Numerical analysis of deep sub-wavelength integrated plasmonic devices based on Semiconductor-Insulator-Metal strip waveguides

Ultracompact Energy Transfer in Anapole-based Metachains

Realization of electromagnetic energy confinement beyond the diffraction limit is crucial for high-performance on-chip devices. Herein we construct an array of nonradiative anapoles that originate from the destructive far-field interference of electric and toroidal dipole modes to achieve ultracompact and high-efficiency electromagnetic energy transfer without the coupler. We experimentally investigate the proposed metachain at mid-infrared frequencies and give the first near-field experimental evidence of anapole-based energy transfer, in which the spatial profile of the anapole mode is also unambiguously identified on the nanoscale. We further demonstrate that the metachain is intrinsically lossless and scalable at infrared wavelengths, realizing a 90° bending loss down to 0.32 dB at the optical communication wavelength. The present scheme bridges the gap between the energy confinement and the transfer of anapoles and opens a new gate for more compactly integrated photonic and energy devices, which can operate in a broad spectral range.

Photothermal modeling and characterization of graphene plasmonic waveguides for optical interconnect

The photothermal properties of graphene plasmonic waveguides (GPWs) are numerically investigated, while most of existing studies focus on their optical properties. A three-dimensional (3D) coupled optical-thermal model based on finite element method (FEM) is presented. The graphene sheet is treated as an graphene equivalent impedance surface. Transient thermal responses and peak temperature of the GPWs are captured using time-domain FEM (TDFEM). The effectiveness of the proposed method is validated by two examples of hybrid GPWs. Numerical results present the main factors that influence the photothermal properties of the GPWs, including the conductivity of graphene, and the wavelength and power density of incident light. The findings unveil that the temperature increase is an underlying factor influencing the maximum integration density of GPWs in optical interconnect.

Theoretical study of bicharacteristic waveguide for fundamental-mode phase-matched SHG from MIR to NIR

In this paper, a bicharacteristic waveguide (BW) is proposed for fundamental-mode phase-matched second harmonic generation (SHG) from mid-infrared (MIR) to near-infrared (NIR). The required phase matching condition (PMC) is satisfied between the fundamental plasmonic mode at 3100 nm and the photonic mode at 1550 nm. With 1 W pump power, the SHG conversion efficiency of 4.173% can be obtained in 90.3 μm length waveguide. Moreover, the SHG conversion can be enhanced by using a microring resonator (MRR). By optimizing the MRR, the SHG conversion efficiency is increased to 8.30%. The proposed waveguide can also provide a promising platform for upconversion detection. By using an on-chip cascaded configuration, a gas sensor with the capability of MIR absorption and NIR detection is proposed. It is found that the detection limit (DL) can reach 1.04 nmol/L with 100 mW pump power, which shows significant enhancement compared with direct MIR absorption and detection.

Field-effect transistor based on surface plasmon polaritons

We investigate the working mechanism of a plasmon field-effect transistor (FET) based on the modulation of surface plasmon polariton (SPP) flow by varying an external electric field. SPP flow propagates at the surface of a metal strip waveguide embedded in a dielectric medium. The external electric field is applied to the metal electrode located under the isolated metal nanoplate placed in the middle of the strip waveguide. In this case, a signal on the control electrode changes the permittivity of the near-surface layer of the nanoplate. Thus, the control signal modulates the intensity of the SPP signal transmitted at the surface of the strip waveguide.

Seeds screening aqueous synthesis, multiphase interfacial separation and in situ optical characterization of invisible ultrathin silver nanowires

We report a multi-step synthetic method to obtain ultrathin silver nanowires (Ag NWs) from an aqueous solution with a ∼17 nm diameter average, and where some of them decreased down to 9 nm. Carefully designed seed screening processes including LED irradiation at high temperature for a short time, and then continuous H2O2 etching, and relative growth mechanisms of high-yield five-twinned pentagonal seeds and ultrathin Ag NWs in aqueous environment are detailed. Then, a rapid and simple multiphase interfacial assembly method particularly suitable for the separation of ultrathin Ag NWs from various by-products was demonstrated with a clear mechanism explanation. Next, a unique optical interaction between light and individual AG NWs, as well as feature structures in the AG NWs film, was investigated by a micro-domain optical confocal microscope measurement in situ together with a theoretical explanation using modal transmission theory. That revealed that the haze problem of AG NWs films was not only arising from the interaction between light and individual or crossed Ag NWs but was also greatly dependent on a weak coupling effect of leaky modes supported by adjacent Ag NWs with large distances which had not been considered before. We then provided direct experimental evidence and concluded how to obtain haze-free films with 100% transparency in the whole visible range based on ultrathin Ag NWs. This breakthrough in diameter confinement and purification of Ag NWs is a highly expected step to overcome the well-focused light diffusion and absorption problems of Ag NWs-based devices applied in various fields such as flexible electronics, high-clarity displays, visible transparent heaters, photovoltaics and various optoelectronic technologies.

Flexible Thermo-Optic Variable Attenuator based on Long-Range Surface Plasmon-Polariton Waveguides

A flexible thermo-optic variable attenuator based on long-range surface plasmon-polariton (LRSPP) waveguide for microwave photonic application was investigated. Low-loss polymer materials and high-quality silver strip were served as cladding layers and core layer of the LRSPP waveguide, respectively. By using finite element method (FEM), the thermal distribution and the optical field distribution have been carefully optimized. The fabricated device was characterized by end-fire excitation with a 1550 nm laser. The transmission performance of high-speed data and microwave modulated optical signal was measured while using a broadband microwave photonics link. The results indicated that the propagation loss of the LRSPP waveguide was about 1.92 dB/cm. The maximum attenuation of optical signal was about 28 dB at a driving voltage of 4.17 V, and the variable attenuation of microwave signals was obviously observed by applying different driving voltage to the heater. This flexible plasmonic variable attenuator is promising for chip-scale interconnection in high-density photonic integrated circuits and data transmission and amplitude control in microwave photonic systems.

Ultralow loss graphene-based hybrid plasmonic waveguide with deep-subwavelength confinement

In this paper, we theoretically propose a novel graphene-based hybrid plasmonic waveguide (GHPW) consisting of a low-index rectangle waveguide between a high-index cylindrical dielectric waveguide and the substrate with coated graphene on the surface. The geometric dependence of the mode characteristics on the proposed structure is analyzed in detail, showing that the proposed GHPW has a low loss and consequently a relatively long propagation distance. For TM polarization, highly confined modes guided in the low-index gap region between the graphene and the high-index GaAs and the normalized modal area is as small as 0.0018 (λ²/4) at 3 THz. In addition to enabling the building of high-density integration of the proposed structure are examined by analyzing crosstalk in a directional coupler composed of two GHPWs. This structure also exhibits ultra-low crosstalk when a center-to-center separation between adjacent GHPWs is 32μm, which shows great promise for constructing various terahertz integrated devices.

Integrated optical readout using a hybrid plasmonic directional coupler in water

The integrated polymer optical readout has been recognized as a promising route to obtain miniaturized cantilever-based sensor works on static mode for various liquid-state chemical and biological measurements in portable lab-on-chip systems. For conventional optical readouts based on end-fire coupling mechanisms, the most sensitive region was limited to a cantilever deflection of several micrometers due to the Gaussian profile of the mode in polymer waveguides. We proposed an integrated optical readout based on a hybrid plasmon directional coupler aiming at improving the sensitivity at the sub-micrometer deflection region (<1  μm). The coupler consists of a short-range surface plasmon waveguide and a polymer waveguide. We show that the coupling length and the propagation loss of the coupler are ultra-sensitive to the deflection, which leads to improved sensitivity of the readout. In addition, the dynamic range can be extended by integrating an array of hybrid plasmon directional couplers onto a single microcantilever. The proposed optical readout is beneficial to high sensitivity cantilever-based sensors for lab-on-chip applications and enables the design of more compact optical waveguide-based sensors in water.

Plasmonic wavelength demultiplexer with a ring resonator using high-order resonant modes

In this paper, we propose a novel wavelength demultiplexer based on metal-insulator-metal plasmonic waveguides with a nanoscale ring resonator. Its transmission characteristics are numerically studied using finite element method (FEM) simulations, and the eigenwavelengths of the ring resonator are theoretically calculated. For the proposed structure, we found that the ratio of the orders of resonant transmittance peaks for two different high-order modes of the ring resonator is close to the ratio of the two communication wavelengths 1310 and 1550 nm. These resonance wavelengths of the demultiplexer are effortlessly tuned by varying the refractive index of the material in the ring resonator and the geometrical parameters of the structure. The results simulated by FEM agree well with those from the resonant theory of the ring resonator. The presented structures will have significant potential applications in highly integrated plasmonic devices.

Hybrid fiber resonator employing LRSPP waveguide coupler for gyroscope

Polarization error and temperature noise are two main limits to the performance of resonant fiber optic gyroscope (RFOG). To overcome these limits, we demonstrated a hybrid resonator consisting of a polymer-based long-range surface plasmon polariton (LRSPP) waveguide coupler and a silica fiber. Single-polarization property of LRSPP waveguide and the offsetting of the opposite thermo-optical characteristics between the polymer-based LRSPP waveguide and the silica fiber can effectively inhibit both the polarization error and the temperature noise of RFOG. The measured resonance spectrum of the hybrid resonator shows the absence of polarization noise. The temperature dependence of wavelength shift (TDWS) of resonator dropped to about 2 pm/°C, or even to 0 pm/°C with optimal structure, which dramatically improves the temperature stability of gyroscope system. In addition, the hybrid resonator also shows tremendous application potential in rate-grade and tactical-grade gyroscopes.

Synthesis of silver nanoplate based two-dimension plasmonic platform from 25 nm to 40 μm: growth mechanism and optical characteristic investigation in situ

We show high-purity synthesis, structural engineering and in situ optical investigation of a 2D plasmonic platform using huge silver nanoplates.

Integrated label-free optical biochemical sensor with a large measurement range based on an angular grating-microring resonator

We propose and design a photonic-integrated optical biochemical sensor, which comprises a microring resonator and angular gratings in a silicon-on-insulator waveguide. With the combination of the angular gratings, the measurement range of the angular grating-microring resonator-based sensor significantly increases without the restriction of a free spectral range. Optimization of the several key structural parameters is investigated to achieve favorable transmission properties. A high-quality factor of more than 1.03×10⁵ can meet the requirements of high sensitivity and low detection limit. The simulation results on the biochemical bulk sensing show that a concentration sensitivity of more than 95.27 pm/% and detection limit of less than 0.329% can be obtained. A large measurement range of 50.2 nm is achieved by the combination of the angular gratings. The investigation on the combination of microring resonator and angular grating is a valuable exploration of the liquid and gas biomedical sensing for the ultra-large measurement range.

Numerical analysis of a near-infrared plasmonic refractive index sensor with high figure of merit based on a fillet cavity

A near-infrared plasmonic refractive index (RI) sensor with figure of merit (FOM) as high as 124.6 is proposed and investigated numerically. The RI sensing is realized by employing the linear relation between resonant wavelength and RI of the material under detecting. Based on the fillet cavity coupled with two metal-insulator-metal waveguides, transmission efficiency (T) and optical resolution (FWHM) of the RI sensor are both improved to a great extent with T = 95% and FWHM = 12nm, keeping acceptable wavelength sensitivity of 1496nm/RIU within the near-infrared region. In addition, a sensitivity as high as 3476nm/RIU is obtained by optimizing the shape and size of fillet cavity. In general, the high FOM, transmittance and sensitivity achieved by our design may get further applications in biomedical science and nanophotonic circuits.

Super low threshold plasmonic WGM lasing from an individual ZnO hexagonal microrod on an Au substrate for plasmon lasers

We demonstrate an individual ZnO hexagonal microrod on the surface of an Au substrate which can become new sources for manufacturing miniature ZnO plasmon lasers by surface plasmon polariton coupling to whispering-gallery modes (WGMs). We also demonstrate that the rough surface of Au substrates can acquire a more satisfied enhancement of ZnO emission if the surface geometry of Au substrates is appropriate. Furthermore, we achieve high Q factor and super low threshold plasmonic WGM lasing from an individual ZnO hexagonal microrod on the surface of the Au substrate, in which Q factor can reach 5790 and threshold is 0.45 KW/cm(2) which is the lowest value reported to date for ZnO nanostructures lasing, at least 10 times smaller than that of ZnO at the nanometer. Electron transfer mechanisms are proposed to understand the physical origin of quenching and enhancement of ZnO emission on the surface of Au substrates. These investigations show that this novel coupling mode holds a great potential of ZnO hexagonal micro- and nanorods for data storage, bio-sensing, optical communications as well as all-optic integrated circuits.

Long range mid-infrared propagation in Si and Ge hybrid plasmonic-photonic nano-ribbon waveguides

We have investigated a hybrid plasmonic-photonic mode in Si and Ge channel waveguides over the 1.55-8.0 μm wavelength range. A 10-nm Cu ribbon was buried midway within a Si₃N₄ "photonic slot" centered in the semiconductor strip. For the TMo mode, propagation lengths L of several millimeters are predicted for a waveguide cross-section of about 0.7λ/n x 0.7λ/n which offers optical confinement mainly within the ~λ²/400-area slot. The L increased strongly with λ. For 0.4λ/n x 0.4λ/n channels, we found multi-centimeter propagation, but there ~60% of the propagating energy had leaked out into the thick, all-around Si₃N₄ cladding.

Highly confined guiding of low-loss plasmon waves in hybrid metal-dielectric slot waveguides

We report the observation of strongly confined plasmon modes in hybridized metal-dielectric slot waveguides, which consist of semiconductor-insulator-semiconductor nanostructures embedded inside the low-index gaps of conventional hybrid plasmonic configurations. Owing to the combined effects induced by the high-refractive-index-contrast dielectric slot and semiconductor-insulator-metal configurations, tight field localization (Aeff ~ λ(2)/1250-λ(2)/55) in conjunction with large propagation distances (L ~ 70-180 μm) can be realized simultaneously at telecommunication wavelength. Through comprehensive numerical simulations, the characteristics of the fundamental hybrid modes are revealed in detail by optimizing key structural parameters of the waveguides. The advantages over their traditional hybrid waveguiding counterparts are unraveled. In addition, the possibilities of extending our current design into other metal/dielectric composites are also discussed. Our studies regarding hybrid metal-dielectric slot structures and their alternatives in this paper are expected to provide effective approaches for the enhancement of traditional hybrid modes' properties and open up new opportunities for the constructions of high-performance plasmon waveguides and devices.

Low-loss light transport at the subwavelength scale in silicon nano-slot based symmetric hybrid plasmonic waveguiding schemes

A hybrid plasmonic structure comprising a silicon slot waveguide separated from an inverse metal ridge by a thin low-index insulator gap is proposed and investigated. Owing to its symmetric hybrid configuration containing closely spaced silicon rails near the metal ridge, the fundamental symmetric hybrid slot mode supported by the structure is demonstrated to be capable of simultaneously achieving low propagation loss and subwavelength field confinement within a wide range of physical dimensions at the telecom wavelength. Comprehensive numerical investigations regarding the effects of key geometric parameters on the guided modes' properties, including the slot sizes, the shape and dimension of the silicon rails, the width of the gap region as well as the height of metallic nanoridge, have been conducted. It is revealed that the propagation distance of the symmetric mode can be more than several millimeters (even up to the centimeter range), while simultaneously achieving a subwavelength mode size and tight field confinement inside the gap region. In addition to the studies on the modal characteristics, excitation strategies of the guided hybrid modes and the conversion between dielectric slot and hybrid slot modes are also numerically demonstrated. The studied platform potentially combines the advantages of silicon slot and plasmonic structures, which might lay important groundwork for future hybrid integrated photonic components and circuits.

Low-loss hybrid plasmonic modes guided by metal-coated dielectric wedges for subwavelength light confinement

The optical characteristics of a metal-coated dielectric wedge structure are investigated at a wavelength of 1550 nm. The effects of the metal/gap layers' thicknesses, as well as the dimension of the dielectric wedge on the guided modes' properties, are systematically analyzed. It is revealed that the characteristics of the fundamental quasi-TE and quasi-TM plasmonic modes supported by the configuration demonstrate similar trends against the variation of the metal layer thickness while exhibiting quite different behaviors with the change of the wedge size. By choosing appropriate physical dimensions, both modes could simultaneously achieve low modal loss and subwavelength field confinement, along with reasonable mode power inside the low-index gap region. Investigations on the directional coupling between adjacent identical waveguides indicate that ultralow crosstalk can be enabled by the quasi-TE mode, with the coupling length more than two orders of magnitude larger than that achieved by the plasmonic mode in conventional hybrid counterparts. The presented metal-coated dielectric wedge structures can be employed as important building blocks for a number of integrated nanophotonic components, and could also enable numerous applications at the subwavelength scale.

Low-loss light transmission in a rectangular-shaped hybrid metal trench at 1550 nm

A hybrid plasmonic waveguide consisting of a high-index dielectric core embedded inside a rectangular-shaped metallic trench is proposed and its guiding properties are investigated at the wavelength of 1550 nm. Numerical simulations based on the finite element method have demonstrated that the introduced dielectric core could greatly reduce the modal loss of the metal trench while maintaining strong confinement of light. The effects of dielectric core size, material of the cladding and the dielectric core on the modal properties have been systematically investigated. The proposed hybrid plasmonic structure can be realized employing fabrication techniques of the traditional metal trench waveguides and could be leveraged as important elements for highly-integrated photonic circuits.

Fabrication and spectroscopic investigation of branched silver nanowires and nanomeshworks

Wide wavelength ranges of light localization and scattering characteristics can be attributed to shape-dependent longitude surface plasmon resonance in complicated nanostructures. We have studied this phenomenon by spectroscopic measurement and a three-dimensional numerical simulation, for the first time, on the high-density branched silver nanowires and nanomeshworks at room temperature. These nanostructures were fabricated with simple light-induced colloidal method. In the range from the visible to the near-infrared wavelengths, light has been found effectively trapped in those trapping sites which were randomly distributed at the corners, the branches, and the junctions of the nanostructures in those nanostructures in three dimensions. The broadened bandwidth electromagnetic field enhancement property makes these branched nanostructures useful in optical processing and photovoltaic applications.

Novel hybrid plasmonic waveguide consisting of two identical dielectric nanowires symmetrically placed on each side of a thin metal film

It is well-known that, a dielectric cylinder on a metal surface offers the advantage of not yielding singular field, which would effectively reduce the propagation loss as opposed to a rectangle-shaped waveguide on a metal surface. In this article, a novel hybrid plasmonic waveguide consisting of two identical dielectric nanowires symmetrically placed on each side of a thin metal film is presented. With the strong interaction between the dielectric cylindrical waveguide mode and long-range surface plasmon polaritons (LRSPP) mode of a thin metal film, deep-subwavelength mode confinement can be achieved. Compared with the hybrid plasmonic mode guided in only one dielectric nanowire above a metal film, a much larger propagation length as well as improved figure of merit (FoM) can be simultaneously realized. A typical propagation length is 434 μm, and optical field is confined into an ultra-small area of approximately 0.0096 μm at 1.55 μm. This structure could enable various applications such as nanophotonic waveguides, high-quality nanolasers, and optical trapping and transportation of nanoparticles and biomolecules.

Self-assembly of large-scale and ultrathin silver nanoplate films with tunable plasmon resonance properties

We describe a rapid, simple, room-temperature technique for the production of large-scale metallic thin films with tunable plasmonic properties assembled from size-selected silver nanoplates (SNPs). We outline the properties of a series of ultrathin monolayer metallic films (8-20 nm) self-assembled on glass substrates in which the localized surface plasmon resonance can be tuned over a range from 500 to 800 nm. It is found that the resonance peaks of the films are strongly dependent on the size of the nanoplates and the refractive index of the surrounding dielectric. It is also shown that the bandwidth and the resonance peak of the plasmon resonance spectrum of the metallic films can be engineered by simply controlling aggregation of the SNP. A three-dimensional finite element method was used to investigate the plasmon resonance properties for individual SNPs in different dielectrics and plasmon coupling in SNP aggregates. A 5-17 times enhancement of scattering from these SNP films has been observed experimentally. Our experimental results, together with numerical simulations, indicate that this self-assembly method shows great promise in the production of nanoscale metallic films with enormous electric-field enhancements at visible and near-infrared wavelengths. These may be utilized in biochemical sensing, solar photovoltaic, and optical processing applications.

Silicon hybrid plasmonic submicron-donut resonator with pure dielectric access waveguides

Characteristic analyses are given for a bent silicon hybrid plasmonic waveguide, which has the ability of submicron bending (e.g., R = 500 nm) even when operating at the infrared wavelength range (1.2 μm~2 μm). A silicon hybrid plasmonic submicron-donut resonator is then presented by utilizing the sharp-bending ability of the hybrid plasmonic waveguide. In order to enable long-distance optical interconnects, a pure dielectric access waveguide is introduced for the present hybrid plasmonic submicron-donut resonator by utilizing the evanescent coupling between this pure dielectric waveguide and the submicron hybrid plasmonic resonator. Since the hybrid plasmonic waveguide has a relatively low intrinsic loss, the theoretical intrinsic Q-value is up to 2000 even when the bending radius is reduced to 800 nm. By using a three-dimensional finite-difference time-domain (FDTD) method, the spectral response of hybrid plasmonic submicron-donut resonators with a bending radius of 800 nm is simulated. The critical coupling of the resonance at around 1423 nm is achieved by choosing a 80 nm-wide gap between the access waveguide and the resonator. The corresponding loaded Q-value of the submicron-donut resonator is about 220.

Hybrid wedge plasmon polariton waveguide with good fabrication-error-tolerance for ultra-deep-subwavelength mode confinement

A novel hybrid plasmonic waveguide consisting of a high-index dielectric nanowire placed above a triangular metal wedge substrate is proposed and analyzed theoretically. The strong coupling between the wedge plasmon polariton and the dielectric nanowire mode results in both the ultra-tight confinement and low propagation loss. Compared to the previous studied hybrid surface plasmon polariton structures without the metal wedge substrate, stronger field enhancement in the low-index gap region as well as improved figure of merit (FOM) could be realized simultaneously. Results of the modal properties considering certain fabrication imperfections show that the proposed structure is also quite tolerant to these errors.

Gain enhancement in a hybrid plasmonic nano-waveguide with a low-index or high-index gain medium

A theoretical investigation of a nano-scale hybrid plasmonic waveguide with a low-index as well as high-index gain medium is presented. The present hybrid plasmonic waveguide structure consists of a Si substrate, a buffer layer, a high-index dielectric rib, a low-index cladding, a low-index nano-slot, and an inverted metal rib. Due to the field enhancement in the nano-slot region, a gain enhancement is observed, i.e., the ratio ∂G/∂g >1, where g and G are the gains of the gain medium and the TM fundamental mode of the hybrid plasmonic waveguide, respectively. For a hybrid plasmonic waveguide with a core width of w(co)=30nm and a slot height of h(slot)=50nm, the intrinsic loss could be compensated when using a low-index medium with a moderate gain of 176dB/cm. When introducing the high-index gain medium for the hybrid plasmonic waveguide, a higher gain is obtained by choosing a wider core width. For the high-index gain case with h(slot)=50nm and w(co)=500nm, a gain of about 200dB/cm also suffices for the compensation of the intrinsic loss.