Metal Stripe Waveguides Research Articles

Generation of entangled waveguided photon pairs by free electrons.

Entangled photons are a key resource in quantum technologies. While intense laser light propagating in nonlinear crystals is conventionally used to generate entangled photons, such schemes have low efficiency due to the weak nonlinear response of known materials and losses associated with in/out photon coupling. Here, we show how to generate entangled polariton pairs directly within optical waveguides using free electrons. The measured energy loss of undeflected electrons heralds the production of counter-propagating polariton pairs entangled in energy and emission direction. For illustration, we study the excitation of plasmon polaritons in metal strip waveguides that strongly enhance light-matter interactions, rendering two-plasmon generation dominant over single-plasmon excitation. We demonstrate that electron energy losses detected within optimal frequency ranges can reliably signal the generation of plasmon pairs entangled in energy and momentum. Our proposed scheme is directly applicable to other types of optical waveguides for in situ generation of entangled photon pairs.

Geometric Control Over the Edge Diffraction of Electrically Excited Surface Plasmon Polaritons by Tunnel Junctions

Plasmonic metal–insulator–metal tunnel junctions (MIM-TJs) readily excite surface plasmon polaritons (SPPs) by inelastic electron tunneling, well below the diffraction limit, eliminating the need for bulky optical elements. The highly confined MIM cavity mode (MIM-SPP) excited by the tunneling electrons dominantly outcouples to photons and single-interface SPPs which further outcouple as photons and give characteristic features in the Fourier plane of observation. Here, we employ SPP waveguides, directly integrated with MIM-TJs, to explore the diffraction of single-interface SPPs at the edges of the metal stripe waveguides. In addition to the leakage of the SPP modes through the metal electrodes, SPP edge diffraction presents as an additional channel for SPP outcoupling, not limited by the thickness of the waveguide. Edge diffraction manifests as a straight-line feature in the Fourier plane of the far-field and by systematically varying the width of the waveguides, we show that the in-plane momentum of the SPPs can be controlled, which in turn leads to control over the edge diffraction. We fabricated MIM-TJs with integrated plasmonic stripe waveguides of different widths, and the propagation and confinement of the SPP modes along the edges of the SPP waveguides are directly identified by the experimental back focal plane imaging, which are further corroborated by numerical simulations. Our findings can be exploited for applications ranging from sensing to nonlinear plasmonics, where focusing and localization of SPP fields are necessary.

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.

Vertically Coupled Plasmonic Racetrack Ring Resonator for Biosensor Applications.

Plasmonic chemical and biological sensors offer significant advantages such as really compact sizes and extremely high sensitivity. Biosensors based on plasmonic waveguides and resonators are some of the most attractive candidates for mobile and wearable devices. However, high losses in the metal and complicated schemes for practical implementation make it challenging to find the optimal configuration of a compact plasmon biosensor. Here, we propose a novel plasmonic refractive index sensor based on a metal strip waveguide placed under a waveguide-based racetrack ring resonator made of the same metal. This scheme guarantees effective coupling between the waveguide and resonator and low loss light transmittance through the long-range waveguide. The proposed device can be easily fabricated (e.g., using optical lithography) and integrated with materials like graphene oxide for providing adsorption of the biomolecules on the sensitive part of the optical elements. To analyze the properties of the designed sensing system, we performed numerical simulations along with some analytical estimations. There is one other interesting general feature of this sensing scheme that is worth pointing out before looking at its details. The sensitivity of the considered device can be significantly increased by surrounding the resonator with media of slightly different refractive indices, which allows sensitivity to reach a value of more than 1 μm per refractive index unit.

Field-effect transistor based on surface plasmon polaritons.

We investigate the working mechanism of a plasmon field-effect transistor (FET) based on the modulation of surface plasmon polariton (SPP) flow by varying an external electric field. SPP flow propagates at the surface of a metal strip waveguide embedded in a dielectric medium. The external electric field is applied to the metal electrode located under the isolated metal nanoplate placed in the middle of the strip waveguide. In this case, a signal on the control electrode changes the permittivity of the near-surface layer of the nanoplate. Thus, the control signal modulates the intensity of the SPP signal transmitted at the surface of the strip waveguide.

Probing Decoherence in Plasmonic Waveguides in the Quantum Regime

We experimentally investigate the decoherence of single surface plasmon polaritons in metal stripe waveguides. In our study we use a Mach-Zehnder configuration previously considered for measuring decoherence in atomic, electronic and photonic systems. By placing waveguides of different length in one arm we are able to measure the amplitude damping time T_1 = 1.90 +/- 0.01 x 10^-14 s, pure phase damping time T_2^* = 11.19 +/- 4.89 x 10^-14 s and total phase damping time T_2 = 2.83 +/- 0.32 x 10^-14 s. We find that decoherence is mainly due to amplitude damping and thus loss arising from inelastic electron and photon scattering plays the most important role in the decoherence of plasmonic waveguides in the quantum regime. However, pure phase damping is not completely negligible. The results will be useful in the design of plasmonic waveguide systems for carrying out phase-sensitive quantum applications, such as quantum sensing. The probing techniques developed may also be applied to other plasmonic nanostructures, such as those used as nanoantennas, as unit cells in metamaterials and as nanotraps for cold atoms.

Nanoscale Schottky contact surface plasmon "point detectors" for optical beam scanning applications.

We propose and investigate a Schottky contact detector where the contact metal is a nanoscale metal stripe waveguide supporting surface plasmons with strong localization along the Schottky contact. We consider Au for the metal stripe, p-doped silicon for the semiconductor, operation in the infrared (at λ0=1550 nm), and internal photoemission as the sub-bandgap detection mechanism. We find that the main surface plasmon mode of operation of the Au stripe exhibits diverging real and imaginary parts of neff with decreasing stripe dimensions, commensurate with increasing confinement. The mode fields are tightly confined to the immediate vicinity of the stripe within a very small area. Coupling efficiencies, responsivities, dark currents, and minimum detectable powers are computed. An application envisaged for such nanoscale "point detectors" is optical beam scanning, motivated by the very small area of the photodetector stripe mode relative to, e.g., a Gaussian beam (∼500× smaller), such that its field distribution approximates a Dirac delta function in comparison, and by the photocurrent originating primarily from selective absorption of the photodetector stripe mode. The proposed application was verified through numerical simulation, demonstrating that a nanoscale "point detector" can scan a tightly focused Gaussian beam (2.5 μm diameter at λ0=1550 nm) with good resolution and signal to noise.

Long-Range Surface Plasmon-Polariton Waveguide Biosensors for Disease Detection

We review recent progress on the application of long-range surface plasmon-polariton waveguide biosensors for disease detection. The biosensors are constructed from metal stripe waveguides cladded in Cytop with etched microfluidic channels to expose the stripe surface to the sensing fluid. The waveguides support long-range surface plasmons in integrated optics geometries when excited in an end-fire manner. Application of straight waveguide biosensors to the detection of leukemia, dengue infection, and urinary tract infection in complex fluids such as blood serum, blood plasma, and urine is reviewed.

High-Resolution Analysis of Leaky Modes in Surface Plasmon Stripe Waveguides

An in-house developed full-vector finite-element imaginary-distance beam propagation method (FV-FE-ID-BPM) is employed to calculate the modes of a surface plasmon metal-stripe waveguide. Leaky modes, which possess less propagation losses compared with corresponding bound modes, are particularly studied. The calculated real and imaginary parts of the modal effective indices are shown to be with local-overshoot characteristics versus the stripe width when fine width steps are used. Using fine enough spatial mesh resolution, the field profile within the metal-stripe region is revealed and examined, and the local-overshoot characteristics could be correlated with the integer number of quasi-periods associated with the field variation along the interface between the metal stripe and the substrate.

Near-field characterization of bound plasmonic modes in metal strip waveguides.

Propagation of bound plasmon-polariton modes along 30-nm-thin gold strips on a silica substrate at the free-space wavelength of 1500 nm is investigated both theoretically and experimentally when decreasing the strip width from 1500 nm down to the aspect-ratio limited width of 30 nm, which ensures deep subwavelength mode confinement. The main mode characteristics (effective mode index, propagation length, and mode profile) are determined from the experimental amplitude- and phase-resolved near-field images for various strip widths (from 30 to 1500 nm), and compared to numerical simulations. The mode supported by the narrowest strip is found to be laterally confined within ~ 100 nm at the air side, indicating that the realistic limit for radiation nanofocusing in air using tapered metal strips is ~ λ/15.

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.

Mapping bound plasmon propagation on a nanoscale stripe waveguide using quantum dots: influence of spacer layer thickness.

In this paper we image the highly confined long range plasmons of a nanoscale metal stripe waveguide using quantum emitters. Plasmons were excited using a highly focused 633 nm laser beam and a specially designed grating structure to provide stronger incoupling to the desired mode. A homogeneous thin layer of quantum dots was used to image the near field intensity of the propagating plasmons on the waveguide. We observed that the photoluminescence is quenched when the QD to metal surface distance is less than 10 nm. The optimised spacer layer thickness for the stripe waveguides was found to be around 20 nm. Authors believe that the findings of this paper prove beneficial for the development of plasmonic devices utilising stripe waveguides.

Observation of Quantum Interference in the Plasmonic Hong-Ou-Mandel Effect

We report direct evidence of the bosonic nature of surface plasmon polaritons (SPPs) in a scattering-based beamsplitter. A parametric down-conversion source is used to produce two indistinguishable photons, each of which is converted into a SPP on a metal-stripe waveguide and then made to interact through a semi-transparent Bragg mirror. In this plasmonic analog of the Hong-Ou-Mandel experiment, we measure a coincidence dip with a visibility of 72%, a key signature that SPPs are bosons and that quantum interference is clearly involved.

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.

Design of a vertically coupled liquid-crystal long-range plasmonic optical switch

An optical switch based on liquid-crystal (LC) tunable long-range metal stripe waveguides is proposed and theoretically investigated. A nematic liquid crystal layer placed between a vertical configuration consisting of two gold stripes is shown to allow for the extensive electro-optic tuning of the coupler's waveguiding characteristics. Rigorous liquid-crystal switching studies are coupled with the investigation of the optical properties of the proposed plasmonic structure, taking into account different excitation conditions and the impact of LC-scattering losses. A directional coupler optical switch is demonstrated, which combines low power consumption, low cross-talk, short coupling lengths, along with sufficiently reduced insertion losses.

Surface plasmon propagation in a metal strip waveguide with biaxial substrate

Surface plasmon propagation in a nano metal strip waveguide with a biaxial substrate material was studied. The dispersion of the four fundamental modes propagating in the strip waveguide structure was analysed using a formalism based on the method of lines. The propagation properties of these modes with respect to the thickness of the metal film was studied, with two different biaxial materials as substrate for the waveguide. The results were then compared with the asymmetric strip waveguide structures with isotropic substrate and a uniaxial anisotropic substrate. Propagation characteristics as a function of longitudinal and transverse anisotropic components of the substrate material were also studied. The characteristics of the fundamental modes are observed to vary depending on the anisotropy of the biaxial material used.

Finite energy Airy-Hermite-Gaussian beam and its paraxial propagation

A new form of laser beam called finite energy Airy-Hermite-Gaussian beam (AHGB) is introduced to describe the out mode of the metal strip waveguide in this paper. In the source plane, this beam can be decomposed into a Hermite-Gaussian function in y-direction and exponentially modified Airy function in x-direction. When the exponential truncation factor is large, the intensity profile in the source plane is an approximate off-axis Hermite-Gaussian distribution. Based on the Collins integral, an analytical expression for the finite energy AHGB passing through a paraxial ABCD optical system has been derived. For special cases, the propagation properties of this kind of beam passing in free space, through a thin lens and a fractional Fourier transformation system are studied and illustrated with numerical calculation. Our results provide an effective and fast way for studying the paraxial propagation properties of the finite energy AHGB.

Metal Stripe Waveguide Based Interferometer-Type Sensor Working in an Aqueous Solution With a Low Refractive Index

This paper reports a metal stripe waveguide based sensor that functions like a Mach-Zehnder interferometer. It consists of three sections. The first and third sections are input and output metal stripe waveguides that support a long-range surface plasmon polariton (LRSPP). The second section is a sensing region; it comprises a substrate, which is common to the first and third sections, an insulator layer with a refractive index larger than 2 (e.g., TiO2), and an Au layer much thicker than the skin depth of gold. For sensing, it is covered by an aqueous solution with a refractive index of about 1.3. Because of the thick Au layer, separate single-interface surface plasmon polaritons (SPPs) propagate along the top and bottom surfaces of the Au layer. Since the top and bottom SPPs rather than an LRSPP are used in the second section, it is not constrained by the condition of supporting an LRSPP, which is that the substrate should have almost the same refractive index as the solution. The top and bottom SPPs play the roles of sensing and reference arms of an interferometer, respectively. In this paper, the sensor is designed, and its bulk-sensing and surface-sensing characteristics are theoretically analyzed. The design results in the compact sensor whose sensing region is ~ 35μm long; the analysis demonstrates that the sensor has sensitivity higher than or comparable to that of previous plasmonic sensors.

Effect of Gyrotropic Substrates on the Surface Plasmon Polaritons Guided by Metal Films of Finite Width

The propagation characteristics of surface plasmon polariton modes in metal-strip plasmonic waveguides with gyrotropic (magneto-optic) substrate material has been numerically studied. The method of line formalism has been adopted for computing the dispersion of fundamental symmetric and antisymmetric modes supported by the structure. The magneto-optic substrate has been chosen such that the resulting propagating modes exhibit nonreciprocal dispersion, enabling their application in many optical devices. Fundamental pair modes of surface plasmon polaritons supported by metal strips, which have degeneracy properties with isotropic dielectric substrates, have been found to be nondegenerate in our case. These modes show different trends in the dispersion with variation of metal film thickness, compared to that in an asymmetric strip waveguide with isotropic substrate. A nonreciprocal phase shift of 0.5-14 rad/mm is achievable for antisymmetric modes that are localized near the gyrotropic interface.

Planar long-period grating filter based on long-range surface plasmon mode of buried metal stripe waveguide

We propose a planar long-period grating filter based on coupling between the long-range surface plasmon mode and a cladding mode of a fully buried metal stripe waveguide. Using a 2.5-mm-long corrugation grating produced along the surface of an epoxy-clad aluminum stripe waveguide, we achieve a rejection band with a contrast of approximately 18 dB at the wavelength approximately 1500 nm, which can be tuned by approximately 25 nm with a temperature change of approximately 30 degrees C. The experimental results agree closely with the simulation results. The filter could find applications in surface-plasmon-based integrated-optic circuits and biosensors.