Chiral Second-Harmonic Generation from Monolayer WS2/Aluminum Plasmonic Vortex Metalens

Ultrafast Time Dynamics of Plasmonic Fractional Orbital Angular Momentum

The creation and manipulation of optical vortices, both in free space and in two-dimensional systems such as surface plasmon polaritons (SPPs), has attracted widespread attention in nano-optics due to their robust topological structure. Coupled with strong spatial confinement in the case of SPPs, these plasmonic vortices and their underlying orbital angular momentum (OAM) have promise in novel light-matter interactions on the nanoscale with applications ranging from on-chip particle manipulation to tailored control of plasmonic quasiparticles. Until now, predominantly integer OAM values have been investigated. Here, we measure and analyze the time evolution of fractional OAM SPPs using time-resolved two-photon photoemission electron microscopy and near-field optical microscopy. We experimentally show the field's complex rotational dynamics and observe the beating of integer OAM eigenmodes at fractional OAM excitations. With our ability to access the ultrafast time dynamics of the electric field, we can follow the buildup of the plasmonic fractional OAM during the interference of the converging surface plasmons. By adiabatically increasing the phase discontinuity at the excitation boundary, we track the total OAM, leading to plateaus around integer OAM values that arise from the interplay between intrinsic and extrinsic OAM.

Tunable Chiral Plasmonic Activities Enabled via Stimuli Responsive Micro-Origami

Chiral plasmonic nanomaterials with distinctive circularly polarized light-dependent optical responses over a broad range of frequency have great potential for photonic and biomedical applications. However, it still remains challenging to fabricate 3D plasmonic chiral micro-constructs with readily modulated chiroptical properties over the magnitude of ellipticity, mode frequency, and switchable handedness, especially in the vis-NIR range. In this study, polymeric micro-origami-based 3D plasmonic chiral structures are constructed through self-rolling of gold  nanospheres (AuNSs)-decorated polymeric micro-sheets. Spherical AuNSs are assembled as highly ordered linear chains on 2D rectangular micro-sheets by polydimethylsiloxane-wrinkle assisted assembly. Upon rolling the micro-sheets to micro-tubules, the AuNS chains transform into 3D helices. The AuNS-assembled helices induce collective plasmonic modes propagating in a helical manner, leading to a strong chiral response over the vis-NIR range. The circular dichroism (CD) is measured to be as high as hundreds of millidegree, and the position and sign of CD peaks are actively modulated by controlling the orientated angle of AuNS chains, enabled by tuning the collective plasmonic modes. This micro-origami-based strategy incorporates the incompatible 2D assembly technique with 3D chiral structures, opening up an intriguing way toward constructing chiral plasmonic structures and modulating chiroptical effects based on responsive polymeric materials.

Plexcitonics: plasmon-exciton coupling for enhancing spectroscopy, optical chirality, and nonlinearity

Plexcitonics is a rapidly developing interdisciplinary field that holds immense potential for the creation of innovative optical technologies and devices. This field focuses on investigating the interactions between plasmons and excitons in hybrid systems. In this review, we provide an overview of the fundamental principles of plasmonics and plexcitonics and discuss the latest advancements in plexcitonics. Specifically, we highlight the ability to manipulate plasmon-exciton interactions, the emerging field of tip-enhanced spectroscopy, and advancements in optical chirality and nonlinearity. These recent developments have spurred further research in the field of plexcitonics and offer inspiration for the design of advanced materials and devices with enhanced optical properties and functionalities.

Enhanced Diffractive Circular Dichroism from Stereoscopic Plasmonic Molecule Array

Artificial nanostructures with large optical chiral responses have been intensively investigated recently. In this work, we propose a diffractive circular dichroism enhancement technique using stereoscopic plasmonic molecule structures. According to the multipole expansion analysis, the z-component of the electric dipole becomes the dominant chiral scattering mechanism during the interaction between an individual plasmonic molecule and the plane wave at a grazing angle. For a periodical structure with the designed plasmonic molecule, large diffractive circular dichroism can be obtained, which can be associated with the Wood-Rayleigh anomaly. Such a diffractive circular dichroism enhancement is verified by the good agreement between numerical simulations and experimental results. The proposed approach can be potentially used to develop enhanced spectroscopy techniques to measure chiral information, which is very important for fundamental physical and chemical research and bio-sensing applications.

Quasi-Bound States in the Continuum Enabled Strong Terahertz Chiroptical Response in Bilayer Metallic Metasurfaces

Bound state in the continuum (BIC) as a novel non-radiating state of light in the continuum of propagating modes has received great attention in photonics. Recently, chiral BICs have been introduced in the terahertz regime. However, strong chiroptical effects of transmitted waves remain challenging to achieve in metallic terahertz metasurfaces, especially for intrinsic chirality at normal incidences. Here, we propose a chiral quasi-BIC by simultaneously breaking the out-of-plane mirror and in-plane C2 rotation symmetries in a bilayer metallic metasurface, in which spin-selective terahertz transmittance is successfully realized. Benefiting from the symmetry-protected nature of our proposed BIC, precise tuning of structural parameters can lead to anticipated chiroptical performance. As a degree of freedom, the rotation angle of the split ring gaps can fully determine the handedness, linewidth, and working frequency with strong circular dichroism. Besides, the sensing performance shows a surrounding refractive index sensitivity of 200 GHz/RIU, which is similar to those of previous works based on terahertz metasurfaces. Taking advantage exclusively of symmetry-protected BICs to realize transmitted terahertz chiroptical response provides fresh insights into the creation of novel BICs, which enables profound advancements in the surging field of novel terahertz devices.

Flatband mode in photonic moiré superlattice for boosting second-harmonic generation with monolayer van der Waals crystals

We theoretically investigate boosting second-harmonic generation (SHG) of monolayer van der Waals crystals by employing flatband modes hosted by photonic moiré superlattices. Such a system with high quality factor and a monolayer crystal accommodated on the top of it, provides a unique opportunity to enhance and manipulate SHG emission. We show that employing a doubly resonant diagram on such a moiré superlattice system not only boosts the SHG, but also tunes the directional emission of the second-harmonic wave. Moreover, we demonstrate that a structured beam illumination could further boost SHG, with the phase structure retrieved through a two-beam second-harmonic interference configuration. These results suggest the flatband modes in moiré superlattice as a promising platform for boosting SHG with monolayer van der Waals crystals, offering new possibilities for developing compact nonlinear photonic devices.

Circular polarization analyzer based on surface plasmon polariton interference

The determination of chirality of circularly polarized light (CPL) is of great significance to the development of various optical techniques. In this paper, a miniature circular polarization analyzer (CPA) based on surface plasmon polariton (SPP) interference is proposed. The proposed CPA consists of a micron scale long sub-wavelength slit and two groups of spatially arranged periodic sub-wavelength rectangular groove pairs, which are etched in a metal layer. Under the illumination of a CPL with a given chirality, the proposed CPA is capable of forming SPP-mediated interference fringes with different periods in far field. The chirality of CPL can be directly and quantitatively differentiated by the frequency value of the far field SPP-mediated interference fringes. Different from the existing SPP-based CPAs, the proposed CPA can directly image the chirality information in far field, avoiding near-field imaging of the SPP field.

Steering Nonlinear Twisted Valley Photons of Monolayer WS2 by Vector Beams

Monolayer transition metal dichalcogenides have intrinsic spin-valley degrees of freedom, drawing broad interests due to their potential applications in information storage and processing. Here, we demonstrate the possibility of using cylindrical vector pumped beams, which are nonseparable in their polarization and spatial modes, to manipulate nonlinear valley-locked twisted-vortex emissions in monolayer tungsten disulfide (WS₂). The second-harmonic (SH) photons from K and K' valleys are encoded with opposite optical vortices, thus allowing the SH beams to emerge as cylindrical vector beams with doubled topological orders compared to the fundamental beams. The conically refracted pumped beams allow us to generate the first-order SH cylindrical vector and full Poincaré beams via tuning the valley-locked emitted light field profiles. With fanshaped WS₂ films breaking the axial symmetry of SH beams, the SH valley photons are routed to opposite directions. Our results pave the way to develop atomically thin nonlinear photonic devices and valleytronic nanodevices.

Tuning of Two-Dimensional Plasmon-Exciton Coupling in Full Parameter Space: A Polaritonic Non-Hermitian System

Non-Hermitian photonic systems with gains and/or losses have recently emerged as a powerful approach for topology-protected optical transport and novel device applications. To date, most of these systems employ coupled optical systems of diffraction-limited dielectric waveguides or microcavities, which exchange energy spatially or temporally. Here, we introduce a diffraction-unlimited approach using a plasmon-exciton coupling (polariton) system with tunable plasmonic resonance (energy and line width) and coupling strength. By designing a chirped silver nanogroove cavity array and coupling a single tungsten disulfide monolayer with a large contrast in resonance line width, we show the tuning capability through energy level anticrossing and plasmon-exciton hybridization (line width crossover), as well as spontaneous symmetry breaking across the exceptional point at zero detuning. This two-dimensional hybrid material system can be applied as a scalable and integratable platform for non-Hermitian photonics, featuring seamless integration of two-dimensional materials, broadband tuning, and operation at room temperature.

Second-harmonic generation enhancement in monolayer transition-metal dichalcogenides by using an epsilon-near-zero substrate

Monolayer transition-metal dichalcogenides (TMDCs) present high second-order optical nonlinearity, which is extremely desirable for, e.g., frequency conversion in nonlinear photonic devices. On the other hand, the atomic thickness of 2D materials naturally leads to low frequency converted intensities, highlighting the importance of designing structures that enhance the nonlinear response for practical applications. A number of methods to increase the pump electric field at 2D materials have been reported, relying on complex plasmonic and/or metasurface structures. Here, we take advantage of the fact that unstructured substrates with a low refractive index naturally maximize the pump field at a dielectric interface, offering a simple means to promote enhanced nonlinear optical effects. In particular, we measured second harmonic generation (SHG) in MoS₂ and WS₂ on fluorine tin oxide (FTO), which presents an epsilon-near zero point near our 1550 nm pump wavelength. Polarized SHG measurements reveal an SHG intensity that is one order of magnitude higher on FTO than on a glass substrate.

Low-loss aluminum epitaxial film for scalable and sustainable plasmonics: direct comparison with silver epitaxial film

Aluminum is a plasmonic material well known for its excellent stability, complementary metal-oxide-semiconductor compatibility and wide availability as compared to popular plasmonic materials such as gold and silver. Aluminum can support surface plasmon resonances in a broad spectral range, including the deep ultra-violet, a regime where no other plasmonic materials can work. However, conventional aluminum films suffer from high losses in the visible region and low fidelity and reproducibility in nanofabrication, making aluminum plasmonics non-ideal for applications. Herein, we report the experimental results of consistent surface plasmon propagation length measurements for epitaxially grown aluminum and silver films (epifilms), using three different methods (white light interferometry, laser scattering and spectroscopic ellipsometry) in the full visible spectrum. In order to avoid losses caused by inferior material quality, we used single-crystalline aluminum and silver films for direct comparison. We found that, on directly comparing with the silver epifilm, the aluminum epifilm possesses reasonably long plasmon propagation lengths in the full visible range and outperforms silver in the deep blue region. These results illustrate the great potential of epitaxial aluminum films for visible-spectrum plasmonic applications, resulting from their superior crystallinity and excellent surface and interface properties.

Deuterogenic Plasmonic Vortices

The optical vortex on a chip is of extreme importance for many applications in nanoscience, and as well-known, the chiral metallic nanostructures like plasmonic vortex lenses (PVLs) can produce a spin-dependent plasmonic vortex (PV) which is governed by plasmonic spin-orbit coupling. The well-established nanophotonic theory and various experimental demonstrations all show a single PV mode in one PVL, when the excitation is fixed. Here, counterintuitively, we report the existence of the nontrivial deuterogenic PVs, besides the one predicted previously. We theoretically reveal a general spin-to-orbit coupling and experimentally demonstrate the surprising existence of multiple PVs in a single PVL even when excited by a fixed circularly polarized vortex beam. This work provides a deeper fundamental understanding of the dynamics and the near-field spin-orbit coupling in nanophotonics, which promises to flexibly manipulate the PV for emerging optical vortex-based nanotechnologies and quantum optical applications on a chip.