Optical metasurfaces are thin-layer subwavelengthpatterned structures that interact strongly with light. Metasurfaces have become the subject of several rapidly growing areas of research, being a logical extension of the field of metamaterials towards their practical applications. Metasurfaces demonstrate many useful properties of metadevices with engineered resonant electric and magnetic optical responses combined with low losses of thin-layer structures.Here we introduce the basic concepts of this rapidly growing research field that stem from earlier studies of frequencyselective surfaces in radiophysics, being enriched by the recent development of metamaterials and subwavelength nanophotonics. We review the most interesting properties of photonic metasurfaces, demonstrating their useful functionalities such as frequency selectivity, wavefront shaping, polarization control, etc. We discuss the ways to achieve tunability of metasurfaces and also demonstrate that nonlinear effects can be enhanced with the help of metasurface engineering.

Optical second harmonic generation (SHG) is studied from multilayer graphene films in the presence of DC electric current flowing in the sample plane. Graphene layers are manufactured by chemical vapour deposition (CVD) technique and deposited on an oxidised Si(001) substrate. SHG intensity from graphene layer is found to be negligible in the absence of the DC current, while it increases dramatically with the application of the electric current. The current-induced change of the SHG intensity rises linearly with the current amplitude and changes its sign under the reversal of the current direction to the opposite. The observed effect is explained in terms of the interference of second harmonic radiation reflected from the Si surface and that induced by the DC current in multilayer graphene.Since its first experimental realisation in 2004 graphene continues to attract enhanced interest as a prospective material for both fundamental and applied science. Fascinating electronic properties which include electric fieldeffect[1], "chiral" quantum Hall effects [2,3], prospects for spintronics [4] and valeytronics [5] immediately pushed graphene research to the cutting edge of modern nanomaterial science and technology. Among the numerous problems currently being studied for graphene is the possible connection between the electron transport and the nonlinear-optical response. The importance of this task is dictated not only by needs of the applied research as allows distant probing of the electron flow in graphene devices but, perhaps, more importantly as a route to gain new comprehensive insight into its fundamental electronic properties.Second harmonic generation (SHG) is among the most ubiquitous methods used for probing surfaces and interfaces of centrosymmetric materials.[6] High sensitivity to the surface and thin film properties arises from SHG being prohibited in the electric dipole approximation in the volume of a centrosymmetric medium. As a result it is generated basically at surfaces and interfaces where the central symmetry is broken. Moreover one can break the inversion symmetry by an external influence such as electric and magnetic field causing so-called field induced second harmonic generation. [7][8][9] It has been demonstrated recently both theoretically and experimentally [10,11] that DC electric current flowing in the plane of a centrosymetric semiconductor can break the symmetry of the electron density distribution, resulting in current-induced SHG (CSHG) which can overwhelm conventional electric-field-induced mechanism if the conductivity of the probed material is sufficiently high. Moreover, theoretical predictions[11] made almost a decade before the advent of graphene demonstrate the possibility of the SHG enhancement by 1∼2 orders of magnitude in case of ballistic electron transport and in case of two-dimensional nature of the investigated electron system. In this paper we report the first investigation of current-induced second harmonic generation in multilayer graphene under ambient conditions...

We study, both experimentally and theoretically, the second-order nonlinear response from resonant metasurfaces composed of metal−dielectric nanodisks. We demonstrate that by exciting the resonant optical modes of the composite nanoparticles we can achieve strong enhancement of the second-harmonic signal from the metasurface. By employing a multipole expansion method for the generated second-harmonic radiation, we show that the observed SHG enhancement is due to the magnetic dipolar and electric quadrupolar second-order nonlinear response of the metasurface.T he nonlinear optical properties of nanostructures are known to differ substantially from those of bulk media because they are affected by strong confinement and local resonances. 1−7 It is well established that the strong field enhancement through formation of "hot spots" can dramatically boost nonlinear effects in metallic nanoparticles. 8−11 Importantly, in the case of metamaterials, the nanopatterning leads not only to more efficient nonlinear interaction but also to completely new nonlinear regimes due to the magnetic optical response of the constituent "meta-atoms". While exciting applications of the linear magnetic response of metamaterials for both metallic 12−16 and dielectric 17−19 structures have been readily achieved at infrared and even optical frequencies, the magnetic nature of nonlinear optical phenomena in metamaterials is still largely unexplored.We focus our attention on the process of second-harmonic generation (SHG), which is an even-order nonlinear process that vanishes in centrosymmetric materials in the electric dipole approximation. 20 For small nanoparticles, efficient SHG may be observed due to several factors, including local field enhancement, deviation of the particle shapes from a symmetric one, and surface effects. 21 Importantly, the resonances in plasmonic and dielectric nanoparticles, combined with a strong enhancement of the optical near-field, as well as the effective overlap of the interacting optical modes, allow for multifold enhancement of SHG. 22−24 The enhancement of efficiency of second-harmonic generation within extremely small nanoscopic volumes is of paramount interest in surface science, 25,26 colloidal chemistry, 27 and catalytic chemistry. 28 Due to its nature, second-harmonic generation is extremely sensitive to surface adsorbents. As such, even a single-molecule layer adsorbed onto a surface can completely change the surface nonlinear susceptibility. This sensitivity to changes of the chemical environment has found many applications to the study of the symmetry properties of surfaces, the nature of adsorbates at surfaces or interfaces, or noninvasive probing of buried interfaces.It was recognized that the local field distribution depends drastically on the size, shape, and mutual orientation of the nanoparticles, thus providing a way for the design of artificial materials with required nonlinear optical properties. 29−31 Such nonlinearity engineering resembles the engineering of nonlinear materials by che...

We demonstrate enhancement of second-harmonic generation efficiency in sub-wavelength resonant nanostructures supporting optically induced magnetic response. This is achieved through simultaneous excitation of electric and magnetic multipoles at the second-harmonic wavelength and their constructive interference.

Excited carrier dynamics in plasmonic nanostructures determines many important optical properties such as nonlinear optical response and photocatalytic activity. Here it is shown that mesoscopic plasmonic covellite nanocrystals with low free‐carrier concentration exhibit a much faster carrier relaxation than in traditional plasmonic materials. A nonequilibrium hot‐carrier population thermalizes within first 20 fs after photoexcitation. A decreased thermalization time in nanocrystals compared to a bulk covellite is consistent with the reduced Coulomb screening in ultrathin films. The subsequent relaxation of thermalized, equilibrium electron gas is faster than in traditional plasmonic metals due to the lower carrier concentration and agrees well with that in a bulk covellite showing no evidence of quantum confinement or hot‐hole trapping at the surface states. The excitation of coherent optical phonon modes in a covellite is also demonstrated, revealing coherent lattice dynamics in plasmonic materials, which until now was mainly limited to dielectrics, semiconductors, and semimetals. These findings show advantages of this new mesoscopic plasmonic material for active control of optical processes.