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Intense efforts have been made in recent years to realize nonlinear optical interactions at the single-photon level. Much of this work has focused on achieving strong third-order nonlinearities, such as by using single atoms or other quantum emitters, while the possibility of achieving strong second-order nonlinearities remains unexplored. Here, we describe a novel technique to realize such nonlinearities using graphene, exploiting the strong per-photon fields associated with tightly confined graphene plasmons in combination with spatially nonlocal nonlinear optical interactions. We show that in properly designed graphene nanostructures, these conditions enable extremely strong internal down-conversion between a single quantized plasmon and an entangled plasmon pair, or the reverse process of second harmonic generation. A separate issue is how such strong internal nonlinearities can be observed, given the nominally weak coupling between these plasmon resonances and free-space radiative fields. On one hand, by using the collective coupling to radiation of nanostructure arrays, we show that the internal nonlinearities can manifest themselves as efficient frequency conversion of radiative fields at extremely low input powers. On the other hand, the development of techniques to efficiently couple to single nanostructures would allow these nonlinear processes to occur at the level of single input photons.

The generation and manipulation of single photons are crucial in advanced quantum technologies, such as quantum communication and quantum computation devices. High-purity single photons can be generated from classical light using the single-photon blockade (1PB). However, the efficiency and purity are exclusive in 1PB, which hinders its practical applications. Here, we show that the resonantly coupled plasmonic-photonic cavity can boost the efficiency of single-photon generation by more than three orders of magnitude compared with that of all-dielectric microcavity. This significant improvement is attributed to two new mechanisms of atom-microcavity coupling after introducing the plasmonic cavity: the formation of a quasi-bound state and the transition to the nonreciprocal regime, due to the destructive interference between the coupling pathways and the nonzero relative phase of the closed-loop coupling, respectively. The quasi-bound state has a relatively small decaying, while its effective coupling strength is significantly enhanced. Suppressing the dissipative component of the effective atom-microcavity coupling in the nonreciprocal regime can further improve single-photon performance, particularly without temporal oscillations. Our study demonstrates the possibility of enhancing the intrinsically low efficiency of 1PB in low excitation regime, and unveils the novel light-matter interaction in hybrid cavities.

Surface plasmons excited by light in metal structures provide a means for manipulating optical energy at the nanoscale. Plasmons are associated with the collective oscillations of conduction electrons in metals and play a role intermediate between photonics and electronics. As such, plasmonic devices have been created that mimic photonic waveguides as well as electrical circuits operating at optical frequencies. We review the plasmon technologies and circuits proposed, modeled, and demonstrated over the past decade that have potential applications in optical computing and optical information processing.

An acoustic plasmon is predicted to occur, in addition to the conventional two-dimensional (2D) plasmon, as the collective motion of a system of two types of electronic carriers coexisting in the same 2D band of extrinsic (doped or gated) graphene. The origin of this novel mode stems from the anisotropy present in the graphene band structure near the Dirac points K and K′. This anisotropy allows for the coexistence of carriers moving with two distinct Fermi velocities along the and directions, which leads to two modes of collective oscillation: one mode in which the two types of carriers oscillate in phase with one another (this is the conventional 2D graphene plasmon, which at long wavelengths ( ) has the same dispersion, , as the conventional 2D plasmon of a 2D free electron gas), and the other mode found here corresponds to a low-frequency acoustic oscillation (whose energy exhibits at long-wavelengths a linear dependence on the 2D wavenumber q) in which the two types of carriers oscillate out of phase. This prediction represents a realization of acoustic plasmons originated in the collective motion of a system of two types of carriers coexisting within the same band.

Strong coupling between plasmons and multiple different exciton states (MESs) enables the creation of multiple hybrid polariton states under ambient conditions. These hybrid states possess unique optical properties different from those of their separate identities, making them ideal candidates for exploiting room-temperature multimode hybridization and multiqubit operation. In this study, we revealed the static spectral response properties of plasmon-MES strong coupling via a fully quantum mechanics approach. These theoretical predictions were experimentally demonstrated in plasmonic nanocavities containing two and three different exciton species. Additionally, the dynamical absorption processes of such strong coupling systems were investigated, and results indicated that the damping of the hybrid polariton states induced by the strong coupling could be markedly modulated by the acoustic oscillations from the plasmonic nanocavities. Our findings contribute a theoretical approach for accurately describing the plasmon-MES interactions and a platform for developing the high-speed active plasmonic devices based on multiqubit strong coupling.

We present a novel wave equation for linearized plasmonic response, obtained by combining the coupled real-space differential equations for the electric field and current density. Nonlocal dynamics are fully accounted for, and the formulation is very well suited for numerical implementation, allowing us to study waveguides with subnanometer cross-sections exhibiting extreme light confinement. We show that groove and wedge waveguides have a fundamental lower limit in their mode confinement, only captured by the nonlocal theory. The limitation translates into an upper limit for the corresponding Purcell factors, and thus has important implications for quantum plasmonics.

We find the low-frequency optical response of highly doped individual and arrayed graphene nanoribbons to be accurately described in terms of plasmon wave functions (PWFs). More precisely, we focus on the lowest-order transverse dipolar mode, for which we define the wave function as the induced charge density associated with the plasmon. We show that a single universal wave function is capable of describing the normal-incidence interaction of paired, co-planar, and stacked arrays of ribbons down to small inter-ribbon distances. Our work provides both intuitive insight into graphene plasmon interactions and a practical way of accurately describing complex graphene geometries based on the PWFs of the individual components.

We investigate the plasmonic excitations and the broadening functions of the plasmon dispersions in multilayer structures consisting of four parallel monolayer graphene (4-MLG) sheets on an inhomogeneous background dielectric within the random-phase approximation. By finding the zeroes of the frequency-dependent dielectric function, we determine one optical and three acoustic plasmon modes in the system. We observed that the dependence of plasmon properties in the inhomogeneous 4-MLG system on the parameters differs significantly from that in the homogeneous one. Once the inhomogeneity of the background dielectric is taken into account, the plasmon frequencies get smaller values, compared to those in the homogeneous situation as well as in the MLG at the same parameters. As the interlayer separation increases, the plasmon branches in the inhomogeneous system move downward while only the optical branch in the homogeneous one does this, the acoustic plasmon branches shift to the opposite direction. With the efficiently large separations, plasmon lines in the homogeneous case become identical while those in the inhomogeneous case separate from each other in the large momentum region. For homogeneous 4-MLG systems, the decrease in carrier density leads to the decrease in plasmon frequency, but for inhomogeneous 4-MLG structures, the decrease in the doping density of the first graphene layer increases remarkably the frequencies of all plasmon branches. Finally, the broadening function of the plasmon dispersions gets the larger values as plasmon lines go far away from the inter single-particle excitation boundary.

We present a semiclassical explanation for the morphology of the surface Fermi arcs of Weyl semimetals. Viewing the surface states as a two-dimensional Fermi gas subject to band bending and Berry curvatures, we show that it is the non-parallelism between the velocity and the momentum that gives rise to the spiral structure of Fermi arcs. We map out the Fermi arcs from the velocity field for a single Weyl point and a lattice with two Weyl points. We also investigate the surface magnetoplasma of Dirac semimetals in a magnetic field, and find that the drift motion, the chiral magnetic effect and the Imbert-Fedorov shift are all involved in the formation of surface Fermi arcs. Our work not only provides an insightful perspective on the surface Fermi arcs and a practical way to find the surface dispersion, but also paves the way for the study of other physical properties of the surface states of topological semimetals, such as transport properties and orbital magnetization, using semiclassical methods.

Although coupled plasmonic systems have been extensively studied in the past decades, their theoretical understanding is still far from satisfactory. Here, based on experimental and numerical studies on a series of symmetry-broken nano-patch plasmonic resonators, we found that Fano’s formula, widely used in modeling such systems previously, works well for one polarization but completely fails for another polarization. In contrast, a two-mode coupled-mode theory (CMT) can interpret all experimental results well. This motivated us to employ the CMT to establish a complete phase diagram for such coupled plasmonic systems, which not only revealed the diversified effects and their governing physics in different phase regions, but more importantly, also justifies the applicabilities of two simplified models (including Fano’s formula) derived previously. Our results present a unified picture for the distinct effects discovered in such systems, which can facilitate people’s understanding of the governing physics and can design functional devices facing requests for diversified applications.