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Due to the topological charge-independent doughnut spatial structure as well as the association of orbital angular momentums, perfect vortex beams promise significant advances in fiber communication, optical manipulation and quantum optics. Inspired by the development of planar photonics, several plasmonic and dielectric metasurfaces have been constructed to generate perfect vortex beams, instead of conventional bulky configuration. However, owing to the intrinsic Ohmic losses and interband electron transitions in materials, these metasurface-based vortex beam generators only work at optical frequencies up to the visible range. Herein, using silicon nitride nanopillars as high-efficiency half-wave plates, broadband and high-performance metasurfaces are designed and demonstrated numerically to directly produce perfect vortex beams in the ultraviolet region, by combining the phase profiles of spiral phase plate, axicon and Fourier transformation lens based on geometric phase. The conversion efficiency of the metasurface is up to 86.6% at the design wavelength. Moreover, the influence of several control parameters on perfect vortex beam structures is discussed. We believe that this ultraviolet dielectric generator of perfect vortex beams will find many significant applications, such as high-resolution spectroscopy, optical tweezer and on-chip communication.

Excitation of ultraviolet (UV) range plasmon resonance with high quality (Q)-factor has been significantly challenging in plasmonics because of inherent limitations in metals like Au and Ag. Herein, we theoretically investigated UV-visible range plasmons in the topological insulator Bi

A plasmonic substrate providing high, reproducible and stable Raman signals should be highly desirable for the development of surface enhanced coherent anti-Stokes Raman spectroscopy (SECARS). In this work, we theoretically present a design of SECARS substrate consisting of five different-sized gold nanodisks and investigate its enhancement properties under different excitation polarizations by using finite element method. The numerical results reveal that the pentamer supports a polarization-independent Fano-resonant scattering spectrum due to its symmetric geometrical arrangement. Multiple electromagnetic hot spots produced by the Fano resonance are overlapped spatially at three characteristic frequencies involved in SECARS process. Consequently, the theoretically estimated overall enhancement factor (EF) of SECARS nearly keeps the same order of magnitude up to ∼1014 for any horizontally excitation polarizations, and the relative root mean square error of the logarithm of the overall EF (Log10EF) is less than 2%. Giant and polarization-insensitive SECARS enhancements enable the pentamer structure to be promising for plasmonic substrates in SECARS as well as other enhanced nonlinear optical process.

Resonant plasmonic nanostructures have attracted much attention due to their exotic optical properties. In this work, we numerically investigate single- and dual-gap ring structures, respectively, and they can be used for refractive index sensing by exciting the gap plasmon mode with sharp resonance spectra. The dual-gap ring plasmonic sensor shows the dual-channel properties and the corresponding sensitivities are 513 nm/RIU and 809 nm/RIU with the refractive indices range of 1.32–1.37, respectively. Furthermore, the gap ring structures are highly symmetrical, and have advantages in fabrication and pave the way for advanced optical devices to detect bio-molecules.

Background The volatile organic compounds (VOCs) produced from avocados during storage may be distinct at different periods, and this difference may be related to their degree of maturity, for which no relevant research has been conducted yet. Results A total of 30 typical target compounds were identified by gas chromatography-ion mobility spectrometry (GC-IMS) combined with principal component analysis (PCA) for the VOCs produced during the post-harvesting process of avocado. With an increase in storage time, the VOCs content produced by avocado due to ripening continued to increase, and the uptrend was particularly obvious on day 13. The storage time of avocado could be distinguished according to the PC1 and PC2 values in the PCA chart. Conclusion GC-IMS detection combined with PCA was used to establish the fingerprints of VOCs in avocado for the first time. The maturity of avocados was determined by identifying the signal strength of characteristic VOCs, and this method could be of great potential to predict the maturity of fruits in the future. © 2020 Society of Chemical Industry.

Metasurfaces, planar arrays of subwavelength optical phase shifters, show unprecedented superiority to convention bulk optical elements in many applications. Until now, most of metasurfaces work at frequencies from the microwave to the visible; material challenges (lack of media supporting resonant responses at higher frequencies) have hampered the realization of controlling light at shorter wavelengths. However, for applications such as high-resolution imaging and high sensitive sensing, they require much higher photon energy to meet the requirement of higher resolution. Herein, using plasmonic effect of topological insulator (Bi 1.5 Sb 0.5 Te 1.8 Se 1.2 , BSTS), we numerically demonstrate that the metasurfaces, composed of BSTS spheroidal nanostructures, operate in the ultraviolet range. Some representative wavefront shaping functions, including anomalous refraction, lensing and optical vortices, are realized by means of geometric phase scheme. This research may inspire more interest in the development of ultraviolet metasurface-based nano-photonic devices with diverse functions such as holographic lithography, subwavelength resolution imaging and quantum information processing.

Plasmon-induced transparency (PIT), an analog of electromagnetically induced transparency, originates from destructive interference of plasmonic resonators with different quality factors and brings about the extreme dispersion within the narrow transparency window, promising remarkable potential for slow light, nonlinear optics and biochemical sensors. However, sometimes a broad transmission frequency band is more desirable for other applications such as bandpass filters. In general, strong coupling between bright and dark plasmon modes in coupled resonant systems leads to wide transparency bandwidth at the PIT resonance. Based on multi-oscillator coupling theory, a metasurface structure consisting of three perpendicularly connected metallic nanobars is purposefully designed and numerically demonstrated to support broadband PIT spectral response. The near-field patterns indicate that the broadening of the transparent band results from the constructive interference of dual excitations of the single non-radiative (dark) resonator by the two radiative (bright) antennas. These results show that this scheme of bright-dark-bright mode coupling is significantly beneficial for designing filters operating over a broad frequency range.

Excitation of ultraviolet (UV) range plasmon resonance with high quality (Q)-factor has been significantly challenging in plasmonics because of inherent limitations in metals like Au and Ag. Herein, we theoretically investigated UV-visible range plasmons in the topological insulator Bi1.5Sb0.5Te1.8Se1.2 (BSTS) nanosphere and nanoshell. In contrast to broad linewidth plasmon absorptions in the BSTS nanospheres, an ultra-sharp absorption peak with the Q-factor as high as 52 is excited at UV frequencies in the BSTS nanoshells. This peak is attributed to Dirac-type plasmon resonance originating from massless Dirac carriers in surface states of the BSTS. Furthermore, a tunable plasmon wavelength of the resonance is demonstrated by varying geometrical parameters of the BSTS nanoshells. This may find applications in surface enhanced Raman spectroscopies, nanolasers and biosensors in the UV regions.

Plasmonic nanostructures have important applications for surface-enhanced Raman chips, optical sensors, perfect absorbers, and so on. In this paper, we theoretically investigate the optical properties of a T-shaped dimer (TD) nanostructure and a TD with nanowire loads (TD/NL) nanostructure. By depositing the NLs around the TD, the optical property of the TD is significantly modified, and the extinction spectrum of composite TD/NL appears the splitting resonance peak and shows the asymmetric characteristic of Fano resonance due to the plasmon hybridization. Besides, a stronger electric field enhancement can be obtained in the composite TD/NL nanostructure, and the calculated results show the field enhancement effect of the dark mode in designed TD/NL nanostructure is three times than that in the TD nanostructure. Furthermore, the Fano resonance in TD/NL nanostructure reveals the narrow spectral linewidth and high quality factor, which is advantageous to optical refractive index sensors, and the corresponding sensing sensitivity is as high as 1083 nm RIU−1.

Gas chromatography-ion mobility spectrometry (GC-IMS) and dynamic quantitative descriptive analysis (D-QDA) were combined to explore the aroma release and perception from the retronasal cavity during bread consumption. D-QDA results elucidated that the sweet, creamy, and roasty notes were the most active attributes during oral processing. The final stage of oral processing had the most complicated changing pattern, followed by the intermediate and initial stages. Thirteen aroma compounds were detected in the retronasal cavity, of which eight had odor activity values (OAVs) greater than 1. The total OAV changing pattern was consistent with the D-QDA results. Addition experiments further confirmed that acetoin, 2,3-butanedione, and 3-(methylthio)propanal were key aroma compounds contributing to retronasal olfaction. 2,3-Butanedione and 3-(methylthio)propanal were both identified as key odorants in the mouth cavity and retronasal cavity during oral processing, but they had 30% loss during the breath delivery from the mouth cavity to the retronasal cavity.

Dried pericarps of Citrus reticulata 'Chachi', named Guangchenpi (GCP), have been used as both food and medicine in China for centuries. Due to its special flavour and taste, the price of GCP is much higher than those of dried pericarps from other Citrus reticulata Blanco varieties (collectively called "Chenpi", CP). Therefore, some CP have been disguised has GCP by dishonest businessmen to obtain a higher profit. In this study, a simple, rapid, effective and economic analytical method based on headspace-gas chromatography-ion mobility spectrometry (HS-GC-IMS) was established to discriminate GCP and CP by their volatile organic compounds (VOCs). Principal component analysis (PCA) was performed to extract meaningful attributes from VOCs based on the GC-IMS chromatograms and to give the analysis results. Twelve, 12 and 14 markers were identified and established as the characteristic fingerprints of GCP analysis of 45 batches of samples via SE-54, OV-1701 and Inter Cap WAX gas chromatographic columns, respectively. The PCA results indicated that GCP and CP could be effectively discriminated. This study confirmed the potential of HS-GC-IMS combined with PCA as a reliable analytical screening technique to discriminate between GCP and CP.

The purpose of this study was to investigate the aroma release and perception from white bread during oral processing by gas chromatography-ion mobility spectrometry (GC-IMS) and dynamic sensory evaluation of temporal dominance of sensations (TDS). TDS curves indicated that two maximum aroma perception signals, fermentation-like and flour-like attributes, were perceived at the beginning and swallowing, respectively. The fermentation-like, flour-like, and sour attributes were the 3 dominant aromas during oral processing. A total of 35 volatile compounds were detected in the mouth cavity during chewing white bread, 19 of them were confirmed and quantified by using the respective external standard. Based on PLSR analysis, 8 aroma compounds were predicted as potent odorants contributing to the aroma perception from chewing white bread. By application of odor activity values analysis and addition experiments, ethyl butanoate, butyl acetate, hexanal, 3-(methylthio)-propanal, 3-methylbutanal, and 2,3-butanedione were confirmed as the key odorants contributing to the aroma perception during chewing of white bread.

A simple and environmentally approach using untargeted imaging of volatile substances combined with chemometrics and markers response was proposed for discriminating different species of honey with headspace gas-chromatography-ion-mobility (HS-GC-IMS). The 3D HS-GC-IMS imaging and their response differences enabled the clear discrimination between winter honey and sapium honey. Principal component analysis (PCA) and partial least-squares discrimination analysis (PLS-DA) were employed to discriminate different honey. Markers of two kinds of honey were identified and confirmed with a user-built imaging database combined with multivariate analysis. Benzaldehyde dimer and phenylacetaldehyde dimer were found to be reliable markers of winter honey, and phenylethyl acetate dimer was of sapium honey. Adulteration identification of the honey samples with different adulteration ratios were subjected to this triple-locked strategy analysis. The results demonstrate that HS-GC-IMS imaging coupled with chemometrics and marker identification is a useful triple-locked strategy to discriminate honey from different floral origins and adulterated honey.

We theoretically investigate optical properties of a plasmonic metasurface consisting of a dipolar wire as the bright antenna stacked above a quadrupolar wire as the dark antenna. It is demonstrated that by adjusting the lateral displacement between the two resonators, the spectral feature of the metasurface can be evolved from the plasmonic electromagnetically-induced transparency to electromagnetically-induced absorption. The extracted physical parameters based on the two-coupled-oscillator model reveal that the near-field coupling strength plays a key role for the transition behavior in the plasmonic metasurface.

Nanohole arrays or individual nanohole oligomers in metallic films have attracted intense attention due to their unique optical properties such as extraordinary optical transmission or Fano resonance. However, the nanohole oligomer array still remains largely unexplored. In this work, we numerically investigate the heptamer-hole arrays in an optically thin silver film, which can support double Fano-type resonances in the transmission spectra. The two Fano-type transmissions arise from the interference between the non-resonant direct transmission through holes and the resonant indirect scatterings based on the excitations of surface plasmons polaritons (SPPs, set up by the array periodicity) and a sub-radiant localized surface plasmon resonance (LSPR, arising from the anti-bonding hybridization between the central and the surrounding holes). Because of their different physical mechanisms, the two Fano resonances can be tuned independently. In addition, the LSPR-related Fano resonance shows an ultra-high se...

We numerically investigated the optical properties of planar resonators metamaterial that exhibits two narrow transmitted dips with quality factors of 23 and 50 in the optical regions. The results show that, both of the two resonances reveal a distinct plasmon shift with respect to a small fluctuation in the refractive index of the surrounding medium, and calculated average refractive index sensitivities are 900 nm/RIU and 493 nm/RIU, and corresponding figure of merits of two modes are 16 and 32 in vacuum, respectively. The sensing performance can be improved by changing the geometric parameters of planar metamaterial due to the plasmon modes coupling effect, which offer an excellent potential for optical nanosensing applications.

Dynamical control of plasmon-induced transparency (PIT) in metamaterials promises essential application opportunities. In this paper, we design a novel disk/rod hybrid metasurface to investigate an actively controlled PIT spectral response through polarization-dependent near-field coupling between the disk (bright) and the rod (dark) resonator. It is found that an on-to-off amplitude modulation of the PIT transparency window is achieved by the rotation of polarization, allowing for dynamically tunable group delays of incident waves. An analytic model based on the dipole-dipole interactions is developed for the proposed configuration, which agrees well with the numerical results. The disk/rod metasurface offers great promise for optical switching and compact slow light devices.

We theoretically demonstrate a kind of plasmon coupled cavity to achieve a nanolaser with high intensity and low threshold. The plasmon cavity is composed of the gold film substrate and gold disk array, and support two strong coupled resonance modes (i.e. surface plasmon polariton (SPP) and localized surface plasmon (LSP)). Compared with the nanolaser with the LSP resonance structure, the scattering cross-section of the LSP-SPP coupled laser is 80 times higher. The coupled cavity nanolaser with different wavelengths can be excited with different plasmon resonance modes by selecting the appropriate gain medium. In addition, a compact tunable dual-wavelength laser can be developed using this coupled structure.

Polyvinylpyrrolidone (PVP)-protected silver nanostructures of various shapes, including nanocubes, nanospheres, and hybrid shapes with nanospheres and nanorods, on the surface of glass or Si substrates (PVP-Ag films) are prepared by using electrostatic self-assembly. With 4-mercaptobenzoic acid (4-MBA) as a probe molecule, it is demonstrated that the PVP-protected silver nanocubes films (PVP-Ag NCs) have better surface-enhanced Raman scattering (SERS) activity with an order of magnitude larger enhancement factors (EF) than the PVP-protected silver nanospheres films and the PVP-protected silver hybrid shapes films, which is confirmed by our numerical simulations. The EF of 4-MBA on the PVP-Ag NCs film are up to ~5.38 × 106, and the detection limit is at least down to ~10−8 M. The uniformity and reproducibility of the SERS signals on PVP-Ag NCs film are tested by point-to-point and batch-to-batch measurements. Meanwhile, the PVP-Ag films are also shown to be an excellent SERS substrate with good biocompatibility for hemoglobin detection. It is shown that the PVP-Ag NCs films can be used as excellent SERS substrate with good activity, uniformity, reproducibility, and biocompatibility and are promising for a myriad of chemical and biochemical sensing applications.

We propose a kind of gradient metasurface based on graphene apertures for wavefront control of terahertz linear polarized waves. By carefully designing the geometric dimensions of the graphene rectangular apertures, the proposed metasurface can realize arbitrary reflection, carpet cloak and reflective focusing, respectively. The simulation results demonstrate that the cloaking and focusing effects can be actively tuned between ON and OFF state by uniformly tuning the graphene Fermi energy. The metalens based on graphene has high-efficiency, broadband and wide-angle off-axis focusing performance, and the focusing efficiency is significantly dependent on graphene Fermi energy and carrier mobility. When the mobility decreases from 1.0 m2/Vs to 0.1 m2/Vs, the focusing efficiency decreases from more than 70% to about 22% at the operating frequency. The tunable and efficient performance makes the graphene aperture based metasurface have great application potential in ultrathin, lightweight and integrated terahertz devices.

Uniform and reproducible substrates for surface enhanced Raman scattering (SERS) are fabricated by self-assembly of Ag nanoparticles (NPs) on 3-aminopropyltrimethoxysilane (APTES) modified glass. Experimental results indicate that the Ag NPs with a narrow size distribution were assembled as a sub-monolayer which exhibits an excellent SERSactivity. The SERS enhancement factor is estimated to be 7.5 × 106 and the detection limit for crystal violet (CV) solution is about ∼10−11 M. The uniformity and reproducibility of the SERS signals are tested by point-to-point and batch-to-batch measurements. It is confirmed that the self-assembled Ag NPs substrates has a high SERS reproducibility and a low standard deviation with respect to the Ag NPs on non-functionalized glass substrates. The self-assembled Ag NPs substrates can be widely used for the application of chemical and biochemical sensing.

The performances of thin film solar cells are considerably limited by the low light absorption. Plasmonic nanostructures have been introduced in the thin film solar cells as a possible solution around this issue in recent years. Here, we propose a solar cell design, in which an ultrathin Si film covered by a periodic array of Ag strips is placed on a metallic nanograting substrate. The simulation results demonstrate that the designed structure gives rise to 170% light absorption enhancement over the full solar spectrum with respect to the bared Si thin film. The excited multiple resonant modes, including optical waveguide modes within the Si layer, localized surface plasmon resonance (LSPR) of Ag stripes, and surface plasmon polaritons (SPP) arising from the bottom grating, and the coupling effect between LSPR and SPP modes through an optimization of the array periods are considered to contribute to the significant absorption enhancement. This plasmonic solar cell design paves a promising way to increase light absorption for thin film solar cell applications.

Correspondence analysis was made for the diatom data from surface sediments in the west of Greenland.A total of 35 samples were divided into five assemblages based on the scores of samples,and the results show that the diatom assemblages are dominated by sea-ice species,Arctic species and cold-water species in the study area,whereas warm-water species mainly exist in the Labrador Sea.Moreover,the environmental variables(e.g.currents、polar waters、melt-water) of the west of Greenland were also discussed in this paper.It is proved that there is a good correlation between the diatom surface sediment assemblages and the environmental variables mentioned above.

Surface plasmons supported by various metallic nanostructures have given rise to several significant breakthroughs in the field of integrated photonic devices due to its ability to effectively confine and enhance optical field in subwavelength volume. In particular, the demand to actively control optical responses of plasmonic systems becomes urgent for the miniaturization of signal processing devices, surface-enhanced Raman scattering (SERS) substrates and biochemical sensors. In this paper, we systematically investigate the plasmon modes as well as their interaction in a layered nanostructure composed of a periodically-arranged radiative nanoring and a metallic ground plane, as well as a thin insulating spacer. A tunable transparent peak on the background of the broadband plasmon resonance emerges in the reflection spectrum as changing the periodicity of nanoparticle array, a plasmonic analogue of electromagnetically induced transparency (EIT). Owing to the structural symmetry of the rings, we demonstrate a new scheme of controlling the interference between localized and delocalized plasmons by means of incident polarization and believe that the proposed metasurface may find applications in optical switching if the polarization-controlled components are introduced.

Since the nucleation and growth of clusters is usually a non-equilibrium condensation process, a distribution of structural isomers for a given cluster size may be encountered even under the same conditions. In this work, molecular dynamics simulations are performed on sets of molten clusters of Cu309 to study their structures at low temperatures while controlling the cooling rate. Several different final structures including perfect icosahedra (ICO), imperfect Mark’ decahedra (MDEC) and imperfect FCC truncated octahedra (TOCT) are obtained even at the same cooling rate. It is calculated that the most favorable structure is icosahedra, which becomes more and more favorable as the cooling rate is slowed. To better understand the process of crystallization, several techniques, including potential-temperature curves, common neighbor analysis (CNA) and radial distribution function (RDF), are used to analyze and study the structural transition. Results show that different structures are obtained under identical conditions due to the stochastic nature of nucleation and relatively small energy difference between isomers. The process of geometrical evolution for icosahedra is given by comparing and analyzing the time evolution of the root-mean-square deviation (RMSD) of atoms located in every shell.

We proposed a facile green synthesis system to synthesize large-scale Ag hemi-mesoparticles monolayer on Cu foil. Ag hemi-mesoparticles have different surface morphologies on their surfaces, including ridge-like, meatball-like, and fluffy-like shapes. In the reaction, silver nitrate was reduced by copper at room temperature in dimethyl sulfoxide via the galvanic displacement reaction. The different surface morphologies of the Ag hemi-mesoparticles were adjusted by changing the reaction time, and the hemi-mesoparticle surface formed fluffy-spherical nanoprotrusions at longer reaction time. At the same time, we explored the growth mechanism of silver hemi-mesoparticles with different surface morphologies. With 4-mercaptobenzoic acid as Raman probe molecules, the fluffy-like silver hemi-mesoparticles monolayer with the best activity of surface enhanced Raman scattering (SERS), the enhancement factor is up to 7.33 × 10(7) and the detection limit can reach 10(-10)M. SERS measurements demonstrate that these Ag hemi-mesoparticles can serve as sensitive SERS substrates. At the same time, using finite element method, the distribution of the localized electromagnetic field near the particle surface was simulated to verify the enhanced mechanism. This study helps us to understand the relationship between morphology Ag hemi-mesoparicles and the properties of SERS.

Surface enhanced coherent anti-Stokes Raman scattering (SECARS) is a sensitive tool and promising for single molecular detection and chemical selective imaging. However, the enhancement factors (EF) were only 10~100 for colloidal silver and gold nanoparticles usually used as SECARS substrates. In this paper, we present a design of SECARS substrate consisting of three asymmetric gold disks and strategies for maximizing the EF by engineering near-field properties of the plasmonic Fano nanoassembly. It is found that the E-field “hot spots” corresponding to three different frequencies involved in SECARS process can be brought to the same spatial locations by tuning incident orientations, giving rise to highly confined SECARS “hot spots” with the EF reaching single-molecule sensitivity. Besides, an even higher EF of SECARS is achieved by introducing double Fano resonances in this plasmonic nanoassembly via further enlarging the sizes of the constituent disks. These findings put an important step forward to the plasmonic substrate design for SECARS as well as for other nonlinear optical processes.

Plasmonic metallic nanostructures have been demonstrated an effective way to enhance the light emission efficiency in LEDs. Here, we propose a design of white LEDs that combining dielectric silicon nanopillar array in the color-converting layer. By investigating theoretically the guided mode caused by the nanopillar array-waveguide system, we demonstrate that the silicon nanopillar arrays enable larger near-field enhancement and more efficient photons emission property than the plasmonic counterparts. These performances make the silicon nanopillar arrays have potential application in light converter for efficient white LEDs. We also show that the guided mode can be controlled by changing the period of nanopillar grating and the thickness of polymer layer. More significant performance can be achieved by further optimizing the shape and size of the silicon nanoparticles. Compared with the square nanoparticle arrays, the hexagonal nanopillar arrays are demonstrated to have larger field enhancement and emission enhancement. Our research is expected to give insights into the design and optimization of the solid-state lighting systems by using silicon nanostructures, and the all-dielectric metamaterials for gaining or lasing devices.

Ag–Fe3O4 nanocomposites were synthesized by the redox reaction between Ag2O and Fe(OH)2 in the absence of additional reductant at moderate temperature and atmospheric condition. The as-synthesized Ag–Fe3O4 nanocomposites are assembled into an orderly arrayed SERS substrate holding clean and reproducible properties with an applied external magnetic field. 4-mercaptobenzoic acid (4-MBA) is chosen as the probe molecule to test the enhancement factors (EF), uniformity and reproducibility of the SERS substrate. Experimental results indicate that the EF of 4-MBA on our proposed SERS substrate is up to 5.2 × 106 and the detection limit is down to ∼10−10 M. The SERS spectra of 4-MBA molecules ranging from 200 cm−1 to 2000 cm−1 were randomly collected from a number of positions on the substrate and six Ag–Fe3O4 nanocomposites substrates are measured with the same procedure. It is shown that the SERS substrate have the good uniformity and reproducibility with low standard deviation, indicating our proposed Ag–Fe3O4 nanocomposites with external magnetic field control abilities have potential applications in the fields of magnetic separation and SERS techniques.

A novel planar plasmonic metamaterial for electromagnetically induced transparency and slow light characteristic is presented in this paper, which consists of nanoring and nanorod compound structures. Two bright modes in the metamaterial are induced by the electric dipole resonance inside nanoring and nanorod, respectively. The coupling between two bright modes introduces transparency window and large group index. By adjusting the geometric parameters of metamaterial structure, the transmittance of EIT window at 385 THz is about 60%, and the corresponding group index and Q factor can reach up to 1.2 × 10³ and 97, respectively, which has an important application in slow-light device, active plasmonic switch, SERS and optical sensing.

We demonstrate theoretically an analogue of electromagnetically induced transparency (EIT) at visible frequencies in an all-dielectric metamaterial-waveguide (ADMW) system that consists of a two-dimensional silicon nanopillar array on top of a dielectric slab waveguide. By varying the lattice period of the array, we show that the transmission feature of the hybridized ADMW system can be efficiently engineered due to the period-dependent guided modes. For ADMW systems with different lattice periods, the EIT-like transparency windows are confirmed either from the two detuned guided modes in the waveguide layer or from the Fano-type interference between the guided modes and the magnetic dipole mode in silicon nanopillars. We have also demonstrated the significant electric field enhancement in the waveguide layer and the remarkable slow-light effect associated with the EIT-like transmission. Compared with the hybrid waveguide-plasmon system including a metal-resonator array, the silicon-based ADMW system enables production of high Q-factor resonances with low absorption losses, thereby having promising applications in low-loss slow-light devices, bio/chemical sensing, enhancing emission rate, and optical modulation.

We theoretically investigate the gain-assisted plasmonic analog of electromagnetically induced transparency (EIT) in a novel planar metamaterial, whose unit cell consists of two perpendicularly connected metallic bars, forming a ?T? configuration. An EIT-like resonance can be achieved by introducing symmetry breaking into its shape. The results show that the group index is greatly enhanced at the optimum value of the gain due to a dramatic amplification of the EIT-like resonance. Furthermore, a trade-off between the group index and the transmittance at the EIT-like resonance, which always exists in passive systems with no gains, can be removed at an approximate gain level. E-field enhancements are used to understand the underlying physics.

A novel plasmonic metamaterial consisting of the solid (bar) and the inverse (slot) compound metallic nanostructure for electromagnetically induced absorption (EIA) is proposed in this paper, which is demonstrated to achieve an ultra-narrow absorption peak with the linewidth less than 8 nm and the absorptivity exceeding 97% at optical frequencies. This is attributed to the plasmonic EIA resonance arising from the efficient coupling between the magnetic response of the slot (dark mode) and the electric resonance of the bar (bright mode). To the best of our knowledge, this is the first time that the plasmonic EIA is used to realize the narrow-band perfect absorbers. The underlying physics are revealed by applying the two-coupled-oscillator model. The near-perfect-absorption resonance also causes an enhancement of about 50 times in H-field and about 130 times in E-field within the slots. Such absorber possesses potential for applications in filter, thermal emitter, surface enhanced Raman scattering, sensing and nonlinear optics.

In this paper, based on the constructive interference of plasmonic dipolar and quadrupolar modes, a classical analogue of electromagnetically induced absorption (EIA) is demonstrated theoretically in a stacked metamaterial consisting of a short metal strip (which acts as a bright resonator) and a long metal strip (acting as a dark resonator), which has been reported to support the electromagnetically induced transparency (EIT) effect. The transition from EIA to EIT can be clearly observed in the absorbance spectra via varying the vertical spacing between two resonant oscillators. With the help of the coupled two-oscillator model, the phase shift between the bright and dark resonance modes is calculated by fitting the simulated absorbance spectra, which reveals the physical mechanisms behind constructive and destructive interference effects in EIT/EIA metamaterials.

The resonance amplification of out-of-plane lattice plasmons (OLPs) in gain-assisted 2D metallic nanoparticle (NP) arrays is investigated theoretically. Due to the angle-dependent near-field optical properties, the gain threshold of OLPs-based spaser can be controlled by adjusting the angle of incident light. OLPs are demonstrated to exhibit lower thresholds for surface plasmon (SP) amplification than the in-plane lattice plasmons (ILPs) for active plasmonic NP arrays. Side-by-side comparisons of the E-field localizations and enhancements for the ILPs and OLPs excited at different incident angle are performed to understand their different spasing performance. The results also indicate that the gain thresholds of lattice plasmons in NP arrays are much lower than those of localized SPs in individual NPs.

A sharp plasmonic Fano resonance is found to appear in a silver nanorod pair structure with broken length symmetry. It is shown that it arises from strong interference between a narrow plasmon mode of inter-nanorod near-field coupling and broad scattering from the nanorod. The inter-nanorod coupling is the result of the magnetic dipole plasmon resonance induced by the anti-parallel current of the nanorod pair, while the broad scattering is caused by electric dipole oscillation. The numerical results show that a giant local field enhancement (197), a high quality factor (52) and a sensing sensitivity of 1426 nm per refractive index unit in the symmetry-broken nanorod pair with a displacement Δl = 40 nm originated from a pronounced Fano resonance in the near-infrared spectrum. Huge local field enhancement and high sensitivity make this simple structure promising for surface enhanced Raman spectroscopy and sensing applications.

We report that the gain threshold of a core–shell nanoparticle-based spaser can be reduced significantly by offsetting the gain-doped dielectric core within the metallic shell. By investigating the optical cross sections of the reduced symmetry core–shell nanoparticle with different levels of gain, we determined the gain threshold of the asymmetric nanoparticle-based spaser fueled by different plasmon modes. The calculation results indicate that when multipolar plasmon oscillations excited in asymmetric core–shell nanostructures are used as lasing modes, the gain threshold of an asymmetric spaser particle can drop by 30% as compared to the case of a perfect or symmetric particle. The underlying physics of low-threshold surface plasmon amplification is explained by investigating the Q factor and the optical field confinement and enhancement associated with the lasing mode.

We theoretically investigate the optical properties in one-dimensional graded soft photonic crystals (1D GSPCs). The proposed structure is constituted of the stacked ferrofluids layer and the dielectric layer. Due to the supermagnetic response of the ferromagnetic nanoparticles, they will align in a line under the influence of the initiated magnetic field, thereby modulating the refractive index of the ferrofluids layer. By resorting to the transfer matrix method, the dispersion relation, transmittance and reflectance in 1D GSPCs were calculated. Numerical results show that a broad photonic band gap appears in such systems, which can even be broadened by increasing the volume fraction of ferromagnetic nanoparticles. Moreover, perfect transmittance of our proposed structure can be realized with an increased number of ferrofluid layers. In comparison with conventional PCs materials, 1D GSPCs composed of liquid material offer a very flexible route to implementation, which can be widely used in the application of optical filters, waveguides, reflectors and so on.

The optical properties of asymmetric ring structures are investigated theoretically by using the discrete dipole approximation method. The numerical results revealed that this kind of structure can achieve a giant localized field enhancement (LFE, 264) and a high LSPR sensitivity (corresponding FOM, 8.28) in the visible spectrum by Fano resonance, whose origin is discussed based on plasmon hybridization theory. Furthermore, the dependence of the Fano resonance on the polarization states of the incident light is also demonstrated. Giant LFE and high LSPR sensitivity enable this structure to be promising for surface enhanced Raman spectroscopy and sensing applications.

A broad-band perfect absorber composing a two-dimensional periodic metal-dielectric-metal sandwiches array on dielectric/metal substrate is designed and numerically investigated. It is shown that the nearly-perfect absorption with a bandwidth of about 50 nm in visible region can be achieved by overlapping of two plasmon resonances: one originating from the coupling of electric dipoles between adjacent unit cells and another arising from magnetic dipole plasmon resonances. A capacitor-inductor circuit description is introduced to explain the dependence of resonance frequencies and band-width on geometrical parameters.