Tunable dielectric metasurfaces by structuring the phase-change material.

Metasurfaces have made great progress in the last decade for generating miniature and integrated optical devices. The optical properties of metasurfaces can be tuned dynamically by integrating with phase-change materials. However, the efficiency of tunable metasurfaces remains a bit low, which is a disadvantage for the realistic applications of metasurfaces. Here, we demonstrate the tunable dielectric metasurfaces by structuring the phase-change material Ge2Sb2Te5. The unit cell of metasurface is composed of several Ge2Sb2Te5 nanopillars with different geometric parameters, and the incident light interacts with different nanopillars at diverse phases of Ge2Sb2Te5, leading to various functions. By elaborately arranging the Ge2Sb2Te5 nanopillars, various tunable optical devices have been realized, including tunable beam steering, reconfigurable metalens and switchable wave plate. The refractive direction, focal length and polarization state can be tuned through the phase transition of Ge2Sb2Te5. The phase-change metasurfaces based on Ge2Sb2Te5 nanostructures could be used in cameras, optical microscopy and adaptive optics.

Switchable transmissive and reflective metadevices based on the phase transition of vanadium dioxide.

Metasurfaces have made great progress in the past decade in generating various planar optical devices. However, most metasurfaces exhibit their functions in either reflection mode or transmission mode, with the other mode unutilized. In this work, we demonstrate switchable transmissive and reflective metadevices by combining metasurfaces with vanadium dioxide. The composite metasurface can work as a transmissive metadevice, with one function for vanadium dioxide in the insulating phase, and is changed to a reflective metadevice with another function for vanadium dioxide in the metallic phase. By carefully designing the structures, the metasurface can be switched from a transmissive metalens to a reflective vortex generator, or between a transmissive beam steering and a reflective quarter-wave plate through the phase transition of vanadium dioxide. The switchable transmissive and reflective metadevices have potential applications in imaging, communication, and information processing.

Microfiber sensor probe integrated with a cascaded Fabry-Perot interferometer.

Developing micrometer-nanometer size optical fiber sensors has promising application prospects in microenvironments, such as biological cells, micro robots, and microfluids. We propose a new strategy to fabricate a microfiber sensor probe (MSP). A femtosecond laser was applied to integrate cascaded Fabry-Perot interferometers (FPIs) into a silica microfiber. And a MSP with diameter of ∼8µm, extinction ratio of 15 dB, fitness of 24.6, and Q-factor of 2310 was demonstrated in the experiment. In addition, the MSP was applied for the refractive index and thermal measurement and the sensitivity was observed to be 10 pm/°C and 18.5 nm/RIU. The two-beam approximation model was applied to analyze the spectrum, and simulations were taken to research the refractive index sensitivity influenced by the fiber size.

Pure magnetic-quadrupole scattering and efficient second-harmonic generation from plasmon-dielectric hybrid nano-antennas.

We theoretically demonstrate that pure magnetic quadrupole (MQ) scattering is achieved via the excitation of anapole modes and Fano resonance in noble metal (Au or Ag) and high refractive index dielectric (AlGaAs) hybrid nano-antennas. In Au-AlGaAs hybrid nano-antennas, electric anapole and magnetic anapole modes are observed, leading to the suppressions of electric and magnetic dipoles. Introducing gain material to AlGaAs nanodisk to increase the strength of electric quadrupole (EQ) Fano resonance leads to the suppression of EQ scattering. Then, ideal MQ scattering is achieved at the wavelength of total scattering cross-section dip. The increase of signal-to-noise ratio of MQ results in the great enhancement of near-field inside AlGaAs nanodisk. Additionally, the strong MQ resonance exhibits great capability for boosting second-harmonic generation by proper mode matching. These findings achieved in subwavelength geometries have important implications for functional metamaterials and nonlinear photonic nanodevices.

Coupling Resonances of Surface Plasmon in Gold Nanorod/Copper Chalcogenide Core-Shell Nanostructures and Their Enhanced Photothermal Effect.

Dual plasmonic Au@Cu2-x S core-shell nanorods (NRs) have been fabricated by using a hydrothermal method and plasmon-coupled effect between the Au core and Cu2-x S shell in the near-infrared (NIR) region. The extinction spectrum of Au@Cu2-x S NRs is dominated by the surface plasmon resonance (SPR) of the Cu2-x S shell, the transverse surface plasmon resonance (TSPR), and the longitudinal surface plasmon resonance (LSPR) of the Au NRs. With the Cu2-x S shell increasing (fixed Au NRs), the TSPR peak slightly redshifts and the LSPR and SPR peaks blueshift, owing to competition between the redshift of the refractive index effect and blueshift from the plasmon coupled effect. Although, for Au@Cu2 S NRs, only TSPR and LSPR peaks can be seen and a redshift arises with the increasing Cu2 S shell thickness, implying that no plasmonic coupling between Au NRs and Cu2 S shell occurred. The extinction spectrum of the Au@Cu2-x S NRs with three coupled resonance peaks is simulated by using the FDTD method, taking into account the electron-transfer effect. The dispersion properties of the coupling of Au@Cu2-x S NRs with the LSPR of the initial Au core are studied experimentally by changing the length of the Au NRs, which are explained theoretically by the coupled harmonic oscillator model. The calculated coupled coefficients between SPR of the Cu2-x S shell and LSPR of the Au NRs is 180 meV, which is much stronger than that of TSPR of Au NRs of 55 meV. Finally, the enhanced photothermal effect of Au@Cu2-x S NRs has been demonstrated.

High-Q Quasi-Bound States in the Continuum in Terahertz All-Silicon Metasurfaces.

Bound states in the continuum (BIC)-based all-silicon metasurfaces have attracted widespread attention in recent years because of their high quality (Q) factors in terahertz (THz) frequencies. Here, we propose and experimentally demonstrate an all-silicon BIC metasurface consisting of an air-hole array on a Si substrate. BICs originated from low-order TE and TM guided mode resonances (GMRs) induced by (1,0) and (1,1) Rayleigh diffraction of metagratings, which were numerically investigated. The results indicate that the GMRs and their Q-factors are easily excited and manipulated by breaking the lattice symmetry through changes in the position or radius of the air-holes, while the resonance frequencies are less sensitive to these changes. The measured Q-factor of the GMRs is as high as 490. The high-Q metasurfaces have potential applications in THz modulators, biosensors, and other photonic devices.

Ultra-Broadband, Omnidirectional, High-Efficiency Metamaterial Absorber for Capturing Solar Energy.

In this study, we investigated an absorber based on a center-aligned tandem nanopillar array for ultra-broadband solar energy harvesting theoretically. A high-efficiency, omnidirectional absorber was obtained by introducing the center-aligned tandem nanopillar array embedded in an Al2O3 dielectric layer. The multi-coupling modes at different wavelengths were interpreted. The strong absorption can be adjusted by changing the radii and heights of nanopillars. According to the simulation results, the average absorptance of the absorber exceeded 94% in the wavelength range from 300 nm to 2000 nm. In addition, the high-efficiency absorption was insensitive to the incident angle and polarization state. The research not only proposed an absorber which possesses a huge potential value for application areas, such as thermal photovoltaic systems, infrared detection, and isotropic absorption sensors, but also pointed out a new way to design an absorber with high efficiency in an ultrabroad wavelength range.

Analogue of Electromagnetically Induced Transparency in an All-Dielectric Double-Layer Metasurface Based on Bound States in the Continuum.

Bound states in the continuum (BICs) have attracted much attention due to their infinite Q factor. However, the realization of the analogue of electromagnetically induced transparency (EIT) by near-field coupling with a dark BIC in metasurfaces remains challenging. Here, we propose and numerically demonstrate the realization of a high-quality factor EIT by the coupling of a bright electric dipole resonance and a dark toroidal dipole BIC in an all-dielectric double-layer metasurface. Thanks to the designed unique one-dimensional (D)-two-dimensional (2D) combination of the double-layer metasurface, the sensitivity of the EIT to the relative displacement between the two layer-structures is greatly reduced. Moreover, several designs for widely tunable EIT are proposed and discussed. We believe the proposed double-layer metasurface opens a new avenue for implementing BIC-based EIT with potential applications in filtering, sensing and other photonic devices.

Total Aqueous Synthesis of Au@Cu2-x S Core-Shell Nanoparticles for In Vitro and In Vivo SERS/PA Imaging-Guided Photothermal Cancer Therapy.

Both accurate tumor navigation and nanostructures with high photothermal (PT) conversion efficiency are important but remain challenging to achieve in current biomedical applications. This study reports an anion exchange-based facile and green approach for synthesizing Au@Cu2-x S core-shell nanoparticles (NPs) in an aqueous system. In addition to the PT effect of the suggested NPs, the surface-enhanced Raman scattering (SERS) is also significantly improved due to the tailored localized surface plasmon resonance coupling between the Au metal core and the Cu2-x S semiconductor shell. Using an epitaxial strategy, Au@Cu2 O NPs are first obtained by the in situ reduction of cupric hydroxide on a cresyl violet acetate-coated Au core; then, Au@Cu2-x S NPs are obtained via anion exchange between the S2- and Cu2 O shell. Both the Cu/S atomic ratio and the Cu2-x S shell thickness can be adjusted conveniently. Hence, the ideal integration of the plasmonic Au core and Cu2-x S shell into a single unit is conducive not only to highly efficient PT conversion but also to the construction of a SERS-based navigator. This new type of SERS-guided NP, with enhanced photoacoustic signals, is an important candidate for both accurate tumor navigation and nondestructive PT treatment guided in vivo by two modes of optical imaging.

Controlled growth and optical response of a semi-hollow plasmonic nanocavity and ultrathin sulfide nanosheets on Au/Ag platelets.

Herein, we report a strategy to construct a semi-hollow plasmonic nanocavity and grow ultrathin sulfide nanosheets inside. The competition and cooperation of Au deposition with Ag etching based on flat Ag nanoplates are proposed. For the establishment of the semi-hollow nanocavity, Au shells are grown on Ag nanoplates, which serve as a stable frame, followed by partial etching of the Ag nanoplates. By controlling the thickness of the initial Ag nanoplates or the injected amount of etchant, the nanocavity size is fine-tuned. Significantly, the remaining unetched Ag layers provide a flat platform for the growth of 2D ultrathin sulfides of Ag2S and CdS inside the semi-hollow plasmonic nanocavity. Strong plasmon resonance and large local field enhancement are exhibited inside the plasmonic cavity where the ultrathin semiconductor sulfides are grown, indicating strong plasmon-exciton interactions in the hybrids. Furthermore, this synthetic approach is extended to grow other metal sulfides such as Bi2S3 and PbS. The combination of a flat plasmonic cavity with ultrathin semiconductor nanosheets in this study provides a new strategy for the development of unique plasmon-based hybrids with excellent optical properties.

Largely enhanced photocatalytic activity of Au/XS2/Au (X = Re, Mo) antenna-reactor hybrids: charge and energy transfer.

An antenna-reactor hybrid coupling plasmonic antenna with catalytic nanoparticles is a new strategy to optimize photocatalytic activity. Herein, we have rationally proposed a Au/XS2/Au (X = Re, Mo) antenna reactor, which has a large Au core as the antenna and small satellite Au nanoparticles as the reactor separated by an ultrathin two-dimensional transition-metal dichalcogenide XS2 shell (∼2.6 nm). Due to efficient charge transfer across the XS2 shell as well as energy transfer via coupling of the Au antenna and Au reactor, the photocatalytic activity has been largely enhanced: Au/ReS2/Au exhibits a 3.59-fold enhancement, whereas Au/MoS2/Au exhibits a 2.66-fold enhancement as compared to that of the sum of the three individual components. The different enhancement in the Au/ReS2/Au and Au/MoS2/Au antenna-reactor hybrid is related to the competition and cooperation of charge and energy transfer. These results indicate the great potential of the Au/XS2/Au antenna-reactor hybrid for the development of highly efficient plasmonic photocatalysts.

Largely enhanced photocatalytic hydrogen production rate of CdS/(Au-ReS2) nanospheres by the dielectric-plasmon hybrid antenna effect.

In this study, we synthesized CdS/(Au-ReS2) nanospheres that have highly efficient photocatalytic hydrogen production activity induced by dielectric-plasmon hybrid antenna resonance. As the diameter (D) of ReS2 nanospheres consisting of 2D nanosheets increases from 114 ± 11 to 218 ± 25 nm, the resonance wavelength of the ReS2 dielectric antenna is tuned from 380 to 620 nm and the hydrogen production rate for the CdS/(Au-ReS2) nanospheres increases by more than 1.85 times and reaches a value as high as 3060 µmol g-1 h-1, with a 9% weight percentage of Au. Due to the enhancements of the local electromagnetic field and excitation energy transfer by the ReS2-Au dielectric-plasmon hybrid antenna, the hydrogen production rate for the CdS/(Au-ReS2) nanospheres (D = 218 ± 25 nm) is 797, 319, 105 and 12 times larger than that for pure ReS2, Au-ReS2, CdS, and CdS-ReS2, respectively. Additionally, the persistence and reusability measurements indicate a favorable stability of CdS/(Au-ReS2). These results provide a strategy to prepare a new class of dielectric-plasmon hybrid antennas consisting of 2D materials and metal nanoparticles, which have promise in applications ranging from photocatalysis to nonlinear optics.

Enhanced Second Harmonic Generation by Mode Matching in Gain-assisted Double-plasmonic Resonance Nanostructure.

We theoretically study the gain-assisted double plasmonic resonances to enhance second harmonic generation (SHG) in a centrosymmetric multilayered silver-dielectric-gold-dielectric (SDGD) nanostructure. Introducing gain media into the dielectric layers can not only compensate the dissipation and lead to giant amplification of surface plasmons (SPs), but also excite local quadrupolar plasmon which can boost SHG by mode matching. Specifically, as the quadrupolar mode dominates SHG in our nanostructure, under the mode matching condition, the intensity of second harmonic near-field can be enhanced by 4.43 × 102 and 1.21 × 105 times when the super-resonance is matched only at the second harmonic (SH) frequency or fundamental frequency, respectively. Moreover, the intensity of SHG near-field is enhanced by as high as 6.55 × 107 times when the nanostructure is tuned to double super-resonances at both fundamental and SH frequencies. The findings in this work have potential applications in the design of nanosensors and nanolasers.

Magnetic Fano resonance-induced second-harmonic generation enhancement in plasmonic metamolecule rings.

The "artificial magnetic" resonance in plasmonic metamolecules extends the potential application of magnetic resonance from terahertz to optical frequency bypassing the problem of magnetic response saturation by replacing the conduction current with the ring displacement current. So far, the magnetic Fano resonance-induced nonlinearity enhancement in plasmonic metamolecule rings has not been reported. Here, we use the magnetic Fano resonance to enhance second-harmonic generation (SHG) in plasmonic metamolecule rings. In the spectra of the plasmonic metamolecule, an obvious Fano dip appears in the scattering cross section, while the dip does not appear in the absorption cross section. It indicates that at the Fano dip the radiative losses are suppressed, while the optical absorption efficiency is at a high level. The largely enhanced SHG signal is observed as the excitation wavelength is adjusted at the magnetic Fano dip of the plasmonic metamolecule rings with stable and tunable magnetic responses. We also compare the magnetic Fano dip with the electric case to show its advantages in enhancing the fundamental and second harmonic responses. Our research provides a new thought for enhancing optical nonlinear processes by magnetic modes.

Strong magnetic resonances and largely enhanced second-harmonic generation of colloidal MoS2 and ReS2@Au nanoantennas with assembled 2D nanosheets.

Colloidal disk-like and sphere-like MoS2 nanoantennas are synthesized. They consist of curly and interlaced 2D nanosheets. The resonance peak of the MoS2 nanoantennas can be tuned from 500 to 900 nm by adjusting the size and shape. The strong magnetic and electric resonances of the dielectric antennas are revealed by theoretical calculations with Mie theory. The second harmonic generation (SHG) of the exfoliated nanosheets and the synthesized nanodisks and nanospheres is investigated and compared by scanning the excitation laser wavelength. SHG enhancement of 52 fold is observed for the spherical nanoantennas at 400 nm, which is attributed to the nanoantenna-enhanced two-photon resonance excitation of the D exciton of MoS2 monolayers. Moreover, ReS2@Au plasmon-dielectric hybrid nanoantennas are also synthesized. The SHG of Au nanoparticles is enhanced 8.5 times by the coupling of the two types of nanoantennas. This new class of optical nanoantennas consisting of 2D materials and exhibiting unique linear and nonlinear optical responses will bring promising applications ranging from nonlinear photonics to photochemistry.

The nonmonotonous shift of quantum plasmon resonance and plasmon-enhanced photocatalytic activity of gold nanoparticles.

The surface plasmon resonance (SPR) of metal nanoparticles exhibits quantum behaviors as the size decreases owing to the transitions of quantized conduction electrons, but most studies are limited to the monotonous SPR blue-shift caused by off-resonant transitions. Here, we demonstrate the nonmonotonous SPR red-shift caused by resonant electron transitions and photocatalytic activity enhanced by the quantum plasmon resonance of colloidal gold nanoparticles. A maximal SPR wavelength and the largest photocatalytic activity are observed in the quantum regime for the first time for the gold nanoparticles with a diameter of 3.6 nm. Theoretical analysis based on a quantum-corrected model reveals the evolution of SPR with quantized electron transitions and well explains the nonmonotonous size-dependencies of the SPR wavelength and absorption efficiency. These findings have profound implications for the understanding of the quantum nature of the SPR of metal nanoparticles and their applications in areas ranging from photophysics to photochemistry.