Ultra-compact directional couplers and Mach-Zehnder interferometers employing surface plasmon polaritons

Optical directional couplers and Mach-Zehnder interferometers employing surface plasmon polaritons are proposed and their characteristics are analyzed numerically using a finite-difference time-domain method. It is shown that these devices can have transverse size smaller than the incident wavelength and can then be regarded as true subwavelength photonic elements. The ultra-compact characteristic of these devices can be utilized as a basis of next generation of integrated photonic circuits. Furthermore, calculations also show that these devices behave differently from their traditional all-dielectric counterparts. For the proposed optical directional coupler, the phase difference between the two output ports will no longer be 90° even when the two ports have the same power outputs.     

Polarization‐free directional coupling of surface plasmon polaritons

Surface plasmon polaritons (SPPs) have sparked enormous interest on nanophotonics beyond the diffraction limit for their remarkable capabilities of subwavelength confinements and strong enhancements. Due to the inherent polarization sensitivity of the SPPs [transverse‐magnetic (TM) polarization], it is a great challenge to couple the s‐polarized free‐space light to the SPPs. Here, an ultrasmall defect aperture (<λ²/2) is designed to directionally couple both the p‐ and s‐polarized incident beams to the single SPP mode in a broad bandwidth, which is guided by a subwavelength plasmonic waveguide. Simulations show that hot spots emerge at the sharp corners of the defect aperture when the incident beams illuminate it from the back side. The strong radiative fields from the hot spots are directionally coupled to the SPP mode because of the symmetry breaking of the defect aperture. By adjusting the structural parameters, both the unidirectional and bidirectional SPP coupling from the two orthogonal linear‐polarization incident beams are experimentally demonstrated. The polarization‐free coupling of the SPPs is of importance in circuits for fully optical processing of information with a deep‐subwavelength footprint.

     

Photonic bandgap structures for long-range surface plasmon polaritons

Propagation of long-range surface plasmon polaritons (LR-SPPs) along periodically thickness-modulated metal stripes embedded in dielectric is studied both theoretically and experimentally for light wavelengths in the telecom range. We demonstrate that symmetric (with respect to the film surface) nm-size thickness variations result in the pronounced band gap effect, and obtain very good agreement between measured and simulated (transmission and reflection) spectra. This effect is exploited to realize a compact wavelength add-drop filter with the bandwidth of 20 nm centered at 1550 nm. The possibilities of achieving a full bandgap (in the surface plane) for LR-SPPs are also discussed.     

Anisotropy‐assisted non‐scattering coherent absorption of surface plasmon‐polaritons

The ability to control propagation of electromagnetic guided modes lies at the heart of integrated nanophotonics. Surface plasmon‐polaritons are a class of guided modes which can be employed in integrated optical systems. Here, we present a theoretical design of a coherent surface plasmon absorber which can perfectly harvest energy of coherently incident surface plasmons without parasitic scattering into free space modes. Excitation of free space modes which usually accompanies scattering of a surface plasmon by an interface boundary is avoided due to specially tailored anisotropy of the absorber. The concept of coherent SPP absorber is analyzed numerically for spatially non‐uniform and finite‐size structures. We believe that our results will be important for the development of integrated nanoplasmonic systems.     

基于金属表面等离子激元控制光束的新进展

表面等离子激元(Surface plasmon polaritons,SPPs)是一种在金属-介质界面上激发并耦合电荷密度起伏的电磁振荡,具有近场增强、表面受限、短波长等特性,在纳米光子学的研究中扮演着重要角色。近年来表面等离子光学和基于SPPs的纳米光子器件的研究引起了国际上科学家们的广泛关注。讨论了SPPs的基本原理和在亚波长结构下的光学特性,介绍了基于亚波长金属结构的表面等离子激元在空间光束准直与聚焦、平面内光束聚焦与传导和在近场纳米光束的控制等方面的研究情况,以及在纳米光子学器件中的潜在应用。     

Surface plasmon polaritons mediated by surface modes in a metal-based photonic crystal complex structure

A hybrid metal–organic–photonic crystal (PC), a gold film with a SU8 film on its top followed by a three-dimensional (3D) PC, was designed to exercise additional control upon the photon behaviors. Attenuated total reflection measurements demonstrate that the 3D PC plays a crucial role in the formation of significant optical properties of the metal-based hybrid and the reflectivity minimum in its reflectivity curve is, mostly, a result of synergetic action of the coupling of surface plasmon polaritons to surface modes and additional guided modes yielded in the resonant cavity composed of the gold film and the PC.     

Surface plasmon polaritons on square-lattice arrays of three-fold symmetric nanostructures

The influence of the geometry of three-fold symmetric rotor shaped nanostructures arranged as a two-dimensional square-lattice on the excitation of surface plasmon polaritons (SPPs) is studied numerically. We consider SPP-related extrema of the far-field reflectivity R(α) as a function of the polarization angle α of the incident light. In agreement with recent experimental work, it is observed that these extrema shift away from α=0° and α=90°, where they are found for rotationally invariant nanostructures. The polarization angle corresponding to the most efficient SPP excitation is found to be independent of the shape of the individual nanostructures. We further investigate the influence of the shape and size of the nanostructures and discuss consequences for optical near- and far-field properties.     

Polarization controlled couplings of surface plasmon with asymmetrical nanoslit pairs

Light-driven surface plasmons offer an opportunity to ultrafast information processing combining the compactness of electric circuits with the bandwidth of photonic networks. For practical applications, the efficient and controllable conversion from signal light to surface plasmons is essential. This leads to the recent developments in the polarization controlled couplings of surface plasmons. Currently, most works only tailor the orientation and arrangement of nanoslits to control the launching of surface plasmons. In this paper, we consider both the orientation and size of each slit in a one-dimensional array of nanoslit dimers. We first realize the unidirectional propagation of surface plasmons with designed wavefronts. Next, the unidirectional coupling and bi-directional coupling of surface plasmons are realized for a pair of orthogonal polarizations, respectively. This is quite different from the conventional opposite propagating surface plasmons excited by two orthogonal polarizations. The manipulation of both orientation and size of nanoslits allows additional freedom in the photon-plasmon conversions.     

Polarization‐controlled surface plasmon holography

The ability of generating arbitrary surface plasmon (SP) profiles in a controllable manner is of particular interest in designing plasmonic imaging, lithography and forcing devices. During the past decades, holography has gained enormous interest and achievements in free‐space three‐dimensional displays. Here, by applying a two‐dimensional version of holography, we experimentally demonstrate a generic method to control the SP profiles. Through controlling the orientation angles of two separated slits under circular polarization incidence, the amplitude and phase of the excited SPs can be freely manipulated, which allows direct generation of the desired SP profiles. A series of controllable SP holography schemes are theoretically and experimentally demonstrated, where the holographic SP profiles with high imaging quality can be dynamically modulated by varying the circular polarization handedness or orientation angle of linear polarization. The universality and simplicity of the proposed design strategies would offer promising opportunities for practical plasmonic applications.

     

Cherenkov terahertz radiation from Dirac semimetals surface plasmon polaritons excited by an electron beam

We demonstrate a physical mechanism for terahertz(THz) generation from surface plasmon polaritons(SPPs). In a structure with a bulk Dirac semimetals(BDSs) film deposited on a dielectric substrate, the energy of the asymmetric SPP mode can be significantly enhanced to cross the light line of the substrate due to the SPP-coupling between the interfaces of the film. Therefore, the SPPs can be immediately transformed into Cherenkov radiation without removing the wavevector mismatch. Additionally, the symmetric SPP mode can also be dramatically lifted to cross the substrate light line when a buffer layer with low permittivity relative to the substrate is introduced. In this case, dual-frequency THz radiation from the two SPP modes can be generated simultaneously. The radiation intensity is significantly enhanced by over two orders due to the field enhancement of the SPPs. The radiation frequency can be tuned in the THz frequency regime by adjusting the beam energy and the chemical potential of the BDSs. Our results could find potential applications in developing room temperature, tunable, coherent, and intense THz radiation sources to cover the entire THz band.     

电子激励表面等离极化激元的粒子模拟

采用极化电流微分方程,对贵金属中自由电子与外电场的共振过程进行描述。将该微分方程与麦克斯韦方程相结合,运用时域有限差分(FDTD)方法,在粒子PIC模拟软件CHIPIC3D的基础上,实现了电子激励表面等离极化激元(SPPs)的模拟。通过对100 keV电子平行于银薄膜表面运动、激励起表面等离极化激元的模拟,观测并分析了SPPs的场强及模式在银薄膜表面的分布,并验证了模拟结果的正确性。     

Multiple beams surface plasmon interference generation: A theoretical analysis

In this paper, multiple beams surface plasmon (SP) interference generation based on prism coupling technique is theoretically analyzed and presented. The proposed multiple beams SP interference configuration is investigated for nanolithography application for two-dimensional feature fabrication. The configuration is then numerically and experimentally analyzed by employing aluminum metal at 364-nm illumination wavelengths to realize high resolution and high aspect ratio two-dimensional periodic nanoscale dot array patterns on the recording medium.     

Numerical investigation of the enhanced unidirectional surface plasmon polaritons generator

A unidirectional surface plasmon polaritons(SPPs) generator with greatly enhanced generation efficiency is proposed. The SPPs generator consists of an asymmetric single nanoslit coated with a polyviny alcohol(PVA) film and a silver rectangle block. The generation efficiency of this SPPs generator is investigated using the finite difference time domain method. Due to the presence of the silver rectangle block, the SPPs generation efficiency of the asymmetric single nanoslit with PVA film can be greatly enhanced and the corresponding wavelength with the maximum enhancement factor can be tuned flexibly. The influence of the structural parameters on the generation efficiency is also investigated for the enhanced unidirectional SPPs generator.     

Grating-assisted generation of 2D surface plasmon interference patterns for nanoscale photolithography

Generation of 2D surface plasmon interference patterns using a 3D metal-dielectric diffraction structure is studied. The potential application field is surface plasmon interference nanolithography aimed at fabrication of 3D periodic structures. The considered structure consists of a 3D dielectric diffraction grating with a metal film applied in the substrate region. The diffraction grating is designed to transform the incident wave into a set of surface plasmons that generate 2D interference pattern underneath the metal film. The configuration of the interference patterns is analyzed theoretically. It is shown by simulations within the rigorous electromagnetic theory that high-contrast interference patterns with the period 2.5-3.5 times smaller than the incident wave length can be produced. The configuration of the calculated patterns coincides with theoretically estimated ones. At the interference maxima electric field intensity exceeds incident wave intensity by an order of magnitude. The ways to control the form and period of the interference pattern by changing polarization and length of the incident wave are presented.     

Modeling of a surface plasmon polariton interferometer

We model the operation of a micro-optical interferometer for surface plasmon polaritons (SPPs) that comprises an SPP beam-splitter formed by equivalent scatterers lined up and equally spaced. The numerical calculations are carried out by using a relatively simple vectorial dipolar model for multiple SPP scattering [Phys. Rev. B 67 (2003) 165405]. The SPP beam-splitter is simulated elucidating the influence of system parameters, such as the angle of SPP beam incidence, scattering particle size, and inter-particle distance, on the splitting efficiency and phase difference between the transmitted and reflected beams. It is found that the splitting efficiency is very sensitive to the size of scatterers and angle of incidence. Comparing our simulations with experimental data available in the literature, we conclude that this approach can be used, with certain limitations, for modelling of SPP components assembled of individual scatterers, e.g., beam-splitters and interferometers, and suggest further improvements of the model used.     

The theoretic analysis of maskless surface plasmon resonant interference lithography by prism coupling

The use of an attenuated total reflection-coupling mode of prism coated with metal film to excite the interference of the surface plasmon polaritons （SPPs） was proposed for periodic patterning with a resolution of subwavelength scale. High intensity of electric field can be obtained because of the coupling between SPPs and evanescence under a resonance condition, which can reduce exposure time and improve contrast. In this paper, several critical parameters for maskless surface plasmon resonant lithography are described, and the preliminary simulation based on a finite difference timedomain technique agrees well with the theoretical analysis, which demonstrates this scheme and provides the theoretical basis for further experiments.     

Hybrid vertical directional coupling between a long range surface plasmon polariton waveguide and a dielectric waveguide

To investigate light coupling between a long range surface plasmon polariton (LRSPP) waveguide and a conventional integrated optical component, a hybrid vertical directional coupler consisting of a LRSPP waveguides and a dielectric waveguide is investigated and fabricated. In the proposed coupler the dielectric waveguide and LRSPP waveguide are vertically configured for dense integration and strong coupling. The characteristics of the even and odd super-modes of the coupler are also analyzed to design the device. The fabricated device exhibits damped sinusoidal behavior along the coupling length due to propagation loss of the LRSPP waveguide. The maximum power transfer of 86% from the LRSPP waveguide to the dielectric waveguide is achieved at the coupling length of 600 μm. The measured characteristics of the device are in relatively good agreement with a theoretical analysis.     

Broadband amplification of spoof surface plasmon polaritons at microwave frequencies

Efficient amplification of spoof surface plasmon polaritons (SPPs) is proposed at microwave frequencies by using a subwavelength‐scale amplifier. For this purpose, a special plasmonic waveguide composed of two ultrathin corrugated metallic strips on top and bottom surfaces of a dielectric substrate with mirror symmetry is presented, which is easy to integrate with the amplifier. It is shown that spoof SPPs are able to propagate on the plasmonic waveguide in broadband with low loss and strong subwavelength effect. By loading a low‐noise amplifier chip produced by the semiconductor technology, the first experiment is demonstrated to amplify spoof SPPs at microwave frequencies (from 6 to 20GHz) with high gain (around 20dB), which can be directly used as a SPP amplifier device. The features of strong field confinement, high efficiency, broadband operation, and significant amplification of the spoof SPPs may advance a big step towards other active SPP components and integrated circuits.

     

Broadband and high‐efficiency conversion from guided waves to spoof surface plasmon polaritons

The conversion from spatial propagating waves to surface plasmon polaritons (SPPs) has been well studied, and shown to be very efficient by using gradient‐index metasurfaces. However, feeding energies into and extracting signals from functional plasmonic devices or circuits through transmission lines require the efficient conversion between SPPs and guided waves, which has not been reported, to the best of our knowledge. In this paper, a smooth bridge between the conventional coplanar waveguide (CPW) with 50 Ω impedance and plasmonic waveguide (e.g., an ultrathin corrugated metallic strip) has been proposed in the microwave frequency, which converts the guided waves to spoof SPPs with high efficiency in broadband. A matching transition has been proposed and designed, which is constructed by gradient corrugations and flaring ground, to match both the momentum and impedance of CPW and the plasmonic waveguide. Simulated and measured results on the transmission coefficients and near‐filed distributions show excellent transmission efficiency from CPW to a plasmonic waveguide to CPW in a wide frequency band. The high‐efficiency and broadband conversion between SPPs and guided waves opens up a new avenue for advanced conventional plasmonic integrated functional devices and circuits.     

表面等离激元的聚焦与波导增强

近年来,表面等离激元学(plasmonics)已经形成一个新的学科热点.电子在金属与介质界面的集体振荡行为形成一种元激发——表面等离激元（surface plasomon polariton,SPP）.由于其具有特殊的耦合与传播性质,与SPP相关的器件设计与应用成为目前纳米光子学领域的国际前沿研究方向.文章介绍了利用微纳加工技术制备的SPP纳米结构,以及利用近场光学表征手段对SPP聚焦、波导、共振增强现象研究的进展.