Long-range Surface Plasmon Polariton Mode 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.

Excitation of "forbidden" guided-wave plasmon polariton modes via direct reflectance using a low refractive index polymer coupling layer.

A surface plasmon polariton (SPP) is an excitation resulting from the coupling of light to a surface charge oscillation at a metal-dielectric interface. The excitation and detection of SPPs is foundational to the operating mechanism of a number of important technologies, most of which require SPP excitation via direct reflectance, commonly achieved via Attenuated Total Reflection (ATR) using the Kretschmann configuration. As a result, the accessible modes are fundamentally high-loss “leaky modes,” presenting a critical performance barrier. Recently, our group provided the first demonstration of “forbidden,” or guided-wave plasmon polariton modes (GW-PPMs), collective modes of a MIM structure with oscillatory electric field amplitude in the central insulator layer with up to an order of magnitude larger propagation lengths than those of traditional SPPs. However, in that work, GW-PPMs were accessed by indirect reflectance using Otto configuration ATR, making them of limited applied relevance. In this paper, we demonstrate a technique for direct reflectance excitation and detection of GW-PPMs. Specifically, we replace the air gap used in traditional Otto ATR with a low refractive index polymer coupling layer, mirroring a technique previously demonstrated to access Long-Range Surface Plasmon Polariton modes. We fit experimental ATR data using a robust theoretical model to confirm the character of the modes, as well as to explore the potential of this approach to enable advantageous propagation lengths. The ability to excite GW-PPMs using a device configuration that does not require an air gap could potentially enable transformative performance enhancements in a number of critical technologies.

Fano Resonance Hybrid Waveguide-Coupled Plasmonic Sensor Using Transparent Conductive Oxide in the Near-Infrared Range

We proposed an ultra-sensitive refractive index sensor by using indium-doped cadmium oxide as a plasmonic material operating in near-infrared based on Fano resonance. The proposed sensor has a hybrid multilayer waveguide structure that supports both a long-range surface plasmon polariton (LRSPP) mode and a dielectric waveguide (DWG) mode. The design strategy of the structure parameters of the inner layers is elaborated in detail through the numerical analysis of the two modes. By suitably tailoring the thickness of the coupling layer, a strong mode coupling between the two modes could be achieved, leading to a sharp asymmetric Fano resonance. With the designed optimal physical parameters, our proposed sensor could achieve a maximum intensity sensitivity of 19,909 RIU−1, a 193-fold enhancement than that of a conventional long-range SPR (LRSPR) based scheme. The proposed design can be a promising platform for biochemical sensing in the near-infrared region.

Compact resonant plasmonic hybrid waveguide refractive index sensors for aqueous medium

Ultra-compact and high sensitivity resonant plasmonic refractive index sensors for aqueous medium are proposed. The sensors are based on the resonant coupling between the waveguide mode and the surface plasmon mode supported by a thin metal layer. For optimized layer thicknesses, the periodic coupling between the two hybrid modes guided by the structure takes place and a resonant absorption peak appears for a particular sensor length. The sensors are designed to operate in the spectral mode at visible wavelengths centred about 0.6328 μm. The sensors have been designed for the long range surface plasmon polaritons (LRSPP) as well as the short range surface plasmon polaritons (SRSPP). This study provides a detailed understanding of designing an ultra-short and efficient sensor for the aqueous medium with high spectral sensitivity (S), ultrahigh resolution (R) and figure of merit (FOM). The attained numerical values show that for optimized layer thicknesses, a sensitivity of 9200 nm/RIU for LRSPP mode and 1800 nm/RIU for SRSPP mode can be achieved for the aqueous medium with the sensor lengths of 83.5 μm and 15 μm (shortest to the best of our knowledge) respectively. Also, the values of R and FOM are 1.52 × 10−7 RIU and 7.7 × 103 RIU−1 for the LRSPP based sensor and 6.7 × 10−7 RIU and 4.5 × 103 RIU−1 for the SRSPP mode based sensor.

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.

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.

Highly Sensitive SPR Sensor Based on Hybrid Coupling Between Plasmon and Photonic Mode

In this letter, we propose a new configuration of surface plasmon resonance sensor based on hybrid coupling between long-range surface plasmon polariton (LRSPP) and dielectric planar waveguide mode (WGM) for ultrasensitive refractive index sensing. The multilayer Otto configuration supports both LRSPP and photonic WGM. Under phase matching condition, these two modes can be dramatically coupled with each other forming hybrid modes and consequently splitting the resonance. By optimizing the structure parameters, an intensity sensitivity as high as 3081/RIU is achieved, which corresponds to 1.2-fold enhancement comparing to the conventional LRSPP sensor. Moreover, the sensor can be modified for detecting the analyte according to different targeting refractive index ranges by simply replacing the dielectric medium.

Band diagrams and field distribution of squarely-modulated slab metallic gratings

The optical transmission properties of squarely-modulated metallic gratings has been widely investigated. This study used rigorous-coupled wave analysis (RCWA) to re-examine in detail the band structure of a laminated metal film with squarely-modulated metallic gratings located at the top. The top structure was shown to modify the long-range surface plasmon polariton modes (LRSPP) at both interfaces of the thin metal film. When the thickness of the intact metal film was altered, the coupling between the two interfaces presented intriguing behavior. If the thickness of the metallic film was 30nm, the field achieved strong coupling similar to that of a two-level system. When the thickness was decreased to 10nm, the band branch possessing negative group velocity was dominant. Our results also verify that the first-order Fourier expansion of the gratings determined the energy position of bands at k||=0, whereas the second-order term caused band gap opening. Introducing an asymmetrical component to the grating profile intensified the opening effect at k||=0 due to an increase in the amplitude of the second-order Fourier component.

Ultrasensitive biosensors based on long-range surface plasmon polariton and dielectric waveguide modes

An ultrasensitive biosensor based on hybrid structure and composed of long-range surface plasmon polariton (LRSPP) and dielectric planar waveguide (PWG) modes is proposed. Both PWG and LRSPP modes have strong resonances to form strong coupling between the two modes, and the two modes can couple to enhance sensitivity of sensors. In the hybrid structure, PWG is composed of cytop–Si–cytop multilayers and the LRSPP configuration is composed of cytop–metal–sensing medium multilayer slabs. The highest imaging sensitivities of 2264 and 3619 RIU−1 were realized in the proposed sensors based on Au and Al-monolayer graphene, respectively, which are nearly 1.2 and 1.9 times larger than the 1910 RIU−1 sensitivity of the conventional LRSPR sensor (LRSPP sensor). Moreover, it is demonstrated that the PWG-coupled LRSPP biosensor is applicable to the sensing medium, with refractive index in the vicinity of 1.34.

Enhancing the Surface Sensitivity of Metallic Nanostructures Using Oblique-Angle-Induced Fano Resonances.

Surface sensitivity is an important factor that determines the minimum amount of biomolecules detected by surface plasmon resonance (SPR) sensors. We propose the use of oblique-angle-induced Fano resonances caused by two-mode coupling or three-mode coupling between the localized SPR mode and long-range surface plasmon polariton modes to increase the surface sensitivities of silver capped nanoslits. The results indicate that the coupled resonance between the split SPR (−kSPR) and cavity modes (two-mode coupling) has a high wavelength sensitivity for small-angle incidence (2°) due to its short decay length. Additionally, three-mode coupling between the split SPR (−kSPR), substrate (+kSub) and cavity modes has a high intensity sensitivity for large-angle incidence due to its short decay length, large resonance slope and enhanced transmission intensity. Compared to the wavelength measurement, the intensity measurement has a lower detectable (surface) concentration below 1 ng/ml (0.14 pg/mm2) and is reduced by at least 3 orders of magnitude. In addition, based on the calibration curve and current system noise, a theoretical detection limit of 2.73 pg/ml (0.38 fg/mm2) can be achieved. Such a surface concentration is close to that of prism-based SPR with phase measurement (0.1–0.2 fg/mm2 under a phase shift of 5 mdeg).

Graphene-based long-range SPP hybrid waveguide with ultra-long propagation length in mid-infrared range.

A graphene-based long-range surface plasmon polariton (LRSPP) hybrid waveguide, which is composed of two identical outer graphene nanoribbons and two identical inner silica layers symmetrically placed on both sides of a silicon layer, is investigated using the finite-difference time-domain method. By combining the simulated results with the coupled mode perturbation theory, we demonstrate that the LRSPP and short-range SPP (SRSPP) modes originate from the coupling of the same modes of the two graphene nanoribbons. For the LRSPP mode, an ultra-long propagation length (~10 μm) and an ultra-small mode area (~10-7A0, where A0 is the diffraction-limited mode area) can be simultaneously achieved. This waveguide can be used for future photonic integrated circuits functional in the mid-infrared range.

Mode size and loss in strongly asymmetric plasmonic waveguide with dielectric cladding

The effect of dielectric cladding on modifying a mode field based on a multilayer planar surface plasmon polariton (SPP) waveguide is investigated at the telecom wavelength of 1.55 μm. Through numerical calculations based on the transfer matrix method and Cauchy integration method, we point out that the mode loss and the mode size can be efficiently engineered via tailoring the dielectric cladding layer. With appropriately optimized thickness and refractive index of the dielectric cladding, the mode size and mode loss of the long-range SPP modes supported by the proposed structure could reach their minima simultaneously such that the figure of merit exhibits a maximum, which breaks the trade-off relationship between confinement and attenuation of SPP waveguide to a certain degree. Furthermore, benefiting from the dielectric cladding effect, the adjustability of field distribution in the dielectric region provides a simple and effective means for improving the light–matter interaction strength for the purpose of SPP signal modulating, detecting or sensing.

Hybrid Plasmonics Waveguide Properties of Dielectric Nanowires Coupling with Metal Substrate

A hybrid plasmonic waveguide system consisting of a high-permittivity cylinder waveguide embedded in a low-permittivity dielectric deposited on a metal substrate is comprehensive investigated. With the strong coupling between dielectric cylindrical waveguide mode and the long-range surface plasmon polaritons (LRSPP) mode of a thin metal film, the basic waveguiding properties, including the effective mode area, the propagation length, the mode character, and the figure of merit (FoM) can be achieved. Our results may provide valuable references for waveguiding metal -supported dielectric nanowires for practical applications. An investigation of the effect of structural perturbations indicates that the disturbance of the dielectric nanowires of our proposed waveguide with different surrounding dielectric layer would have a great influence on the experiment results.

Long-range hybrid wedge plasmonic waveguide.

We design a novel long-range hybrid wedge plasmonic (LRHWP) waveguide composed of two identical dielectric nanowires symmetrically placed on two opposed wedges of a diamond shaped metal wire. With strong coupling between the dielectric nanowire mode and long-range surface plasmon polariton (SPP) mode, both deep subwavelength mode confinement and low propagation loss are achieved. On one hand, when compared to the previous long-range hybrid SPP waveguide, LRHWP waveguide can achieve smaller mode size with similar propagation length; on the other hand, when compared to the previous hybrid wedge SPP waveguide, LRHWP waveguide can provide an order of magnitude longer propagation length with similar level of mode confinement. The designed LRHWP waveguide also features an overall advantage of one-order improvement of Figure of Merit. We further evaluate in detail the impacts of possible practical fabrication imperfections on the mode properties. The obtained results of mode properties show that the proposed LRHWP waveguide with an optimized wedge tip angle of 140 degree is fairly tolerant to practical fabrication errors in geometry parameters such as misalignment in the horizontal direction, asymmetry in the vertical direction, variation of wedge tip angle, tilt or rotation of metal wire, and variation of wedge tip curvature radius.

Photo-generated THz plasmonic waveguides

We propose a novel method to actively control the propagation of THz surface plasmon polaritons in plasmonic waveguides. This method relies on the excitation of long range surface plasmon polaritons (LRSPPs) in thin slabs of semiconductors. The characteristics of LRSPPs (electromagnetic field confinement and mode propagation length) can be precisely tuned by optically pumping charge carriers to the conduction band of the semiconductor. This excitation can be local by illuminating a flat semiconductor slab with an expanded optical beam that is structured with the shape of the THz waveguide. In this way, it may be possible to design active and integrated THz circuits by simply changing the illumination pattern on the surface.

Fabrication of long-range surface plasmon polaritons waveguide by wet chemical etching

The fabrication of long-range surface plasmon polaritons (LRSPPs) waveguides based on a thin Au stripe embedded in poly(methyl-methacrylate-glycidly-methacrylate) polymers was investigated. By patterning the photoresist, a wet chemical etching technique was used to avoid sharp pin-like and shark-fin-like structures on the edges of the Au stripe. The surface morphology of the Au film and polymer cladding were studied by atomic force microscopy (AFM), as well as by using the waveguide configuration of the Au stripe. AFM images proved the elimination of parasitic structures. A 2 cm long, 4 μm wide, and 25 nm thick Au stripe waveguide exhibited a propagation loss of approximately 4.3 dB cm−1 measured by the cut-back method and end-fire excitation of LRSPP mode guiding at 1550 nm. The demonstration of optical signal transmission indicates that the LRSPP waveguide fabricated by wet chemical etching is a potential solution to on-chip optical interconnections.

Variable Optical Attenuator Based on Long‐Range Surface Plasmon Polariton Multimode Interference Coupler

The fabrication and characterization of a thermal variable optical attenuator based on long‐range surface plasmon polariton (LRSPP) waveguide with multimode interference architecture were investigated. The surface morphology and waveguide configuration of Au stripe were studied by atomic force microscopy. The fluctuation of refractive index of poly(methyl‐methacrylate‐glycidyl‐methacrylate) polymer cladding was confirmed to be less than 3 × 10−4 within 8 h curing at 120°C. The end‐fire excitation of LRSPP mode guiding at 1550 nm along Au stripe indicated that the extinction ratio of attenuator was about 12 dB at a driving power of 69 mW. The measured optical rise time and fall time are 0.57 and 0.87 ms, respectively. These favorable properties promise potentials of this plasmonic device in the application of optical interconnection.

Grating-Assisted Excitation of Short-Range Surface Plasmon Polariton Mode

We propose an anti-symmetric grating structure for effective excitation of the short-range surface plasmon polariton (SR-SPP) mode of a metal stripe waveguide. The structure consists of two corrugated Bragg gratings offset by half of a period formed on the two sides of the metal stripe, respectively. The SR-SPP mode is excited in the backward direction by the long-range surface plasmon polariton mode launched into the waveguide. We investigate the effects of the metal stripe thickness, the grating corrugation depth, and the number of grating periods on the transmission and reflection characteristics of the grating with the finite-difference time-domain method in detail. Our numerical results show that a peak reflection of ~57% in the SR-SPP mode can be achieved with a 50-μm long grating on a 40-nm thick gold stripe.

Large Optical Nonlinearity of Surface Plasmon Modes on Thin Gold Films

We investigate the optical nonlinear effects of a long-range surface plasmon polariton mode propagating on a thin gold film. These effects may play a key role in the design of future nanophotonic circuits as they allow for the realization of active plasmonic elements. We demonstrate a significant enhancement of the transmission on a timescale below a millisecond as well as a phase shift exceeding 2π already for modest peak powers of 150 mW. On the contrary, slow effects suppress the transmission on a millisecond timescale.