures by combining Maxwell's stress tensor with the surface integral equation method (SIE). Conventional time averaging of Maxwell’s stress tensor allows obtaining an elegant form in terms of surface currents for the force exerted on nanostructures. Unfortunately, the information about the time dependence of the force – which can be very important in ultrafast photonics experiments and in nano-manipulation applications – is lost in such an approach. To overcome this, we have developed a time-domain method based on the inverse Fourier transform of the frequency-domain SIE. The calculations in the frequency domain allow accurately taking into account the dispersion of the permittivity function of the system and the use of surface currents enables the rigorous treatment of intricate geometries for the scatterer. Furthermore, the integration of Maxwell’s stress tensor directly on the scatterer’s boundary significantly reduces the required computation time and increases the accuracy of the method. We show quite unusual sum frequency-like terms in the dynamics of the force appearing in Maxwell’s stress tensor, which normally vanish for the time-averaged force. To illustrate this effect, we study how the pulse duration influences the dynamics of optical force in the case of a rectangular shape and Gaussian pulses illuminating thin film at normal incidence. In the framework of the developed numerical method, we study the influence of the sum-frequency-like terms on the dynamics of optical forces in the case of a spherical scatterer.
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