Plasmonic Slot Research Articles

Plasmonic devices – equivalent circuit representations

An equivalent circuit model for plasmonic slot waveguide-based devices is presented. Taking advantage of the high mode confinement provided by this waveguide geometry, we express plasmonic waveguide geometries using transmission line parameters and express T-junctions using lumped equivalent circuit elements. By combining these fundamental building blocks, we subsequently introduce equivalent circuit models for stub filters and branch-line couplers. We show that plasmonic circuits, if designed with sharp discontinuities, feature low losses that are comparable to losses from RF circuits and even the corresponding photonic circuits. The framework presented here gives insight into the design of novel microwave-inspired plasmonic devices and circuits and significantly speeds up the design time, as a large part of the geometry optimization can be performed in the equivalent circuit domain. For instance, we use this framework in a follow-up paper to design ultra-compact plasmonic hybrids, such as those needed for coherent detection.

Ultrafast Silicon/Graphene Optical Nonlinear Activator for Neuromorphic Computing

AbstractOptical neural networks (ONNs) have shown great promise in overcoming the speed and efficiency bottlenecks of artificial neural networks. However, the absence of high‐speed, energy‐efficient nonlinear activators significantly impedes the advancement of ONNs and their extension to ultrafast application scenarios like real‐time intelligent signal processing. In this work, a novel silicon/graphene ultrafast all‐optical nonlinear activator, leveraging the hybrid integration of silicon slot waveguides, plasmonic slot waveguides, and monolayer graphene is demonstrated. Exploiting the exceptional picosecond‐scale photogenerated carrier relaxation time of graphene, the response time of the activator is markedly reduced to ≈93.6 ps, establishing all‐optical activator as the fastest known in silicon photonics to knowledge. Moreover, the all‐optical nonlinear activator holds a low threshold power of 5.49 mW and a corresponding power consumption per activation of 0.51 pJ. Its feasibility and capability for use in ONNs, manifesting performance comparable with commonly used activation functions are experimentally confirmed. This breakthrough in speed and energy efficiency of all‐optical nonlinear activators opens the door to significant improvements in the performance and applicability of ONNs.

Nonvolatile multilevel adjustable optical switch based on plasmonic slot waveguide and GST segmented structure

Optical computing has gradually demonstrated its efficiency in handling increasingly complex computational demands, attracting widespread attention. Optical switches can effectively control and modulate optical signals, providing flexibility and efficiency for optical computing systems. However, traditional optical switches face performance issues such as power consumption, switching speed, and compactness, severely limiting the implementation of large-scale photonic integrated circuits and optical neural networks. This paper proposes an innovative design structure for a non-volatile multi-level adjustable optical switch by combining a plasmonic slot waveguide with segmented phase-change materials. Modulation of waveguide light transmission is achieved by adjusting the phase state of Ge2Sb2Te5(GST). At a wavelength of 1550 nm, a low insertion loss of 0.5dB has been achieved, with approximately an 85% difference in optical transmittance between amorphous state (aGST) and crystalline state (cGST). The high transmittance difference contributes to achieving a wide range of weight variations and supports precise weight updates. Based on this design, we successfully implemented a handwritten digit recognition task with an accuracy of 95%, laying the foundation for future more efficient memory computing neural morphic networks.

Plasmonic‐Enhanced Polymer‐Stabilized Liquid Crystals Switching for Integrated Optical Attenuation

AbstractLiquid crystals are widely used in photonics because of their profound electro–optic properties. However, the slow switching speeds (milliseconds) and the fluid nature of liquid crystals restrict their potential use in integrated photonics. In this work, polymer‐stabilized liquid crystals are utilized as the functional material to improve the response time with a polymer network that helps pre‐orientate the liquid crystal. Additionally, plasmonic slot structures are simultaneously employed to minimize the switch voltage by confining the optical field within the electrode spacing at subwavelength scales. Thanks to the large overlap of the optical and electric fields, the scattering states of the polymer‐stabilized liquid crystal can be effectively manipulated to modulate the loss of the propagating wave, resulting in strong optical attenuation in the integrated photonic platform. Specifically, a plasmonic enhanced polymer‐stabilized liquid crystal photonic device for operation at 1550 nm, which has a length of only 10 µm and shows an extinction ratio of 0.38 dB µm−1, with a power consumption of less than 6 µW and a response time ≈20 µs, resulting in a figure‐of‐merit of 0.012 mW.

Plasmonic equi-triangular slot loaded bowtie nano-antenna for quantum optical wireless communication

In this manuscript, the plasmonic equi-triangular slot loaded bowtie aluminium dipole nano-antenna is investigated on a glass substrate for quantum optical wireless communication The optical characteristics of aluminium, such as permittivity and refractive index, are analyzed using the Drude dispersive model, which exhibits the required criteria for plasmonic resonance. A nanocircuit model for bowtie nano-antennas using lumped circuit elements is proposed and validated through CST simulation. The four equi-triangular slot loaded bowtie nanostructures are designed to tune the resonating frequency from 111 THz to 292 THz. Further, detailed investigation of the equi-triangular slot loaded bowtie nano-antenna is performed through the design parameter Tc (thickness) to obtain the optimum characteristics appropriate for quantum optical wireless communication. The proposed single bowtie nano-antenna exhibits a directivity of 7.606 dBi and a field enhancement factor of 289.5 at the resonating frequency of 292 THz.

Numerical evaluation of an external environment effectiveness to the surface plasmon mode in Au slot waveguide for design the sensor simulation

The effective permittivity of gold thin films with an external thin dielectric layer on the top and core of the slot waveguide was evaluated as an effective compound semi-metal layer of the surface plasmonic polariton slot waveguide that will vary due to some environmental factors. By uses simple parallel and series permittivity of compound material, this value need for the computation of the effective propagating vector βsp. This takes for a sensor model designing which the external environment effectiveness was changed by the surface plasmonic mode in its slot waveguide. So the resonance length and grating period are evaluated for the sensor model in the first. Further, the propagation of the SPP surface wave mode along the interface between the compound cladding and Si dielectric waveguide is computed, while the effective refractive index of the compound cladding of the slot waveguide is increased from 1 − 1.50. The computation results showed that the TM0 mode power transfer and propagation length seem to vary with this effective refractive index of the compound cladding on the surface plasmonic polariton waveguide. The 140 nm gap width and 3.70 mm long waveguide is also simulated by the finite different time domain method in the optiwave program, where the TM0 mode transition output is corrected, while the refractive index of the environment layer is changed. The results of the simulation showed good agreement with the computation in that the TM0 mode depends on the external refractive of index and input wavelength. This result shows that loss peak ships vary with the external refractive index thin film layer and input wavelength. Thus, they seem to be useful for sensor applications such as the solution concentration analytic.

Grating-assisted hybrid plasmonic slot resonator for on-chip SERS sensor with built-in filter

A grating-assisted hybrid plasmonic slot resonator is proposed and theoretically investigated as an on-chip surface-enhanced Raman scattering (SERS) sensor. A dielectric Bragg grating and a gold baffle are introduced into the hybrid plasmonic slot waveguide as two mirrors to form a plasmonic–dielectric cavity, in which the constructive interference of the input pump light greatly enhances the electric field. The grating reflects the wasted pump light for reverse secondary coupling and acts as a built-in notch filter for filtering out pump light from Raman scattered light simultaneously, improving the coupling efficiency and realizing a compact and miniaturized on-chip SERS sensor. Both the volumetric electric field enhancement and Raman collection efficiency are taken into account. The proposed scheme provides a simple, effective and universal method for improving the performance of the plasmonic-antennas-type waveguide-based SERS sensors.

Plasmonic Slot Waveguide Propagation Analysis

Plasmonic slot waveguides provide extreme light confinement with the benefits of having naturally present electrodes for switching and high thermal conductivity of the metal layers to remove excess heat. Past works relied on numerical computation for these structures, which is time-consuming and lacks physical insight. Here, we present an analytical model of plasmonic slot waveguides to determine the modal properties based on single-mode matching to continuum. The model is accurate to within 3% of rigorous numerical simulations. The theory allows for rapid design and provides physical insight into mode propagation in plasmonic slot waveguides for information processing, optical manipulation, and sensing applications.

Improving the transmittance of the plasmonic slot filter by using a single reflector.

A scheme to improve the transmittance of the metal-insulator-metal (MIM) plasmonic slot filter is proposed and numerically studied. Using this scheme, the transmittance of all channels in the MIM slot filter can be significantly improved by using only one reflector. The simulation results show that the transmittance of all channels with this scheme is almost 160% higher than without it. A single-channel filter at 980 nm and a three-channel filter are both demonstrated using this scheme. All the work above is completed by the finite-element analysis method. The model and characteristics of the structure with periodical stubs are also introduced and analyzed. For the first time, to our knowledge, the expressions of all four elements of the transfer matrix together with the reflectance of the periodical stubs are given. It is believed that our research will help to provide new ideas for improving transmittance and promote the application of plasmonic filters.

Design, Analysis, and Optimization of a Plasmonic Slot Waveguide for Mid-Infrared Gas Sensing.

In this work, we investigated the optimization of a plasmonic slot waveguide (PSWG) in the mid-IR region particularly for a representative wavelength of 4.26 µm, which is the absorption line of CO2 and thus particularly relevant for applications. We analysed the mode features associated with metal-dielectric-metal (MDM), dielectric-metal-dielectric (DMD), and truncated metal film (TMF) structures with respect to the considered PSWG. Subsequently, the mode features of the PSWG were considered based on what we outlined for MDM, DMD, and TMF structures. Furthermore, as confinement factor and propagation length are two crucial parameters for absorption sensing applications, we optimized the PSWG based on a figure of merit (FOM) defined as the product of the aforementioned quantities. To characterize the propagation length, the imaginary part of the effective mode index of a guided mode was considered, leading to a dimensionless FOM. Finally, we investigated the PSWG also for other wavelengths and identified particularly attractive wavelengths and geometries maximizing the FOM.

Broadband graphene modulator with high modulation depth based on tip plasmonic waveguide

Broadband operation is a critical feature for high-speed electro-optic modulators in optical signal processing. To overcome the obstacle of material absorption-induced optical bandwidth shrinkage, graphene optical modulators based on the tip plasmonic concept have been proposed. The modulator is composed of buried double-layer graphene and Ag-SiO2-Si-SiO2-Ag hybrid structure with taper slots. Due to the tightened field confinement induced by the tip plasmonic effect, the light-graphene coupling is significantly enhanced. The direct metal layers contact with graphene is exploited to serve as the electrode. The simulation results based on the finite element method present a graphene modulator with a high modulation depth (MD) of 1.1 dB/μm and a relatively low propagation loss of 0.14 dB/μm. The power consumption is about 43.80 fJ/bit at the modulation length of 20 µm. The 3 dB-bandwidth is estimated to be around 340 GHz. Comparison with reported graphene modulators adopting hybrid plasmonic slot waveguide proves favorable modulation depth of proposed design, which promise its potentials in hybrid integration and on-chip signal processing.

Numerical analysis of an infrared gas sensor utilizing an indium-tin-oxide-based plasmonic slot waveguide

Abstract. Plasmonic waveguides have attracted much attention owing to the associated high field intensity at the metal–dielectric interface and their ability to confine the modes at the nanometer scale. At the same time, they suffer from relatively high propagation loss, which is due to the presence of metal. Several alternative materials have been introduced to replace noble metals, such as transparent conductive oxides (TCOs). A particularly popular TCO is indium tin oxide (ITO), which is compatible with standard microelectromechanical systems (MEMS) technology. In this work, the feasibility of ITO as an alternative plasmonic material is investigated for infrared absorption sensing applications: we numerically design and optimize an ITO-based plasmonic slot waveguide for a wavelength of 4.26 µm, which is the absorption line of CO2. Our optimization is based on a figure of merit (FOM), which is defined as the confinement factor divided by the imaginary part of the effective mode index (i.e., the intrinsic damping of the mode). The obtained optimal FOM is 3.2, which corresponds to 9 µm and 49 % for the propagation length (characterizing the intrinsic damping) and the confinement factor, respectively.

Graphene photodetector employing double slot structure with enhanced responsivity and large bandwidth

Silicon photonics integrated with graphene provides a promising solution to realize integrated photodetectors operating at the communication window thanks to graphene’s ultrafast response and compatibility with CMOS fabrication process. However, current hybrid graphene/silicon photodetectors suffer from low responsivity due to the weak light-graphene interaction. Plasmonic structures have been explored to enhance the responsivity, but the intrinsic metallic Ohmic absorption of the plasmonic mode limits its performance. In this work, by combining the silicon slot and the plasmonic slot waveguide, we demonstrate a novel double slot structure supporting high-performance photodetection, taking advantages of both silicon photonics and plasmonics. With the optimized structural parameters, the double slot structure significantly promotes graphene absorption while maintaining low metallic absorption within the double slot waveguide. Based on the double slot structure, the demonstrated photodetector holds a high responsivity of 603.92 mA/W and a large bandwidth of 78 GHz. The high-performance photodetector provides a competitive solution for the silicon photodetector. Moreover, the double slot structure could be beneficial to a broader range of hybrid two-dimensional material/silicon devices to achieve stronger light-matter interaction with lower metallic absorption.

Hybrid plasmonic slot waveguide with a metallic grating for on-chip biosensing applications.

Designing reliable and compact integrated biosensors with high sensitivity is crucial for lab-on-a-chip applications. We present a bandpass optical filter, as a label-free biosensor, based on a hybrid slot waveguide on the silicon-on-insulator platform. The designed hybrid waveguide consists of a narrow silicon strip, a gap, and a metallic Bragg grating with a phase-shifted cavity. The hybrid waveguide is coupled to a conventional silicon strip waveguide with a taper. The effect of geometrical parameters on the performance of the filter is investigated by 3D finite-difference time-domain simulations. The proposed hybrid waveguide has potential for sensing applications since the optical field is pulled into the gap and outside of the silicon core, thus increasing the modal overlap with the sensing region. This biosensor offers a sensitivity of 270 nm/RIU, while it only occupies a compact footprint of 1.03µm×17.6µm.

Millimeter-Wave-to-Terahertz Superconducting Plasmonic Waveguides for Integrated Nanophotonics at Cryogenic Temperatures.

Plasmonics, as a rapidly growing research field, provides new pathways to guide and modulate highly confined light in the microwave-to-optical range of frequencies. We demonstrated a plasmonic slot waveguide, at the nanometer scale, based on the high-transition-temperature (Tc) superconductor Bi2Sr2CaCu2O8+δ (BSCCO), to facilitate the manifestation of chip-scale millimeter wave (mm-wave)-to-terahertz (THz) integrated circuitry operating at cryogenic temperatures. We investigated the effect of geometrical parameters on the modal characteristics of the BSCCO plasmonic slot waveguide between 100 and 800 GHz. In addition, we investigated the thermal sensing of the modal characteristics of the nanoscale superconducting slot waveguide and showed that, at a lower frequency, the fundamental mode of the waveguide had a larger propagation length, a lower effective refractive index, and a strongly localized modal energy. Moreover, we found that our device offered a larger SPP propagation length and higher field confinement than the gold plasmonic waveguides at broad temperature ranges below BSCCO’s Tc. The proposed device can provide a new route toward realizing cryogenic low-loss photonic integrated circuitry at the nanoscale.

Compact hybrid plasmonic slot waveguide sensor with a giant enhancement factor for surface-enhanced Raman scattering application.

In this paper, a surface-enhanced Raman scattering (SERS) sensor with a giant field enhancement factor based on the coupling of surface plasmon polaritons (SPPs) is designed and studied theoretically. The proposed sensor adopts a metal-dielectric layered hybrid slot waveguide structure, combining thin metal (gold) layers and silicon nitride strip waveguides. Unlike other similar sensors, the silicon nitride waveguide structure does not serve as an excitation signal channel, conventionally loaded with the guided modes, but as an auxiliary layer, making it easier to concentrate the light field in the slot. Therefore, the sensor has a higher enhancement factor compared to the pure metal or dielectric slot structure. The results exhibit that we can obtain a maximum enhancement factor exceeding 10^6 under the compact configuration of 510 × 300 × 225nm^3 at the wavelength of 785 nm. By analyzing the dependence of the sensor performance on the structural parameters, we show that the structure of such sensor can directly be applied to SERS spectroscopic analysis as well as integrated with micro-and nano-photonic platform to perform on-chip detection system.

Design and Simulation of Optical Logic Gates Based on (MIM) Plasmonic Waveguides and slot cavity resonator for Optical Communications

In the field of optics the tinier devices are the better; therefore, the diffraction limit of light seems like an essential limitation in the way of that field. In return, new methods have appeared to resolve this issue. One of these methods is the plasmonic technology which allows light pressure into nanostructures. The current study proposes all-optical logic gates based on metal insulator metal structures (mim) waveguide. This waveguide has an important characteristic which is restricting the applied light strongly far from the diffraction limit. The proposed structure is small compared to the applied wavelength. The optical plasmonic gates proposed are (OR, NOR, AND, NAND, NOT). The comsol multiphysics 5.5 software was used for simulation by the 2-D FDTD method. Hence, these five gates will be obtained by optical interference between the propagating signals through the input ports and the control ports, whose positions can be altered according to the gate needed. The implementation and simulation of the proposed gates were all in the same structure, with the same dimensions, the same wavelength and the same transmission threshold, with applicable wavelength of (1550 nm). The performance of the proposed plasmonic gates was tested by two criteria; the optical transmission ratio and the contrast ratio, which is the ratio between the ON and OFF states of the proposed gate..

Broadband Metallic Fiber-to-Chip Couplers and a Low-Complexity Integrated Plasmonic Platform.

We present a plasmonic platform featuring efficient, broadband metallic fiber-to-chip couplers that directly interface plasmonic slot waveguides, such as compact and high-speed electro-optic modulators. The metallic gratings exhibit an experimental fiber-to-slot coupling efficiency of −2.7 dB with −1.4 dB in simulations with the same coupling principle. Further, they offer a huge spectral window with a 3 dB passband of 350 nm. The technology relies on a vertically arranged layer stack, metal–insulator–metal waveguides, and fiber-to-slot couplers and is formed in only one lithography step with a minimum feature size of 250 nm. As an application example, we fabricate new modulator devices with an electro-optic organic material in the slot waveguide and reach 50 and 100 Gbit/s data modulation in the O- and C-bands within the same device. The devices’ broad spectral bandwidth and their relaxed fabrication may render them suitable for experiments and applications in the scope of sensing, nonlinear optics, or telecommunications.

Designing Mid-Infrared Gold-Based Plasmonic Slot Waveguides for CO2-Sensing Applications.

Plasmonic slot waveguides have attracted much attention due to the possibility of high light confinement, although they suffer from relatively high propagation loss originating from the presence of a metal. Although the tightly confined light in a small gap leads to a high confinement factor, which is crucial for sensing applications, the use of plasmonic guiding at the same time results in a low propagation length. Therefore, the consideration of a trade-off between the confinement factor and the propagation length is essential to optimize the waveguide geometries. Using silicon nitride as a platform as one of the most common material systems, we have investigated free-standing and asymmetric gold-based plasmonic slot waveguides designed for sensing applications. A new figure of merit (FOM) is introduced to optimize the waveguide geometries for a wavelength of 4.26 µm corresponding to the absorption peak of CO2, aiming at the enhancement of the confinement factor and propagation length simultaneously. For the free-standing structure, the achieved FOM is 274.6 corresponding to approximately 42% and 868 µm for confinement factor and propagation length, respectively. The FOM for the asymmetric structure shows a value of 70.1 which corresponds to 36% and 264 µm for confinement factor and propagation length, respectively.

Ultra-compact Universal Linear-Optical Logic Gate Based on Single Rectangle Plasmonic Slot Nanoantenna

Optical logic gates are key elements in all-optical circuits and computing. However, the footprints of current optical logic gates are still on micrometer scale. There remains challenging for further miniaturization of the logic gates to nanometer scale. In this paper, a simple scheme for realizing ultra-compact universal linear-optical logic gates is proposed. All common logic gates (AND, OR, NOT, NAND, NOR, XOR, and XNOR) encoded via binary phase shift keying are achieved by using single rectangle plasmonic slot nanoantenna with a footprint of 200 nm by 50 nm, which considerably shrinks the dimensions of the device.