Optical Spin Waves.

Chirality is inherent to a broad range of systems, including solid-state and wave physics. The precession (chiral motion) of the magnetic moments in magnetic materials, forming spin waves, has various properties and many applications in magnetism and spintronics. We show that an optical analogue of spin waves can be generated in arrays of plasmonic nanohelices. Such optical waves arise from the interaction between twisted helix eigenmodes carrying spin and orbital angular momenta. We demonstrate that these optical spin waves are reflected at the interface between successive domains of enantiomeric nanohelices, forming a heterochiral lattice regardless of the wave propagation direction within the lattice. Optical spin waves may be applied in techniques involving photon spin, ranging from data processing and storage to quantum optics.

Controllable Chiral Light Generation and Vortex Field Investigation Using Plasmonic Holes Revealed by Cathodoluminescence.

Control of the angular momentum of light is a key technology for next-generation nano-optical devices and optical communications, including quantum communication and encoding. We propose an approach to controllably generate circularly polarized light from a circular hole in a metal film using an electron beam by coherently exciting transition radiation and light scattering from the hole through surface plasmon polaritons. The circularly polarized light generation is confirmed by fully polarimetric four-dimensional cathodoluminescence mapping, where angle-resolved spectra are simultaneously obtained. The obtained intensity and Stokes maps show clear interference fringes as well as almost fully circularly polarized light generation with controllable parities by the electron beam position. By applying this approach to a three-hole system, a vortex field with a phase singularity is visualized in the middle of three holes.

Coherent random fiber laser emission from CdSe/ZnS quantum dots.

We experimentally demonstrate the coherent random laser emission by combining CdSe/ZnS colloidal quantum dots with hollow optical fiber. Through the localized surface plasmon resonance induced by Ag nanoparticles (NPs), well-distinguished discrete spikes are observed from the Ag modified hollow fiber loaded with CdSe/ZnS QDs solution. In addition, coherent random laser action with low threshold is also realized in the hollow optical fiber filled with high packing-density CdSe/ZnS QDs even if the Ag NPs is not introduced. The self-assembled clusters of CdSe/ZnS QDs serve as the optical gain media as well as the strong scattering centers. The angle measurement experiments show that the directional emission of random laser can be adjusted by using different pumping manners. This facile, inexpensive, low pump threshold random laser could be widely used in photonic devices and display imaging.

Ti3C2TxMXene as surface-enhanced Raman scattering substrate.

The electromagnetic field enhancement mechanisms leading to surface-enhanced Raman scattering (SERS) of R6G molecules near Ti3C2TxMXene flakes of different shapes and sizes are analyzed theoretically in this paper. In COMSOL simulations for the enhancement factor (EF) of SERS, the dye molecule is modeled as a small sphere with polarizability spectrum based on experimental data. It is demonstrated, for the first time, that in the wavelength range of500 nm-1000 nm, the enhancement of Raman signals is largely conditioned by quadrupole surface plasmon (QSP) oscillations that induce a strong polarization of the MXene substrate. We show that the vis-NIR spectral range quadrupole SP resonances are strengthened due to interband transitions (IBTs), which provide EF values of the order of 105-107in agreement with experimental data. The weak sensitivity of the EF to the shape and size of MXene nanoparticles (NPs) is interpreted as a consequence of the low dependence of the absorption cross-section of QSP oscillations and IBT on the geometry of the flakes. This reveals a new feature: the independence of EF on the geometry of MXene substrates, which allows to avoid the monitoring of the shape and size of flakes during their synthesis. Thus, MXene flakes can be advantageous for the easy manufacturing of universal substrates for SERS applications. The electromagnetic SERS enhancement is determined by the 'lightning rod' and 'hot-spot' effects due to the partial overlapping of the absorption spectrum of the R6G molecule with these MXene resonances.

Study of Photophysical Properties of Thiol-capped CdS Quantum Dots Doped with Gold Nanoparticles.

In this study, CdS quantum dots (QDs) capped with benzyl mercaptan (thiol) were prepared by microwave irradiation technique. The shape, size, morphology, and spectral properties of thiol-capped CdS QDs were characterized with transmission electron microscopy (TEM), scanning electron microscopy (SEM), UV-vis absorption, and photoluminescence (PL) spectrometry. The photophysical properties of synthesized thiol-capped CdS QDs in the presence of different amounts of gold nanoparticles (AuNPs) have been investigated, exhibiting strong PL quenching. The amount of fluorescence quenching was found to be dependent on the concentration of metal nanoparticles. A Stern-Volmer kinetics model was used to analyze the observed quenching mechanism as a function of the quencher (AuNPs) concentration. The Stern-Volmer plot along with the absorption spectra of thiol-capped CdS QDs in the absence and presence of AuNPs suggest the dynamic (collision) nature of quenching and rule out the possibility of static quenching. The energy transfers from QDs to Au NPs, and hence the quenching of QDs emissions provides new insight into designing novel optical-based materials and the development of FRET-based bionanosensors and phototherapeutic applications.

Metallic Heterostructures for Plasmon-Enhanced Electrocatalysis.

Metallic heterogeneous nanostructures with plasmonic functionality have attracted great attention in the field of plasmon-enhanced electrocatalysis, where surface plasmons produced under light excitation could facilitate the overall electrocatalytic performances. Owing to their controllability, multifunctionality, and complexity, heterogeneous metallic nanostructures take advantages of the properties from individual components and synergistic effects from adjacent components, thus may achieve remarkable electrocatalytic performances. This review highlights the state-of-the-art progress of the application of metallic heterostructures for plasmon-enhanced electrocatalysis. First, a brief introduction to plasmonic heterogeneous nanostructures is demonstrated. Then, fundamental principles of localized surface plasmon resonance and the underlying mechanisms of plasmonic heterogeneous nanostructures in catalysis are discussed. This is followed by a discussion of recent advances of plasmonic heterogeneous nanostructures in plasmon-enhanced electrocatalysis, in which the enhanced activity, selectivity, and stability are particularly emphasized. Finally, an outlook of remaining challenges and future opportunities for plasmonic heterogeneous nanomaterials and plasmon-related electrocatalysis is presented.

Directional emission of nanoscale chiral sources modified by gap plasmons.

Efficient manipulation of the emission direction of a chiral nanoscale light source is significant for information transmission and on-chip information processing. Here, we propose a scheme to control the directionality of nanoscale chiral light sources based on gap plasmons. The gap plasmon mode formed by a gold nanorod and a silver nanowire realizes the highly directional emission of chiral light sources. Based on the optical spin-locked light propagation, the hybrid structure enables the directional coupling of chiral emission to achieve a contrast ratio of 99.5%. The emission direction can be manipulated by tailoring the configuration of the structure, such as the positions, aspect ratios, and orientation of the nanorod. Besides, a great local field enhancement exists for highly enhanced emission rates within the nanogap. This chiral nanoscale light source manipulation scheme provides a way for chiral valleytronics and integrated photonics.

Quasicylindrical Waves for Ordered Nanostructuring.

Laser-induced self-organization of periodic nanostructures on highly absorbing materials is widely understood to be due to interference between laser and surface plasmon polaritons (SPPs) that are excited by initial surface roughness. The structure order naturally emerges from the propagation phase of SPPs. Here, we reveal an unexplored mechanism that is predominantly induced by quasicylindrical waves (QCWs) with negligible contributions from SPPs. This mechanism features a new principle of order emergence in growth of periodic nanostructures through short-range electromagnetic interactions between QCWs and marginal nanofringes. In this scenario, the periodicity of nanostructures is not simply determined by the electromagnetic wavelength. With suppressed long-range interactions, the formation of nanostructures shows a domino-like growth process, thus significantly improving structure uniformity. An in situ microscopic observation is performed to characterize the temporal dynamics of structural growth and verify the new mechanism. Further, the QCWs are directly observed in experiments, which are theoretically supported by a scattering model.

Uncovering the Evolution of Low-Energy Plasmons in Nanopatterned Aluminum Plasmonics on Graphene.

We report adjusting the charge-transfer-plasmon (CTP) resonances of aluminum (Al) bowties on suspended monolayer graphene via controlled nanofabrication and focused electron-beam irradiation. CTP resonances of bowties with a conductive junction blue-shift with an increase in junction width, whereas their 3λ/2 and λ resonances barely red-shift. These plasmon modes are derived and confirmed by an LC circuit model and electromagnetic simulations performed with boundary-element and frequency-domain methods. A monotonic decay of the CTP lifetime is observed, while the junction width is extended. Instead, the lifetimes of 3λ/2 and λ resonances are nearly independent of junction width. When the junction is shrunk by electron-beam irradiation, all antenna resonances red-shift. Having created an electron-beam-induced sub 5 nm gap in bowties, we monitor the unambiguous transition of a CTP into a bonding-type gap mode, which is highly sensitive to the separation distance. Meanwhile, the 3λ/2 and λ resonances evolve into dipolar bright and dipolar dark modes.

Hydrogel-Based, Dynamically Tunable Plasmonic Metasurfaces with Nanoscale Resolution.

Flat metasurfaces with subwavelength meta-atoms can be designed to manipulate the electromagnetic parameters of incident light and enable unusual light-matter interactions. Although hydrogel-based metasurfaces have the potential to control optical properties dynamically in response to environmental conditions, the pattern resolution of these surfaces has been limited to microscale features or larger, limiting capabilities at the nanoscale, and precluding effective use in metamaterials. This paper reports a general approach to developing tunable plasmonic metasurfaces with hydrogel meta-atoms at the subwavelength scale. Periodic arrays of hydrogel nanodots with continuously tunable diameters are fabricated on silver substrates, resulting in humidity-responsive surface plasmon polaritons (SPPs) at the nanostructure-metal interfaces. The peaks of the SPPs are controlled reversibly by absorbing or releasing water within the hydrogel matrix, the matrix-generated plasmonic color rendering in the visible spectrum. This work demonstrates that metasurfaces designed with these spatially patterned nanodots of varying sizes benefit applications in anti-counterfeiting and generate multicolored displays with single-nanodot resolution. Furthermore, this work shows system versatility exhibited by broadband beam-steering on a phase modulator consisting of hydrogel supercell units in which the size variations of constituent hydrogel nanostructures engineer the wavefront of reflected light from the metasurface.