Long-range Surface Plasmon Polariton Research Articles

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).

Unidirectional Bragg Gratings Using Parity-Time Symmetry Breaking in Plasmonic Systems

Optical systems following concepts of parity-time (PT) symmetry have attracted significant attention because of their extraordinary behavior such as unidirectional reflectance or power oscillation. PT symmetric optical systems are realized by judiciously manipulating the complex refractive index to produce even- and odd-symmetric distributions for the real and imaginary indices, respectively. We propose two PT symmetric Bragg gratings based on step-in-width metal stripes and dielectric-loaded metal stripes operating with long-range surface plasmon polaritons. The gratings are designed to operate near 880 nm because optical gain can be conveniently provided by IR140-doped PMMA. Asymmetric reflectance is predicted in the proposed gratings based on modal and transfer matrix method computations. Moreover, we analyzed pulse reshaping and energy transport in generic gratings, and in the proposed plasmonic gratings, in terms of group and energy velocities. It is found that the group and energy velocities are dispersionless at the PT symmetry breaking point. Also, the group velocity dispersion can be inverted by changing the PT symmetric state from broken to unbroken or vice versa. Our designs are practical because a large left-right asymmetric reflectance contrast is produced for a wide range of physical dimensions. The proposed gratings are suitable as on-chip devices for optical processing providing new functionality such as switching and controlling the time delay of a data pulse without distortion.

Analysis on vertical directional couplers with long range surface plasmons for multilayer optical routing

A structure featuring vertical directional coupling of long-range surface plasmon polaritons between strip waveguides at λ = 1.55 μm is investigated with the aim of producing efficient elements that enable optical multilayer routing for 3D photonics. We have introduced a practical computational method to calculate the interaction on the bent part. This method allows us both to assess the importance of the interaction in the bent part and to control it by a suitable choice of the fabrication parameters that helps also to restrain effects due to fabrication issues. The scheme adopted here allows to reduce the insertion losses compared with other planar and multilayer devices.

Bulk sensing using a long-range surface-plasmon triple-output Mach–Zehnder interferometer

A triple-output Mach–Zehnder interferometric sensor operating with long-range surface plasmon–polaritons at free-space wavelengths near 1310 nm was constructed by etching a microfluidic channel through the top cladding (Cytop) of one arm of the interferometer to expose the Au stripe embedded therein. Optical bulk (refractometric) sensing was conducted by sequentially flowing sample solutions with different refractive indices through the microfluidic channel and measuring the optical powers of the three outputs, which responded sinusoidally and were separated by ∼2π/3 rad as expected in theory. Three detection schemes are analyzed and compared, demonstrating that the device benefits from a 3× larger dynamic range and the ability to suppress common perturbations relative to its single-output counterpart, thus improving the detection limit. The device is promising for biosensing applications.

Bulk Sensing Using a Long-Range Surface-Plasmon Dual-Output Mach–Zehnder Interferometer

Optical bulk (refractometric) sensing of sample solutions is demonstrated using dual-output Mach-Zehnder interferometers built from long-range surface-plasmon polariton waveguides operating at a free-space wavelength of 1375 nm. The device of interest was constructed by embedding Au stripes in Cytop claddings and etching a fluidic channel through the top cladding of one arm of the interferometer to expose the Au stripe. Bulk sensing was carried out by flowing sequentially a series of solutions of different refractive index through the microfluidic channel. The optical powers of the two outputs responded sinusoidally and were complimentary, as expected in theory. Three detection schemes aiming at improving the detection limit are analyzed showing that the device benefits from a 2× larger dynamic range, and has the ability to suppress common perturbations, relative to a single output. A detection limit of ~4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> RIU is demonstrated, which can be further improved by lengthening the sensing channel. The device is promising for application as a biosensor.

Parity-time symmetry-broken Bragg grating operating with long-range surface plasmon polaritons

A parity-time symmetry-broken Bragg grating concept, comprising step-in-width IR-140 doped PMMA stripes covering step-in-width Au stripes on a fused silica substrate and operating with long-range surface plasmon polaritons, is reported. The design methodology is outlined, structures are modelled using a combination of modal analysis and the transfer matrix method, and reflectance and transmittance responses are computed. The design robustness with respect to deviations in the dimensions due to fabrication error is also investigated. The structure is capable of producing a reflectance contrast ratio of 1 over optical bandwidths of 879.96–880.04 nm.

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.

Semianalytical approach to study the loss characteristics of a symmetrical multilayered plasmonic waveguide.

A metal waveguide of finite width and thickness surrounded by a dielectric provides increased propagation length of the surface plasmon polariton and is therefore called long-range surface plasmon polaritons (LR-SPP). In this work, a new structure is proposed by modifying the refractive index of the dielectric surrounding the metal waveguide, leading to further improvement of the propagation length. It is shown that, when the single dielectric surrounding is replaced by a multilayered surrounding where each layer has different thicknesses and refractive index, the propagation loss gets reduced, leading to increased propagation length of the LR-SPP. The propagation loss is calculated semianalytically from the FWHM of the Lorentzian peak obtained in the plot of excitation efficiency of such waveguide for different values of the propagation constant. Before doing this calculation, the 2D variation of refractive index is first converted into a 1D effective refractive index. All the steps of analysis are discussed in detail, and, wherever necessary, the calculated results are matched with similar results of other researchers.

Third-order susceptibility of gold for ultrathin layers.

This Letter presents an experimental study of nonlinear plasmonic effects in gold-stripe waveguides. The optical characterization is performed by a picosecond laser and reveals two nonlinear effects related to propagation of long-range surface plasmon polaritons: nonlinear power transmission of plasmonic modes and spectral broadening of plasmonic modes. The experimental values of the third-order susceptibility of the gold layers are extracted. They exhibit a clear dependence on layer thickness.

Hydrogen sensing with Pd-coated long-range surface plasmon membrane waveguides.

A low power, reusable optical hydrogen sensor using long-range surface plasmon polariton (LRSPP) cladded membrane waveguides is demonstrated. The sensor incorporates a 5 μm wide, 20 nm thick gold stripe embedded in a 160 nm thick free-standing Cytop membrane with a 5 nm thick Pd over-layer. Input and output coupling is achieved with directly integrated broadside grating couplers. The sensor is tested with hydrogen concentrations up to 3% and demonstrates a strong response with an estimated detection limit of 290 ppm, and a response time of 7 s to a 0.6% H2 step - this level of performance is among the best reported for optical H2 sensors. Cycling of the hydrogen exposure shows a significant hysteresis response, however no film deformation or delamination was observed over many cycles. The high stress that is induced in clamped films during hydrogenation is relaxed in due to the film being deposited on the flexible and elastic Cytop membrane. This could result in improved lifetimes for this sensor and increased uptake ability.

Transmission of long-range surface plasmon-polaritons across gap in Au waveguide

Optical loss induced by the arbitrary structure discontinuity in the Au stripe that supports long-range surface plasmon-polaritons (LRSPPs) propagation is investigated in this paper. A broad range of arbitrary gap sizes, 4 to 20 μm, is reported for 25 nm thick Au stripe with different widths embedded in 5 μm thick optically symmetric polymer SU-8. The simulations and experimental data find a high tunneling efficiency of the long-range mode power transmission of larger than 50% over a 10 μm gap at a wavelength of 1550 nm. Accordingly, a thermally activated in-line Mach–Zehnder interferometer switch based on the guiding of LRSPPs along Au stripe with gaps is designed and fabricated. Switching characteristics are analyzed upon heating one arm of the interferometer through the passage of current therein. The fabricated switch exhibits an extinction ratio of 17 dB with a driving power of 13 mW. The results prove that LRSPPs are insensitive to technological imperfections and waveguide interruptions, which is of benefit to active plasmonic device applications.

Fabrication of metal strip waveguides for optical and microwave data transmission

Metal strip waveguides and devices suitable for high-speed digital signal transmission at both microwave and optical frequencies are fabricated and demonstrated in this work. The waveguide structure consists of three metal strips forming a coplanar waveguide (CPW) to support a microwave mode. In the proposed structure, the signal line consists of a copper strip sandwiched between two thin gold layers. The CPW ground planes are thin gold strips supporting long-range surface plasmon polaritons (LRSPPs) at optical frequencies. Thus, the proposed structure can simultaneously support LRSPPs at optical frequencies and microwave signals up to at least 40 GHz. The metalizations are patterned using bilayer photolithography followed by thermal evaporation. Then, to create the signal waveguide, an O2 plasma etch of the cladding and copper E-beam evaporation are used. The fabrication process steps are verified through experimental characterization. Microwave and optical transmission through the fabricated devices is demonstrated using radio frequency probes applied to the top of the device and optical fibers in an end-fire coupling configuration, respectively.

Long-Range Surface Plasmon Polaritons for Efficient Optical Coupling in AlGaN/GaN Quantum Well Infrared Photodetector

In this paper, the supermodes, long-range surface plasmon polaritons (LRSPPs), have been theoretically studied to enhance the optical coupling of AlGaN/GaN quantum well infrared photodetector (QWIP) based on gold–Si3N4 hybrid architecture. The electromagnetic field, energy flow, and current density are analyzed by finite element method (FEM). In time domain, the electric field component E z and current density J z perpendicular to the multi-quantum wells (MQWs) are symmetric and asymmetric distributions over the gold grating, respectively, which precisely prove the existence of LRSPPs. The averaged |E z |2 across the whole quantum well region reaches 1.51 (V/m)2 when the electric field intensity (|E 0|2) of normal incidence is 1 (V/m)2 at 4.65 μm. Extraordinarily low loss of the LRSPPs results in a coupling efficiency enhancement ratio of 2.23 in AlGaN/GaN QWIP compared with that obtained via bare gold grating with different polarized sources, exhibiting great potential for application in the focal plane arrays.

Graphene-based hybrid plasmonic modulator

A graphene-based hybrid plasmonic modulator is designed based on an asymmetric double-electrode plasmonic waveguide structure. The photonic device consists of a monolayer graphene, a thin metal strip, and a thin dielectric layer that is inserted between the grapheme and the metal strip. By electrically tuning the graphene’s refractive index, the propagation loss of the hybrid long-range surface plasmon polariton strip mode in the proposed graphene-based hybrid plasmonic waveguide is switchable, and hence the intensity of the guided modes is modulated. The highest modulation depth is observed at the graphene’s epsilon-near-zero region. The device characteristics are characterized over the entire C-band (1.530–1.565 μm).

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.

Dielectric supported bimetal layer configuration for long-range surface plasmon polariton interference–based subwavelength lithography

Long-range surface plasmon polariton (LRSPP) interference in a dielectric supported bimetal layer configuration is analyzed for nanoscale periodic feature fabrication. The single metal layer in a conventional configuration has been split into a bimetal layer configuration supported by a dielectric layer to enable LRSPP interference in different layers. It is shown that the configuration can be implemented in two different ways to record the interference pattern with high exposure depth and high contrast. Subwavelength periodic pattern having a half-pitch resolution of 65 nm at 446-nm wavelength is illustrated with the proposed two-beam and four-beam LRSPP interference configurations.

Plasmonic gain in long-range surface plasmon polariton waveguides bounded symmetrically by dye-doped polymer

The plasmonic gain of a top-pumped active symmetric metal slab waveguide is investigated theoretically and experimentally. The structure consists of a thin Ag film cladded above and below by gain media (IR140-doped poly (methyl methacrylate)), and operating with long-range surface plasmon polaritons (LRSPPs) at near-infrared wavelengths. We consider the spatial distribution of the pump intensity and the position dependence of the dipole lifetime within the claddings when computing the LRSPP gain. We find that the bottom cladding provides significant gain to the LRSPP, despite the low pump transmittance through the Ag film, as long as the pump intensity is strong enough to saturate the gain material (∼4 MW/cm2). In this situation, the LRSPP gain is doubled compared to the case where the top cladding only is active. The LRSPP gain was measured in a fabricated structure using the variable stripe length method, yielding gmod = 16.7 cm−1 at a pump intensity of ∼4 MW/cm2. The measured LRSPP gain agrees very well with the computed value, implying that the bottom cladding provides significant gain to the mode. Active plasmonic devices based on the symmetric dielectric-metal-dielectric structure can be significantly more efficient by using gain layers as both the top and bottom claddings.

Broadband silicon optical modulator using a graphene-integrated hybrid plasmonic waveguide

Graphene is an excellent electronic and photonic material for developing electronic–photonic integrated circuits in Si-based semiconductor devices with ultra wide operational bandwidth. As an extended application, here we propose a broadband silicon optical modulator using a graphene-integrated hybrid plasmonic waveguide, and investigate the optical characteristics numerically at a wavelength of 1.55 μm. The optical device is based on the surface plasmon polariton absorption of graphene. By electrically tuning the graphene’s refractive index as low as that of a noble metal, the hybrid plasmonic waveguide supports a strongly confined highly lossy hybrid long-range surface plasmon polariton strip mode, and hence light coupled from an input waveguide experiences significant power attenuation as it propagates along the waveguide. Over the entire C-band from 1.530 to 1.565 μm wavelengths, the on/off extinction ratio is larger than 13.7 dB. This modulator has the potential to play a key role in realizing graphene–Si waveguide-based integrated photonic devices.

Single-mode lasers and parity-time symmetry broken gratings based on active dielectric-loaded long-range surface plasmon polariton waveguides.

Single-mode distributed feedback laser structures and parity-time symmetry broken grating structures based on dielectric-loaded long-range surface plasmon polariton waveguides are proposed. The structures comprise a thin Ag stripe on an active polymer bottom cladding with an active polymer ridge. The active polymer assumed is PMMA doped with IR140 dye providing optical gain at near infrared wavelengths. Cutoff top ridge dimensions (thickness and width) are calculated using a finite element method and selected to guarantee single-mode operation of the laser. Several parameters such as the threshold number of periods and the lasing wavelength are determined using the transfer matrix method. A related structure based on two pairs of waveguides of two widths, which have the same imaginary part but different real part of effective index, arranged within one grating period, is proposed as an active grating operating at the threshold for parity-time symmetry breaking (i.e., operating at an exceptional point). Such "exceptional point" gratings produce ideal reflectance asymmetry as demonstrated via transfer matrix computations.

Selective detection of bacteria in urine with a long-range surface plasmon waveguide biosensor.

Experimentation demonstrates long-range surface plasmon polariton waveguides as a useful biosensor to selectively detect gram negative or gram positive bacteria in human urine having a low concentration of constituents. The biosensor can detect bacteria at concentrations of 10(5) CFU/ml, the internationally recommended threshold for diagnostic of urinary tract infection. Using a negative control urine solution of bacterial concentration 1000☓ higher than the targeted bacteria, we obtain a ratio of 5.4 for the positive to negative signals.