Motivated by the recent emergence of a new class of anisotropic 2D materials, we examine their electromagnetic modes and demonstrate that a broad class of the materials can host highly directional hyperbolic plasmons. Their propagation direction can be manipulated on the spot by gate doping, enabling hyperbolic beam reflection, refraction, and bending. The realization of these natural 2D hyperbolic media opens up a new avenue in dynamic control of hyperbolic plasmons not possible in the 3D version.

Black phosphorus has been the subject of growing interest due to its unique band structure that is both layer dependent and anisotropic. While many have studied the linear optical response of black phosphorus, the nonlinear response has remained relatively unexplored. Here we report on the observation of third-harmonic generation in black phosphorus using an ultrafast near-IR laser and measure χ (3) experimentally for the first time. It was found that the third-harmonic emission is highly anisotropic, dependent on the incident polarization, and varies strongly with the number of layers present due to signal depletion and phase-matching conditions. Black phosphorus (BP) recently emerged as a new two dimensional material with highly unique optical and electrical properties, including high carrier mobility, 1,2 optical and electrical anisotropy, 3-5 and importantly, a tunable, direct bandgap. 6-9 Particularly, for thicknesses greater than a few nanometers, BP's bandgap bridges the technically important mid-infrared (mid-IR) spectral range that is not currently covered by other two-dimensional semiconductors. 4 These attributes of BP make it very promising for optoelectronic devices that cover a broad spectral range from near-to mid-IR for both communication and optical sensing. 10,11 In addition to linear optical properties, the quantum confinement in 2D materials also leads to many novel nonlinear optical properties. For example, MoS2, 12-14 WSe2, 15,16 and hBN,13 all with non-centrosymmetric lattice structures, have shown strong second-order nonlinear optical effects, such as second-harmonic generation (SHG). SHG in these materials has shown strong enhancement at the exciton resonances, 15 can be electrically tuned by a local back gate, 16 and has been utilized for optically probing the crystal orientation 12 and thickness. 13,14 Additionally, third-order optical nonlinearity such as third-harmonic generation (THG) has been observed to be strong in graphene, 17,18 as well as in MoS2 thin films. 19 In terms of nonlinear optics in BP, its centrosymmetric crystalline structure only permits third-order nonlinearity but its strong anisotropy and layer dependent band structure should lead to very intriguing nonlinear optical effects. Research to date, however, has been primarily limited to the saturable absorption effect in BP, studied with z-scan 20,21 and ultrafast pump-probe 22 techniques for liquid exfoliated BP suspensions and utilized for application in mode-locked lasers. [23][24][25][26][27] The intrinsic optical nonlinearity of crystalline BP has not been experimentally investigated.In this paper, we investigate BP's optical nonlinearity by measuring both the polarization and thickness dependence of THG in multilayer BP samples. We find that the THG in BP is

Ultrafast electrically driven nanoscale light sources are critical components in nanophotonics. Compound semiconductor-based light sources for the nanophotonic platforms have been extensively investigated over the past decades. However, monolithic ultrafast light sources with a small footprint remain a challenge. Here, we demonstrate electrically driven ultrafast graphene light emitters that achieve light pulse generation with up to 10 GHz bandwidth across a broad spectral range from the visible to the near-infrared. The fast response results from ultrafast charge-carrier dynamics in graphene and weak electron-acoustic phonon-mediated coupling between the electronic and lattice degrees of freedom. We also find that encapsulating graphene with hexagonal boron nitride (hBN) layers strongly modifies the emission spectrum by changing the local optical density of states, thus providing up to 460% enhancement compared to the gray-body thermal radiation for a broad peak centered at 720 nm. Furthermore, the hBN encapsulation layers permit stable and bright visible thermal radiation with electronic temperatures up to 2000 K under ambient conditions as well as efficient ultrafast electronic cooling via near-field coupling to hybrid polaritonic modes under electrical excitation. These high-speed graphene light emitters provide a promising path for on-chip light sources for optical communications and other optoelectronic applications.

We study the electromagnetic response and surface electromagnetic modes in a generic gapped Dirac material under pumping with circularly polarized light. The valley imbalance due to pumping leads to a net Berry curvature, giving rise to a finite transverse conductivity. We discuss the appearance of nonreciprocal chiral edge modes, their hybridization and waveguiding in a nanoribbon geometry, and giant polarization rotation in nanoribbon arrays. DOI: 10.1103/PhysRevB.93.041413 Introduction. The Berry curvature is a topological property of the Bloch energy band and acts as an effective magnetic field in momentum space [1][2][3]. Hence, topological materials may exhibit anomalous Hall-like transverse currents in the presence of an applied electric field, in the absence of a magnetic field. Examples includes topological insulators [4] with propagating surface states that are protected against backscattering from disorder and impurities and transition metal dichalcogenides where the two valleys carry opposite Berry curvature giving rise to bulk topological charge neutral valley currents [5,6]. These bulk topological currents were also experimentally investigated in other Dirac materials, such as a gapped graphene and bilayer graphene system [7,8]. The electromagnetic response of these gapped Dirac systems, particularly that due to its surface electromagnetic modes (i.e., plasmons), are relative unexplored.In gapped graphene or monolayer transition metal dichalcogenides, electrons in the two valleys have opposite Berry curvature, ensured by time-reversal symmetry (TRS) of their chiral Hamiltonians [5]. Hence, far field light scattering properties of these atomically thin systems does not differentiate between circularly polarized light, i.e., zero circular dichroism in the classical sense. Optical pumping with circularly polarized light naturally breaks TRS, and a net planar chirality ensues. However, under typical experimental conditions, the transverse conductivity due to Berry curvature is less than the quantized conductivity e 2 / , and the associated optical dichroism effect is not prominent. These effects, however, can potentially be amplified through enhanced light-matter interaction with plasmons [9][10][11][12][13].In this Rapid Communication, we discuss the emergence of chiral electromagnetic plasmonic modes and their associated optical dichroism effect. We consider a gapped Dirac system under continuous pumping with circularly polarized light. We discuss the appearance of edge chiral plasmons and how they can allow launching of one-way propagating edge plasmons in a semi-infinite geometry. We also consider the hybridization of these chiral edge modes in a nanoribbon geometry and the possibility of nonreciprocal waveguiding. Their far-field optical properties reveal resonant absorption accompanied by sizable polarization rotation.

The ability to transform two-dimensional (2D) materials into a three-dimensional (3D) structure while preserving their unique inherent properties might offer great enticing opportunities in the development of diverse applications for next generation micro/nanodevices. Here, a self-assembly process is introduced for building free-standing 3D, micro/nanoscale, hollow, polyhedral structures configured with a few layers of graphene-based materials: graphene and graphene oxide. The 3D structures have been further modified with surface patterning, realized through the inclusion of metal patterns on their 3D surfaces. The 3D geometry leads to a nontrivial spatial distribution of strong electric fields (volumetric light confinement) induced by 3D plasmon hybridization on the surface of the graphene forming the 3D structures. Due to coupling in all directions, resulting in 3D plasmon hybridization, the 3D closed box graphene generates a highly confined electric field within as well as outside of the cubes. Moreover, since the uniform coupling reduces the decay of the field enhancement away from the surface, the confined electric field inside of the 3D structure shows two orders of magnitude higher than that of 2D graphene before transformation into the 3D structure. Therefore, these structures might be used for detection of target substances (not limited to only the graphene surfaces, but using the entire volume formed by the 3D graphene-based structure) in sensor applications.