Enhanced optical nonlinearities in 2D-3D heteromaterials

Summary form only given. Remarkable optical and electrical properties of graphene [1] and other two-dimensional (2D) materials provide significant potential for novel optoelectronic applications and devices, many of which depend on nonlinear optical effects in these 2D materials [2, 3]. In this paper, we will review some recent results [4, 5] pertaining to nonlinear optical effects in nanostructured bulk optical media containing homogeneous or nanopatterned 2D materials

Polarization control using passive and active crossed graphene gratings

Graphene gratings provide a promising route towards the miniaturization of THz metasurfaces and other photonic devices, chiefly due to remarkable optical properties of graphene. In this paper, we propose novel graphene nanostructures for passive and active control of the polarization state of THz waves. The proposed devices are composed of two crossed graphene gratings separated by an insulator spacer. Because of specific linear and nonlinear properties of graphene, these optical metasurfaces can be utilized as ultrathin polarization converters operating in the THz frequency domain. In particular, our study shows that properly designed graphene polarizers can effectively select specific polarization states, their thickness being about a tenth of the operating wavelength and size more than 80× smaller than that of similar metallic devices. Equally important, we demonstrate that the nonlinear optical properties of graphene can be utilized to actively control the polarization state of generated higher harmonics

Tunable and dual-broadband giant enhancement of second-harmonic and third-harmonic generation in an optimized graphene-insulator-graphene metasurface

We demonstrate a scheme to dramatically enhance both the second- and third-harmonic generation (SHG, THG) in a graphene-insulator-graphene metasurface. The key underlying feature of our approach is the existence of a double-resonance phenomenon, namely, the metasurface is designed to possess fundamental plasmon resonances at both the fundamental frequency and the higher harmonic. This dual resonant field enhancement, combined with a favorable spatial overlap of the optical near fields, lead to the increase of the THG and SHG by ∼ 10 9 and ∼ 10 6 , respectively. We also demonstrate that by tuning the Fermi energy of the graphene gratings the dual-resonance property can be locked in over a remarkably broad spectral range of ∼ 20 THz , which is more than three orders of magnitude larger than the spectral tunability achievable in metal-based plasmonic systems. Importantly, the enhanced nonlinear frequency generation process can be readily switched in the same system between the second and third harmonic. This type of graphene metasurface could open up new avenues towards the development of novel ultracompact and multifrequency active photonic nanodevices

Double-resonant enhancement of third-harmonic generation in graphene nanostructures

Intriguing and unusual physical properties of graphene offer remarkable potential for advanced, photonics-related technological applications, particularly in the area of nonlinear optics at the deep-subwavelength scale. In this study, we use a recently developed numerical method to illustrate an efficient mechanism that can lead to orders of magnitude enhancement of the third-harmonic generation in graphene diffraction gratings. In particular, we demonstrate that by taking advantage of the geometry dependence of the resonance wavelength of localized surface-plasmon polaritons of graphene ribbons and discs one can engineer the spectral response of graphene gratings so that strong plasmonic resonances exist at both the fundamental frequency and third-harmonic (TH). As a result of this double-resonant mechanism for optical near-field enhancement, the intensity of the TH can be increased by more than six orders of magnitude

Homogenization and nonlinearity enhancement of 2D graphene-based metasurfaces

A general homogenization technique is developed to study the optical properties of 2D graphene-based metasurfaces. The results show the effective nonlinear susceptibilities of graphene metasurfaces can be enhanced by more than two orders of magnitude

All-optically Control of Light Propagation in Valley-Hall Topological Waveguides of Graphene Metasurfaces

We study the influence of graphene Kerr effect on valley-Hall topological modes of a graphene plasmonic crystal waveguide. Extra air holes are introduced to break the spatial-inversion symmetry of the plasmonic metasurface, which can be performed using e-beam lithography. As a result, a gapless Dirac cone and topologically protected edge modes form inside the nontrivial frequency bandgap. Taking advantage of the fact that graphene is a nonlinear optical material possessing an extremely large Kerr coefficient, we demonstrate that an all-optical switch can be implemented in this topological photonic system by controlling an optical signal propagating in the waveguide via a pump beam injected into the bulk modes of the metasurface. This work may lead to new graphene-based active topological photonic nanodevices

Valley-Hall Topological Transport in Graphene Plasmonic Crystal Waveguides

Due to immunity to disorder and structural imperfections, topologically-protected plasmonic modes have recently attracted increasing attention. Here, we introduce two different mechanisms to construct valley-Hall domain-wall interface waveguides in graphene plasmonic crystal to mimic the quantum valley-Hall effect. In the first case, we break the in-plane spatial inversion symmetry of a single-layer graphene plasmonic crystal waveguide to achieve valley-Hall topological characteristics, whereas in the second case, we break the out-of-plane spatial inversion symmetry of a bi-layer graphene plasmonic crystal waveguide to implement the analog quantum valley-Hall effect. A molecular sensor based on this valley-Hall topological transport phenomenon is also be presented

T-matrix method for calculation of second-harmonic generation in clusters of spherical particles

In this article, we present a T-matrix method for numerical computation of second-harmonic generation from clusters of arbitrarily distributed spherical particles made of centrosymmetric optical materials. The electromagnetic fields at the fundamental and second-harmonic (SH) frequencies are expanded in series of vector spherical wave functions, and the single sphere T-matrix entries are computed by imposing field boundary conditions at the surface of the particles. Different from previous approaches, we compute the SH fields by taking into account both local surface and nonlocal bulk polarization sources, which allows one to accurately describe the generation of SH in arbitrary clusters of spherical particles. Our numerical method can be used to efficiently analyze clusters of spherical particles made of various optical materials, including metallic, dielectric, semiconductor, and polaritonic materials

Enhanced Second-Harmonic Generation in Monolayer MoS2 Driven by a BIC-based Nonlinear Metasurface

Dielectric metasurfaces have opened novel routes for nonlinear optics in recent years. In this work, we integrate a nonlinear metasurface with monolayer molybdenum disulfide (MoS2) to enhance second-harmonic generation (SHG) from atomically thin MoS2. By utilizing bound states in the continuum, we achieve about 600× of SHG enhancement from monolayer MoS2 on a resonant metasurface relative to suspended monolayer MoS2. Moreover, an eigenmode expansion approach is exploited to express second-harmonic power and the corresponding analytical results agree well with the rigorous calculations