Gap Surface Plasmon Research Articles

High-efficiency reflection phase tunable metasurface at near-infrared frequencies* *Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFA0200400, 2016YFA0200800, and 2016YFA0300601), the National Natural Science Foundation of China (Grant Nos. 61888102,11674387, 11974386, and 61905274), the Strategic Priority Research Program and Key Research Program of Frontier Sciences of Chinese Academy of Sciences (Grant Nos. XDB33000000, XDB28000000,

The realization of active modulation of reflection phase based on metasurfaces is of great significance for flexible control of electromagnetic wavefront, which makes metasurfaces have practical application values in polarization conversion, beam steering, metalens, etc. In this paper, a reflection phase tunable gap-surface plasmon (GSP) metasurface based on phase change materials Ge2Sb2Te5 (GST) is designed and experimentally demonstrated. By virtue of the characteristics of large permittivities difference before and after GST phase transition and the existence of stable intermediate states, the continuous modulation of near-infrared reflection phase larger than 200° has been realized. At the same time, through the reasonable design of the structure sizes, the reflection has been maintained at about 0.4 and basically does not change with the GST phase transition, which improved the working efficiency of the metasurface significantly. In addition, the coupled-mode theory (CMT) is introduced to make a full analysis of the modulation mechanism of the reflection phase, which proves that the phase transition of GST can induce the transition of metasurface working state from overcoupling mode to critical coupling mode. The improvement of the metasurface working efficiency has practical values for wavefront modulation.

Topological-Insulator-Based Gap-Surface Plasmon Metasurfaces

Topological insulators (TIs) have unique highly conducting symmetry-protected surface states while the bulk is insulating, making them attractive for various applications in condensed matter physics. Recently, topological insulator materials have been tentatively applied for both near- and far-field wavefront manipulation of electromagnetic waves, yielding superior plasmonic properties in the ultraviolet (UV)-to-visible wavelength range. However, previous reports have only demonstrated inefficient wavefront control based on binary metasurfaces that were digitalized on a TI thin film or non-directional surface plasmon polariton (SPP) excitation. Here, we numerically demonstrated the plasmonic capabilities of the TI Bi2Te3 as a material for gap–surface plasmon (GSP) metasurfaces. By employing the principle of the geometric phase, a far-field beam-steering metasurface was designed for the visible spectrum, yielding a cross-polarization efficiency of 34% at 500 nm while suppressing the co-polarization to 0.08%. Furthermore, a birefringent GSP metasurface design was studied and found to be capable of directionally exciting SPPs depending on the incident polarization. Our work forms the basis for accurately controlling the far- and near-field responses of TI-based GSP metasurfaces in the visible spectral range.

Enhanced nonlinear optical effects based on strong coupling between epsilon-near-zero mode and gap surface plasmons

Optical nonlinear effect plays an important role in optical communication, optical detection, quantum information and other areas. However, it is constrained by the weakness of the nonlinear optical response of the common materials. The enhancement of the optical nonlinear response on a nanoscale becomes a critical challenge. Over the years, several ways to enhance the optical nonlinear effects have been suggested. In fact, these technologies can slightly enhance the optical nonlinear response. Recently, some research groups focused on the materials with vanished permittivity, which is called epsilon-near-zero (ENZ) material, showing that it can exhibit large optical nonlinearity due to the field enhancement in the material of this type. However, the ENZ material only holds a large optical nonlinear response in a limited spectral range. In order to overcome this limitation, here in this paper we report the ENZ mode which is excited by the ITO film and strongly coupled to the gap surface plasmons excited by the metal-dielectric-metal structure. To acquire the nonlinear refractive index <i>n</i><sub>2</sub>, we first calculate the ITO permittivity through the Drude-Lorentz model and find the wavelength of the ENZ material. Then we calculate the time-dependent electron temperature and lattice temperature of ITO by the two-temperature model. According to the elevated electron temperature, we can calculate the plasma frequency <inline-formula><tex-math id="M1">\begin{document}${\omega _{\rm p}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M1.png"/></alternatives></inline-formula>, and by taking it into the Drude-Lorentz model, we can obtain a new permittivity of ITO compared with the initial one. Finally, we can calculate the variation of the refractive index <inline-formula><tex-math id="M2">\begin{document}$ \Delta n $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M2.png"/></alternatives></inline-formula>, and the nonlinear refractive index <inline-formula><tex-math id="M3">\begin{document}$ {n_2} = \Delta n/{I_0} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20210290_M3.png"/></alternatives></inline-formula>. In this paper, our coupled structure exhibits a broadband (～1000 nm bandwidth) enhancement of the nonlinear optical effect in the near-infrared spectrum, a maximum nonlinear refractive index <i>n</i><sub>2</sub> as large as 3.02 cm<sup>2</sup>·GW<sup>–1</sup>, which is nearly 3 orders larger than the previously reported nonlinear refractive index of bare ITO film. As a result, it is possible to realize a dramatically large variation of nonlinear refractive index under a low-power optical field. It is expected to be used in the nano photonic devices such as optical storage, all-optical switches, etc.

Enhancing plasmonic hot-carrier generation by strong coupling of multiple resonant modes.

Plasmon-induced hot carriers have recently attracted considerable interest, but the energy efficiency in visible light is often low due to the short lifetime of hot carriers and the limited optical absorption of plasmonic architectures. To increase the generation of hot carriers, we propose to exert multiple plasmonic resonant modes and their strong coupling using a metal-dielectric-metal (MDM) nanocavity that comprises an Au nanohole array (AuNHA), a TiO2 thin film and an Au reflector. Unlike common MDM structures, in addition to the Fabry-Pérot mode in the dielectric layer, AuNHA as the top layer is special because it excites the localized surface plasmon resonance (LSPR) mode in the Au nanoholes and launches the gap surface plasmon polariton (GSPP) mode in the Au reflector surface. The spatial field overlapping of the three resonance modes enables strong mode coupling by optimizing the TiO2 thickness, which leads to notably enhanced average IPCE (∼1.5%) and broadband photocurrent (170 μA·cm-2). This MDM structure would be useful for photochemistry and photovoltaics using sunlight.

Demonstration of > 2\u03c0 reflection phase range in optical metasurfaces based on detuned gap-surface plasmon resonators

Plasmonic metasurfaces, representing arrays of gap-surface plasmon (GSP) resonators and consisting of arrays of metal nanobricks atop thin dielectric layers supported by thick metal films, constitute an important subclass of optical metasurfaces operating in reflection and enabling the realization of numerous, diverse and multiple, functionalities. The available phase variation range is however limited to being <! 2pi, a circumstance that complicates the metasurface design for functionalities requiring slowly varying phases over the whole range of 2pi, e.g., in holographic applications. The available phase range also determines the wavelength bandwidth of metasurfaces operating with linearly polarized fields due to the propagation (size-dependent) nature of the reflection phase. We suggest an approach to extend the phase range and bandwidth limitations in the GSP-based metasurfaces by incorporating a pair of detuned GSP resonators into a metasurface elementary unit cell. With detailed simulations related to those for conventional single-resonator metasurfaces and proof-of-concept experiments, we demonstrate that the detuned-resonator GSP metasurfaces designed for beam steering at {900},,hbox {nm} wavelength exhibit the extended reflection phase and operation bandwidth. We believe that the considered detuned-resonator GSP metasurfaces can advantageously be exploited in applications requiring the design of arbitrary phase gradients and/or broadband operation with linearly polarized fields.

Direct Visualization of Gap-Plasmon Propagation on a Near-Touching Nanowire Dimer.

Dimers of metallic nanowires (NWs) with nanometric gaps could be an alternative to overcome the limitations of existing plasmonic waveguides. The gap-surface plasmon polaritons (gap-SPPs) of the dimers may propagate along the NW without crosstalk and greatly enhance the coupling efficiency with an emitter, enabling ultracompact optical circuits. Such a possibility has not been realized, and we experimentally show its possibility. The gap-SPPs of the AgNW-molecule-AgNW structure, with a gap of 3-5 nm defined by the molecules, are visualized using the surface-enhanced Raman scattering (SERS) of the molecules. The SERS images, representing the gap-field intensity distribution, reveal the decay and beating of the monopole-monopole and dipole-dipole gap modes. The propagation lengths of the two (l1 = 0.5-2 μm and l2 = 5-8 μm) closely follow the model prediction with a uniform gap, confirming that the scattering loss induced by the gap irregularities is surprisingly low.

Characterization of gap-plasmon based metasurfaces using scanning differential heterodyne microscopy

Optical phase-gradient metasurfaces, whose unique capabilities are based on the possibility to arbitrarily control the phase of reflected/transmitted light at the subwavelength scale, are seldom characterized with direct measurements of phase gradients. Using numerical simulations and experimental measurements, we exploit the technique of scanning differential heterodyne microscopy (SDHM) for direct phase and amplitude characterization of gap-plasmon based optical metasurfaces. Two metasurface configurations utilizing the third-order gap surface plasmon (GSP) resonance, representing a binary grating and linear phase gradient, are experimentally characterized with the SDHM operating at the light wavelength of 633 nm. Comparing the experimental performances of these GSP metasurfaces with those expected from the phase and amplitude profiles reconstructed from the SDHM measurements, we verify the efficiency and accuracy of the developed SDHM characterization approach for direct inspection of GSP reflective metasurfaces.

High-efficiency focused optical vortex generation with geometric gap-surface plasmon metalenses

Optical vortices (OVs) carrying orbital angular momentum have been an emerging research area due to their widespread applications ranging from optical communication to particle trapping. However, conventional OV generators suffer from bulky configurations and limited performance. Here, we design and experimentally demonstrate high-efficiency focused OV generation by using geometric gap-surface plasmon metalenses with spin-flipped focal planes, where the polarization states, positions, and corresponding topological charges of the OVs can be controlled by changing the spin of incident circularly polarized light, reproducing thereby the combined functionalities of a lens and a q-plate. The fabricated dual-polarity OV metalens features the polarization conversion ratios above 88% and efficiencies exceeding 50% for both right-handed and left-handed circularly polarized light in a wide wavelength range of 850 to 1000 nm. We further extend this approach to realize a multiple-focal OV metalens with distinct and spin-multiplexed OVs at four different longitudinal focal planes. Compared to conventional OV generators, the proposed OV metalenses are compact, ultrathin, and multiplexed, therefore opening an innovative avenue for advanced research and applications aiming at multiple functionalities and dense integration in photonics.

Color Gamut Behavior in Epsilon Near‐Zero Nanocavities during Propagation of Gap Surface Plasmons

AbstractThis work reports on numerical and experimental results obtained in plasmonic metal–insulator nanocavities. The systems are composed of silver as metal, and different materials as insulator, namely polyvinylpyrrolidone (PVP), indium tin oxide (ITO), and zinc oxide (ZnO). The proposed nanocavities exhibit extraordinary optical effects as tunable color hue, highlighted in gamut maps, depending on incident/viewing angles, extraordinary transmission and zero reflection at resonant wavelengths, for different incident polarizations. These phenomena are related to the formation of surface plasmon polaritons (SPPs) and gap surface plasmons (GSPs) whose presence is evidenced by a remarkable sigmoidal behavior of the pseudo dielectric function, with epsilon‐near‐zero singularities in its real and imaginary parts. This function is directly calculated from the measured ellipsometric parameters Ψ and Δ and allows probing, in a fast and effective way, the existence of the plasmonic modes. Moreover, in presence of these singularities, the ellispometric analysis of the systems also shows a pronounced dephasing between p‐ and s‐reflected beams that can lead to a Goos–Hänchen shift effect to be exploited for sensing applications. Thanks to their unusual optical properties, the proposed nanocavities open a wide scenario of applications in fields like tunable color filters, optics, photonics, physical security, and sensing.

A Thermal Tuning Meta-Duplex-Lens (MDL): Design and Characterization.

Multifunctional metasurfaces play an important role in the development of integrated optical paths. However, some of the realizations of current multifunctional metasurface devices depend on polarization selectivity, and others change the polarization state of the outgoing light. Here, based on vanadium dioxide (VO2) phase change material, a strategy to design a meta-duplex-lens (MDL) is proposed and numerical simulation calculations demonstrate that at low temperature (about 300 K), VO2 behaves as a dielectric so that the MDL can act as a transmission lens (transmission efficiency of 87.6%). Conversely, when VO2 enters the metallic state (about 355 K), the MDL has the ability to reflect and polymerize electromagnetic waves and works as a reflection lens (reflection efficiency of 85.1%). The dielectric waveguide and gap-surface plasmon (GSP) theories are used in transmission and reflection directions, respectively. In order to satisfy the coverage of the phase gradient in the range of 2π in both cases, we set the antenna as a nanopillar with a high aspect ratio. It is notable that, via symmetrical antennas acting in concert with VO2 phase change material, the polarization states of both the incident light and the outgoing light are not changed. This reversible tuning will play a significant role in the fields of imaging, optical storage devices, communication, sensors, etc.

Gap-surface plasmon metasurfaces for linear-polarization conversion, focusing, and beam splitting

Gap-surface plasmon (GSP) metasurfaces have attracted progressively increasing attention due to their planar configurations, ease of fabrication, and unprecedented capabilities in manipulating the reflected fields that enable integrating diverse bulk-optic-based optical components into a single ultrathin flat element. In this work, we design and experimentally demonstrate multifunctional metalenses that perform simultaneous linear-polarization conversion, focusing, and beam splitting, thereby reproducing the combined functionalities of conventional half-wave plates, parabolic reflectors, and beam splitters. The fabricated single-focal metalens incorporates properly configured distinct half-wave-plate-like GSP meta-atoms and exhibits good performance under linearly polarized incidence in terms of orthogonal linear-polarization conversion ( &gt; 75 % ) and focusing (overall efficiency &gt; 22 % ) in the wavelength spectrum ranging from 800 to 950 nm. To further extend the combined functionalities, we demonstrate a dual-focal metalens that splits and focuses a linearly polarized incident beam into two focal spots while maintaining the capability of orthogonal linear-polarization conversion. Furthermore, the power distribution between two split beams can readily be controlled by judiciously positioning the incident beam. The demonstrated multifunctional GSP-based metalenses mimic the combined functionalities of a sequence of discrete bulk optical components, thereby eliminating the need for their mutual alignment and opening new perspectives in the development of ultracompact and integrated photonic devices.

Subwavelength high-performance polarizers in the deep ultraviolet region.

This study shows that high-performance metasurface polarizers are possible in the deep ultraviolet (DUV) region. A metasurface consisting of a trilayer of an Al (Aluminum) grating, an Al2O3 (Alumina) thin film, and an Al thin film serves as a mirror with high reflectance for light with polarization parallel to the grooves. The reflectance for polarization perpendicular to the grooves vanishes owing to destructive interference between the propagating and gap surface plasmon polaritons. As a result, the metasurface plays the role of a polarizer with a high extinction ratio exceeding 6.2 × 106. This study also shows that the bandwidth is improved by adjusting the incident angle, period, and the Al2O3 layer of the metasurface. The performance of the metasurface polarizer is discussed experimentally. Our finding paves the way for realizing high-performance polarizers in the DUV region.

Polarization-controlled bifunctional metasurface for structural color printing and beam deflection.

We propose a polarization-controlled bifunctional metasurface composed of arrayed trapezoidal nanoantennas. Under orthogonal-polarized incidence, different types of gap-surface plasmons are generated, regulating the intensity and phase, respectively. Thus, structural color printing and beam deflection functions are achieved on a miniaturized chip. The color printing function works from 400 to 800 nm, exhibiting a subwavelength-scale chromatic image with a broad gamut. The beam deflection function works from 360 to 540 nm, mapping light to the first diffraction order with the anomalous angle from 40.4° to 76.6°. The proposed bifunctional metasurface could serve as a key component in integrated optics systems and will find many other wide-ranging applications in optical and biological areas.

Accessing the Hotspots of Cavity Plasmon Modes in Vertical Metal–Insulator–Metal Structures for Surface Enhanced Raman Scattering

AbstractIn metal–insulator–metal structures such as Au nanodiscs on an Au film separated by a thin oxide spacer layer, the metal/insulator interfaces can form a Fabry–Perot cavity in which gap surface plasmons are confined. As a result, these cavity plasmon modes show the advantageous properties of strong field enhancement and very narrow resonance line shapes due to the confinement in a cavity. In this work, the hotspots of the electromagnetic near‐field of cavity plasmon modes are exposed to make the strongly enhanced field accessible to Raman molecules. This is achieved by creating an undercut in the spacer between the Au discs and the gold film by wet etching of the spacer. The electromagnetic field distributions as well as the resonance wavelengths of two different cavity plasmon modes in such cavities both with and without undercut are studied by simulations based on the finite element method. The size of the undercut is estimated by relating the measured spectral shift before and after reducing the spacer disc to the one obtained in simulations. Surface enhanced Raman scattering measurements are conducted on arrays of cavities. The effect of the undercut on the Raman scattering intensity is investigated and a notable increase is found.

Nanoscale quantum plasmon sensing based on strong photon–exciton coupling

We propose a scheme of quantum plasmon sensing system based on strong photon–exciton coupling in the gap surface plasmon nanostructure. The system’s sensitivity is characterized as Rabi splitting, which is sensitive to a slight change in environmental permittivity and determined by the coupling coefficient and detuning between the emitter and plasmon nanocavity. By increasing the dipole moment of the emitter, the sensitivity can exceed that of a traditional plasmon sensing system while only depending on the resonance spectral shift. Quantum plasmon sensing provides a unique mechanism in the application of bio-sensing, opto-chemical sensing, and quantum photonics at the nanoscale.

Dual-Band Metasurfaces Using Multiple Gap-Surface Plasmon Resonances.

Metasurfaces operating at multiple spectral ranges with integrated diversified functionalities while retaining the flexible design strategy are highly desired within the area of modern flat optics. Here, we propose and demonstrate the use of multiple gap-surface plasmon (GSP) resonances for the realization of dual-band multifunctional metasurfaces by designing GSP meta-atoms that would resonate at two different wavelengths. By tailoring nanobrick dimensions of a simple GSP meta-atom so as to enable both the first-order resonance at 1450 nm and the third-order one at 633 nm, we design phase-gradient GSP metasurfaces for polarization-independent beam steering and polarization-splitting, simultaneously, at telecom (1350-1550 nm) and visible (575-675 nm) wavelengths. The fabricated metasurfaces show good performance with >65% diffraction efficiency at the first-order resonant wavelength of 1450 nm and over 50% efficiency within the telecom range of 1350-1550 nm, while at the third-order resonant wavelength of 633 nm, the diffraction efficiency is 20 and >10% within the visible range of 575-675 nm. Our findings, therefore, demonstrate a flexible and robust approach for the realization of efficient dual-band GSP metasurfaces that can readily be combined with complex integrated designs to implement multiple functionalities highly sought after for diverse applications.

Focused vortex-beam generation using gap-surface plasmon metasurfaces

Abstract In spite of a wide range of applications ranging from particle trapping to optical communication, conventional methods to generate vortex beams suffer from bulky configurations and limited performance. Here, we design, fabricate, and experimentally demonstrate orthogonal linear-polarization conversion and focused vortex-beam generation simultaneously by using gap-surface plasmon metasurfaces that enable high-performance linear-polarization conversion along with the complete phase control over reflected fields, reproducing thereby the combined functionalities of traditional half-wave plates, lenses, and q-plates. The fabricated metasurface sample features the excellent capability of orthogonal linear-polarization conversion and focused vortex-beam generation within the wavelength range of 800–950 nm with an averaged polarization conversion ratio of ~80% and absolute focusing efficiency exceeding 27% under normal illumination with the x-polarized beam. We further show that this approach can be extended to realize a dual-focal metasurface with distinctly engineered intensity profiles by using segmented metasurfaces, where an orthogonal-polarized beam with Gaussian-distributed intensity and a vortex beam with intensity singularity have been experimentally implemented. The proposed multifunctional metasurfaces pave the way for advanced research and applications targeting photonics integration of diversified functionalities.

Gate-Tunable Emission of Exciton–Plasmon Polaritons in Hybrid MoS2-Gap-Mode Metasurfaces

The advance in designing arrays of ultrathin two-dimensional optical nano-resonators, known as metasurfaces, is currently enabling a large variety of novel flat optical components. The remarkable control over the electromagnetic fields offered by this technology can be further extended to the active regime in order to manipulate the light characteristics in real-time. In this contribution, we couple the excitonic resonance of atomic thin MoS2 monolayers with gap-surface-plasmon (GSP) metasurfaces, and demonstrate selective enhancement of the exciton-plasmon polariton emissions. We further demonstrate tunable emissions by controlling the charge density at interface through electrically gating in MOS structure. Straddling two very active fields of research, this demonstration of electrically tunable light-emitting metasurfaces enables real-time manipulation of light-matter interactions at the extreme subwavelength dimensions.

Tunable Hybrid Gap Surface Plasmon Polariton Waveguides with Ultralow Loss Deep-Subwavelength Propagation

Exploring hybrid gap surface plasmon polariton waveguides (HGSPPWs) is an important milestone in developing the next-generation, nanoscale integrated photonic circuit technology. To advance their potential applications, HGSPPWs are required to have tunable capability, highly reliable, simple fabrication process, and feasible integration. In this paper, we propose two tunable HGSPPWs fulfilling the requirements. The proposed HGSPPWs consist of a metallic wedge laterally coupled with a dielectric waveguide. The modal characteristics of HGSPPWs are investigated at the optical telecommunication wavelength, which shows the modal characteristics could be effectively controlled by tuning the key geometry parameters and structure of HGSPPWs. The propagation length could achieve the centimeter scale while maintaining the propagation mode size at the deep-subwavelength scale (~ λ2/105). The studies on fabrication tolerance and waveguide crosstalk show their robust property for practical implementations. The effective tunable mechanism is also proposed and studied, which shows remarkable feasibility to realize multifunctional plasmon-based photonic components. Compared with the conventional HGSPPWs, the proposed HGSPPWs exhibit superior features in ultralow loss deep-subwavelength light guiding, are highly reliable, and are easy to integrate.

Plasmonic Metasurfaces with Tunable Gap and Collective Surface Plasmon Resonance Modes

Optical properties of a plasmonic metasurface made of a monolayer of gold nanoparticles in close proximity to an aluminum thin film were studied numerically and experimentally. Extinction spectra of the plasmonic metasurface were studied as functions of the thickness of a dielectric spacer between the monolayer of gold nanoparticles and the aluminum film in the visible wavelength range. The goal was to understand the excitation of a collective surface plasmon resonance (SPR) mode and a gap plasmon mode as well as their dependence on the spacer thickness, nanoparticles spacing, and their size. By using finite-difference-time-domain (FDTD) calculations, we find that the SPR extinction peak first red-shifts and then splits into two peaks. The first extinction peak is associated with the collective SPR mode of the monolayer, and it shifts to shorter wavelengths as the spacer layer decreases. As the spacer layer decreases from 35 to 7.5 nm, the second peak gradually appears in the extinction spectra of the metasurface. We assign the second peak to the gap mode. The gap mode first appears at around 620 nm or greater, and it shifts to larger wavelength for larger nanoparticle spacing and size. The FDTD simulations are confirmed by an experimental examination of the dispersion curves of a similar multilayer system. The computational results match the experimental results and confirm the excitation of the two modes.