Long-range Surface Plasmon Polariton Waveguide Research Articles

Double-layer graphene-based wedge and groove SPP waveguides with ultra-long propagation length

This study investigates long-range surface plasmon polariton (LRSPP) waveguides based on graphene in wedge and groove configurations, comprised of two graphene layers embedded in silica wedges and grooves, respectively. The finite-difference time-domain method is employed for numerical simulations. By combining coupled-mode perturbation theory with numerical results, we demonstrate the excitation of both LRSPP modes and short-range SPP modes when two wedge-shaped or two groove-shaped graphene layers are coupled. Our findings emphasize the significant advantage of the LRSPP fundamental mode, simultaneously achieving an ultra-long propagation length (∼10 μm) and an ultra-deep subwavelength confinement (∼10−7–10−6 times the diffraction-limited mode area). This novel waveguide emerges as a promising candidate for guiding mid-infrared electromagnetic waves in future photonic integrated circuits.

Tunable Narrow-Band Filter Based on Long-Range Surface Plasmon Polariton Waveguide Bragg Grating

A narrow-band Bragg grating filter based on a long-range surface plasmon polariton (LRSPP) waveguide is theoretically demonstrated. The three-dimensional Au stripe that is embedded in polymer SU-8 acts as both the waveguide and the heating electrode. With the eigen mode expansion and finite element method optimizations, the proposed filter shows a reflectivity of 0.578 and a 3 dB bandwidth of 1.1 nm. The central wavelength can be tuned from 1549.9 nm to 1544.3 nm by varying temperature from 25 °C to 75 °C, while maintaining the optical return loss at −2.5 dB. This proposed tunable filter has potential in on-chip light signal processing.

Excitation of Surface Plasmon Polariton Modes with Double-Layer Gratings of Graphene.

A long-range surface plasmon polariton (SPP) waveguide, composed of double-layer graphene, can be pivotal in transferring and handling mid-infrared electromagnetic waves. However, one of the key challenges for this type of waveguide is how to excite the SPP modes through an incident light beam. In this study, our proposed design of a novel grating, consisting of a graphene-based cylindrical long-range SPP waveguide array, successfully addresses this issue using finite-difference time-domain simulations. The results show that two types of symmetric coupling modes (SCMs) are excited through a normal incident light. The transmission characteristics of the two SCMs can be manipulated by changing the interaction of the double-layer gratings of graphene as well as by varying various parameters of the device. Similarly, four SCMs can be excited and controlled by an oblique incident light because this light source is equivalent to two orthogonal beams of light. Furthermore, this grating can be utilized in the fabrication of mid-infrared optical devices, such as filters and refractive index sensors. This grating, with double-layer graphene arrays, has the potential to excite and manipulate the mid-infrared electromagnetic waves in future photonic integrated circuits.

Integration of on-chip perovskite nanocrystal laser and long-range surface plasmon polariton waveguide with etching-free process.

Perovskite materials prepared in the form of solution-processed nanocrystals and used in top-down fabrication techniques are very attractive to develop low-cost and high-quality integrated optoelectronic circuits. Particularly, integrated miniaturized coherent light sources that can be connected to light-guiding structures on a chip are highly desired. To control light propagating on a small footprint with low-loss optical modes, long-range surface plasmon polariton (LRSPP) waveguides are employed. Herein, we demonstrate an on-chip fabricated photonic-plasmonic hybrid system consisting of a perovskite lasing structure coupled to an LRSPP waveguide achieving a low lasing threshold and a propagation length over 100 μm. Preventing perovskite material degradation and the formation of surface roughness of the laser cavity during fabrication is made possible by designing a fabrication technique without any etching step.

Continuous quantum walk in a 1-dimensional plasmonic lattice structure based on metal strip waveguides.

We experimentally studied a continuous time evolution of a "plasmonic" walker in a 1-dimensional lattice structure based on long-range surface plasmon polariton waveguides. The plasmonic walker exhibited a typical time evolution of a 1-dimensional quantum walk, which indicates that the plasmonic system is a potential platform to construct quantum walk simulators. By comparing experimental results to numerical simulations, the fidelity of the plasmonic quantum walk simulator is estimated to be > 0.96, which demonstrates that the plasmonic system can be a feasible platform for large-scale and high dimensional quantum walk simulators.

Coupling effect between semiconductor diodes and polarization-maintaining fibers

An innovative polarization coupler based on a long-range surface plasmon-polariton waveguide, which demonstrates efficient coupling between a semiconductor laser diode and a polarization-maintaining fiber, is proposed and analyzed. The proposed coupler—with different coupling facet sizes—can simultaneously match the mode sizes of the semiconductor laser diode and the polarization-maintaining fiber and can achieve polarization light coupling between different devices according to the features of the long-range surface plasmon-polariton waveguide. The coupling efficiency and alignment tolerance of the proposed coupler are investigated using the numerical-analysis method, and the design is optimized to obtain the best properties of the coupler. The calculated optimal coupling efficiency and the insertion loss are 89.96% and 0.97 dB, respectively. Further, the device exhibits a good alignment tolerance and has widespread potential applicability in the field of optical interconnections.

Modal analysis of thin long-range plasmonic waveguides

In this work, we present a comprehensive modal analysis, by means of numerical simulations, of a long-range surface plasmon polariton waveguide consisting of a metal stripe sandwiched between two dielectric claddings. Our results complement those from previous reports where similar structures have been studied and optimized for attenuation-based measurements. Specifically, we found conditions where a significant phase change can be induced while the losses of the structure vary minimally. Around these regions, an attenuation-based measurement would be insensitive for sensing, however, a high-sensitivity interferometric measurement could be performed without a significant penalty on the contrast of the interference signal. Our structure is designed specifically for biologically relevant situations and it can be fabricated with commercial-grade materials that are commonly available in clean room facilities.

Plasmonic-dielectric hybrid interferometric structures for high-sensitivity refractive index sensing

In this work, we present a parametric study, by means of numerical simulations, of an integrated Mach-Zehnder interferometer, where the sensing arm consists of a long-range surface plasmon polariton waveguide. Our hybrid dielectric-plasmonic structure is designed specifically for biologically relevant situations, where high-sensitivity sensing of refractive index is critical for tracing small changes in analytes which are commonly presented in aqueous solution form. The design of our device is realistic as it can be fabricated with commercial-grade materials that are commonly available in clean room facilities. Overall, our results validate the use of the proposed structure for high-sensitivity refractive index sensing, based on the sensitivity obtained in our study.

Wafer-bonded surface plasmon waveguide sensors with in-plane microfluidic interfaces

Sensors exploiting long-range surface plasmon polariton (LRSPP) waveguides comprised of Au stripes embedded in Cytop with integrated and encapsulated microfluidic channels are fabricated and demonstrated. A fabrication approach was devised where the lower cladding and recessed Au stripes are fabricated on a Si supporting substrate, and the upper cladding and microfluidic channels are fabricated on a glass substrate, followed by wafer bonding to assemble the wafers into complete sealed structures. The bond is centered over the full length of the optical path, yet no evidence of optical scattering or excess loss due to the bond could be observed, and no evidence of a bonding interface could be discerned from high-magnification cross-sectional images. We also demonstrate wafer-scale fabrication of in-plane microfluidic inlets and outlets, along with a fixture for fluidic edge coupling that provides sealed interfaces to external fluidic tubing and components. In-plane microfluidic interfaces are automatically defined along chip facets upon wafer dicing, precluding the need to drill through holes in lids. The performance of the chips was assessed by measuring the attenuation of LRSPPs on fully cladded reference waveguides, on waveguides passing through microfluidic channels, and by measuring the response of sensors to changes in refractive index produced by injecting various sensing solutions. Our fabrication approach based on wafer bonding and in-plane fluidic interfacing is compelling for low-cost high-volume manufacturing.

Straight Long-Range Surface Plasmon Polariton Waveguide Sensor Operating at l0 = 850 nm.

A bulk refractive index sensor based on a straight long-range surface plasmon polariton (LRSPP) waveguide is theoretically designed. The waveguide sensor consists of an Au stripe that is embedded in ultraviolet sensitive polymer SU-8. The geometric parameters are optimized by finite difference eigenmode method at the optical wavelength of 850 nm. The sensitivity of 196 dB/RIU/mm can be obtained with a 1.5 μm wide, 25 nm thick Au stripe waveguide. Straight LRSPP waveguides are fabricated by a double layer lift-off process. Its optical transmission is characterized to experimentally prove the feasibility of the proposed design. This sensor has potential for the realization of a portable, low-cost refractometer.

Ultrafast Plasmonic Graphene Photodetector Based on the Channel Photothermoelectric Effect

We propose an on-chip CMOS compatible graphene plasmonic photodetector based on the photo-thermoelectric effect (PTE) that occurs across an entire homogeneous graphene channel. The proposed photodetector incorporates the long-range dielectric-loaded surface plasmon polariton (LR-DLSPP) waveguide with a metal stripe serving simultaneously as a plasmon supporting metallic material and one of the metal electrodes. Large in-plane component of the transverse magnetic (TM) plasmonic mode can couple efficiently to the graphene causing large temperature increases across an entire graphene channel with a maximum located at the metal stripe edge. As a result, the electronic temperatures exceeding 12000K at input power of only a few tens of {\mu}W can be obtained at the telecom wavelength of 1550nm. Even with limitations such as the melting temperature of graphene (T= 4510 K), a responsivity exceeding at least 200 A/W is achievable at telecom wavelength of 1550 nm. It is also shown that under certain operation conditions, the PTE channel photocurrent can be isolated from photovoltaic and p-n junction PTE contributions providing an efficient way for optimizing the overall photodetector performance.

Asymmetric Long-Range Surface Plasmon Polariton Waveguides for Sensing Applications

A plasmonic sensing device based on an asymmetric long-range surface plasmon polaritons waveguide is proposed and investigated. The sensing structure consists of a microchamber and a gold strip with a cover layer, which is located on a dielectric supported by a silicon substrate. The optical and sensing properties of the device were studied using a finite element method. The sensitivity of the designed structure approaches 7.74 W/RIU for optimal sensing length and the limit of detection is 5.17 × 10−7 RIU. The proposed device is appropriate for a wide range of sensing applications, in fields that include biochemistry, environmental science, food safety, and medicine.

Straight Long-Range Surface Plasmon Polariton Waveguides with a Gap.

In straight long-range surface plasmon polariton (LR-SPP) waveguides with a gap (G-SPPWs), an excited input SPP propagates, jump overs the gap with low coupling loss, and propagates again, despite a micrometers-long gap. The micrometers long gap can be filled with various materials to control the LR-SPP mode in the G-SPPWs. Here, the optical characteristics of fabricated 20-nm-thick G-SPPWs with a fixed 2 µm gap length are experimentally evaluated at a wavelength of 1.55 µm. The gap coupling losses in the G-SPPWs were determined to be less than ~0.22 dB for various widths of the SPP waveguide (SPPW) up to ~8 µm, and the insertion losses were determined to be less than ~0.89 dB for the same design range. A minimum insertion loss of ~0.45 dB was obtained from the G-SPPW with a fixed 2 µm gap length and a 7 µm SPPW width. In the 2.5-Gbps optical signal transmission experiment, the G-SPPW exhibits excellent eye opening and the G-SPPW transfers the carrier wave as well as the data signal. The G-SPPW has potential to serve as a new plasmonic modulation element offering control of the guided SPP through interaction with an applied force in the gap.

Tapered Long-Range Surface Plasmon Polariton Waveguides with a Gap.

We propose tapered long-range surface plasmon polariton (LR-SPP) waveguides with a gap (tapered G-SPPWs) for improved tunneling efficiency in G-SPPWs. The optical characteristics of the fabricated tapered G-SPPWs were experimentally measured and compared with simulation results. The proposed tapered structure can improve tunneling efficiency by reducing insertion loss of the tapered G-SPPWs. The insertion losses of the straight G-SPPW with an 8-µm gap length and a 2-µm SPPW width and the tapered G-SPPW with an 8-µm gap length, a 2-µm SPPW width, a 6-µm taper width, and a 3-µm taper length were measured to be ~1.03 and ~0.74 dB, respectively. The tapered G-SPPW has potential as an efficient plasmonic modulation device element, offering control of the guided SPP through interaction with an applied force in the gap.

Integrated multichannel Young’s interferometer sensor based on long-range surface plasmon waveguides

Two integrated Young's interferometer (YI) sensors based on long-range surface plasmon polariton (LRSPP) waveguides are presented. The first sensor is single-channel and based on a Y-junction splitter, and the other is multi-channel and based on a corporate feed structure. The multichannel YI enables simultaneous and independent phase-based monitoring of refractive index changes in multiple channels. The diverging output beams from the waveguides are overlapped in the far field to form interference patterns which are then post-processed using the fast Fourier transform (FFT) algorithm to extract phase values. The sensing capability of these YIs was demonstrated through sequential injection of solutions with increasing refractive index into the sensing channels. A detection limit of ∼ 1 × 10-6 RIU was obtained for both LRSPP based YIs, a significant improvement over measurements from similar structures using attenuation-based sensing.

High-bandwidth and high-responsivity waveguide-integrated plasmonic germanium photodetector

Here we propose a waveguide-integrated germanium plasmonic photodetector that is based on a long-range dielectric-loaded surface plasmon polariton waveguide configuration. As this configuration ensures a long propagation distance, i.e., small absorption into metal, and a good mode field confinement, i.e., high interaction of the electric field with a germanium material, it is perfect for a realization of plasmonic germanium photodetectors. Such a photodetector even without optimization provides a responsivity exceeding 1 A/W for both wavelengths of 1310 nm and 1550 nm. To achieve such a responsivity, only a 5 {\mu}m-long waveguide is required for 1310 nm and 30 {\mu}m-long for 1550 nm. With optimization this value can be highly improved. In the proposed arrangement a metal stripe simultaneously supports a propagating mode and serves as one of the electrodes, while the second electrode is located a short distance from the waveguide. As a propagating mode is tightly confined to the germanium ridge, the external electrode can be placed very close to the waveguide without disturbing it. As such, the distance between electrodes can be smaller than 350 nm which allows one to achieve a bandwidth exceeding 100 GHz. However, as most of the carriers are generated inside a distance of 100 nm from a stripe, a bandwidth exceeding 150 GHz can be achieved for a bias voltage of -4 V.

Plasmonic Schottky photodetector with metal stripe embedded into semiconductor and with a CMOS-compatible titanium nitride

Here we propose an original waveguide-integrated plasmonic Schottky photodetector that takes full advantage of a thin metal stripe embedded entirely into a semiconductor. The photodetector is based on the long-range dielectric-loaded surface plasmon polariton waveguide with a metal stripe deposited on top of a semiconductor rib and covered by another semiconductor. As the metal stripe is entirely surrounded by semiconductor, all hot electrons with appropriate k-vectors can participate in transitions that highly enhances the electron transfer, and consequently the internal quantum efficiency. In addition, a high coupling efficiency from the photonic waveguide to the photodetector is simulated exceeding 90 % which enhances the external quantum efficiency. Calculations show that a responsivity exceeding 0.5 A/W can be achieved at telecom wavelength of 1550 nm and the bandwidth can exceed 100 GHz. Furthermore, it is shown that titanium nitride is a perfect material for the photodetector as it provides a low Fermi energy and long electron mean free path that enhance the hot electron transfer to the semiconductor. In addition, it shows reasonable metallic behavior and CMOS compatibility. Measurements showed that the Schottky barrier height between titanium nitride and p-doped silicon reaches 0.69–0.70 eV that matches the optimum signal-to-noise ratio operation calculated at 0.697 eV.

Grating couplers fabricated by e-beam lithography for long-range surface plasmon waveguides embedded in a fluoropolymer.

Long-range surface plasmon polariton waveguides consisting of Au stripes integrated with input and output grating couplers embedded in thick Cytop claddings are proposed and demonstrated experimentally. Under the right conditions, grating couplers enable broadside (top) coupling with good efficiency while producing a low level of background light. The scheme does not require high-quality input and output edge facets, and it simplifies optical alignments. We demonstrate coupling using a cleaved bow-tie fiber and a lensed fiber, and we determine the grating coupling efficiencies in both cases over a broad operating wavelength range. The lensed fiber produces a better overlap with the long-range surface plasmon mode of interest and thus results in a better coupling efficiency with essentially no background light as observed on an infrared camera. The measurements are compared with theoretical results obtained using a realistic model of the structures, including out-of-plane curvature in the grating profile resulting from our fabrication process. The coupling scheme along with the surface plasmon waveguides hold strong potential for biosensing applications.

Suspended LRSPP for the development of highly integrated active plasmonic devices.

We present a novel long-range surface plasmon polariton (LRSPP) device consisting of a suspended dielectric matrix in which an electrically active, millimeter-long metallic waveguide is embedded. We show that, by opening an air gap under the lower cladding, the influence of the substrate is suppressed and the symmetry of the thermo-optical distribution around the LRSPP waveguide is preserved over extended ranges of applied electrical current with minimal optical losses. Experimental results show that, compared to a standard nonsuspended structure, our device allows either the induction of a phase change that is three times larger, for a fixed electrical power, or, equivalently, a scaling down of the device to one-tenth of its original length, for a fixed phase change.

Long-Range Surface Plasmon-Polariton Waveguide Biosensors for Human Cardiac Troponin I Detection.

Straight long-range surface plasmon-polariton (LRSPP) waveguides as biosensors for label-free detection are discussed. The sensors consist of 5-μm-wide 35-nm-thick gold stripes embedded in a low-index optical-grade fluoropolymer (CYTOPTM) with fluidic channels etched to the Au surface of the stripes. This work demonstrates the application of the LRSPP biosensors for the detection of human cardiac troponin I (cTnI) protein. cTnI is a biological marker for acute myocardial infarction (AMI), often referred to as a heart attack, which can be diagnosed by elevated levels of cTnI in patient blood. Direct and sandwich assays were developed and demonstrated over the concentration range from 1 to 1000 ng/mL, yielding detection limits of 430 pg/mL for the direct assay and 28 pg/mL for the sandwich assay (1 standard deviation), the latter being physiologically relevant to the early detection or onset of AMI. In addition, a novel approach for data analysis is proposed, where the analyte response is normalized to the response of the antibody layer.