Multiband and bidirectional multiplexing asymmetric optical transmission empowered by nanograting-coupled defective multilayer photonic crystal.

Asymmetric optical transmission (AOT) has been an enduring hot topic of interest in various fields, including optical communication, information processing, and so on. Particularly, the development of reciprocal micro-nanostructures achieving AOT further facilitates and accelerates the miniaturization and integration of traditional optical components. However, most of these optical components merely consider a single AOT band and transmission in a specified direction, limiting the development of their versatile functions. In this paper, we theoretically propose an all-dielectric metamaterial consisting of a nanograting and a defective multilayer photonic crystal, exhibiting multi-band and bidirectional multiplexing AOT. More specifically, the proposed metamaterial demonstrates both narrowband and wideband AOT for incidence from the nanograting to the photonic crystal, and a completely different narrowband AOT for the opposite incidence, namely, from the photonic crystal to the nanograting. These distinctive AOT spectral features are achieved by matching the diffraction effect of the nanograting with the special energy band of the defective multilayer photonic crystal. Remarkably, the device exhibits a transmittance difference of up to 0.974 and a contrast ratio of up to 0.997 (transmittance ratio of up to 673), with a transmission bandwidth of 62.7 nm for incident light with a wavelength of 624 nm illuminating from the nanograting to the defective multilayer photonic crystal. Furthermore, the bandwidth and number of transmission bands can be flexibly tuned by changing the polarization angle of the incident light, showcasing its excellent polarization multiplexing characteristics. The designed metamaterial provides an effective strategy for the realization of versatile AOT devices and is conducive to expanding the application scenarios of AOT devices.

Photoemission Enhancement of Plasmonic Hot Electrons by Au Antenna-Sensitizer Complexes.

The photoemission of surface plasmon decay-produced hot electrons is usually of very low efficiencies, hindering the practical utilization of such nonequilibrium charge carriers in harvesting photons with less energy than the semiconductor band gap for more efficient solar energy collection and photodetection. However, it has been demonstrated that the photoemission efficiency of small metal clusters increases as the particle size decreases. Recent studies have also shown that the photoemission efficiency of surface plasmon-yielded hot carriers can be intrinsically improved through proper material construction. In this paper, we report that the photoemission efficiency of hot electrons on the Au nanodisk-cluster complex/TiO2 interface can be dramatically enhanced under optical nanoantenna-sensitizer design. Such an enhancement is dominantly attributed to three factors. First, the large plasmonic nanodisk antennas provide a significantly enhanced optical near field, which largely increases light absorption in the small Au clusters that are acting as hot electron injection sensitizers. Second, the sub-3 nm size of the Au clusters facilitates the collection of delocalized spreading charges by the semiconductor. Third, the hybrid interface and molecule-like energy level of the Au cluster result in a much longer lifetime of excited electrons. Our results provide a promising approach for the effective harvesting of solar energy with plasmonic antenna-sensitizer complexes.

Asymmetric transmission devices empowered by a cascaded structure of a dielectric metasurface-photonic crystal.

In this Letter, we theoretically propose an all-dielectric quasi-three-dimensional subwavelength structure constructed by a dielectric metasurface cascaded with a multilayer photonic crystal (PC) to achieve a high-performance asymmetric optical transmission (AOT). The desired optical control of the AOT is realized by combining the predetermined anomalous beam steering of a phase gradient metasurface with a unique bandgap as well as transmission characteristics of the multilayered stacked PC. The simulated results demonstrate that the proposed AOT device operating at the center wavelength of 633 nm with a circularly polarized state exhibits a high transmission of up to 62.4% with a contrast ratio exceeding 606. The excellent performance of AOT is achieved by making disassembled transverse magnetic and transverse electric polarized light under the same deflection angle concurrently match with respective high-efficient transmission bands in the multilayer PC. Furthermore, dependence of the performance of the proposed device on structural dimensions is also explored. Fortunately, the designed AOT structure is applicable to any linearly polarized light but is accompanied by double diffraction channels as compared to the circularly polarized light case. Owing to its planar configuration, passive operation, and compelling performance under various polarization states, the proposed strategy for achieving AOT paves a new road for realizing high-performance optical metadevices in compact optical systems.

Plasmonic Resonance Coupling of Nanodisk Array/Thin Film on the Optical Fiber Tip for Integrated and Miniaturized Sensing Detection.

Fiber-optic surface plasmon resonance (FOSPR) sensing technology has become an appealing candidate in biochemical sensing applications due to its distinguished capability of remote and point-of-care detection. However, FOSPR sensing devices with a flat plasmonic film on the optical fiber tip are seldom proposed with most reports concentrating on fiber sidewalls. In this paper, we propose and experimentally demonstrate the plasmonic coupled structure of a gold (Au) nanodisk array and a thin film integrated into the fiber facet, enabling the excitation of the plasmon mode on the planar gold film by strong coupling. This plasmonic fiber sensor is fabricated by the ultraviolet (UV) curing adhesive transferring technology from a planar substrate to a fiber facet. The experimental results demonstrate that the fabricated sensing probe has a bulk refractive index sensitivity of 137.28 nm/RIU and exhibits moderate surface sensitivity by measuring the spatial localization of its excited plasmon mode on Au film by layer-by-layer self-assembly technology. Furthermore, the fabricated plasmonic sensing probe enables the detection of bovine serum albumin (BSA) biomolecule with a detection limit of 19.35 µM. The demonstrated fiber probe here provides a potential strategy to integrate plasmonic nanostructure on the fiber facet with excellent sensing performance, which has a unique application prospect in the detection of remote, in situ, and in vivo invasion.

SERS Monitored Kinetic Process of Gaseous Thiophenol Compound in Plasmonic MOF Nanoparticles.

Benefiting from the electromagnetic enhancement of noble metal nanoparticles (NPs) and the capture ability of organic frameworks, plasmonic metal-organic framework (MOF) structures have greatly promoted the development of gas detection by surface-enhanced Raman spectroscopy (SERS). In those detections, the kinetic process of gaseous molecules in plasmonic-MOF structures has a great influence on SERS spectra, which is still lacking intensive investigation in previous reports. In this work, the kinetic processes of gaseous thiophenol compounds (TPC) in the plasmonic Zeolitic Imidazolate Framework (Ag@ZIF) core-shell NPs are studied by SERS spectra. The experimental data demonstrate that the SERS intensities of gaseous TPC could be enhanced once more in an H2 mixed gas environment with different functional groups of TPC. Further results reveal that the two-step enhancement of SERS intensities is not only related to the thicknesses of the MOF shell but also affected by the ambient mixed gas. To understand this novel phenomenon, the binding energy between the gaseous molecule and ZIF is calculated based on first-principles computation. In combination with the plasmonic properties of the Ag core, a molecular collision model is introduced here to show the distribution of gaseous TPC molecules in ZIF, which could be responsible for this interesting two-step enhancement of SERS intensities. Furthermore, the H2 assisted kinetic process of gaseous p-aminothiophenol (PATP) is also analyzed by the classical pseudo-first-order kinetic model, which is consistent with our experimental SERS data. Our work not only reveals the novel phenomenon of plasmonic-MOF structures to improve the gas detection by SERS spectra but also enriches the understanding of the microcosmic process of gaseous molecules in the mixed gas environment to optimize MOF structures for gas capture and storage.

Robust Heterostructures in Two-Dimensional Perovskites by Threshold-Dominating Anion Exchange.

Heterostructures play an irreplaceable role in high-performance optoelectronic devices. However, the preparation of robust perovskite heterostructures is challenging due to spontaneous interdiffusion of halogen anions. Herein, a vapor-phase anion exchange method universally suitable for the preparation of robust 2D Ruddlesden-Popper perovskite (RPP) heterostructures is developed. A variety of heterostructures are fabricated based on exfoliated RPP microplates (MPs). Depending on the specific organic cations, the heterostructures can be either sharp and uniform, or broad and gradient, suggesting a new anion diffusion behavior different from that in 3D perovskites. Further experimental studies reveal that the lateral transport of anions follows a threshold-dominating mechanism, while the vertical transport can be partially or completely suppressed by organic cations. Subsequently, quantitative investigation of anion diffusion in 2D perovskites is conducted. The lateral diffusion coefficient of halogen anions is calculated to be 6 to 7 orders of magnitude larger than the vertical coefficient, consistent with the observed highly anisotropic anion diffusion. In addition, it is shown that the anion exchange threshold can also enhance the thermodynamic stability of the heterostructures at elevated temperature. These results provide a general method to fabricate robust lateral RPP heterostructures, and offer important insights into anion behavior in low-dimensional perovskites.

Anion-Exchange Driven Phase Transition in CsPbI3 Nanowires for Fabricating Epitaxial Perovskite Heterojunctions.

Anion-exchange in halide perovskites provides a unique pathway of bandgap engineering for fabricating heterojunctions in low-cost photovoltaics and optoelectronics. However, it remains challenging to achieve robust and sharp perovskite heterojunctions, due to the spontaneous anion interdiffusion across the heterojunction in 3D perovskites. Here, it is shown that the anionic behavior in 1D perovskites is fundamentally different, that the anion exchange can readily drive an indirect-to-direct bandgap phase transition in CsPbI3 nanowires (NWs) and greatly lower the phase transition temperature. In addition, the heterojunction created by phase transition is epitaxial in nature, and its chemical composition can be precisely controlled upon postannealing. Further study of the phase transition dynamics reveals a threshold-dominating anion exchange mechanism in these 1D NWs rather than the gradient-dominating mechanism in 3D systems. The results provide important insights into the ionic behavior in halide perovskites, which is beneficial for applications in solar cells, light-emitting diodes (LEDs), and other semiconductor devices.

Strong confinement of gap modes induced by the film modified electric and magnetic modes in dielectric nanoparticle dimers.

Nanogaps are one of the most useful systems in nanooptics. The gap modes in a film coupled dielectric nanoparticle dimer system are influenced by both of the film and the electric and magnetic modes of the particles. In this work, strong confinement of gap modes of dielectric (Si) nanoparticle dimer on Au/Si film is investigated. The results show an abnormal electric field enhancement obtained between Si nanoparticle dimer on metal film, which is attributed to film coupled electric and magnetic dipole modes in dielectric nanoparticle dimer. The results are further analyzed with mode hybridization theory. Furthermore, the surface enhanced Raman spectroscopy (SERS) is performed to demonstrate these theoretical analyses. The film induced electromagnetic field redistribution in dielectric nanoparticle dimer not only extend the knowledge of dielectric gap modes but also has tremendous applications, e.g. light manipulating in subwavelength, light harvest, surface enhanced spectrum, etc.

Self-Constructed Multiple Plasmonic Hotspots on an Individual Fractal to Amplify Broadband Hot Electron Generation.

Plasmonic nanoparticles are ideal candidates for hot-electron-assisted applications, but their narrow resonance region and limited hotspot number hindered the energy utilization of broadband solar energy. Inspired by tree branches, we designed and chemically synthesized silver fractals, which enable self-constructed hotspots and multiple plasmonic resonances, extending the broadband generation of hot electrons for better matching with the solar radiation spectrum. We directly revealed the plasmonic origin, the spatial distribution, and the decay dynamics of hot electrons on the single-particle level by using ab initio simulation, dark-field spectroscopy, pump-probe measurements, and electron energy loss spectroscopy. Our results show that fractals with acute tips and narrow gaps can support broadband resonances (400-1100 nm) and a large number of randomly distributed hotspots, which can provide unpolarized enhanced near field and promote hot electron generation. As a proof-of-concept, hot-electron-triggered dimerization of p-nitropthiophenol and hydrogen production are investigated under various irradiations, and the promoted hot electron generation on fractals was confirmed with significantly improved efficiency.

Chiral surface plasmon-enhanced chiral spectroscopy: principles and applications.

In this review, the development context and scientific research results of chiral surface plasmons (SPs) in recent years are classified and described in detail. First, the principle of chiral SPs is introduced through classical and quantum theory. Following this, the classification and properties of different chiral structures, as well as the superchiral near-field, are introduced in detail. Second, we describe the excitation and propagation properties of chiral SPs, which lays a good foundation for the application of chiral SPs and their chiral spectra in various fields. After that, we have summarized the recent research results of chiral SPs and their applications in the areas of biology, two-dimensional materials, topological materials, analytical chemistry, chiral sensing, chiral optical force, and chiral light detection. Chiral SPs are a new type of optical phenomenon that have useful application potential in many fields and are worth exploring.

Electrical tuning effect for Schottky barrier and hot-electron harvest in a plasmonic Au/TiO2 nanostructure.

Schottky barrier controls the transfer of hot carriers between contacted metal and semiconductor, and decides the performance of plasmonic metal-semiconductor devices in many applications. It is immensely valuable to actively tune the Schottky barrier. In this work, electrical tuning of Schottky barrier in an Au-nanodisk/TiO2-film structure was demonstrated using a simple three-electrode electrochemical cell. Photocurrents excited at different wavelength significantly increase as the applied bias voltage increases. Analyzing and fitting of experimental results indicate that the photocurrent is mainly affected by the bias tuning position of Schottky barrier maximum, which shifts to metal-semiconductor interface as applied voltage increases, and enhances the collection efficiency of the barrier for plasmonic hot electrons. The conduction band curvature of 0.13 eV was simultaneously obtained from the fitting. This work provides a new strategy for facile tuning of Schottky barrier and hot-electron transfer across the barrier.

Hot-carrier generation from plasmons in an antenna-spacer-mirror nanostructure.

By introducing Au-nanodisk antennas, we conveniently got hot carriers from decay of surface plasmons (SPs) on planar interface in an Au-antennas/TiO2-spacer/Au-mirror (ASM) structure without an additional phase-matching process for SP generation. The presence of hot carriers from SPs is distinguished by opposite photocurrents compared with a similar structure without an Au mirror. Analyzed by extinction spectra and electrodynamics simulations, reflection between an Au nanodisk layer and an Au mirror induces an optical response of cavity mode, which excites SPs on an Au-mirror interface and significantly enhances the light harvesting, thus leading to a relatively high hot-carrier density from SP decay. The peak of incident photon-to-electron conversion efficiencies at different wavelength also well matches the optical response of the structure.

Fano-like chiroptical response in plasmonic heterodimer nanostructures.

Plasmonic chirality has attracted more and more attention recently due to the enhanced chiroptical response and its potential applications in biosensing. Plasmonic Fano resonance arises from the interference between a dark narrow resonance and a bright broad resonance, and it provides a new paradigm to control the plasmon mode interactions. Even though a strong circular dichroism (CD) effect has been predicted in chiral nanostructures with a Fano resonance, there are few experimental studies, and the correlation between the two effects is unclear. In this research, we investigate these two effects in plasmonic heterodimer nanorods in the same spectral range. We find that the heterodimer nanostructure exhibits a Fano-like resonance and Fano-like chiroptical response, both of which are correlated with the coupling between a super-radiant electric dipole and a sub-radiant magnetic dipole mode. Due to the interference nature of the Fano resonance, the Fano-like chiroptical response exhibits distinctively sharp features in a narrow spectral range. This Fano-like chiroptical response can be explained by a modified chiral molecule theory and a simplified coupled electric-magnetic dipole model. This research may provide new insight into the physics picture of plasmonic chirality and paves the way for the development of sensitive plasmonic sensors.

Shape-engineered silver nanocones for refractive index plasmonic nanosensors.

Silver nanocones with tunable plasmon resonances and high refractive index (RI) sensitivity have attracted much attention. Herein, through systematic measuring of the RI sensitivities of silver nanocones with different geometric parameters, the size and shape effects are investigated. The results show that RI sensitivities increase as silver nanocones become longer and the widths of their heads become smaller. Through engineering of the outline symmetry, the silver nanocones exhibit RI sensitivity as high as 910 nm/RIU (RI unit) and the figure of merit arrives at 3.8.

Near-Infrared-Plasmonic Energy Upconversion in a Nonmetallic Heterostructure for Efficient H2 Evolution from Ammonia Borane.

Plasmonic metal nanostructures have been widely used to enhance the upconversion efficiency of the near-infrared (NIR) photons into the visible region via the localized surface plasmon resonance (LSPR) effect. However, the direct utilization of low-cost nonmetallic semiconductors to both concentrate and transfer the NIR-plasmonic energy in the upconversion system remains a significant challenge. Here, a fascinating process of NIR-plasmonic energy upconversion in Yb3+/Er3+-doped NaYF4 nanoparticles (NaYF4:Yb-Er NPs)/W18O49 nanowires (NWs) heterostructures, which can selectively enhance the upconversion luminescence by two orders of magnitude, is demonstrated. Combined with theoretical calculations, it is proposed that the NIR-excited LSPR of W18O49 NWs is the primary reason for the enhanced upconversion luminescence of NaYF4:Yb-Er NPs. Meanwhile, this plasmon-enhanced upconversion luminescence can be partly absorbed by the W18O49 NWs to re-excite its higher energy LSPR, thus leading to the selective enhancement of upconversion luminescence for the NaYF4:Yb-Er/W18O49 heterostructures. More importantly, based on this process of plasmonic energy transfer, an NIR-driven catalyst of NaYF4:Yb-Er NPs@W18O49 NWs quasi-core/shell heterostructure, which exhibits a ≈35-fold increase in the catalytic H2 evolution from ammonia borane (BH3NH3) is designed and synthesized. This work provides insight on the development of nonmetallic plasmon-sensitized optical materials that can potentially be applied in photocatalysis, optoelectronic, and photovoltaic devices.

Electromagnetic Energy Redistribution in Coupled Chiral Particle Chain-Film System.

Metal nanoparticle-film system has been proved that it has the ability of focusing light in the gap between particle and film, which is useful for surface-enhanced Raman scattering and plasmon catalysis. The rapid developed plasmonic chirality can also be realized in such system. Here, we investigated an electromagnetic energy focusing effect and chiral near-field enhancement in a coupled chiral particle chain on gold film. It shows large electric field enhancement in the gap between particle and film, as well as chiral near field. The enhancement properties at resonant peaks for the system excited by left circularly polarized light and right circularly polarized light are obviously different. This difference resulted from the interaction of circularly polarized light and the chiral particle-film system is analyzed with plasmon hybridization. The enhanced optical activity can provide promising applications for the enhancement of chiral molecule sensor for this chiral particle chain-film system.

Strong up-conversion luminescence of rare-earth doped oxide films enhanced by gap modes on ZnO nanowires.

Up-conversion luminescence (UCL) from rare-earth doped oxide (RE) films has great potential for application in fields such as solar cells, bioanalysis, or display technologies. However, the relatively high phonon energy of oxide matrices usually facilitates nonradiative relaxation leading to low UCL efficiency. Herein, we report a three-layer hierarchical structure of Ag/ZnO nanowires (nw-ZnO)/RE composite films, which enhances the UCL of rare-earth doped oxide films. An optimization of the geometric structure of the composite films demonstrated an increase of UCL by up to almost two orders of magnitude in Er3+ and Tm3+ doped YbMoO4 films. This UCL enhancement is attributed to the formation of a very strong electric field in the tips of the nw-ZnO creating a highly effective electric field at the composite films, combined with reflection at the silver layer. Furthermore, we use the UCL properties of these novel Ag/nw-ZnO/RE composite films to demonstrate their possible use in ZnO-based photocatalytic processes to enhance the utilization of near-infrared sunlight in these devices.

Searching the Theoretical Ultimate Limits of Probing Surface-Enhanced Raman Optical Activity.

The single-molecule Raman detection has been realized for a long time because of the enhancement effect of surface plasmons. However, the small cross section of Raman optical activity (ROA) makes it so hard to detect the ROA of even a few molecules; and a normal surface-enhanced ROA (SE-ROA) is also very time consumable even with strong laser power. Detecting ROA in an economic way is an important issue. In this paper, we discuss the ultimate limit of the SE-ROA and provide the enhancement factor formula for SE-ROA. Following the formula, we proposed a structure with both huge Raman enhancement and ROA enhancement, which is helpful for single-molecule ROA detection.

Analyzing intrinsic plasmonic chirality by tracking the interplay of electric and magnetic dipole modes.

Plasmonic chirality represents significant potential for novel nanooptical devices due to its association with strong chiroptical responses. Previous reports on plasmonic chirality mechanism mainly focus on phase retardation and coupling. In this paper, we propose a model similar to the chiral molecules for explaining the intrinsic plasmonic chirality mechanism of varies 3D chiral structures quantitatively based on the interplay and mixing of electric and magnetic dipole modes (directly from electromagnetic field numerical simulations), which forms mixed electric and magnetic polarizability.

A Nonmetal Plasmonic Z-Scheme Photocatalyst with UV- to NIR-Driven Photocatalytic Protons Reduction.

Ultrabroad-spectrum absorption and highly efficient generation of available charge carriers are two essential requirements for promising semiconductor-based photocatalysts, towards achieving the ultimate goal of solar-to-fuel conversion. Here, a fascinating nonmetal plasmonic Z-scheme photocatalyst with the W18 O49 /g-C3 N4 heterostructure is reported, which can effectively harvest photon energies spanning from the UV to the nearinfrared region and simultaneously possesses improved charge-carrier dynamics to boost the generation of long-lived active electrons for the photocatalytic reduction of protons into H2 . By combining with theoretical simulations, a unique synergistic photocatalysis effect between the semiconductive Z-scheme charge-carrier separation and metal-like localized-surface-plasmon-resonance-induced "hot electrons" injection process is demonstrated within this binary heterostructure.