Surface Plasmon Polariton Modes Research Articles

Multidimensional structural coloration from hierarchically designed plasmonic structures

We propose a photonic strategy for multidimensional structural coloration that responds to varying illumination conditions. Both theoretical and experimental results verified that the combination of a metal/dielectric/metal cavity with a one-dimensional (1D) metal grating can increase the degree of freedom of structural coloration. We fabricated Ag/MgF2/Ag cavities coupled to 1D Ag gratings (pitches of 400, 570, and 800 nm) and obtained their photonic band dispersions (i.e., angle-resolved reflectance spectra) to unravel their mode characteristics. The 1D Ag grating and Ag/MgF2/Ag cavity induced surface plasmon polariton (SPP) and Fabry–Perot cavity modes, respectively. Notably, only a cavity mode emerged in the s-polarization and gradually blueshifted with increasing angle, whereas in the p-polarization, both SPP and cavity modes appeared and shifted oppositely with changes in the angle, leading to intricate color patterns at different wavelengths, angles, and polarizations. To further understand the effect of each mode upon structural coloration, we obtained the distribution of electric field intensity using rigorous coupled-wave analysis simulation, which identified multiple photonic bands in the p-polarization as various SPP-mediated hybrid modes.

High Performance Dual-Core D-Shaped PCF Refractive Index Sensor Coated with Gold Grating

In this study, a dual−core D-shaped photonic crystal fiber (PCF) surface plasmon resonance sensor coated with gold grating is designed and analyzed using the finite-element method (FEM). The surface plasmon resonance (SPR) effect between the fiber core modes and surface plasmon polariton (Spp) modes is used to measure the analyte refractive index. The effects of the PCF structure parameters (polishing depths, large holes, and small holes) and grating parameters (grating heights, grating periods, and grating duty) are discussed, and a two-feature interrogation method that combines wavelength and intensity interrogations is introduced to enhance the resolution. The results show that the grating and dual-core play important roles in enhancing the sensor properties. The proposed sensor achieves an average wavelength sensitivity of 994.5 nm/RIU when the analyte refractive index increases from 1.33 to 1.37. Furthermore, a maximum amplitude sensitivity of 181.049 RIU−1 is obtained. The two-feature interrogation is determined to have a resolution of 2.03 × 10−6 RIU, which is better than the wavelength and amplitude interrogations. The proposed sensor has a good sensing performance and is highly suitable for practical applications.

Tunable Layered Gold Nanochips for High Sensitivity and Uniformity in SERS Detection

Reasonable design and controllable fabrication of plasmonic substrates are the keys to achieving surface-enhanced Raman spectroscopy (SERS) detection with high sensitivity and uniformity. The coupling of the two main modes of surface plasmon, the localized surface plasmon resonance (LSPR) mode and the surface plasmon polariton (SPP) mode, has been proven to be an appealing way to obtain excellent SERS responses. Nevertheless, it remains an obstacle to further improve the LSPR-SPP coupling efficiency due to the dramatic nonuniformity of the nanostructure obtained by traditional wet chemical methods. Here, we report a controllable electron-beam lithography (EBL) process for the fabrication of layered Au nanochips. For the optimized Au CFN (cap and film nanostructure) structure, benefiting from the remarkable electric field enhancement induced by the effective LSPR-SPP coupling at the junction of the Au cap and Au film structure, an improved SERS performance with an enhancement factor (EF) of 2.8 × 106 and a low detection limit of 10–13 M are achieved using 4-nitrothiophenol (4-NTP) as the probe molecule. In addition, the Au CFN substrate exhibits a high uniformity over a large area. Such a new strategy for the design of plasmonic substrates with superior LSPR-SPP coupling can be applied in various SERS detection scenarios.

All-optical control of high-purity trions in nanoscale waveguide

The generation of high-purity localized trions, dynamic exciton–trion interconversion, and their spatial modulation in two-dimensional (2D) semiconductors are building blocks for the realization of trion-based optoelectronic devices. Here, we present a method for the all-optical control of the exciton-to-trion conversion process and its spatial distributions in a MoS2 monolayer. We induce a nanoscale strain gradient in a 2D crystal transferred on a lateral metal–insulator–metal (MIM) waveguide and exploit propagating surface plasmon polaritons (SPPs) to localize hot electrons. These significantly increase the electrons and efficiently funnel excitons in the lateral MIM waveguide, facilitating complete exciton-to-trion conversion even at ambient conditions. Additionally, we modulate the SPP mode using adaptive wavefront shaping, enabling all-optical control of the exciton-to-trion conversion rate and trion distribution in a reversible manner. Our work provides a platform for harnessing excitonic quasiparticles efficiently in the form of trions at ambient conditions, enabling high-efficiency photoconversion.

Modulation of Cathodoluminescence by Surface Plasmons in Silver Nanowires.

The waveguide modes in chemically-grown silver nanowires on silicon nitride substrates are observed using spectrally- and spatially-resolved cathodoluminescence (CL) excited by high-energy electrons in a scanning electron microscope. The presence of a long-range, travelling surface plasmon mode modulates the coupling efficiency of the incident electron energy into the nanowires, which is observed as oscillations in the measured CL with the point of excitation by the focused electron beam. The experimental data are modeled using the theory of surface plasmon polariton modes in cylindrical metal waveguides, enabling the complex mode wavenumbers and excitation strength of the long-range surface plasmon mode to be extracted. The experiments yield insight into the energy transfer mechanisms between fast electrons and coherent oscillations in surface charge density in metal nanowires and the relative amplitudes of the radiative processes excited in the wire by theelectron.

Strongly coupled Raman scattering enhancement revealed by scattering-type scanning near-field optical microscopy

Abstract Recent advances in near-field technology with an ultrahigh spatial resolution breaking optical diffraction limit, make it possible to further identify surface-enhanced Raman scattering (SERS) enhancement theories, and to monitor the SERS substrates. Here we verify the electromagnetic enhancement mechanism for SERS with a close-up view, using scattering-type scanning near-field optical microscopy. The array of metal-insulator-metal (MIM) subwavelength structures is studied, in which the field enhancement comes from the strong coupling between gap plasmon polariton and surface plasmon polariton modes. The near-field optical measurements reveal that SERS enhancement factor (EF) varies from one MIM subwavelength unit to another in a finite array. Besides the enhancement of isolated unit, the loss exchange phenomenon in strong coupling with a large Rabi splitting can give rise to an additional enhancement of more than 2 orders of magnitude in periodic arrays and close to 3 orders of magnitude in finite arrays. The SERS EF of the array composed of only 5 units is demonstrated to yield the best SERS performance. Our near-field optical measurements show evidence that finite-size structures embodied with strong coupling effect are a key way to develop practical high-performance SERS substrates.

Broadband cavity architecture for ultra-thin type-II superlattice mid-infrared detectors

A broadband cavity architecture for ultra-thin type-II superlattice (T2SL) mid-infrared detectors is designed by exploiting coordinated coupling of the surface plasmon polariton mode and cavity mode in an Au-antenna/detector/highly doped semiconductor ground-plane configuration. By optimizing the doping concentration of the doped semiconductor ground-plane and the size of the antenna, the desired extent of coupling between the modes can be achieved, resulting in enhanced absorption over a broad infrared wavelength band. The absorption in the T2SL active layer in the proposed cavity architecture can be enhanced by nearly 10 times compared with that in the reference structure (without ground-plane and antenna). The cavity architecture is also studied by investigating angular and polarization dependence. This cavity architecture offers potential benefits to type-II superlattice detector performance with minimal growth cost.

Fundamental optical logic gate operations using optically controlled two surfaces plasmonic polariton mode coupler based on graphene clad waveguide

Here, an optical pulse controlled 2 × 2 surface–plasmon–polariton (SPP) mode interference coupler based on graphene clad is designed for the operation of optical fundamental gates. Exploiting the light–matter interactions on graphene–silicon interface and nonlinear optical property of graphene, coupling characteristics are obtained as a function of extra phase change between the excited SPP modes (fundamental and first order) modes with incidence of optical pulse. The device with a very small coupling length of ∼4.3 μm shows optical NOT, AND, and OR gate operation. Our compact design with high fabrication tolerance opens an avenue for the development of a complex optical processor.

Terahertz spoof surface plasmon polariton gradient index lens

Plasmonics takes advantage of surface plasmon polaritons (SPPs), which have the characteristics of strong subwavelength field confinement and enhancement. Plasmonic devices that confine light in regions with dimensions that are smaller than the wavelength have important application prospects in on-chip integrated circuits. Nonetheless, at terahertz frequencies, the weak confinement of SPPs and immature near-field technology hinder the development of related devices. Here, a highly confined SPP mode (spoof SPP) is implemented by structuring metal surfaces. Gradient index lenses with variable indices are constructed by fine-tuning the size of the pillars, and the characteristics of several different lenses are examined and explored by scanning near-field terahertz microscopy. Thus, terahertz waves are regulated and more possibilities are provided for further miniaturization and compactness of THz devices.

Ultra-Wideband Planar End-Fire Antenna Based on High-Order Modes of Spoof Surface Plasmon Polaritons

In this article, an ultrawideband planar end-fire antenna based on high-order modes of spoof surface plasmon polaritons (SSPPs) is proposed. First, a quad-band end-fire antenna is designed by using the first four high-order SSPP modes. An SSPP transmission line with gradually decreasing groove depth is used to radiate the high-order modes. Moreover, to improve the gain and sidelobe level at high frequencies, the SSPP units in the quad-band antenna are tilted. Then, an SSPP structure consisting of a periodic array of metal strips is proposed. By adding two arrays of tilted I-shaped strip resonators to the quad-band antenna, the first two bandgaps are eliminated. The proposed antenna achieves a fractional bandwidth of 140.8% from 6.9 to 39.7 GHz. Minimum and maximum measured gains of 6.4 and 15.2 dBi are achieved at 6.9 and 38 GHz, respectively. The proposed antenna has a great potential for applications in imaging and wireless communication.

High-Quality Surface Plasmon Polaritons in Large-Area Sodium Nanostructures.

Sodium (Na) is predicted to be an ideal plasmonic material with ultralow optical loss across visible to near-infrared (NIR). However, there has been limited research on Na plasmonics. Here we develop a scalable fabrication method for Na nanostructures by combining phase-shift photolithography and a thermo-assisted spin-coating process. Using this method, we fabricated Na nanopit arrays with varying periodicities (300-600 nm) and with tunable surface plasmon polariton (SPP) modes spanning visible to NIR. We achieved SPP resonances as narrow as 9.3 nm. In addition, Na nanostructures showed line width narrowing from visible toward NIR, showing their prospect operating in the NIR. To address the challenges associated with the high reactivity of Na, we designed a simple encapsulation strategy and stabilized the Na nanostructures in ambient conditions for more than two months. As a low-cost and low-loss plasmonic material, Na offers a competitive option for nanophotonic devices and plasmon-enhanced applications.

Effect of multi-interface patterns on the light-outcoupling of perovskite light-emitting diodes

Patterned structures in perovskite light-emitting diodes (PeLEDs) contribute to improved external quantum efficiency; however, unsatisfactory light outcoupling resulting from multi-interface optical loss remains a primary and unsolved challenge. This study investigated the effect of multi-interface patterns on the light outcoupling of PeLEDs using a SiO2-microcolumn pattern (SMP). It was found that the optical loss of PeLEDs decreased as the number of patterned interfaces increased. This was attributed to the strong scattering effect and the utilization of waveguide and surface plasmon polariton modes at the bottom interface and the electron transmission layer of the top electrode. A PeLED with a five-interface structure based on an SMP substrate was prepared to verify the improved light outcoupling, indicating that a synergistic design in light coupling and perovskite film formation should be considered to fabricate efficient devices with multi-interface patterns.

Frequency Converters for the Terahertz and Infrared Ranges

A method for solving the problem of nonlinear diffraction on two-dimensional periodic gratings of graphene ribbons has been developed. The third-order nonlinear conductivity of graphene under the action of two waves is taken into account, which is determined by the field of the pump wave, for which we use the field on graphene ribbons obtained by solving the linear diffraction problem. Numerical analysis shows the efficiency of nonlinear frequency conversion in the terahertz and infrared ranges when the frequencies of the incident pump and signal waves coincide with the resonant frequencies of the fundamental and higher order modes of surface plasmon polaritons in graphene ribbons.

Highly sensitive plasmonic sensor based on eccentric-core photonic crystal fibers.

To further reduce the fabrication difficulty of optical fiber sensors and improve the sensing performance, this study introduced the surface plasmon resonance (SPR) effect into optical fiber sensing technology and designed an eccentric-core photonic crystal fiber (EC-PCF). We investigated the characteristics of the two fundamental modes in the fiber core and the surface plasmon polariton (SPP) modes on the surface of the gold film. We also investigated the influence of the structural parameters, such as gold film coating area and thickness, air hole diameter, and eccentricity, on the confinement loss and achieved a refractive index (RI) sensitivity of 31.25 μm RIU-1 in the RI range of 1.29-1.43, corresponding to a figure of merit (FOM) of 521.6 per RIU. When the resolution of the optical spectrum analyzer was 0.1 nm, the EC-PCF could achieve a refractive index resolution of 3.2 × 10-6 RIU. Moreover, we performed tests with two typical sensing types, one in which the sensor was directly in contact with adulterated gasoline to achieve kerosene-concentration detection, and another in which the sensor was coated with a layer of polydimethylsiloxane (PDMS), whose RI is sensitive to the temperature field, to achieve temperature sensing. The EC-PCF demonstrated excellent sensing performance and offers obvious manufacturing advantages, providing a new and easily fabricated structural design idea for optical fiber sensing.

Broadband plasmon-induced transparency to an anapole mode induced absorption conversion Janus metastructure by a waveguide structure in the terahertz region.

A Janus metastructure (MS) assisted by a waveguide structure (WGS) resting on anapole modes and exhibiting direction-dependent behavior has been developed in the terahertz (THz) region. Ultra-broadband absorption is formed by the destructive interference through the anapole as well as Janus trait and is shaped by nested WGS. In this design, vanadium dioxide (VO2) is expected to attain functional transformation from plasmon-induced transparency (PIT) to absorption. The insulating nature of the VO2 results in the creation of the PIT, which is characterized by a wide and high transmission window ranging from 1.944 THz to 2.284 THz, corresponding to the relative bandwidth of 7.4% above 0.9. However, when the VO2 reaches the metallic state, a high absorptivity of 0.921 at 2.154 THz can be implemented in the -z-direction owing to the excitement of the toroidal dipole and electric dipole moments in the near-infrared region. And in the +z-direction, the broadband absorption above 0.9 in the 1.448-2.497 THz range takes shape in virtue of surface plasmon polariton modes, in which intensely localized oscillation of free electrons is confined to the metal-dielectric interface supported by the WGS. Noting that the MS is equipped with a favorable sensitivity to the incidence angle, we develop an ultra-broadband backward absorption in the TM mode from 0.7-10 THz nearly all above 0.9 when the incidence angle changes from 30°-70°. Moreover, owing to the highly symmetrical structure, the MS exhibits exotic polarization angular stability. All the awesome properties make this MS a good candidate for various applications such as in electromagnetic wave steering, spectral analysis, and sensors.

Floquet engineering of dressed surface plasmon polariton modes in plasmonic waveguides

We present a comprehensive study surface plasmon polariton (SPP) propagation on planar metallic waveguides under a dressing field. We perform a set of analytical calculations of the periodic time-dependent Schr\"odinger equation to study the interaction of an intense electromagnetic field with a metallic system. We model the strong light coupling to the metallic system using a nonperturbative procedure. In this paper, we show that, by introducing Floquet theory into the dressed metal, the behavior of the Floquet states are similar to the phase-modulated signals in the communication system. This will reveal a new perspective on the underlying science in an intricate, dressed quantum system. Furthermore, we examine the impurity scattering effects on charge transport in disordered plasmonic metals using the generalized Floquet-Fermi golden rule and provide a novel approach to diminish the losses in plasmonic materials using the dressing field. To gauge the effectiveness of our results, we introduce a figure of merit to compare the performance of plasmonic metals, subjective to a dressing field. The calculated dressed SPP characteristics suggest that high-efficiency SPP propagation can be achieved in practice. Our findings open up new directions for state-of-the-art nanoplasmonic devices.

Dynamical mean-field study of a photon-mediated ferroelectric phase transition

The interplay of light and matter gives rise to intriguing cooperative effects in quantum many-body systems. This is even true in thermal equilibrium, where the electromagnetic field can hybridize with collective modes of matter, and virtual photons can induce interactions in the solid. Here, we show how these light-mediated interactions can be treated using the dynamical mean-field theory formalism. We consider a minimal model of a two-dimensional material that couples to a surface plasmon polariton mode of a metal-dielectric interface. Within the mean-field approximation, the system exhibits a ferroelectric phase transition that is unaffected by the light-matter coupling. Bosonic dynamical mean-field theory provides a more accurate description and reveals that the photon-mediated interactions enhance the ferroelectric order and stabilize the ferroelectric phase.

Coupling tandem MOFs in metal-insulator-metal resonator advanced chemo-sieving sensing

Metal-insulator-metal (MIM) resonator allows concentrating light into deep-subwavelength volumes for filtering by accessing surface plasmon polariton mode, which is typically constructed by using inorganic materials and polymers as an insulator. Although MIM filters can manipulate light, dynamic color change upon environmental stimuli is still challenging. Here, we report a metal-organic frameworks (MOFs) based MIM resonator with tunable bandwidth that can realize visualized sensing of chemicals. Different bandwidths of the resonator show distinct color reflection that can be tailored by different types of MOFs and thickness of the MOFs layers. Chemical sensing on the MOFs-based MIM resonators shows breakthrough enhancement of sensitivity. Moreover, we configure tandem MOFs thin films in the MIM to form a metal-insulator-insulator-metal (MIIM) resonator for promoting selective sensing of chemicals. Also, molecular dynamics simulations demonstrated that the MIIM resonator achieved selective adsorption of xylenes in a mixture of xylene and methanol due to the chemo-sieving effect. The MIIM resonators show not only host-guest chemistry and sieving performance of tandem MOFs with high durability, but also anti-interference selective sensing of analyte in the mixture.

Mid-infrared refractive index photonic crystal fiber sensor based on surface plasmon resonance for ultra-high sensitivity

An ultra-high sensitivity photonic crystal fiber mid-infrared refractive index (RI) sensor based on surface plasmon resonance is proposed, whose properties are investigated by the finite element method. Gold can be utilized to create excitation between the fundamental mode and surface plasmon polaritons mode. The simulation results indicate that, at the wavelength of 1.7–4.0 μm, the proposed sensor could detect a RI in the range of 1.00–1.31, a maximum sensitivity of 37 000 nm RIU−1, and an optimal resolution of 2.70 × 10−6 RIU is achieved. The result of curve fitting is 0.98185. The proposed mid-infrared RI sensor plays an important role in organic chemistry detection, medical detection, and related fields.

Perfect absorption and phase singularities induced by surface lattice resonances for plasmonic nanoparticle array on a metallic film.

The formation of pairs of perfect absorption associated with phase singularities in the parameter space using the hybridized structure constructed with a metallic nanoparticle array and a metallic film is promising to enhance light-mater interactions. However, the localized plasmon resonances of the array possess strong radiative losses, which is an obstacle to improve the performances for many applications. On the contrary with the subwavelength array hybridized structure, this study shows that by enlarging the lattice spacing, the oscillator strength of the nanoparticles can be enhanced with the formation of surface lattice resonance, thereby leading to similar but much narrower pairs of perfect absorption due to the interactions with the Fabry-Pérot cavity modes. Furthermore, when the surface plasmon polariton mode shift to the same spectral range associated with the enlarged lattice spacing, the coupling and mode hybridization with the surface lattice resonance result in an anticrossing in the spectra. Although the resonance coupling does not enter the strong coupling regime, the quality factors (∼ 134) and near-field enhancements (∼ 44) are strongly enhanced for the hybridized resonance modes due to the effectively suppressed radiative losses compared with that of the localized plasmon resonances, which make the hybridized structure useful for the design of functional nanophotonic device such as biosensing, multi-model nanolasing, and high-quality imaging.