Plasmonic crystal demultiplexer and multiports

We report the realization of two-dimensional optical wavelength demultiplexers and multiports for surface plasmons polaritons (SPPs) based on plasmonic crystals, i.e., photonic crystals for SPPs. These SPP elements are built up of lithographically fabricated gold nanostructures on gold thin films. We show by direct imaging of laterally confined SPP beams in the visible spectral range by leakage radiation microscopy that SPPs of different wavelengths are efficiently rerouted into different directions. In addition we demonstrate the generation of three output SPP beams from one input beam.     

A nonvolatile plasmonic switch employing photochromic molecules

We demonstrate a surface plasmon-polariton (SPP) waveguide all-optical switch that combines the unique physical properties of small molecules and metallic (plasmonic) nanostructures. The switch consists of a pair of gratings defined in an aluminum film coated with a 65 nm thick layer of photochromic (PC) molecules. The first grating couples a signal beam consisting of free space photons to SPPs that interact effectively with the PC molecules. These molecules can reversibly be switched between transparent and absorbing states using a free space optical pump. In the transparent (signal "on") state, the SPPs freely propagate through the molecular layer, and in the absorbing (signal "off") state, the SPPs are strongly attenuated. The second grating serves to decouple the SPPs back into a free space optical beam, enabling measurement of the modulated signal with a far-field detector. In a preliminary study, the switching behavior of the PC molecules themselves was confirmed and quantified by surface plasmon resonance spectroscopy. The excellent (16%) overlap of the SPP mode profile with the thin layer of switching molecules enabled efficient switching with power densities of approximately 6.0 mW/cm2 in 1.5 microm x 8 microm devices, resulting in plasmonic switching powers of 0.72 nW per device. Calculations further showed that modulation depths in access of 20 dB can easily be attained in optimized designs. The quantitative experimental and theoretical analysis of the nonvolatile switching behavior in this letter guides the design of future nanoscale optically or electrically pumped optical switches.     

CMOS-Compatible Electronic–Plasmonic Transducers Based on Plasmonic Tunnel Junctions and Schottky Diodes

To develop methods to generate, manipulate, and detect plasmonic signals by electrical means with complementary metal–oxide–semiconductor (CMOS)-compatible materials is essential to realize on-chip electronic–plasmonic transduction. Here, electrically driven, CMOS-compatible electronic–plasmonic transducers with Al–AlO_X–Cu tunnel junctions as the excitation source of surface plasmon polaritons (SPPs) and Si–Cu Schottky diodes as the detector of SPPs, connected via plasmonic strip waveguides of Cu, are demonstrated. Remarkably, the electronic–plasmonic transducers exhibit overall transduction efficiency of 1.85 ± 0.03%, five times higher than previously reported transducers with two tunnel junctions (metal–insulator–metal (MIM)–MIM transducers) where SPPs are detected based on optical rectification. The result establishes a new platform to convert electronic signals to plasmonic signals via electrical means, paving the way toward CMOS-compatible plasmonic components.     

A dynamic plasmonic manipulation technique assisted by phase modulation of an incident optical vortex beam

A novel phase modulation method for dynamic manipulation of surface plasmon polaritons?(SPPs) with a phase engineered optical vortex?(OV) beam illuminating on nanoslits is experimentally demonstrated. Because of the unique helical phase carried by an OV beam, dynamic control of SPP multiple focusing and standing wave generation is realized by changing the OV beam's topological charge constituent with the help of a liquid-crystal spatial light modulator. Measurement of SPP distributions with near-field scanning optical microscopy showed an excellent agreement with numerical predictions. The proposed phase modulation technique for manipulating SPPs features has seemingly dynamic and reconfigurable advantages, with profound potential for development of SPP coupling, routing, multiplexing and high-resolution imaging devices on plasmonic chips.     

Liquid Plasmonics: Manipulating Surface Plasmon Polaritons via Phase Transitions

This paper reports the manipulation of surface plasmon polaritons (SPPs) in a liquid plasmonic metal by changing its physical phase. Dynamic properties were controlled by solid-to-liquid phase transitions in 1D Ga gratings that were fabricated using a simple molding process. Solid and liquid phases were found to exhibit different plasmonic properties, where light coupled to SPPs more efficiently in the liquid phase. We exploited the supercooling characteristics of Ga to access plasmonic properties associated with the liquid phase over a wider temperature range (up to 30 °C below the melting point of bulk Ga). Ab initio density functional theory-molecular dynamic calculations showed that the broadening of the solid-state electronic band structure was responsible for the superior plasmonic properties of the liquid metal.     

Surface plasmon polariton analogue to Young's double-slit experiment

When a light wave strikes a metal film it can, under appropriate conditions, excite a surface plasmon polariton (SPP)--a surface electromagnetic wave that is coupled to the free electrons in the metal. Such SPPs are involved in a wide range of phenomena, including nanoscale optical waveguiding, perfect lensing, extraordinary optical transmission, subwavelength lithography and ultrahigh-sensitivity biosensing. However, before the full potential of technology based on SPPs (termed 'plasmonics') can be realized, many fundamental questions regarding the interaction between light and matter at the nanoscale need to be answered. For over 200 years, Young's double-slit experiment has been a valuable pedagogical tool for demonstrating the wave nature of light. Here, we perform a double-slit experiment with SPPs to reveal the strong analogy between SPP propagation along the surface of metallic structures and light propagation in conventional dielectric components (such as glass waveguides). This allows us to construct a general framework to describe the propagation, diffraction and interference of SPPs. It also suggests that there is an effective diffraction limit for the lateral confinement of SPPs on metal stripe waveguides, and justifies the use of well-developed concepts from conventional optics and photonics in the design of new plasmonic devices.     

Compact antenna for efficient and unidirectional launching and decoupling of surface plasmons

Controlling the launching efficiencies and the directionality of surface plasmon polaritons (SPPs) and their decoupling to freely propagating light is a major goal for the development of plasmonic devices and systems. Here, we report on the design and experimental observation of a highly efficient unidirectional surface plasmon launcher composed of eleven subwavelength grooves, each with a distinct depth and width. Our observations show that, under normal illumination by a focused Gaussian beam, unidirectional SPP launching with an efficiency of at least 52% is achieved experimentally with a compact device of total length smaller than 8 μm. Reciprocally, we report that the same device can efficiently convert SPPs into a highly directive light beam emanating perpendicularly to the sample.     

Periodical concentration of surface plasmon polaritons by wave interference in metallic film with nanocavity array

Metallic thin films with nanocavity arrays provide ideal platforms for plasmonics, non-linear optics, surface chemistry and corresponding applications. A general understanding of electromagnetic (EM) field distributions is needed for further creation, manipulation and designation of near-field enhancements. Herein, we study the distribution of plasmonic hot spots over Ag thin films with triangular nanocavities in hexagonal arrays with a variable of lattice parameters. We propose that the concentration and interference of surface plasmon polaritons (SPP) dominates the distribution of plasmonic hot spots. The localized surface plasmonic resonance (LSPR) at nanocavities excites SPPs to propagate on the thin film, whose concentration and interference lead to an extremely strong near-field enhancement at the surface of the thin film, the location of which can also be termed as plasmonic hot spot. For this model, the calculation results of the physical formula are in excellent agreements with both the experimental results and the electrodynamic simulations with 3D finite element method (FEM). Moreover, the plasmonic hot spots distribute periodically within the nanocavity arrays, determined by the geometric symmetry of the array as well as with the polarization state of the incident field. The periodicity of plasmonic hot spots on flat surface illustrates a new way to concentrate SPPs in an extendable area, which has potential applications in localized non-linear optics, sensing, plasmonic logical circuit and optical computing.     

Multiple-wavelength focusing of surface plasmons with a nonperiodic nanoslit coupler

A novel type of multiple-wavelength focusing plasmonic coupler based on a nonperiodic nanoslit array is designed and experimentally demonstrated. An array of nanoslits patterned on a thin metal film is used to couple free-space light into surface plasmon polaritons (SPPs) and simultaneously focus different-wavelength SPPs into arbitrary predefined locations in the two-dimensional plane. We design and fabricate a compact triplexer on a glass substrate with an integrated silicon photodetector. The photocurrent spectra demonstrate that the incident light is effectively coupled to SPPs and routed into three different focal spots depending on the wavelength. The proposed scheme provides a simple method of building wavelength-division multiplexing and spectral filtering elements, integrated with other plasmonic and optoelectronic devices.     

A triangular shaped channel MIM waveguide filter

A compact plasmonic branch-shaped metal–insulator–metal (MIM) waveguide with a triangular-annular channel is proposed and investigated. The results show that all-band waves can transmit through the structure when it has transmission symmetry. As the symmetry is broken, a typical filtering function is obtained and the transmissivity of the resonance frequency depends on the destroyed degree of symmetry, which results from interference between the two branched surface plasmon polaritons waves (SPPs) propagating along the branch-shaped waveguide. Moreover, the filtering wavelength can be selected by changing the length of the triangular-annular channel. These asymmetric waveguide structures may open up new avenues toward the control of SPPs and the development of filters based on plasmonic designs.     

Plasmonic-induced transparency in a metallic stub with two cuts and transmission line model

We investigate a metal–insulator–metal (MIM) structure, which is composed of a bus MIM waveguide and a stub modified by two cuts. A transmission line (TL) model is proposed to depict the propagation characteristics of surface plasmon polaritons (SPPs). The finite element method is conducted to calculate the transmission spectrum of SPPs. Plasmonic-induced transparency spectral response can be achieved when different cuts are introduced. Under different parameters, theoretical results based on the TL model match with the simulation results very well. It is believed that our findings provide a smart way to design MIM-based plasmonic sensors and slow light devices.     

Optical control of plasmonic Bloch modes on periodic nanostructures

We study and actively control the coherent properties of surface plasmon polaritons (SPPs) optically excited on a nanohole array. Amplitude and phase of the optical excitation are externally controlled via a digital spatial light modulator (SLM) and SPP interference fringe patterns are designed and observed with high contrast. Our interferometric observations reveal SPPs dressed with the Bloch modes of the periodic nanostructure. The momentum associated with these dressed plasmons (DP) is highly dependent on the grating period and fully matches our theoretical predictions. We show that the momentum of DP waves can, in principle, exceed the SPP momentum. Actively controlling DP waves via programmable phase patterns offers the potential for high field confinement applicable in lithography, surface enhanced Raman scattering, and plasmonic structured illumination microscopy.     

Low-temperature plasmonics of metallic nanostructures

The requirements for spatial and temporal manipulation of electromagnetic fields on the nanoscale have recently resulted in an ever-increasing use of plasmonics for achieving various functionalities with superior performance to those available from conventional photonics. For these applications, ohmic losses resulting from free-electron scattering in the metal is one major limitation for the performance of plasmonic structures. In the low-frequency regime, ohmic losses can be reduced at low temperatures. In this work, we study the effect of temperature on the optical response of different plasmonic nanostructures and show that the extinction of a plasmonic nanorod metamaterial can be efficiently controlled with temperature with transmission changes by nearly a factor of 10 between room and liquid nitrogen temperatures, while temperature effects in plasmonic crystals are relatively weak (transmission changes only up to 20%). Because of the different nature of the plasmonic interactions in these types of plasmonic nanostructures, drastically differing responses (increased or decreased extinction) to temperature change were observed despite identical variations of the metal's permittivity.     

Plasmonic Luneburg and Eaton lenses

Plasmonics takes advantage of the properties of surface plasmon polaritons, which are localized or propagating quasiparticles in which photons are coupled to the quasi-free electrons in metals. In particular, plasmonic devices can confine light in regions with dimensions that are smaller than the wavelength of the photons in free space, and this makes it possible to match the different length scales associated with photonics and electronics in a single nanoscale device. Broad applications of plasmonics that have been demonstrated to date include biological sensing, sub-diffraction-limit imaging, focusing and lithography and nano-optical circuitry. Plasmonics-based optical elements such as waveguides, lenses, beamsplitters and reflectors have been implemented by structuring metal surfaces or placing dielectric structures on metals to manipulate the two-dimensional surface plasmon waves. However, the abrupt discontinuities in the material properties or geometries of these elements lead to increased scattering of surface plasmon polaritons, which significantly reduces the efficiency of these components. Transformation optics provides an alternative approach to controlling the propagation of light by spatially varying the optical properties of a material. Here, motivated by this approach, we use grey-scale lithography to adiabatically tailor the topology of a dielectric layer adjacent to a metal surface to demonstrate a plasmonic Luneburg lens that can focus surface plasmon polaritons. We also make a plasmonic Eaton lens that can bend surface plasmon polaritons. Because the optical properties are changed gradually rather than abruptly in these lenses, losses due to scattering can be significantly reduced in comparison with previously reported plasmonic elements.     

Double Fano-type resonances in heptamer-hole array transmission spectra with high refractive index sensing

Nanohole arrays or individual nanohole oligomers in metallic films have attracted intense attention due to their unique optical properties such as extraordinary optical transmission or Fano resonance. However, the nanohole oligomer array still remains largely unexplored. In this work, we numerically investigate the heptamer-hole arrays in an optically thin silver film, which can support double Fano-type resonances in the transmission spectra. The two Fano-type transmissions arise from the interference between the non-resonant direct transmission through holes and the resonant indirect scatterings based on the excitations of surface plasmons polaritons (SPPs, set up by the array periodicity) and a sub-radiant localized surface plasmon resonance (LSPR, arising from the anti-bonding hybridization between the central and the surrounding holes). Because of their different physical mechanisms, the two Fano resonances can be tuned independently. In addition, the LSPR-related Fano resonance shows an ultra-high sensitivity to surrounding dielectric medium with a figure of merit of 25 due to its sub-radiant feature, far larger than the SPP-related Fano resonance, offering tremendous potentials for plasmonic biosensors.     

An Electrically-Driven GaAs Nanowire Surface Plasmon Source

Over the past decade, the properties of plasmonic waveguides have extensively been studied as key elements in important applications that include biosensors, optical communication systems, quantum plasmonics, plasmonic logic, and quantum-cascade lasers. Whereas their guiding properties are by now fairly well-understood, practical implementation in chipscale systems is hampered by the lack of convenient electrical excitation schemes. Recently, a variety of surface plasmon lasers have been realized, but they have not yet been waveguide-coupled. Planar incoherent plasmonic sources have recently been coupled to plasmonic guides but routing of plasmonic signals requires coupling to linear waveguides. Here, we present an experimental demonstration of electrically driven GaAs nanowire light sources integrated with plasmonic nanostrip waveguides with a physical cross-section of 0.08λ(2). The excitation and waveguiding of surface plasmon-polaritons (SPPs) is experimentally demonstrated and analyzed with the help of full-field electromagnetic simulations. Splitting and routing of the electrically generated SPP signals around 90° bends are also shown. The realization of integrated plasmon sources greatly increases the applicability range of plasmonic waveguides and routing elements.     

Near-Field Photothermal Heating with a Plasmonic Nanofocusing Probe

Noble metal nanostructures support plasmon resonances—collective oscillation of charge carriers at optical frequencies—and serve as effective tools to create bright light sources at the nanoscale. These sources are useful in broad application areas including, super-resolution imaging and spectroscopy, nanolithography, and near-field optomechanical transducers. The feasibility of these applications relies on efficient conversion of free-space propagating light to plasmons. Recently, we demonstrated a hybrid nanofocusing scheme for efficient coupling of light to plasmons at the apex of a scanning probe. In the approach, free-space light is coupled to propagating surface plasmon polaritons (SPPs) on the tapered shaft of the scanning probe. The SPPs propagate adiabatically towards the probe tip where they are coupled to localized plasmons (LSPs). The nanofocusing scheme was explored in a near-field scanning optical microscope for super-resolution imaging, near-field transduction of nanomechanical vibrations, and local detection of ultrasound. Owing to the strong concentration of light at the probe, significant heating of the tip and a sample positioned in the optical near-field is expected. This paper investigates the local heating produced by the plasmonic nanofocusing probe under steady-state conditions using the tip-enhanced Raman scattering approach. In addition, a finite element model is explored to study the coupling of free propagating light to LSPs, and to estimate the temperature rise expected in a halfspace heated by absorption of the LSPs. This study has implications for exploring the plasmonic nanofocusing probe in heat-assisted nanofabrication and fundamental studies of nanoscale heat transport in materials.     

Modeling of a plasmonic nanosensor based on an open box-like metal cavity

We propose a plasmonic nanosensor based on an open box-like metal nanocavity. Surface plasmon polaritons (SPPs) excited at the metal/dielectric interface oscillate in the cavity, and then, plasmonic resonance modes are formed. Since the cavity is open, a part of the resonance light of the SPPs is scattered to light. By monitoring the shift in the scattering spectrum, the refractive index change of the sensed material can be derived. Because of the high reflectivity of the metallic walls, the sensitivity and figure of merit (FOM) are higher than those using single nanoparticle or nanoantenna. A sensitivity of 1046 nm/RIU (RIU denotes refractive index unit) and a FOM of 23.4 are derived for a 700 nm long and 350 nm high square cavity. Furthermore, the sensing area of the proposed sensor is smaller than 1 μm² and the performance of the nanosensor can be further tuned by varying the cavity dimensions. The proposed sensor is well suited for observing small changes in biological and chemical reactions.     

Electrical detection of surface plasmon polaritons by 1G0 gold quantum point contacts

Electrical detection of surface plasmons polaritons (SPPs) is essential for realization of integrated fast nanoscale plasmonic circuits. We demonstrate electrical detection of SPPs by measuring their remote gating effect on 1G(0) metal quantum point contacts (MQPC) made of gold. Gating is argued to take place by a photoassisted transport mechanism with nonmonotonic behavior of its magnitude as a function of distance between the MQPCs and the position of SPPs creation.