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We report the observation of an analog of double electromagnetically induced absorption (EIA) in a plasmonic system consisting of two disk resonators side-coupled to a discrete metal-insulator-metal (MIM) waveguide. The finite-difference time-domain (FDTD) simulation calculations show that two absorption windows are obtained and can be easily tuned by adjusting the parameters of the two resonance cavities. The consistence between the coupled-model theory and FDTD simulation results verify the feasibility of the proposed system. Since the scheme is easy to be fabricated, our proposed configuration may thus be applied to narrow-band filtering, absorptive switching, and absorber applications. [ABSTRACT FROM AUTHOR]

We investigate numerically the modification of the nonlinear optical properties of a nanoantenna in the trapping of nanoparticles (NPs) by using both the discrete dipole approximation method and the finite-difference time-domain technique. The nanoantenna, which is formed by two gold nanorods (GNRs) aligned end to end and separated by a small gap, can emit strong two-photon-induced luminescence (TPL) under the excitation of a femtosecond laser light which is resonant with its longitudinal surface plasmon resonance. In addition, the excited antenna can stably trap small NPs which in turn induce modifications in the emitted TPL. These two features make it a promising candidate for building highly sensitive detectors for NPs of different materials and sizes. It is demonstrated that sensors built with antennas possess higher sensitivities than those built with single GNRs and nanorod-based antennas are more sensitive than nanoprism-based antennas. In addition, it is found that the trapping probability for a second NP is significantly reduced for the antenna with a trapped NP, implying that trapping of NPs may occur sequentially. A relationship between the TPL of the system (antenna + NP) and the optical potential energy of the NP is established, enabling the extraction of the information on the optical potential energy and optical force by recording the TPL of the system. It is shown that the sequential trapping and releasing of NPs flowing in a microfluid channel can be realized by designing two different antennas arranged closely. [ABSTRACT FROM AUTHOR]

The toroidal dipole (TD) has attracted growing attention due to its unique properties. Here, we propose and demonstrate almost pure TD resonance in the visible region in a silicon hollow cylinder. The enhanced optical coupling to TD resonance is implemented using a focused radially polarized beam illumination matching, well-designed individual silicon nanostructure resonator. The polarization of the longitudinal electric field in the silicon hollow cylinder that breaks space-inversion symmetry is critical to the formation of enhanced TD resonance. Additionally, the pure TD resonance can be achieved in a wide spectral range by tuning the geometrical parameters of the structure. The proposed pure TD resonator may provide potential applications in the local enhancement of electromagnetic fields and the design of all-dielectric nanoantennas. [ABSTRACT FROM AUTHOR]

In this study, the photothermal effect and up-conversion florescence imaging effect of gold nanobipyramids in liver cancer cells are investigated theoretically and experimentally to explore the photothermal ablation tumor therapy with higher photothermal conversion efficiency, shorter laser action time, smaller action range and lower laser power. The small-size gold nanobipyramids with good biocompatibility and infrared absorption peak located in the first biological window are synthesized. Femtosecond laser is focused on the nanobipyramids clusters in cells and the cells die after being irradiated for 20 s at a power as low as 3 mW. In contrast, the control cells die after irradiation with 30 mW laser for 3 min. The theoretical simulation results show that: under femtosecond laser irradiation, the local thermal effect of gold nanoclusters is produced in the range of hundreds of square nanometers and the temperature rises by 516 °C in 106 picoseconds. This therapy reduces the treatment time to seconds level, and the treatment range to square micrometer level, the power to milliwatt level. In this treatment, cells die by apoptosis rather than necrosis, which reduces inflammation. This result opens up a new way to develop photothermal ablation therapy with less side effects and more minimally invasive. [ABSTRACT FROM AUTHOR]

Metasurfaces, employed to simultaneously generate nanoprinting and holographic images, have been extensively explored recently. Among them, multi-wavelength multiplexing in a single metasurface is often accompanied by dispersion and crosstalk, which hinder the display of multicolor patterns. Here, we propose an efficient phase method to decouple the wavelength and realize a three-channel display operating at different wavelengths. Holographic images appear in the far field with the illumination of two different circularly polarized lights while a nanoprinting image is reconstructed by inserting an orthogonal optical path with the illumination of linear polarization light. The proposed metasurface is only composed of four types of unit cells, which significantly decreases the complexity of fabrication and improves the information capacity. Benefiting from its different decoding strategies and capability of multi-wavelength control, this approach may develop broad applications in information encryption, security, and color display. [ABSTRACT FROM AUTHOR]

Nanostructures with appropriate sizes can limit light-matter interaction and support electromagnetic multipole resonance. The interaction between light and nanostructures is intimately related to manipulating the direction of scattered light in the far field as well as the electromagnetic field in the near field. In this paper, we demonstrate dual-wavelength directional forward-scattering enhancement in an individual open-hole silicon nanoblock (OH-SiNB) and simultaneously achieve bulk and surface electromagnetic field localization. The second harmonic generation is enhanced using electromagnetic field localization on the square hole surface. Numerical simulations reveal that the resonance modes, at λ1 = 800 nm and λ2 = 1190 nm, approximately satisfy the Kerker condition. In the near field, the magnetic dipole modes at dual wavelength all satisfy the boundary condition that the normal component of the electric displacement is continuous on the square holes surface, thus obtaining the surface electromagnetic field localization. Moreover, highly efficient second harmonic generation can be achieved at dual wavelengths using the surface electromagnetic field localization and the increased surface area of the square holes. Our results provide a new strategy for the integration of nanoantennas and nonlinear optoelectronic devices in optical chips. [ABSTRACT FROM AUTHOR]

The spin-polarized conversion and splitting of beam are highly important for photonic researches and applications. Although the photonic spin Hall effect (PSHE) realized by the Pancharatnamâ€"Berry (PB) metasurface has shown unprecedented capabilities to control spin-polarized light, spin-polarized beam splitting metadevices suffer from the limitations of low-efficiency. Here, we present a highly efficient spin-polarized beam splitter (SPBS) based on PB metasurface comprising silicon nano elliptical cylinder (Si-NEC) arrays. Because of the electromagnetic multipole resonance inside the designed Si-NECs, the PB metasurface can achieve high transmittance and enhanced PSHE. Therefore, the SPBS based on the PB metasurface can achieve a high spin conversion efficiency of nearly 100%, while ensuring a transmittance of 87% at 622 nm wavelength. It can also maintain a good working effect within the bandwidth of 600â€"660 nm. Furthermore, by introducing spatial shift between the two reverse Si-NEC arrays, the SPBS can also be used to realize 45° polarization rotation of an incident linearly polarized light, avoiding the input polarization angle dependence. Our design may have potential applications in high-performance and broadband spin-photonic devices. [ABSTRACT FROM AUTHOR]

• Transformation of BICs to high-Q quasi-BICs: By maintaining the C 2 symmetry of the structure while perturbing the structural parameters, we achieve the transformation from BICs to quasi-BICs, reducing loss to enhance the utilization of quasi-BICs. • Effective mode distribution with high Q-factor: Design a dimer structure to achieve a range of effective modes. Patterns such as TD, MD and EQ are generated and distributed in and around the structure, amplifying the electromagnetic field. • Advancement in metasurface and material interaction: The longitudinal radiation modes effectively enhance the interaction between light and matter. This work substantially improves the SHG on the two-dimensional materials. Quasi-bound states in the continuum (Quasi-BIC) possess a high quality-factor (Q-factor) that can amplify local electromagnetic fields and nonlinear effects such as second harmonic generation (SHG). In this paper, we propose a silicon metasurface with C 2 symmetry. By tuning the structural parameters without breaking the C 2 symmetry, three symmetric protected BICs (SP-BICs) are transformed into sharp quasi-BICs dominated by different modes in the nearinfrared spectrum. The quasi-BICs exhibit Q-factors as high as 10370 in C 2 symmetric metasurface. Numerical simulations reveal that the toroidal dipole (TD) and magnetic dipole (MD) enhance intercellular coupling and local electromagnetic fields, laying the foundation for generating metasurface with high Q-factor. Furthermore, the longitudinal radiative component of electric quadrupole (EQ) outside the facet promotes enhanced light interaction with the 2D material. Leveraging the robust local electromagnetic fields and radiation characteristics of these quasi-BICs, we have achieved substantial SHG enhancement across multiple wavelengths under continuous wave (CW) operation. The SHG from 2D GaSe flake reaches 14189-fold enhancement. Our results not only offer insights into designing metasurface structures with high Q-factors but also explore the methods of enhancing the interaction between metasurface and 2D materials. The proposed nanostructure has numerous potential applications in nonlinear optics, optoelectronics and signal processing. [ABSTRACT FROM AUTHOR]

Excellent characteristics and promising application prospects promote the rapid development of metamaterials. We have numerically proposed and demonstrated a novel subwavelength broadband metamaterial perfect absorber (BMPA) based on diamond dielectric arrays. The proposed absorber is composed of an ultra-thin two-layer structure covering the dielectric periodic array on a metal substrate. The materials of dielectric silicon (Si) and gold (Au) substrate are discussed in detail. In addition, different dielectric and refractory materials are also applied to achieve broadband absorption, which will make the proposed absorber greatly broaden the application field. A perfect absorption window (i.e., absorption rate exceeding 90%) can be obtained from near-ultraviolet to the visible range. The average absorption rate of 93.3% is achieved in the visible range. The results of multipole decomposition show that broadband absorption is mainly caused by electromagnetic dipole resonance and lattice resonance in a periodic array of Si. The proposed absorber can be extended freely by adjusting the structural parameters. The polarization-independent and incident angle insensitivity are proved. The proposed absorber may well be used in light energy acquisition, as well as for the scalability of optoelectronic and sensing devices. [ABSTRACT FROM AUTHOR]

In this work, we proposed an all-dielectric nanostructure composed of silicon concentric nanoring resonator dimers (Si CNRD). We studied the scattering properties of Si CNRD and demonstrated the magnetic Fano resonance resulting from the interference between a narrow magnetic dipole mode and a broad magnetic dipole mode coupled into Si CNRD excited by an azimuthally polarized beam (APB). The spectral characteristics of the magnetic Fano resonance are characterized quantitatively by a coupled oscillator model. By changing the geometric parameters of the Si CNRD, the wavelength and intensity of the magnetic Fano resonance can be effectively adjusted. The nanostructure we proposed exhibits potential applications in low-loss sensors, nonlinearity, and laser devices. [ABSTRACT FROM AUTHOR]

Artificial control of colors based on plasmonic and dielectric nanostructures is not only interesting for fundamental research, but also important for practical applications. Apart from spatial resolution, chromaticity, angular independence, and dynamical color-tuning have emerged as the most desirable functions for structural color. Here, it is shown theoretically that the orientations of the electric or magnetic dipoles induced in a silicon nanoparticle can be manipulated by using an evanescent wave excited by s- or p-polarized light, and it is experimentally demonstrated that the scattering light color can be controlled by simply varying the polarization of the illumination light. It is revealed that the excitation of a silicon nanoparticle using an evanescent wave is equivalent to that of using both the incident and reflected light, enabling the manipulation of the scattering light color by controlling the orientations of the electric and magnetic dipoles. In addition, an enhanced scattering light intensity and a completely dark background are achieved by using evanescent wave excitation. More importantly, a color-tuning display with a spatial resolution close to the optical diffraction limit, and a good chromaticity on an array of silicon nanopillars fabricated on a silica substrate is demonstrated, which are compatible with the current fabrication technology of silicon chips. [ABSTRACT FROM AUTHOR]

The nonlinear optical properties of single gold nanorods (GNRs) with a large diameter of ∼200 nm and a long length of ∼800 nm were investigated by using a focused femtosecond (fs) laser light with tunable wavelength. While the linear and nonlinear optical properties of small-sized GNRs have been extensively studied, the nonlinear optical properties of large-sized GNRs and the effects of high-order surface plasmon resonances remain unexplored. Second harmonic generation (SHG) or/and two-photon-induced luminescence (TPL) were observed in the nonlinear response spectra, and their dependences on excitation wavelength and polarization were examined. The scattering and absorption spectra of the small- and large-sized GNRs were compared by using the discrete dipole approximation method. It was found that the extinction of large-sized GNRs is dominated by scattering rather than absorption, which is dominant in small-sized GNRs. In addition, it was revealed that the excitation wavelength-dependent SHG of a GNR is governed by the linear scattering of the GNR and the maximum SHG is achieved at the valley of the scattering spectrum. In comparison, the excitation wavelength dependence of TPL is determined by the absorption spectrum of the GNR. The polarization-dependent SHG of a GNR exhibits a strong dependence on the dimension of the GNR, and it may appear as bipolar distributions parallel or perpendicular to the long axis of the GNR or multipole distributions. [ABSTRACT FROM AUTHOR]

Abstract: Ripples with a subwavelength period were induced on the surface of a stainless steel (301 L) foil by femtosecond laser pulses. By optimizing the irradiation fluence of the laser pulses and the scanning speed of the laser beam, ripples with large amplitude (∼150nm) and uniform period could be obtained, rendering vivid structural colors when illuminating the surface with white light. It indicates that these ripples act as a surface grating that diffracts light efficiently. The strong dependence of the ripple orientation on the polarization of laser light offers us the opportunity of decorating different regions of the surface with different types of ripples. As a result, different patterns can be selectively displayed with structural color when white light is irradiated on the surface from different directions. More interestingly, we demonstrated the possibility of decorating the same region with two or more types of ripples with different orientations. In this way, different patterns with spatial overlapping can be selectively displayed with structural color. This technique may find applications in the fields of anti-counterfeiting, color display, decoration, encryption and optical data storage. [Copyright &y& Elsevier]

A novel multifunctional device based on a hybrid metal–graphene Electromagnetically induced transparency (EIT) metamaterial at the terahertz band is proposed. It is composed of a parallel cut wire pair (PCWP) that serves as a dark mode resonator, a vertical cut wire pair (VCWP) that serves as a bright mode resonator and a graphene ribbon that serves as a modulator. An ultra-broadband transmission window with 1.23 THz bandwidth can be obtained. The spectral extinction ratio can be tuned from 26% to 98% by changing the Fermi level of the graphene. Compared with previous work, our work has superior performance in the adjustable bandwidth of the transmission window without changing the structure of the dark and bright mode resonators, and has a high extinction ratio and dynamic adjustability. Besides, we present the specific application of the device in filters and optical modules. Therefore, we believe that such a metamaterial structure provides a new way to actively control EIT-like, which has promising applications in broadband optical filters and photoelectric intensity modulators in terahertz communications. [ABSTRACT FROM AUTHOR]

The photothermal kinetics of hollow gold nanorod (HGNR) under femtosecond laser irradiation are studied numerically with finite-element methods and a two-temperature model. Compared with solid gold nanorod (SGNR) with the same aspect ratio (AR), the localized surface plasmon resonance (LSPR) peak of HGNR can be red-shifted to the second near-infrared window, and the absorption cross-section of HGNR can be larger than that of SGNR. In addition, under the influence of an applied numerically electromagnetic field (simulated femtosecond laser irradiation), the heat generated by HGNR makes the temperature rise of the surrounding medium faster and higher. Compared with SGNR with the same resonance wavelength, HGNR has a slightly smaller absorption cross-section but can achieve a higher temperature rise of the external medium. In addition, the laser energy, required to achieve the critical temperature for selective photothermal damage of tumor cells, is also significantly reduced. Moreover, with the same incident laser energy, the decreasing of HGNR shell thickness leads to an increase of the temperature rise of the external medium, while the change of femtosecond laser pulse width will not significantly change the temperature rise of its lattice and the external medium. In short, this study aims to provide some useful insights for the applications of HGNR in photothermal tumor therapy. [ABSTRACT FROM AUTHOR]

Gallium (Ga) is not only a metal possessing surface plasmon resonance in the ultraviolet region but also a phase‐change material with very low melting point. Spherical Ga nanoparticles composed of liquid Ga cores and solid Ga2O3 shells, which exhibit surface plasmonic resonances spanning the ultraviolet to near‐infrared spectral region, are fabricated by using femtosecond laser ablation. Under the excitation of femtosecond laser light, only second harmonic generation is observed in the Ga nanospheres placed on a glass substrate. In sharp contrast, white light emission can be generated in such Ga nanospheres when they are placed on a thin silver film. It is revealed numerically and confirmed experimentally that a Fano resonance originating from the interference between the mirror‐image‐induced magnetic dipole mode and the gap plasmon mode is formed in the backward scattering spectra of the Ga nanospheres. Efficient white light emission can be achieved by resonantly exciting either the magnetic dipole resonance or the Fano resonance. Based on spectral analysis, the white light emission is identified as the hot‐electron intraband luminescence. The fabricated core–shell Ga nanospheres provide an ideal platform on which the optical properties and practical applications of liquid Ga nanoparticles can be systematically investigated. [ABSTRACT FROM AUTHOR]

We proposed the use of the evanescent wave generated in a total internal reflection configuration to excite large gold nanospheres and investigated the radiations of the high-order plasmon modes supported in gold nanospheres. It was revealed that the evanescent wave excitation is equivalent to the excitation by using both the incident and reflected light, offering us the opportunity to control the orientation of the electric field used to excite nanoparticles. In addition, it was found that the scattering light intensity is greatly enhanced and the background noise is considerably suppressed, making it possible to detect the radiations from high-order plasmon modes. Moreover, the influence of the mirror images on the scattering induced by a metal substrate is eliminated as compared with the surface plasmon polariton excitation. By exciting a gold nanosphere with s-polarized light and detecting the scattering light with a p-polarized analyzer, we were able to reveal the radiation from the electric quadrupole mode of the gold nanosphere in both the spatial and the frequency domains. Our findings are important for characterizing the radiations from the high-order modes of large nanoparticles and useful for designing nanoscale photonic devices. [ABSTRACT FROM AUTHOR]

In this paper, the plasmon resonance effects of gold nanorods was used to achieve rapid photothermal therapy for malignant melanoma cells (A375 cells). After incubation with A375 cells for 24 h, gold nanorods were taken up by the cells and gold nanorod clusters were formed naturally in the organelles of A375 cells. After analyzing the angle and space between the nanorods in clusters, a series of numerical simulations were performed and the results show that the plasmon resonance coupling between the gold nanorods can lead to a field enhancement of up to 60 times. Such high energy localization causes the temperature around the nanorods to rise rapidly and induce cell death. In this treatment, a laser as low as 9.3 mW was used to irradiate a single cell for 20 s and the cell died two h later. The cell death time can also be controlled by changing the power of laser which is focused on the cells. The advantage of this therapy is low laser treatment power, short treatment time, and small treatment range. As a result, the damage of the normal tissue by the photothermal effect can be greatly avoided. [ABSTRACT FROM AUTHOR]