We study the effective linear and nonlinear optical parameters of composites containing noble metal nanoparticles and their dependence on the shape and size of the particles. Our numerical approach is based on the effective medium approximation combined with discrete dipole approximation, which results in a fast and accurate numerical method. The results demonstrate the possibility to achieve large enhancements of the linear and nonlinear optical parameters by tuning the plasmon resonance to a desired frequency by changing the size and the shape of the nanoparticles.

Anapole modes are known to be excited only in high-index nanoparticles. Here we show that the nanodisks made of dielectric materials with much lower refractive indices, such as LiNbO3, can generate anapole modes, provided that they are supported by a metallic or near-zero-index substrate. The important parameter for exciting an anapole mode is the index contrast between the nanodisk and substrate, rather than the refractive index of the nanodisk. We numerically demonstrate highly efficient coherent generation of vacuum ultraviolet with an efficiency higher than 10–2 % based on second-harmonic generation via the excitation of anapole modes in LiNbO3 nanodisks supported by an aluminum substrate. The highly efficient vacuum ultraviolet nanosource proposed in this work can find important applications for spectroscopic and sensing purposes at the nanoscale.

Mode locking of solid-state lasers using metal nanocomposites as slow saturable absorbers (SAs) is studied theoretically. The derived equation for the transient nonlinear response of metal nanocomposites is based on the semiclassical two-temperature model. The model is confirmed experimentally by pump-probe measurements on Au nanoparticles (NPs). The theory was applied to study passive mode locking of a solid-state laser containing Au NPs as SA in the green spectral range. Pulse durations as short as 100 fs are predicted, and design criteria of metal NP SAs are derived.

inversion symmetry, local Chern invariants have the values of ±1/2 at around K and K' valleys, leading to the existence of valley-locked edge states at the boundaries between two valley-Hall insulators with valley-Chern numbers of opposite signs at K (K') valleys [20,21] . Due to the robustness of the edge states against the structural disorder and the feasibility of fabricating such structures, valley-Hall photonic topological insulators have found promising applications for beam splitting, [31] topological lasing, [32][33][34][35] and frequency conversion. [36] Although the various hexagonal photonic lattices have been revealed to exhibit valley-Hall topological effects, such effects have been rarely investigated in square lattices, since they have non-zero Berry curvatures at non-symmetric points in the first Brillouin zone.Recently, higher-order topological insulating phases have emerged as an important research topic in topological physics, including topological photonics [37][38][39][40][41] and phononics. [42] When a parameter of a photonic topological insulator is changed, gapless edge bands are open above a certain value. For further parameter change, gapped edge states appear accompanying with corner or hinge states, which have dimensions lower than the edge states. As an example, the 2D second-order topological insulators generate zero-dimensional corner states as well as 1D edge states immune to the structural defects, enabling robust photonic nanocavities [43] and nanolasers. [44] In photonic practices, one needs multiband edge and corner states, in particular for nonlinear topological photo nics which frequently requires operating over wide spectral ranges. Up to date, multiband edge states [45,46] in photonic topological systems and their applications for nonlinear optical frequency conversion [36] have been reported, while multiband corner states have still not been investigated. In this work, we reveal 2D photonic crystals with square lattices of triangular dielectric rods to exhibit multiband edge and corner states depending on the structural parameters. For small constituent dielectric rods, there exist gapless multiband edge states immune to the structural disorder. With increasing their sizes, multiple photonic bandgaps appear, resulting in simultaneous occurrence of multiband edge and corner states. By evaluating the eigenstates and their excitation characteristics, we show that the multiband corner states are robust against structural defects. The results presented in this work can find important applications for nonlinear topological frequency conversion.

Most of preceding works for quasi-BICs in all-dielectric metasurfaces dealt with in-plane inversion symmetry-broken photonic systems (see, e.g., refs. [8,14]) rather than mirror symmetry-broken ones.Chiral metamaterials [17][18][19] and metasurfaces, [20][21][22][23][24][25][26][27][28][29] which are characterized by broken mirror symmetry, [30][31][32] have found broad photonic applications for circular dichroic nonlinear generations, [33][34][35][36] spinselective wavefront shaping and manipulating, [37][38][39] and biosensing. [40][41][42] Up to date, a number of results for chiral metasurfaces and metamaterials have been reported, while they dealt with wide bandwidth circular dichroism. For photonic applications, circular dichroism accompanying with narrow bandwidth and high quality factor is required, for example, for chiral sensing with high spectral resolution, simultaneous spectral, and spin selective wavefront shaping and secure optical communications, [43] efficient circular dichroic nonlinear generations, and circularly polarized lasing with low threshold. Employing quasi-BIC for mirror symmetry-broken systems might be a potential solution. Recent reports for metasurfaces exhibiting chiral quasi-BIC, however, have proposed to use double layered structure [43] or to introduce out-of-plane perturbations for obtaining high-Q chiroptical responses, [44] requiring complicated fabricating processes.In this work, we numerically show that chiral quasi-BIC with high Q-factor can be obtained by simultaneously breaking the in-plane inversion and mirror symmetries in dielectric metasurfaces. Taking metasurfaces with unit cells containing dielectric meta-atoms of different shapes as examples, we reveal that strong circular dichroism can be obtained for such a simultaneous in-plane symmetry breaking, sustaining high Q-factor and highly enhanced local field. When the in-plane inversion and mirror asymmetries gradually increase, the circular dichroism for the transmission and reflection of the metasurface increases and the Q-factor decreases. Dielectric metasurfaces with mirror symmetry supporting symmetry-protected BIC ubiquitously exhibit chiral quasi-BIC by simultaneously broken in-plane inversion and mirror symmetries. The results presented in this work can find important photonic applications such as chiral biosensing with high spectral resolution, Bound states in the continuum (BIC) have recently attracted great attention in photonics. Metasurfaces with broken inversion symmetry exhibit high-Q resonances via quasi-BIC, enabling low threshold lasing, sensing, and efficient nonlinear generation. Recently, chiral BICs have been investigated by using double layer structure or out-of-plane perturbation, requiring complicated fabrication processes. This work presents chiral quasi-BIC by simultaneously breaking the in-plane inversion and mirror symmetries in dielectric metasurfaces. For such a simultaneous symmetry breaking, the metasurfaces exhibit quasi-BIC with strong chirality, resulting in near-unity circular dich...

We study saturable absorption and the nonlinear contribution to the refractive index of metal-nanoparticle composites by using a modifie self-consistent Maxwell-Garnett formalism for spherical nanoparticles and a generalization of the discrete-dipole formalism for particles of arbitrary shape and size. The results for fused silica doped with silver nanoparticles show that the saturation of loss of the composites is strongest near the surface plasmon resonance and the saturation intensity is in the range of 10 MW/cm(2). The nonlinear refraction index decrease with increasing intensity and its sign depends on frequency and fillin factor. The predictions show that metal-nanoparticle composites can be used for mode locking of lasers in a broad spectral range down to 400 nm, where attractive saturable absorbers are still missing.

We develop the time-domain discrete dipole approximation (DDA), describing the temporal evolution of electric field in plasmonic nanostructures. The main equation is obtained by taking the inverse Fourier transform of the Taylor expansion of the frequency-domain DDA in terms of frequency deviation from the central frequency. Thus we assume that incident wavefronts of different frequencies accumulate relatively small phase difference when passing the particle. This assumption is always valid for nanoparticles much smaller than the wavelength. Being the time-domain method, the proposed approach also requires an analytic frequency dependence of electric permittivity, e.g. the Drude model. We present numerical results of application of the time-domain DDA to silver nanosphere, rod, and disk, which agree well with that obtained with its frequency-domain counterpart and the finite-difference time-domain method. Moreover, the time-domain DDA is the fastest of the three methods for incident pulses of several-femtoseconds width. Thus, it can effectively be applied for modeling the temporal responses of plasmonic nanostructures.

Coherent frequency conversions using multiband topological edge states in photonic structures made of nonlinear optical materials have been extensively investigated. In practice, however, coherent sources localized at the subwavelength scale are of great importance. Herein, efficient second‐harmonic generation (SHG) using dual‐band second‐order topological corner states is reported. By properly adjusting the structural parameters, dual‐band corner states can be obtained in the first and second bandgaps, when the eigenfrequency of the corner state in the higher bandgap is twice that of the corner one in the lower gap. In a square lattice made of tellurium as an example, high SHG efficiency in the order of 10−1% can be obtained for a peak pump intensity of around 20 GW cm−2. The results presented provide the realizability of subwavelength‐scale coherent sources, which are robust against the structural disorder or defects.