Short-range Surface Plasmon Polaritons Research Articles

Amplification and excitation of surface plasmon polaritons via four-wave mixing process

We suggest a scheme for the excitation and amplification of surface plasmon polaritons (SPPs) along the interface between metal and semiconductor quantum well (SQW), employing a four-wave mixing (FWM) process. The SQW consists of four-level asymmetric double quantum wells that exhibit quantum interference effects, which leads to the coupler-free excitation of SPPs. In our proposed system, the inherent losses of SPPs are compensated by introducing gain through the FWM process, resulting in a significant enhancement in the propagation length and penetration depth of SPPs. We further analyze the effect of gain on the long-range and short-range SPPs and observe that the propagation distance and lifetime of both types of SPPs are increased.

Harnessing Short-Range Surface Plasmons in Planar Silver Films via Disorder-Engineered Metasurfaces.

Short-range surface plasmon polaritons (SR-SPPs) can arise due to the hybridization of surface plasmon polaritons propagating along the two interfaces of a thin metal slab. In optics, they have gained particular interest for imaging and sensing applications because of their short wavelengths at optical frequencies along with strong field enhancement. However, mediating the interaction of SR-SPPs with photons in planar films is difficult because of the large momentum mismatch. For efficient coupling, nanostructuring such thin films (∼20 nm thickness), or placing metallic nanostructures in close proximity to the planar film, is technologically challenging and can strongly influence the SR-SPP properties. In this article, harnessing SR-SPPs in planar silver films is demonstrated using disorder-engineered metasurfaces. Disorder-engineering is realized by the light-controlled growth of silver nanoparticles. The dispersion of the hybrid modes with the silver thickness is measured and compared with simulations. We anticipate these results to introduce a facile method for harnessing SR-SPPs in planar optical systems and make use of their promising properties for imaging, sensing, and nonlinear optics.

Rigorous Analysis and Systematical Design of Double-Layer Metal Superlens for Improved Subwavelength Imaging Mediated by Surface Plasmon Polaritons.

A double-layer metal superlens was rigorously analyzed and systematically designed to improve subwavelength imaging ability. It was revealed that transmission properties of the imaging system could be accurately interpreted by the five-layer waveguide mode theory—each amplification peak among the spatial frequency range of evanescent waves was associated with a corresponding surface plasmon polariton (SPP) mode of an insulator-metal-insulator-metal-insulator (IMIMI) structure. On the basis of such physical insight, evanescent waves of higher spatial frequency were effectively amplified via increasing propagation constants of symmetrically coupled short-range SPP (s-SRSPP) and antisymmetrically coupled short-range SPP (a-SRSPP), and evanescent waves of lower spatial frequency were appropriately diminished by approaching to cut off symmetrically coupled long-range SPP (s-LRSPP). A flat and broad optical transfer function of the imaging system was then achieved, and improved subwavelength imaging performance was validated by imaging an ideal thin object of two slits with a 20-nm width distanced by a 20-nm spacer, under 193-nm illumination. The resolution limit of the designed imaging system with double-layer superlens was further demonstrated to be at least ~λ/16 for an isolated two-slit object model. This work provided sound theoretical analysis and a systematic design approach of double-layer metal superlens for near-field subwavelength imaging, such as fluorescent micro/nanoscopy or plasmonic nanolithography.

Resonance modes of a metal-semiconductor-metal multilayer mediated by electric charge

Electromagnetic fields around metal–semiconductor–metal (MSM) multilayers with square island top layers were numerically simulated to elucidate the difference in physics between the circuit resonance and Fabry–Pérot interference mediated by the surface plasmon polaritons (SPP). In the current study, the top and bottom metal layers were made of gold, and the intermediate semiconductor layer was a gallium antimony (GaSb). The lumped-element and Fabry–Pérot interference models showed less accuracy when the island width of the MSM multilayer was comparatively smaller. Since the capacitor and SPP could not be supported between the top and bottom gold layers, the anti-reflection mode of the gold–GaSb bilayer mainly affected the absorptance. However, when the width of the island was sufficiently large, the time-lapse development of the electromagnetic fields at resonant wavelengths showed strong electric and magnetic responses relating to the circuit resonance. Simultaneously, the electric fields depicted the movement of the electric charge, which coupled to the short-range surface plasmon polariton (SRSP) existing at the thin GaSb layer sandwiched by two gold layers. The wavelength of the SRSP approximately corresponded to that of the Fabry–Pérot interference. It was revealed that the lumped-element and Fabry–Pérot interference models indicated the same resonant mode from two different perspectives in physics.

Periodic Nanostructure Formation Induced by Short-range Surface Plasmon Polaritons Excited with Few-cycle Laser Pulses

Periodic Nanostructure Formation Induced by Short-range Surface Plasmon Polaritons Excited with Few-cycle Laser Pulses

Sub-100-nm periodic nanostructure formation induced by short-range surface plasmon polaritons excited with few-cycle laser pulses

We have demonstrated that intense 7-fs, ∼810-nm laser pulses can produce a much finer periodic nanostructure on a diamond-like carbon film through ablation in air than that formed with intense 100-fs, ∼800-nm laser pulses. The minimum period size is ∼60 nm, corresponding to ∼1/13 of the center wavelength. To understand the physical mechanism responsible for the finer nanostructuring, we have observed a bonding structural change in the nanostructure with micro-Raman spectroscopy and a scanning transmission electron microscope. It has been found that the modified layer thickness with the 7-fs pulses is much thinner than that with the 100-fs pulses. The results show that the 7-fs pulses create a few-nm-thick layer with high-density electrons and excite short-range surface plasmon polaritons, which have a large wave number around the layer and induce the plasmonic near-field nanoablation. The period size estimated by using a model target reproduces well the observed size of nanostructures.

Experimental observation of the short-range surface plasmon polariton mode and its longitudinal adiabatic compression in a metallic wedge.

In this study, we explore analytically and experimentally long- and short-range surface plasmon polariton (LR-SPP and SR-SPP, respectively) modes in gold wedges. Especially, we aim to observe the 2-dimensional confinement of the electromagnetic field in gold wedges as it could enhance the light-matter interaction by offering a local density of states which depends on the propagation constant, consequently on the wedge height. The LR-SPP mode can propagate over a long distance, but the real part of the propagation constant remains relatively insensitive to the decreasing wedge height. This mode also experiences cut-off at a wedge height of about 50 nm in our experimental condition. Meanwhile, the SR-SPP mode has a large propagation constant that increases further with decreasing wedge height. As a result, the effective wavelength of the mode shrinks confining the electromagnetic wave longitudinally along the propagation direction in addition to enhancing the transverse confinement of SR-SPP. In the experiment, we use gold wedges with different edge heights to excite each SPP mode individually and image the electromagnetic near field by using a pseudo-heterodyne scattering scanning near-field optical microscope. By imaging the LR-SPP mode field, we demonstrate that the theoretical and measured values of the effective wavelength agree quite well. By using short wedges, we measure the SR-SPP mode field and demonstrate that the effective wavelength decreases to 47% in about half a micrometer of propagation distance. This corresponds to a 3.5 times decrease of the vacuum wavelength or an effective index of 3.5. It is important to note that this value is, by no means, the limit of the electromagnetic field's longitudinal confinement in a gold wedge. Rather, we were only able to measure the electromagnetic field up to this point due to our measurement limitations. The electromagnetic field will be propagating further, and the longitudinal confinement will increase as well. In conclusion, we measured the SR-SPP in a gold wedge and demonstrate the electromagnetic field confinement in the visible spectrum in gold wedges.

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.

Diode like high-contrast asymmetric transmission of linearly polarized waves based on plasmon-tunneling effect coupling to electromagnetic radiation modes

We present a narrow-band optical diode with a high-contrast forward-to-backward ratio at the near-infrared region. The design has a forward transmission of approximately , and a backward one of less than , yielding a contrast ratio of greater than dB at a wavelength of nm. The structure is composed of a one-dimensional diffraction grating on top of a dielectric slab waveguide, both of which are made of silicon nitride (Si3N4), and all together are placed over a silver (Ag) thin film embedded on a dielectric substrate. Utilizing a dielectric-based diffraction grating waveguide on a thin silver layer leads to the simultaneous excitation of two surface plasmon modes known as long- and short-range surface plasmon polaritons (SPPs) at both interfaces of the metallic layer. The plasmon-tunneling effect, which is the result of the coupling of SPPs excited at the upper interface of the metallic layer to the radiation modes, provides a high asymmetric transmission (AT) property. The spectral response of the proposed high-contrast AT device is verified using both rigorous coupled-wave analysis as an analytical approach and finite difference time domain as a numerical one.

Ultrahigh resolution integrated optic short range surface plasmon-polariton based sensor for aqueous environment

We propose a short length plasmonic integrated optic sensor with ultrahigh sensitivity and resolution for aqueous environment. The sensor section is a simple multilayer structure consisting of a dielectric planar waveguide separated from a thin metal film by a low index buffer layer. The sensor is based on the hybrid coupling between the guided mode and the short range plasmonic mode supported by the thin metal film. Periodic coupling between these modes takes place along the length of the sensor. For optimized layer thicknesses, resonant peak appears at very short length of sensing section. The spectral position of resonant peak is strongly dependent on the analyte index. We show theoretically that a spectral sensitivity of 3.2 × 104 nm RIU−1 with FWHM of 2 nm and resolution of 6.2 × 10–8 RIU can be achieved for aqueous environment.

Micro-spectroscopy of Buried Short-Range Surface Plasmon Polaritons Supported by Thin Polycrystalline Gold Films

The dispersive properties of short-range surface plasmon polaritons are investigated at the buried interfaces in vacuum/Au/fused silica and vacuum/Au/SiO2/Si multilayer systems for different gold film thicknesses of up to 50 nm using two-photon photoemission electron microscopy. The experimental data agrees excellently with results of transfer matrix method simulations, emphasizing the sensitivity of the plasmonic wave vector to the thickness of the gold film and an ultrathin native substrate oxide layer. The results furthermore illustrate the exceptional qualification of low-energy electron photoemission techniques in studying electronic excitations at buried interfaces.

Effect of buffer layer index on short range surface plasmon polariton based TE pass polarizer

We investigate theoretically the dependence of the extinction ratio, metal layer thickness and buffer thickness of a short length plasmonic polarizer on the buffer layer index. The polarizer section consists of a thin metal layer above the single mode dielectric waveguide separated by a low index dielectric buffer layer. Strong coupling between the surface plasmon polariton (SPP) mode and the guided mode takes place when their phase constants are matched. The SPP mode being of TM type, interacts with the guided TM mode and is completely absorbed by metal film due to resonant coupling between the two modes. The TE guided mode does not couple to the SPP mode and is passed un-attenuated through the polarizer. The buffer layer thickness can be optimized to maximize the TM mode attenuation and reduce the TE losses, which provides a large extinction ratio. Since the polarizer is based on the interference of the SPP mode and the waveguide mode, a periodic coupling of the field takes place between these two modes. It is shown that for properly optimized layer thicknesses, an extinction ratio exceeding 200 dB can be achieved for various buffer layer indices. Effect of buffer layer index on the polarizer length is also presented.

Photonic Spin Hall Effect Modified by Ultrathin Au Films and Monolayer Transition Metal Dichalcogenides in One-Dimensional Photonic Crystal

When the metal film becomes far thinner than the electron mean free path (EMFP) in their bulk counterpart, many exotic optical properties accompanied with numbers of novel applications may emerge. Herein, the photonic spin Hall effect (PSHE) occurring on the surface of one-dimensional photonic crystal consisting of ultrathin Au films, monolayer transition metal dichalcogenides (TMDCs), and one defective layer is investigated. The Au film thickness ranges from 2.4 to 40 nm which covers the thickness range below the EMFP (37.7 nm) in bulk Au. Results show that ultrathin Au films are more competent to enhance the PSHE than bulk Au film. In particular, the maximum spin-dependent transverse displacement (37.94 times of the incident wavelength) at the Au film thickness of 4.4 nm is more than 12-fold higher than that at the bulk Au film with thickness of 40 nm. Besides it may be due to the coupling between the short-range surface plasmon polaritons and the B excitonic resonance in monolayer WS2, among four monolayer TMDCs (i.e., MoS2, MoSe2, WS2, WSe2), the photonic crystal with monolayer WS2 gives the strongest PSHE. Additionally, evolutions of photonic spin Hall shifts with the refractive index of defective layer, incident wavelength, and incident angle are also studied in detail. These findings shed light on the superiority of ultrathin metal films for light manipulation in spinoptics and may provide new routes to tailor the optical helicity in nanophotonic devices.

Enhancement of Long-Range Surface Plasmon Excitation, Dynamic Range and Figure of Merit Using a Dielectric Resonant Cavity.

In this paper, we report a theoretical framework on the effect of multiple resonances inside the dielectric cavity of insulator-insulator-metal-insulator (IIMI)-based surface plasmon sensors. It has been very well established that the structure can support both long-range surface plasmon polaritons (LRSPP) and short-range surface plasmon polaritons (SRSPP). We found that the dielectric resonant cavity under certain conditions can be employed as a resonator to enhance the LRSPP properties. These conditions are: (1) the refractive index of the resonant cavity was greater than the refractive index of the sample layer and (2) when light propagated in the resonant cavity and was evanescent in the sample layer. We showed through the analytical calculation using Fresnel equations and rigorous coupled wave theory that the proposed structure with the mentioned conditions can extend the dynamic range of LRSPP excitation and enhance at least five times more plasmon intensity on the surface of the metal compared to the surface plasmon excited by the conventional Kretschmann configuration. It can enhance the dip sensitivity and the dynamic range in refractive index sensing without losing the sharpness of the LRSPP dip. We also showed that the interferometric modes in the cavity can be insensitive to the surface plasmon modes. This allowed a self-referenced surface plasmon resonance structure, in which the interferometric mode measured changes in the sensor structure and the enhanced LRSPP measured changes in the sample channel.

Magnetoplasmonic crystal waveguide.

We propose a perspective type of insulator-metal-insulator magnetoplasmonic crystal waveguide, composed of a gold grating placed between two garnet layers. Using an original non-perturbing method for the deposition of the upper magneto-dielectric layer, we fabricate the samples and provide experimental results evidencing the coupling of surface plasmon-polaritons propagating on the opposite Au/garnet interfaces. In contrast to traditional Au/garnet magnetoplasmonic crystals, spectra of the magneto-optical effect measured in transmission through this waveguide demonstrate rather specific features: a high-quality resonance for the long-range surface plasmon-polariton and a broad 60 nm wide resonance for the short-range surface plasmon-polariton. Our findings open new routes towards the development of high-sensitivity robust magnetoplasmonic sensors.

Polarization-Insensitive Ultra-Narrow Plasmon-Induced Transparency and Short-range Surface Plasmon Polariton Bloch Wave in Ultra-thin Metallic Film Nanostructures

We demonstrate that polarization-insensitive ultra-narrow double plasmon-induced transparency (PIT) can be achieved in the thin-metal-film nanostructures. Ultra-narrow PIT resonance with a bandwidth of 2.5 nm at central wavelength of 768 nm was obtained (~ 1/307 of the peak wavelength). Multispectral PIT can be obtained, and its linewidth and the “on-off” state of the PIT peaks in the system can be adjusted by the metallic nanostructure thickness and particularly the waveguide layer thickness. The narrow transparency window is exhibited to be generated by the coupling and interference between a delocalized hybrid-waveguide photonics mode and the localized surface plasmon (LSP) mode. The angular-dependent dispersion of the system with double PITs is examined and investigated, where the high-order short-range surface plasmon polariton (SRSPP) Bloch waves excited for large incidence angles are indicated and revealed. The multispectral PITs with high quality in the large-area plasmonic system are promising for practical and compact nanophotonics applications such as optical buffers, ultra-sensitive sensor, plasmonic filters, and switches.

Modeling of enhanced spontaneous parametric down-conversion in plasmonic and dielectric structures with realistic waves

A numerical study of the enhancement of the spontaneous parametric down-conversion in plasmonic and dielectric structures is considered. The modeling is done using a nonlinear transfer-matrix method which is extended to include vacuum fluctuations and realistic waves (e.g. Gaussian beam). The results indicate that in the case of short-range surface plasmon polaritons, the main limiting factor of the enhancement is the short length of the coherent buildup. In the case of long-range surface plasmon polaritons or dielectric guided waves, the very narrow resonances are the main limiting factor instead.

Excitation of Multiple Surface Plasmon-Polaritons by a Metal Layer Inserted in an Equichiral Sculptured Thin Film

Excitation of multiple surface plasmon-polaritons(SPPs) by an equichiral sculptured thin film with a metal layer defect was studied theoretically in the Sarid configuration, using the transfer matrix method. Multiple SPP modes were distinguished from waveguide modes in optical absorption for p- polarized plane wave. The degree of localization of multiple SPP waves was investigated by calculation of the time averaged Poynting vector. The results showed that the long-range and short-range SPP waves can simultaneously be excited at both interfaces of metal core in this proposed structure which may be used in a broad range of sensing applications.

High-Quality Ultrathin Gold Layers with an APTMS Adhesion for Optimal Performance of Surface Plasmon Polariton-Based Devices.

A low-absorption adhesion layer plays a crucial role for both localized and propagating surface plasmons when ultrathin gold is used. To date, the most popular adhesion layers are metallic, namely, Cr and Ti. However, to the best of our knowledge, the influence of these adhesion layers on the behavior of propagating plasmon modes has not been thoroughly investigated nor reported in the literature. It is therefore important to study the effect of these few- to several-nanometers-thick adhesion layers on the propagating plasmons because it may affect the performance of plasmonic devices, in particular, when the Au layer is not much thicker than the adhesion layers. We experimentally compared the performances of the ultrathin gold films to show the pivotal influence of adhesion layers on highly confined propagating plasmonic modes, using Cr and 3-aminopropyl trimethoxysilane (APTMS) adhesion layers and without any adhesion layer. We show that the gold films with the APTMS adhesion layer have the lowest surface roughness and the short-range surface plasmon polaritons supported on the Au surface exhibit properties close to the theoretical calculations, considering an ideal gold film.

Extremely Short Length and High Extinction Ratio Plasmonic TE Mode Pass Polarizer

Extremely short length and ultrahigh extinction ratio TE pass polarizer is proposed for wavelength $1.55\mu \text{m}$ . The polarizer section consists of an optical dielectric waveguide separated from the thin metal film plasmonic guide by a low index buffer layer. The thin film metal layer supports only Short Range Surface Plasmon Polariton (SRSPP) because effective indices below and above the metal film are different. Periodic coupling between the SRSPP mode and the optical waveguide mode takes place and varies with the length of the polarizer. It is shown that for a properly optimized metal and the buffer layer thicknesses, an extinction ratio of 223 dB can be achieved for a polarizer of length $4.6~\mu \text{m}$ . Dependence of polarizer length on the optimized buffer and metal layers is also discussed.