Long-range Surface Plasmon Polariton Research Articles

Grating couplers for broadside input and output coupling of long-range surface plasmons

Metal gratings for in-coupling a Gaussian beam incident from broadside to the long-range surface plasmon polariton (LRSPP) propagating in one direction along a membrane-supported Au slab bounded by air or water are proposed and modeled by the finite-difference time-domain method. Grating couplers for out-coupling the propagating LRSPP into free radiation directed along broadside are also investigated. Short grating designs consisting of a small number of Au bumps yield 15% to 20% in-coupling efficiencies, and about 60% out-coupling efficiencies. LRSPP back-reflections along the membrane waveguide caused by the out-coupling grating are also calculated and discussed.

Amplification of long-range surface plasmons by a dipolar gain medium

Surface plasmon–polaritions, collective electron oscillations coupled to light waves at the surface of a metal, show unique properties that are valuable in a broad range of scientific fields. However, the intrinsic propagation loss of these waves poses a fundamental problem to many potential applications. To overcome this drawback, researchers have explored the possibility of loss compensation by means of surface plasmon–polarition amplification. Here we provide the first direct measurement of gain in propagating plasmons using the long-range surface plasmon–polariton supported by a symmetric metal stripe waveguide that incorporates optically pumped dye molecules in solution as the gain medium. The measured mode power gain is 8.55 dB mm−1. Furthermore, it is shown that this new class of amplifier benefits from reduced spontaneous emission into the amplified mode, potentially leading to low-noise optical amplification. Researchers overcome the propagation loss of surface-plasmon polaritons, with this demonstration being the first direct gain measurement of propagating plasmons. Low-loss long-range modes of a metal stripe waveguide are amplified by using optically pumped dye molecules in solution as the gain medium. The mode power gain was measured to be 8.55 dB mm−1.

One-dimensional long-range plasmonic-photonic structures

We have fabricated line gratings from periodically etched fused silica on which a thin silver film is deposited that is in turn covered with a silica index-matched fluid. This dielectrically symmetric geometry supports an independent long-range surface plasmon-polariton (LRSPP) and a short-range surface plasmon polariton, and the associated plasmonic band structure has been probed. Coupling to external light is achieved via the patterned grating, and an ultrasharp LRSPP linewidth of 4 nm is observed. The experimental results are compared with finite-difference time-domain simulations.

Hybrid plasmonic waveguide for low-loss lightwave guiding

A hybrid plasmonic waveguide structure is proposed and fabricated for low-loss lightwave guiding along a metal stripe core. By embedding Au stripe in dual slab waveguides with high refractive-index contrast, the field of the guided mode is confined more in the two dielectric core layers. Thus, the propagation loss is significantly reduced. The guided mode is like a combination of a fundamental long-range surface plasmon polariton strip mode and a dual symmetric dielectric slab mode. We fabricate 5 nm-thick Au stripe optical waveguides and measure the optical properties at a wavelength of 1.31 microm. The propagation loss is less than 1.0 dB/cm with the metal stripe width of 1-5 microm.

Improving imaging performance of a metallic superlens using the long-range surface plasmon polariton mode cutoff technique

The metallic superlens is based on excitation and amplification of coupled surface plasmon polariton (SPP) modes through a metal slab. However, the narrow and too-high peaks of the SPP resonance modes in the transfer function can jeopardize imaging performance, such that high sidelobes occur in the image of isolated subwavelength patterns. We propose to design a metallic superlens by approaching the cutoff condition of the long-range SPP mode to flatten the transfer function and to improve imaging performance significantly.

Plasmonic mode-gap waveguides using hetero-metal films

We propose a novel waveguide structure using a hetero-metal film which is composed of two metals of different plasma frequencies. In the proposed waveguide, a long-range surface plasmon-polariton (LR-SPP) mode in the central metal film of a higher plasma frequency are horizontally confined since no propagation mode is allowed in the outer films of a lower plasma frequency for a certain frequency range which is dubbed a plasmonic mode-gap (PMG). The propagation characteristics of the proposed PMG waveguide are numerically analyzed. The proposed waveguide shows tight horizontal confinement close to the diffraction limit and notable suppression of radiation loss in bendings due to the PMG effect. It seems that the PMG guiding can improve integration densities of optical devices based on the LR-SPPs without exacerbating their propagation losses.

Modelling of Polarization-Dependent Loss in Plasmonic Nanowire Waveguides

We have investigated the potential of using gold nanowires embedded in a dielectric cladding environment as polarization-independent long-range surface plasmon polariton waveguides at telecom wavelengths. We performed finite-element analysis on various symmetric and close-to-symmetric cross-sectional geometries and evaluated the effects of cladding thickness on the propagation and coupling loss. The calculations confirm that fabrication of polarization-independent waveguides with reasonable tolerances is feasible and that straight-waveguide insertion losses around 1.5 dB for short (0.5 mm) devices can be realized when coupling to and from conventional dielectric waveguide geometries.

Broadside Excitation of Long-Range Surface Plasmons via Grating Coupling

A metal grating for coupling a Gaussian beam incident from broadside to the long-range surface plasmon-polariton (LRSPP) propagating along a membrane-supported Au slab is proposed and modelled by the finite-difference time-domain method. A short grating design consisting of a small number of Au bumps yields a 19.5% coupling efficiency at ¿ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = 1310 nm into the LRSPP propagating in a single direction when the grating is excited off-center near one of its edges. The grating coupler is easy to fabricate and it facilitates experimental alignments.

Tunable long-range surface plasmon polaritons taking advantage of nonlinear surface waves

We present an improved Kretschmann configuration with nonlinear dielectric as substrate, in which long-range surface plasmon polaritons (SPPs) may appear, the modes and propagation lengths are tunable. We numerically simulated the solutions of SPP modes and their propagations. The results show that the SPP propagation length can range from tens of micrometers to tens of centimeters or even longer. The configuration proposed in this letter makes a breakthrough of the intrinsic restriction in conventional Kretschmann configuration and there exist the continuous steady SPP modes also in the range of k0ε31/2&amp;gt;β&amp;gt;k0ε11/2. It allows for optimization of SPP properties and makes SPP application more flexible in practices.

Multilayered structures for p- and s-polarized long-range surface-plasmon-polariton propagation

A normalized admittance diagram assists in describing and designing multilayered structures to excite long-range surface-plasmon-polariton (LRSPP) waves of either the p- or the s-polarization state. These structures comprise symmetric periodic multilayers on one or both sides of a metal thin film in either the Kretschmann or the Sarid configuration. The normalized admittance diagram even assists in designing structures that can be used to excite LRSPP waves of both polarization states simultaneously.

Long-range surface plasmon polaritons

Long-range surface plasmon polaritons (LRSPPs) are optical surface waves that propagate along a thin symmetric metal slab or stripe over an appreciable length (centimeters). Vigorous interest in LRSPPs has stimulated a large number of studies over three decades spanning a broad topical landscape. Naturally, a good segment of the literature covers fundamentals such as modal characteristics, excitation, and field enhancement. But a large portion also involves the LRSPP in diverse phenomena, including nonlinear interactions, molecular scattering, fluorescence, surface-enhanced Raman spectroscopy, transmission through opaque metal films and emission extraction, amplification and lasing, surface characterization, metal roughness and islandization, optical interconnects and integrated structures, gratings, thermo-, electro- and magneto-optics, and (bio)chemical sensing. Despite the breadth and depth of the research conducted to date, much remains to be uncovered, and the scope for future investigations is broad. We review the properties of the LRSPP, survey the literature involving this wave, and discuss the prospects for applications. Avenues for further work are suggested.

Hybrid vertical directional coupling between a long range surface plasmon polariton waveguide and a dielectric waveguide

To investigate light coupling between a long range surface plasmon polariton (LRSPP) waveguide and a conventional integrated optical component, a hybrid vertical directional coupler consisting of a LRSPP waveguides and a dielectric waveguide is investigated and fabricated. In the proposed coupler the dielectric waveguide and LRSPP waveguide are vertically configured for dense integration and strong coupling. The characteristics of the even and odd super-modes of the coupler are also analyzed to design the device. The fabricated device exhibits damped sinusoidal behavior along the coupling length due to propagation loss of the LRSPP waveguide. The maximum power transfer of 86% from the LRSPP waveguide to the dielectric waveguide is achieved at the coupling length of 600 μm. The measured characteristics of the device are in relatively good agreement with a theoretical analysis.

Extremely high efficient coupling between long range surface plasmon polariton and dielectric waveguide mode

To connect the surface plasmon polariton (SPP) based devices with the conventional dielectric devices is a significant issue for the further development of SPP and related applications. In this paper, extremely high efficient coupling (&amp;gt;99%) between long range SPP (LRSPP) waveguide mode and TM mode of dielectric waveguide has been demonstrated from a hybrid coupler, which composed of an Au (LRSPP) waveguide and a SiN (dielectric) waveguide. Based on this hybrid coupler, a polarization splitter with pure TM mode output from one arm and TE mode output from the other arm with high TE/TM extinction ratio has been realized.

Refractive index dependence of the coupling characteristics between long-range surface-plasmon-polariton and dielectric waveguide modes

The coupling characteristics of a hybrid coupler comprised of a long-range surface-plasmon-polariton (LRSPP) waveguide and a dielectric waveguide is analyzed with different detecting layers. Calculation results show that the coupling strength between the LRSPP mode and the dielectric-waveguide mode is rather sensitive to the refractive index of the detecting layer. This is promising to realize an integrated refractive index sensor with high resolution better than 4 x 10(-7) refractive index units or a modulator with rather low driving power and insert loss.

Theoretical Analysis of Interference Nanolithography of Surface Plasmon Polaritons without a Match Layer

Interference nanolithography techniques based on long-range surface plasmon polaritons (LR-SPP) are hardly ever achieved by experiments at present. One key reason is that suitable liquid materials are difficult to find as the match layer connects the metal film and the resist. We redesign a Kretschmann−Raether structure for interference lithography. A polymer layer is coated under the metal film, and an air layer is placed between the polymer layer and the resist layer. This design not only avoids the above-mentioned question of the match layer, but also can form a soft contact between the polymer layer and the resist layer and can protect the exposure pattern. Simulation results confirm that a device with an appropriately thick polymer layer can form high intensity and contrast interference fringes with a critical dimension of about λ/7 in the resist. In addition, the fabrication of the device is very easy.

Long-range surface plasmon polaritons with subwavelength mode expansion in an asymmetrical system

Long-range surface plasmon polariton (LRSPP) modes in an asymmetrical system, in which the thin metal film is sandwiched between a semi-infinite substrate and a high permittivity polymer film with a finite thickness, are theoretically calculated and analyzed. Due to the high permittivity of the polymer film, at proper polymer film thicknesses, the index-matching condition of the dielectrics at both sides of the metal can be satisfied for supporting LRSPP modes, and the electromagnetic field above the metal can be localized well. It is found that these LRSPP modes have both long propagation lengths and subwavelength mode expansion above the metal at the optimal polymer film thicknesses. Furthermore, the requirements on the refractive index and the thickness of the polymer film to support LRSPP modes at the optimal thicknesses are found to be not critical.

Demonstration of Quadrature-Squeezed Surface Plasmons in a Gold Waveguide

We report on the efficient generation, propagation, and reemission of squeezed long-range surface-plasmon polaritons in a gold waveguide. Squeezed light is used to excite the nonclassical surface-plasmon polaritons, and the reemitted quantum state is fully characterized by complete quantum tomographic reconstruction of the density matrix. We find that the plasmon-assisted transmission of nonclassical light in metallic waveguides can be described by a beam splitter relation. This result is explained theoretically.

Long-range surface plasmon-polariton waveguide sensors with a Bragg grating in the asymmetric double-electrode structure

We propose a Bragg grating resonance sensor based on long-range surface plasmon-polaritons (LRSPP) excited on an asymmetric double-electrode waveguide structure. The proposed LRSPP waveguide sensor utilizes spectral resonance of the asymmetric double-electrode structure by adding a Bragg grating layer on the top surface of the metal slab. We have numerically estimated the bulk index resolution and thickness detection limit of a target biomolecule layer under 30 dB total propagation loss. The sub-mum fluidic channel between the two metal layers always experiences a highly confined LRSPP mode excited by end-fire coupling with optical fibers, therefore the proposed LRSPP sensor platform can be applied to a variety of integrated sensor-chip scenarios.

Low-loss silicide/silicon plasmonic ribbon waveguides for mid- and far-infrared applications

We report low-loss silicide/silicon plasmonic ribbon waveguides for mid- and far-IR applications. The composite modes in silicide ribbon waveguides offer a low-loss and highly confined mode profile, giving excellent plasmon waveguiding for long-wavelength applications. The calculated propagation loss of the composite long-range surface-plasmon polariton mode at a wavelength of 100 microm is 2.18 dB/cm with a mode height of less than 30 microm. The results presented provide important design guidelines for silicide/Si plasmon waveguides.

Long-range surface-plasmon-polariton excitation at the quantum level

We provide the quantum-mechanical description of the excitation of long-range surface-plasmon polaritons (LRSPPs) on thin metallic strips. The excitation process consists of an attenuated-reflection setup, where efficient photon-to-LRSPP wave-packet transfer is shown to be achievable. For calculating the coupling, we derive the first quantization of LRSPPs in the polaritonic regime. We study quantum statistics during propagation and characterize the performance of photon-to-LRSPP quantum state transfer for single-photons, photon-number states, and photonic coherent superposition states.