Off-Normal Polarized Single-Photon Emission with Anisotropic Holography Metasurfaces.

Solid-state quantum emitters (QEs) with arbitrary direction emission and well-defined polarization are critical for scalable single-photon sources and quantum information processing. However, the design strategy for on-chip generation of off-normal photon emission with high-purity polarization characteristics has so far remained elusive. Here, we introduce the anisotropic holography metasurfaces for efficiently manipulating the emission direction and polarization of QE. The proposed method offers a flexible way to realize phase matching in surface plasmon scattering with spatially varying filling factors and provides an efficient route for designing advanced QE-coupled metasurfaces. By nonradiatively coupling nanodiamonds with metasurfaces, we experimentally demonstrate on-chip generation of well-collimated single-photon emission propagating along off-normal directions (i.e., 20° and 30°) featuring a divergence angle lower than 2.5°. The experimental average degree of linear polarization attains up to >0.98, thereby revealing markedly high polarization purity. This study facilitates applications of QEs in the deployment of integrated quantum networks.

Intracavity spatially modulated metasurfaces for a wavelength-tunable figure-9 vortex fiber laser.

Intracavity optical metasurfaces with compact and flexible light manipulation capabilities, effectively enrich the implementation of miniaturized and user-friendly orbital angular momentum (OAM) laser sources. Here we demonstrate a wavelength-tunable figure-9 Yb-doped vortex fiber laser solely with standard non-polarization-maintaining single-mode fibers, which utilizes a gap-surface plasmon (GSP) metasurface as the intracavity mode regulation component to generate OAM beams, extending the avenues and related applications for cost-effective OAM laser sources. Gained by the broadband operation range of the metasurface, the figure-9 fiber laser could emit OAM light with center wavelength tunable from 1020 nm to 1060 nm and of high mode purity (about 90%). OAM beams with different topological charges such as l = ±1 have been obtained by changing the metasurface design. The proposed fiber laser with the intracavity GSP metasurface provides a reliable and customized output of OAM beams at the laser source, holding great promise for a wide range of applications in optical communications, sensing, and super-resolution imaging.

Near-Field Observation of the Photonic Spin Hall Effect.

The photonic spin Hall effect, referring to the spatial separation of photons with opposite spins due to spin-orbit interactions, has enabled potential for various spin-sensitive applications and devices. Here, using scattering-type near-field scanning optical microscopy, we observe spin-orbit interactions introduced by a subwavelength semiring antenna integrated in a plasmonic circuit. Clear evidence of unidirectional excitation of surface plasmon polaritons is obtained by direct comparison of the amplitude- and phase-resolved near-field maps of the plasmonic nanocircuit under excitation with photons of opposite spin states coupled to a plasmonic nanoantenna. We present details of the antenna design and experimental methods to investigate the spatial variation of complex electromagnetic fields in a spin-sensitive plasmonic circuit. The reported findings offer valuable insights into the generation, characterization, and application of the photonic spin Hall effect in photonic integrated circuits for future and emerging spin-selective nanophotonic systems.

Ultimate Limit for Optical Losses in Gold, Revealed by Quantitative Near-Field Microscopy.

We report thorough measurements of surface plasmon polaritons (SPPs) running along nearly perfect air-gold interfaces formed by atomically flat surfaces of chemically synthesized gold monocrystals. By means of amplitude- and phase-resolved near-field microscopy, we obtain their propagation length and effective mode index at visible wavelengths (532, 594, 632.8, 729, and 800 nm). The measured values are compared with the values obtained from the dielectric functions of gold that are reported in literature. Importantly, a reported dielectric function of monocrystalline gold implies ∼1.5 times shorter propagation lengths than those observed in our experiments, whereas a dielectric function reported for properly fabricated polycrystalline gold leads to SPP propagation lengths matching our results. We argue that the SPP propagation lengths measured in our experiments signify the ultimate limit of optical losses in gold, encouraging further comprehensive characterization of optical material properties of pure gold as well as other plasmonic materials.

Anapole States in Gap-Surface Plasmon Resonators.

Anapole states associated with the destructive interference between dipole and toroidal moments result in suppressed scattering accompanied by strongly enhanced near fields. In this work, we comprehensively examine the anapole state formation in metal-insulator-metal configurations supporting gap surface-plasmon (GSP) resonances that are widely used in plasmonics. Using multipole decomposition, we show that in contrast to the common case of dielectric particles with out-of-phase superposition of electric and toroidal dipoles anapole states in GSP resonators are formed due to the compensation of magnetic dipole moments. Unlike anapole states in dielectric particles, magnetic anapole states in GSP resonator does not provide a pronounced suppression of scattering, but it features huge electric field enhancement, which we verify by numerical simulations and two-photon luminescence measurements. This makes the GSP resonator configuration very promising for use in a wide range of applications, ranging from nonlinear harmonic generation to absorption enhancement and sensing.

Plasmonic Lithium Niobate Mach-Zehnder Modulators.

Lithium niobate Mach-Zehnder modulators (MZMs) are present in a wide range of technologies and though fulfilling the performance and reliability requirements of present applications, they are becoming progressively too bulky, power inefficient, and slow in switching to keep pace with future technological demands. Here, we utilize plasmonics to demonstrate the most efficient (VπL = 0.23 Vcm) lithium niobate MZM to date, consisting of gold nanostripes on lithium niobate that guide both plasmonic modes and electrical signals that control their relative optical phase delay, thereby enabling efficient electro-optic modulation. For high linearity (modulation depth of >2 dB), the proposed MZM inherently operates near its quadrature point by shifting the relative phase of the signal in the interferometric arms. The demonstrated lithium niobate MZM manifests the benefits of employing plasmonics for applications that demand compact (<1 mm2) and fast (>10 GHz) photonic components operating reliably at ambient temperatures.

MEMS Tunable Metasurfaces Based on Gap Plasmon or Fabry-Pérot Resonances.

Tunable metasurfaces promise to enable adaptive optical systems with complex functionalities. Among possible realizations, a recent platform combining microelectromechanical systems (MEMS) with gap-surface plasmon (GSP) metasurfaces offers high modulation efficiency, broadband operation, and fast response. We compare tunable metasurfaces operating in GSP and Fabry-Pérot (FP) regions by investigating polarization-independent blazed gratings both numerically and experimentally. Peak efficiency is calculated to be ∼75% in both cases (∼40% in measurements), while the operation bandwidth is found larger when operating in the GSP region. Advantages of operating in the FP region include relaxed assembly requirements and operation tolerances. Additionally, simulation and experimental results show that coupling between neighboring unit cells increases for larger air gaps, resulting in deteriorated efficiency. We believe the presented analysis provides important guidelines for designing tunable metasurfaces for diverse applications in miniaturized adaptive optical systems.

On-Chip Ge Photodetector Efficiency Enhancement by Local Laser-Induced Crystallization.

Metal-semiconductor-metal plasmonic nanostructures enable both on-chip efficient manipulation and ultrafast photodetection of strongly confined modes by enhancing local electrostatic and optical fields. The latter is achieved by making use of nanostructured thin-film germanium (Ge) plasmonic-waveguide photodetectors. While their sizes and locations can be accurately controlled during the nanofabrication, the detector efficiencies are significantly reduced due to deposited Ge amorphous nature. We demonstrate that the efficiency of waveguide-integrated Ge plasmonic photodetectors can be increased significantly (more than 2 orders of magnitude) by spatially controlled laser-induced Ge crystallization. We investigate both free-space and waveguide-integrated Ge photodetectors subjected to 800 nm laser treatment, monitoring the degree of crystallization with Raman spectroscopy, and demonstrate the efficiency enhancement by detecting the telecom radiation. The demonstrated local postprocessing technique can be utilized in various nanophotonic devices for efficient and ultrafast on-chip radiation monitoring and detection, offering significantly improved detector characteristics without jeopardizing the performance of other components.

High-Speed Plasmonic Electro-Optic Beam Deflectors.

Highly integrated active nanophotonics addressing both device footprint and operation speed demands is a key enabling technology for the next generation optical networks. Plasmonic systems have proven to be a serious contender to alleviate current performance limitations in electro-optic devices. Here, we demonstrate a plasmonic optical phased array (OPA) consisting of two 10 µm long plasmonic phase shifters, utilized to control the far-field radiation pattern of two subwavelength-separated emitters for aliasing-free beam steering with an angular range of ±5° and flat frequency response up to 18 GHz (with the potential bandwidth of 1.2 THz). Extreme optical and electrostatic field confinement with great spatial overlap results in high phase modulation efficiency (VπL = 0.24 Vcm). The demonstrated approach of using plasmonic lithium niobate technology for optical beam manipulation offers inertia-free, robust, ultracompact, and high-speed beam steering.

Anisotropic second-harmonic generation from monocrystalline gold flakes.

Noble metals with well-defined crystallographic orientation constitute an appealing class of materials for controlling light-matter interactions on the nanoscale. Nonlinear optical processes, being particularly sensitive to anisotropy, are a natural and versatile probe of crystallinity in nano-optical devices. Here we study the nonlinear optical response of monocrystalline gold flakes, revealing a polarization dependence in second-harmonic generation from the {111} surface that is markedly absent in polycrystalline films. Our findings confirm that second-harmonic microscopy is a robust and non-destructive method for probing the crystallographic orientation of gold, and can serve as a guideline for enhancing nonlinear response in plasmonic systems.