Long-range Surface Plasmon Polariton Research Articles

Near-infrared long-range surface plasmon resonance in a D-shaped honeycomb microstructured optical fiber coated with Au film.

Long-range surface plasmon resonances (LRSPRs) are featured with longer propagation and deeper penetration, compared with conventional surface plasmon resonances (SPRs). Thus, LRSPR-based fiber sensors are considered to have great potential for highly sensitive detection in chemistry or biomedicine areas. Here, we propose and demonstrate a near-infrared LRSPR sensor based on a D-shaped honeycomb microstructured optical fiber (MOF) directly coated with gold film. Although there is no additional heterogeneous buffer layer, the optical field of the long-range surface plasmon polariton (LRSPP) mode penetrates strongly into the analyte region. Thus the effective refractive index of the LRSPP mode depends highly on the analyte's material refractive index and an abnormal dispersion relationship between the LRSPP mode and MOF's y-polarized core mode is observed. The mechanism of the LRSPR excitation in the coupling zone is attributed to an avoided crossing effect between these two modes. It also results in the generation of a narrow-bandwidth peak in the loss spectrum of the core mode. Further discussion shows that the resonance wavelength is mainly determined by the core size that is contributed by the MOF's cladding pitch, silica-web thickness and planar-layer-silica thickness together. It indicates that the operation wavelength of the proposed LRSPR device can be flexibly tuned in a broadband wavelength range, even longer than 2 µm, through appropriately designing the MOF's structural parameters. Finally, the proposed LRSPR sensor shows the highest wavelength sensitivity of 14700 nm/RIU and highest figure of merit of 475 RIU-1 for the analyte refractive index range from 1.33 to 1.39.

All-epitaxial long-range surface plasmon polariton structures with integrated active materials

We demonstrate all-epitaxial structures capable of supporting short- and long-range surface plasmon polariton (SRSPP and LRSPP) modes in the long-wave infrared region of the electromagnetic spectrum. The SRSPP and LRSPP modes are bound to the interfaces of a buried heavily doped (n++) semiconductor layer and surrounding quantum-engineered type-II superlattice (T2SL) materials. The surrounding T2SLs are designed to allow optical transitions across the frequency dispersion of the SPP modes. We map the SPP dispersion in our structure using grating-coupled angle- and polarization-dependent reflection and photoluminescence spectroscopy. The epitaxial structures are analytically described using a simplified three-layer system (T2SL/n++/T2SL) and modeled using rigorous coupled wave analysis with excellent agreement to our experimental results. The presented structures offer the potential to serve as long-range interconnects or waveguides in all-epitaxial plasmonic/optoelectronic systems operating in the long-wave infrared.

Synthesis of polyurethane-imids and application in surface plasmon polaritons waveguide

The crosslinkable electro-optic (EO) polymers polyurethane-imides (PUI) were designed and synthesized with monomers azo chromophore, phenyl diisocyanate and aromatic dianhydrides. Molecular structure characteristics for the polymers were evidenced by nuclear magnetic resonance (1HNMR) and fourier transform infrared (FTIR) spectroscopy. PUI as cladding, the strip long-range surface plasmon polaritons (LRSPPs) waveguides were designed and fabricated. The LRSPPs waveguiding along 18-nm-thin and 8-µm-wide gold stripes buried in EO PUI polymers were characterized at the light wavelength of 1310 nm and 1550 nm. The results indicated that the PUI can be used as preparation surface plasmon polaritons EO polymer optical waveguide materials, due to the merits of crosslinkable property, thermal stability, good film-forming ability, high processability and low optical waveguide propagation loss in near-infrared wave band.

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.

Long-range Graphene Surface Plasmon Polariton induced Tunable Optical Bistability in Terahertz Range

An idea towards the low threshold optical bistability and tunability of bistable thresholds at THz frequency under realistic situation is analyzed through a proposed multilayered configuration. The proposed configuration comprises a coupling prism of silicon, and monolayer graphene sheets spatially separated through 35 nm thick polymer which is sandwiched between an air gap of thickness 4.6 μm and a kerr polymer (GaAs) at 300 K temperature and at an incident light frequency of 2 THz. Utilizing the large nonlinear effect of the configuration obtained by the nonlinearity of the kerr polymer and the high local field effect due to long-range graphene surface plasmon resonance, the switching-down and switching-up threshold values for optical bistability have been markedly reduced upto 1.21×105 V/m and 1.24×105 V/m respectively thereby rendering a minimum threshold intensity of 7.2 kW/cm2 than the articles reported till date. Tunability of the switching-down and switching-up threshold values have also been realized from 1.21×105 V/m to 5.18×105 V/m and 1.24×105 V/m to 1.424×106 V/m respectively through electrical or chemical modification of the charge carrier density and hence fermi energy of the graphene from 0.23 eV to 0.41 eV and from 1.21×105 V/m to 1.56×105 V/m and 1.24×105 V/m to 2.12×105 V/m respectively for the variation in incident angle from 70° to 88°. The effect of damping rate of graphene on optical bistability behavior has also been observed. Therefore the proposed optical bistable configuration finds potential applications in the field of all-optical switching, optical transistor, optical logic, nano-illumination and optical memory with low input threshold at THz frequency and can also be used to realize tunable optical bistable device for future THz optical communication technology.

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.

Sensing self-referenced fiber optic long-range surface plasmon resonance sensor based on electronic coupling between surface plasmon polaritons.

A type of hollow gold nanoparticle (HGNP)-modified fiber optic long-range surface plasmon resonance (LRSPR) sensor with sensing self-reference is proposed and demonstrated. HGNPs have a stronger plasmonic field compared to solid GNPs because of the coupling between the inner and outer walls of HGNPs. The intense near-field electronic coupling between long-range surface plasmon polaritons associated with the LRSPR gold layer and localized surface plasmon polaritons of HGNPs leads to localized electromagnetic-field enhancement and LRSPR response signal amplification. Therefore, the HGNP-modified LRSPR sensor possesses a more excellent sensing property compared with the unmodified LRSPR sensor. The long-range resonance dip in the transmission spectrum is shown to shift in response to ambient refractivity change, and the characteristic absorption peak is fixed, allowing to regard it as a reference to improve detection accuracy of the sensors. The mode-field distribution of the sensors is simulated by using the finite element method, and the simulation results show that the electric-field intensity on the HGNP surface is significantly enhanced compared with that of the gold layer surface of the unmodified LRSPR sensor. 1874.79 nm/RIU improvement in sensitivity, 1.42 times improvement in figure of merit (FOM), and approximately 50% reduction in limit of detection (LOD) are achieved for the refractivity measurement of a low-concentration biological solution with the employment of HGNPs in LRSPR sensing experiments. The HGNP-modified LRSPR sensor proposed in this paper has high detection accuracy and FOM and low LOD, and can realize remote real-time online monitoring. Therefore, it has important research value and broad application prospects in the field of biochemical detection.

Highlighting recent progress in long-range surface plasmon polaritons: guest editorial

I discuss advances in the field of long-range surface plasmon polaritons in the ten years since publication of my review on the topic in the first volume of Advances in Optics and Photonics.

Grating couplers for (Bloch) long-range surface plasmons on metal stripe waveguides

Gratings capable of incoupling an incident Gaussian beam into long-range surface plasmon polaritons (LRSPPs), propagating along a thin narrow Au stripe on a low-index cladding (fluoropolymer), or on a truncated 1D photonic crystal as a Bloch LRSPP, are investigated for potential biosensing application. The gratings are optimized, initially, through 2D modeling using the vectorial finite element method. Different 3D grating designs are then investigated through 3D modeling using the vectorial finite difference time domain method, including wide gratings on adiabatic and nonadiabatic flared stripes. Wide gratings on flared stripes have a larger area enabling size matching and coupling to larger incident beams, which facilitates optical alignments. We also investigate the effects of changing the size of the beam incident on the structures. Incoupling efficiencies of up to 25% at λ0=1310 are predicted for optimal gratings—such coupling efficiencies are among the highest reported to date for a structure of this kind. Similar grating designs are also examined as outcoupling structures, capable of coupling (Bloch) LRSPPs into a perpendicularly emerging Gaussian beam. The absorptance, reflectance, and transmittance of the structure were also calculated. Structures were fabricated on a multilayer wafer supporting Bloch LRSPPs to verify design concepts.

A D-Shaped Fiber Long-Range Surface Plasmon Resonance Sensor With High Q-Factor and Temperature Self-Compensation

A D-shaped fiber based long-range surface plasmon resonance (LRSPR) sensor with high quality factor ( $Q$ -factor) of 67.75 RIU−1 and temperature self-compensation was proposed and demonstrated. The multilayered architecture of the LRSPR sensor consisted of fiber/dielectric buffer layer (DBL)/Au film/analyte, and the deep penetration depth and long propagation distance of long-range surface plasmon polaritons made the LRSPR sensor possess high $Q$ -factor. The temperature-sensitive photoabsorption characteristic of the Terbium(III) complexes was utilized to provide temperature self-compensation for the refractive index (RI) measurement of the LRSPR sensor, and the LRSPR sensor realized simultaneous measurement of RI and temperature at the exact same location, which could eliminate the measurement error introduced by temperature to RI measurement. The simulation analysis, supported by finite element analysis (FEA) based on coupled-mode theory, was presented in detail. The simulation results showed that the electric field intensity on the surface of the LRSPR sensor was 2.52 times higher than that of the surface plasmon resonance (SPR) sensor and the sensitivity of the LRSPR sensor increased by 947.31 nm/RIU compared with the SPR sensor, implying the LRSPR sensor possessed more excellent sensing performance. An SPR sensor and a Terbium(III) complex-based LRSPR sensor were developed, and their RI sensing performance was systematically studied and tabulated. The experimental results showed that the $Q$ -factor of the LRSPR sensor was 2.56 times higher than that of the SPR sensor, which agreed well with the simulation results. The LRSPR sensor proposed in this paper possessed higher detection precision and presented greatly promising application prospects in the field of biochemistry.

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.