Propagating Surface Plasmon Polariton Modes Research Articles

Optical Manipulation of Incident Light for Enhanced Photon Absorption in Ultrathin Organic Photovoltaics.

We attempted to improve the photon absorption of the photoactive layer in organic photovoltaic (OPV) devices by device engineering without changing their thickness. Soft nanoimprinting lithography was used to introduce a 1D grating pattern into the photoactive layer. The increase in photocurrent caused by the propagating surface plasmon-polariton mode was quantitatively analyzed by measuring the external quantum efficiency in transverse magnetic and transverse electric modes. In addition, the introduction of an ultrathin substrate with a refractive index of 1.34 improved photon absorption by overcoming the mismatched optical impedance at the air/substrate interface. As a result, the power conversion efficiency (PCE) of an ultrathin OPV with a 400 nm grating period was 8.34%, which was 11.6% higher than that of an unpatterned ultrathin OPV, and the PCE was 3.2 times higher at a low incident light angle of 80°, indicating very low incident light angle dependence.

Scalable‐Manufactured Plasmonic Metamaterial with Omnidirectional Absorption Bandwidth across Visible to Far‐Infrared

AbstractBroadband, omnidirectional light absorption in the infrared range is critical for emerging aerospace and ground applications, such as machine vision, autonomous vehicle technology, and aerospace telescope. Broadband absorbers (BBAs) need to possess strong absorption through thin structures for high signal‐to‐noise ratios, as well as manufacturing scalability and service reliability in harsh environment for practical applications. Such requirements rule out many known absorbing materials such as carbon nanotubes that are intrinsically lossy and fragile. Achieving strong light‐matter interaction in the mid‐ and long‐wavelength infrared ranges has been extremely challenging despite long‐standing research efforts. Here, an aerospace‐grade, ≈2.0 µm‐thick hierarchical coral‐structured titanium nitride (coral‐TiN) plasmonic metamaterial is experimentally realized with over 90% omnidirectional absorption across the visible to the long‐wavelength infrared range (0.25–25 µm) using a scalable fabrication method. The broadband optical control of the coral‐TiN BBA is achieved by the superposition of the hybridized plasmonic mode in the visible and near‐infrared, the cavity mode in the short‐wavelength infrared as well as the propagating surface plasmon polariton mode in the mid‐ and long‐wavelength infrared. With conformal alumina coating, the plasmonic absorber demonstrates outstanding reliability in rigorous aerospace‐grade tests under harsh mechanical and thermal environmental conditions.

Correlations between incident and emission polarization in nanowire-particle coupled junctions.

Plasmonic nanostructures with subwavelength confinement are of great importance for the development of integrated nanophotonic circuits and devices. Here, we experimentally investigate how the polarization of the emitted light from nanowire-particle junction relies on the incident polarization. We demonstrate that the correlations can be effectively modulated by the particle position relative to the wire. By varying the wire-particle gap with only several nanometers, the nanowire-particle junction can be changed from polarization maintainer to rotator. Then, by moving the particle along the wire within half of the surface plasmon polariton (SPP) beat, the polarization behaviors can be tuned from positive to negative correlation. The mechanism can be well understood by the hybridization of wire-particle coupled mode and propagating SPP modes, which is verified by finite-difference time-domain simulations. These findings would provide a new degree of freedom for manipulating light polarization at the nanometer scale and additional flexibility for constructing nanophotonic devices.

Coherence Between Different Propagating Surface Plasmon Polariton Modes Excited by Quantum Mechanical Tunnel Junctions

AbstractCoherence between different surface plasmon polariton (SPP) modes excited by inelastically tunneling electrons in biased metal–insulator–metal tunnel junctions (MIM‐TJs) is demonstrated. By employing a dedicated SPP stripe waveguide with MIM‐TJ, an effective double‐slit configuration similar to the Young's experiment is realized for an electrically biased SPP source. The spatial correlation between different SPP modes originates from a single inelastic tunneling event and leads to strong interference in the far‐field, observed as alternate bright and dark fringes in the Fourier plane. The measured fringe‐spacing inversely follows the stripe waveguide length, with upper limit dictated by the SPP propagation length, confirming the SPP mediated spatial correlation. Finite difference time domain simulations support the experimental findings. Also, the experimental and simulation results unambiguously demonstrate the two‐step plasmonic decay process in plasmonic MIM‐TJs. The results presented here provide a simple and robust demonstration of the inherent coherence existing between different decay channels (plasmons and photons) of the inelastically tunneling electrons and can be exploited for plasmonic applications with tailored spatial coherence with implications in plasmon amplification and quantum information processing.

Monitoring the surface quality of silver plasmon waveguides with nonlinear photoemission electron microscopy and in-situ ion sputtering

We use photoemission electron microscopy (PEEM) with single- and two-photon excitation to study the properties of surface plasmon polaritons (SPPs) in tapered silver stripe waveguides. Arrays of waveguides with different widths, grating coupler geometries, and taper angles are fabricated using electron beam lithography (EBL) on Si and indium-tin-oxide (ITO)-covered glass substrates. The presence of propagating SPP modes is indicated by modulations of the non-linear photoemission intensity which run along the length of the waveguide - the result of interfering SPPs reflected from the edges. Surface roughness also results in the presence of random emission hot spots and strong edge diffraction, which hinders an analysis of the effect of waveguide geometry on SPP propagation length and nanofocusing. Accordingly, we develop a relatively simple, in-situ method of reducing the surface roughness by argon ion sputtering using low-energy (∼ 300eV) ions. Surface roughness is qualitatively assessed by the ratio of the photoemission from the central interference fringes to that from the edges, and from the hot spot intensity. The improvement of the surface quality is a critical step for reducing radiative losses and therefore increasing the propagation length of plasmonic waveguides.

Dynamics of Surface Plasmon Polaritons in Metal Nanowires

Metal nanostructures have found extensive use in a variety of applications in chemistry, including as substrates for molecular sensing and surface enhanced spectroscopy and as nanoscale heaters for photothermal therapy. These applications depend on the strong absorption and field enhancements associated with localized surface plasmon resonance (LSPR). This has led to a number of studies of how the LSPR line width, which measures energy losses for the coherent electron motion, depends on the size and shape of different types of metal nanoparticles, and the environment around the particle. Extended metal nanostructures, such as nanowires and nanoplates, display propagating surface plasmon modes, termed surface plasmon polaritons (SPPs), in addition to LSPRs. These modes are important for applications where metal nanostructures are used as waveguides. However, less is known about the damping of the propagating SPPs compared to the LSPRs. The energy losses for the propagating SPP modes can be investigated by ...

Fundamental limitations in spontaneous emission rate of single-photon sources

Rate of single-photon generation by quantum emitters (QEs) can be enhanced by placing a QE inside a resonant structure. This structure can represent an all-dielectric micro-resonator or waveguide and thus be characterized by ultra-low loss and dimensions on the order of wavelength. Or it can be a metal nanostructure supporting localized or propagating surface plasmon-polariton modes that are of subwavelength dimensions, but suffer from strong absorption. In this work, we develop a physically transparent analytical model of single-photon emission in resonant structures and show unambiguously that, notwithstanding the inherently high loss, the external emission rate can be enhanced with plasmonic nanostructures by two orders of magnitude compared to all-dielectric structures. Our analysis provides guidelines for developments of new plasmonic configurations and materials to be exploited in quantum plasmonics.

Multiphoton Photoemission Microscopy of High-Order Plasmonic Resonances at the Ag/Vacuum and Ag/Si Interfaces of Epitaxial Silver Nanowires

Understanding the physics of surface plasmons and related phenomena requires knowledge of the spatial, temporal, and spectral distributions of the total electromagnetic field excited within nanostructures and their interfaces, which reflects the electromagnetic mode excitation, confinement, propagation, and damping. We present a microscopic and spectroscopic study of the plasmonic response in single-crystalline Ag wires grown in situ on Si(001) substrates. Excitation of the plasmonic modes with broadly tunable (UV–IR) femtosecond laser pulses excites ultrafast multiphoton photoemission, whose spatial distribution is imaged with an aberration-corrected photoemission electron microscope, thereby providing a time-integrated map of the locally enhanced electromagnetic fields. We show by tuning the wavelength, polarization, and k-vector of the incident laser light that for a few micrometers long wires we can selectively excite either the propagating surface plasmon polariton modes or high-order multipolar reso...

Tunable Plasmonic Dispersion and Strong Coupling in Graphene Ribbon and Double Layer Sheets Structure

Based on the interplay between propagating surface plasmon polaritons (PSPs) in graphene ribbon and double layer sheets structure, we theoretically demonstrate a tunable strong coupling mechanism significantly different from reported conventional noble metal nanostructures. The strong electromagnetic coupling between the low order antisymmetric and high order symmetric PSPs modes occurs due to the intersections of dispersion curves, which leads to a modification of plasmonic dispersion and multiple significant anti-crossing regions. Of particular, this strong coupling is controllable through external gate voltage of graphene sheets or ribbon. The results offer an effective regime to dynamically tune the interaction of graphene PSPs, which may find applications in the field of nanophotonic devices in the mid-infrared range.

Resonant Enhancement of Magneto-Optical Activity Induced by Surface Plasmon Polariton Modes Coupling in 2D Magnetoplasmonic Crystals

Magnetoplasmonic crystals are spatially periodic nanostructured magnetic surfaces combining the features of surface plasmon polariton excitation and magneto-optical tunability. Here we present a comprehensive experimental and theoretical work demonstrating that in magnetoplasmonic crystals the coupling of free space radiation to surface plasmon polariton modes in conjunction with the inherent magneto-optical activity, enable cross-coupling of propagating surface plasmon polariton modes. We have explored the consequences of this unique magnetoplasmonic crystal optical feature by studying the light reflected from a two-dimensional periodic array of cylindrical holes in a ferromagnetic layer illuminated at oblique incidence and magnetized in the sample plane, namely, in the so-called longitudinal Kerr effect geometry. We observe that the magneto-optical spectral response arises from all the excitable surface plasmon polariton modes in the magnetoplasmonic crystal irrespective of the incoming light polarizati...

Effect of substrate discontinuities on the propagating surface plasmon polariton modes in gold nanobars.

The surface plasmon polariton (SPP) modes of gold nanobars (nanowires with rectangular dimensions) have been investigated by scanning pump-probe microscopy. In these experiments the nanobars were suspended over trenches cut in glass coverslips, and propagating SPP modes were launched in the supported portion of the nanobar by focusing a near-IR pump laser beam at the end of the nanobar. Transient absorption images were then collected by scanning the probe laser over the nanobar using a galvo-mirror system. The images show that the trench has a large effect on the SPP modes, specifically, for approximately half the nanowires the propagation length is significantly reduced after the trench. Finite element calculations were performed to understand this effect. The calculations show that the pump laser excites bound and leaky modes (modes that have their fields localized at the nanobar/glass or nanobar/air interfaces, respectively) in the supported portions of the nanobars. These modes propagate along the nanobar. When they meet the trench their field distributions are altered. The modes that derive from the bound mode are strongly damped over the trench. Thus, the bound mode is not reconstituted on the opposite side of the trench, and only the leaky mode contributes to the signal. Because the bound and leaky modes can have different propagation lengths, the propagation lengths measured in our experiments can change from one side of the trench to the other. The results show how the substrate can be engineered to control the SPP modes in metal nanostructures.

Highly efficient broadband ultrafast plasmonics

To date, considerable experimental and theoretical focus has been placed on the spatial control of Surface Plasmon Polaritons (SPPs) using nanostructured surfaces; however, research aimed toward accessing the ultrafast dynamics of SPPs remains vastly unexplored. Despite this, SPPs have the potential to exhibit some of the fastest possible optical processes, while maintaining the advantage of nanoscale spatial manipulation. Here, we present an experimental and computational investigation of a system that provides access to the efficient excitation of broadband, propagating SPP modes. To achieve this, a surface array of tailor designed, reduced symmetry nanostructures has been fabricated to enable the required control of the plasmon dispersion map to match sub 20 fs pulses in the near infra-red. Using a combination of optical spectroscopy and frequency resolved optical gating techniques, complimented by finite element computational analysis, the efficient excitation of propagating broadband plasmonic modes is demonstrated.

Imaging the extent of plasmon excitation in Au nanowires using pump-probe microscopy

Knowledge of how energy and charge carriers move in nanoscale systems is essential for engineering efficient devices. In this Letter, we demonstrate a technique to directly image dynamics in nanostructures based on laser scanning transient absorption microscopy, which provides near diffraction-limited spatial resolution and ultrafast time resolution. The capabilities of the technique are demonstrated by experiments on propagating surface plasmon polariton modes of Au nanowires, although these measurements can be used to study a variety of fluorescent and nonfluorescent systems.

Tuning the Fano Resonance Between Localized and Propagating Surface Plasmon Resonances for Refractive Index Sensing Applications

Localized and propagating surface plasmon resonances are known to show very pronounced interactions if they are simultaneously excited in the same nanostructure. Here, we study the Fano interference that occurs between localized surface plasmon resonance (LSPR) and propagating surface plasmon polariton (SPP) modes by means of phase-sensitive spectroscopic ellipsometry. The sample structures consist of periodic gratings of gold nanodisks on top of a continuous gold layer and a thin dielectric spacer, in which the structural dimensions were tuned in such a way that the dipolar LSPR mode and the propagating SPP modes are excited in the same spectral region. We observe pronounced anti-crossing and strongly asymmetric line shapes when both modes move to each other’s vicinity, accompanied of largely increased phase differences between the respective plasmon resonances. Moreover, we show that the anti-crossing can be exploited to increase the refractive index sensitivity of the localized modes dramatically, which result in largely increased values for the figure-of-merit which reaches values between 24 and 58 for the respective plasmon modes.

Investigation on the Second Part of the Electromagnetic SERS Enhancement and Resulting Fabrication Strategies of Anisotropic Plasmonic Arrays

In general, the electromagnetic mechanism is understood as the strongest contribution to the overall surface-enhanced Raman spectroscopy (SERS) enhancement. Due to the excitation of surface plasmons, a strong electromagnetic field is induced at the interfaces of a metallic nanoparticle leading to a drastic enhancement of the Raman scattering cross-section. Furthermore, the Raman scattered light expierences an emission enhancement due to the plasmon resonances of the nanoantennas. Herein, this second part of the electromagnetic enhancement phenomenon is investigated for different Raman bands of crystal violet by utilizing the anisotropic plasmonic character of gold nanorhomb SERS arrays. We aim at evaluating the effects of localized and propagating surface plasmon polariton modes as well as their combination on the scattered SERS intensity. From that point of view, design and fabrication strategies towards the fabrication of SERS arrays for excitation wavelengths in the visible and near-infrared (NIR) spectral region can be given, also using a double-resonant electromagnetic enhancement.

Coupling to light, and transport and dissipation of energy in silver nanowires

Transient absorption experiments with diffraction-limited spatial resolution have been used to study the optical absorption properties and dynamics of isolated, single silver nanowires. The images and polarization analysis show that the near-IR pump and near-UV probe beams couple to fundamentally different electron motions. The near-IR pump laser excites the propagating surface plasmon polariton (SPP) modes of the wires when focused at the ends, and multipolar plasmon modes (antenna modes) for medial excitation. The images show that these two modes have comparable optical absorption cross-sections. In contrast, the near-UV probe couples to the transverse plasmon resonance of the wire independent of the spatial position. For either end-on or medial excitation, pump laser absorption causes lattice heating and coherently excites the breathing vibrational mode of the nanowires. The vibrational quality factors depend on the acoustic impedance mismatch between the nanowire and the environment, and are similar to those recently measured for silver nanocubes. Experiments performed with spatially separated pump and probe beams, with the pump beam focused at one end to excite the propagating SPP modes, show that the amplitudes of the initial transient absorption signal and the breathing motion decrease with distance along the wire. This arises because the propagating SPP mode decays as it moves down the wire, which reduces the number of electronic excitations and, therefore, the signal level in the experiments. The measured length scale for the SPP decay is similar to that obtained in previous light scattering experiments.

Slow Propagation, Anomalous Absorption, and Total External Reflection of Surface Plasmon Polaritons in Nanolayer Systems

I predict that a nanoscopic, high-permittivity layer on the surface of a plasmonic metal can cause total external reflection of surface plasmon polaritons (SPPs). Such a layer can be used as a mirror in nanoplasmonics, in particular for resonators of nanolasers and spasers and can also be used in adiabatic nanooptics. I also show that the earlier predicted slow propagating SPP modes, especially those with negative refraction, are highly damped.