Nanoscale Plasmonic Devices Research Articles

Optical Introduction and Manipulation of Plasmon–Exciton–Trion Coupling in a Si/WS2/Au Nanocavity

Strong plasmon-exciton coupling, which has potential applications in nanophotonics, plasmonics, and quantum electrodynamics, has been successfully demonstrated by using metallic nanocavities and two-dimensional materials. Dynamical control of plasmon-exciton coupling strength, especially by using optical methods, remains a big challenge although it is highly desirable. Here, we report the optical introduction and manipulation of plasmon-exciton-trion coupling realized in a dielectric-metal hybrid nanocavity, which is composed of a silicon (Si) nanoparticle and a thin gold (Au) film, with an embedded tungsten disulfide (WS2) monolayer. We employ scattering and photoluminescence spectra to characterize the coupling strength between plasmons and excitons in Si/WS2/Au nanocavities constructed by using Si nanoparticles with different diameters. We enhance the plasmon-exciton and plasmon-trion coupling strength by injecting excitons and trions into the WS2 monolayer with a 488 nm laser beam. It is revealed that the emission intensities of excitons and trions with respect to the reference WS2 monolayer can be modified through the change in the coupling strength induced by the laser light. Interestingly, the coupling strength between the plasmons and the excitons/trions can be manipulated from weak to strong coupling regime by simply increasing the laser power, which is clearly resolved in the scattering spectra of Si/WS2/Au nanocavities. More importantly, the plasmon-exciton-trion coupling induced by the laser light is confirmed by the energy exchange between excitons and trions. Our findings indicate the possibility for optically manipulating plasmon-exciton interaction and suggest the practical applications of dielectric-metal hybrid nanocavities in nanoscale plasmonic devices.

Airy-Talbot plasmon: an accelerating self-imaging surface wave.

In this Letter, a new class of an accelerating self-imaging surface plasmonic wave, the Airy-Talbot plasmon, is introduced for the first time, to the best of our knowledge. Our research shows that such a surface wave propagates at the interface between metal (silver) and a dielectric material (air) and causes a strong interference along curved trajectories, which generates the Talbot effect in the surface. The propagation properties have potential value in nanoscale plasmonic devices. A scheme for generating this novel plasmon theoretically is proposed, and we prove it by finite difference time-domain numerical simulations.

Localized surface plasmons in platinum aluminide semi-shells

The dielectric function of the intermetallic compound PtAl2 is assessed and found to be comparable to that of titanium nitride, suggesting that nanostructures of PtAl2 may be suitable for plasmonic devices. In order to probe this further, the optical properties of experimentally produced arrays of nanoscale PtAl2 semi-shells of about 300 nm diameter were examined and compared to the results of numerical simulations. The structures showed a broad localized surface plasmon resonance centered on ∼1.3 eV (∼950 nm), which matched the simulations. Calculations showed that a ten-fold enhancement of the electric field of the incident light will be achieved around the rim of suitably oriented PtAl2 semi-shells. The phase of the oscillation induced by 1060 nm light will be retarded by π/2 relative to the incident light. This is indicative of a resonant condition. These observations suggest that it could be worthwhile to investigate possible applications for this and other intermetallic compounds in nanoscale plasmonic devices.

Nanoscale temperature of plasmonic HAMR heads by polymer imprint thermal mapping

ABSTRACT Polymer imprint thermal mapping (PITM) is a high-resolution thermal mapping technique that is especially valuable for nanoscale plasmonic devices. PITM leverages a ∼50 nm polymer film coating that crosslinks irreversibly with temperature, which records the peak temperature rise of the surface in the local, linear reduction of polymer film thickness. Using AFM to measure topography before and after heating, but not during operation, PITM sidesteps plasmonic artifacts seen in other near-field thermometries, where the probe tip disturbs and is heated directly by the near- and far-field radiation around the plasmonic device. This is notably troublesome for characterizing heat-assisted magnetic recording (HAMR) heads for next-generation hard disk drives. HAMR heads use near-field transducers (NFTs) to focus light on a magnetic media, heating a nanoscale region to its Curie temperature to enable magnetic writing. The PITM proof-of-concept was introduced at The Magnetic Recording Conference (TMRC) in 2015: here, we present a mature technique capable of benchmarking finite-element thermal simulations of nanoscale devices.

Dielectric Metasurface as a Platform for Spatial Mode Conversion in Nanoscale Waveguides.

We experimentally demonstrate a nanoscale mode converter that performs coupling between the first two transverse electric-like modes of a silicon-on-insulator waveguide. The device operates by introducing a nanoscale periodic perturbation in its effective refractive index along the propagation direction and a graded effective index profile along its transverse direction. The periodic perturbation provides phase matching between the modes, while the graded index profile, which is realized by the implementation of nanoscale dielectric metasurface consisting of silicon features that are etched into the waveguide taking advantage of the effective medium concept, provides the overlap between the modes. Following the device design and numerical analysis using three-dimensional finite difference time domain simulations, we have fabricated the device and characterized it by directly measuring the modal content using optical imaging microscopy. From these measurements, the mode purity is estimated to be 95% and the transmission relative to an unperturbed strip waveguide is as high as 88%. Finally, we extend this approach to accommodate for the coupling between photonic and plasmonic modes. Specifically, we design and numerically demonstrate photonic to plasmonic mode conversion in a hybrid waveguide in which photonic and surface plasmon polariton modes can be guided in the silicon core and in the silicon/metal interface, respectively. The same method can also be used for coupling between symmetric and antisymmetric plasmonic modes in metal-insulator-metal or insulator-metal-insulator structures. On the basis of the current demonstration, we believe that such nanoscale dielectric metasurface-based mode converters can now be realized and become an important building block in future nanoscale photonic and plasmonic devices. Furthermore, the demonstrated platform can be used for the implementation of other chip scale components such as splitters, combiners couplers, and more.

Single-crystalline aluminum film for ultraviolet plasmonic nanolasers.

Significant advances have been made in the development of plasmonic devices in the past decade. Plasmonic nanolasers, which display interesting properties, have come to play an important role in biomedicine, chemical sensors, information technology, and optical integrated circuits. However, nanoscale plasmonic devices, particularly those operating in the ultraviolet regime, are extremely sensitive to the metal and interface quality. Thus, these factors have a significant bearing on the development of ultraviolet plasmonic devices. Here, by addressing these material-related issues, we demonstrate a low-threshold, high-characteristic-temperature metal-oxide-semiconductor ZnO nanolaser that operates at room temperature. The template for the ZnO nanowires consists of a flat single-crystalline Al film grown by molecular beam epitaxy and an ultrasmooth Al2O3 spacer layer synthesized by atomic layer deposition. By effectively reducing the surface plasmon scattering and metal intrinsic absorption losses, the high-quality metal film and the sharp interfaces formed between the layers boost the device performance. This work should pave the way for the use of ultraviolet plasmonic nanolasers and related devices in a wider range of applications.

Exploiting the interaction between a semiconductor nanosphere and a thin metal film for nanoscale plasmonic devices.

The interaction of silicon (Si) nanospheres (NSs) with a thin metal film is investigated numerically and experimentally by characterizing their forward scattering properties. A sharp resonant mode and a zero-scattering dip are found to be introduced in the forward scattering spectrum of a Si NS by putting it on a 50-nm-thick gold film. It is revealed that the sharp resonant mode arises from a new magnetic dipole induced by the electric dipole and its mirror image while the zero-scattering dip originates from the destructive interference between the new magnetic dipole and the original one together with its mirror image. A significant enhancement in both electric and magnetic fields is achieved at the contact point between the Si NS and the metal film. More interestingly, the use of a thin silver film can lead to vivid scattering light with different color indices. It is demonstrated that a small change in the surrounding environment of Si NSs results in the broadening of the resonant mode and the disappearance of the zero-scattering dip. Our findings indicate that dielectric-metal hybrid systems composed of semiconductor NSs and thin metal films act as attractive platforms on which novel nanoscale plasmonic devices can be realized.

Plasma-mediated photothermal effects in ultrafast laser irradiation of gold nanoparticle dimers in water.

The intention of this paper is to study the physical mechanism underlying the response of gold nanoparticle (AuNP) dimers to a near-infrared off-resonance femtosecond pulse laser in aqueous medium. We show that the strongly localized field enhancement in the gap distance and around nanoparticles significantly reduces the laser fluence threshold to achieve an optical breakdown in comparison with an AuNP monomer. This optical breakdown results from highly localized plasma in surrounding media where the nanoparticles stay intact. Also the impact of the gap distance, field polarization, laser fluence and pulse duration on the energy deposition in plasma is presented. These results can be used to make nanoscale plasmonic devices for variety of absorption-based applications.

Localization, Hybridization, and Coupling of Plasmon Resonances in an Aluminum Nanomatryushka

In this study, we investigate the plasmon response of two concentric aluminum (Al) nanoshells as a nanomatryushka unit to introduce a novel compositional structure that has a strong potential to employ in designing practical nanoscale plasmonic devices. Herein, we employed Al nanoshells with a coverage of oxide (Al2O3) layer with certain and homogenous size of thickness in inner and outer sides. Using plasmon hybridization theory and finite-difference time-domain (FDTD) method as numerical model, we calculated and sketched the optical response and energy level diagram for the studied structure. Strong plasmon resonances are reported in the UV and visible wavelengths that can be supported efficiently by using the proposed nanomatryushka unit composed of Al/Al2O3 on a SiO2 surface. Utilizing presented nanomatryushka in designing an artificial dimer configuration, the possibility of appearing of dark modes and formation of Fano resonances in such a symmetric structure in the UV and visible spectra are verified numerically. Immersing the presented dimer in various liquids with different refractive indices, the behavior of Fano dip is investigated and corresponding figure of merit (FoM) is quantified based on the plasmon resonance energy shifts over the refractive index variations. This understating opens novel avenues to obtain sharp and deep Fano resonances in simple and low-cost structures that have strong potentials in fabrication of biochemical sensors, superlensing, and biological agents.

Lossless propagation in active channel plasmon polariton waveguides

In this paper, we have analyzed and simulated the propagation of lightwave in three different types of active channel plasmon polariton waveguides (CPPWs), V-groove, trench and step-trench at 1550nm wavelength based on a kind of available semiconductor as the active gain medium. By using the finite-difference frequency-domain method, strong plasmon modes confinement is observed in these waveguides and the required gains for achieving lossless propagations are derived. Due to the strong optical confinement, strong nonlinearity effects can be acquired in CPPWs. The dependence of the required gains on the geometrical parameters of the structures and wavelength changes for acquiring lossless propagation are analyzed and discussed in all three waveguides. The simulation results show that the calculated gain values are achievable by the existing semiconductor technology. Results show that the required gains for lossless propagation in nanoscale plasmonic devices can be well controlled by adjusting their geometrical parameters.

Direct Temperature Mapping of Nanoscale Plasmonic Devices

Side by side with the great advantages of plasmonics in nanoscale light confinement, the inevitable ohmic loss results in significant joule heating in plasmonic devices. Therefore, understanding optical-induced heat generation and heat transport in integrated on-chip plasmonic devices is of major importance. Specifically, there is a need for in situ visualization of electromagnetic induced thermal energy distribution with high spatial resolution. This paper studies the heat distribution in silicon plasmonic nanotips. Light is coupled to the plasmonic nanotips from a silicon nanowaveguide that is integrated with the tip on chip. Heat is generated by light absorption in the metal surrounding the silicon nanotip. The steady-state thermal distribution is studied numerically and measured experimentally using the approach of scanning thermal microscopy. It is shown that following the nanoscale heat generation by a 10 mW light source within a silicon photonic waveguide the temperature in the region of the nanotip is increased by ∼ 15 °C compared with the ambient temperature. Furthermore, we also perform a numerical study of the dynamics of the heat transport. Given the nanoscale dimensions of the structure, significant heating is expected to occur within the time frame of picoseconds. The capability of measuring temperature distribution of plasmonic structures at the nanoscale is shown to be a powerful tool and may be used in future applications related to thermal plasmonic applications such as control heating of liquids, thermal photovoltaic, nanochemistry, medicine, heat-assisted magnetic memories, and nanolithography.

Approaching the strong coupling limit in single plasmonic nanorods interacting with J-aggregates

We studied scattering and extinction of individual silver nanorods coupled to the J-aggregate form of the cyanine dye TDBC as a function of plasmon – exciton detuning. The measured single particle spectra exhibited a strongly suppressed scattering and extinction rate at wavelengths corresponding to the J-aggregate absorption band, signaling strong interaction between the localized surface plasmon of the metal core and the exciton of the surrounding molecular shell. In the context of strong coupling theory, the observed “transparency dips” correspond to an average vacuum Rabi splitting of the order of 100 meV, which approaches the plasmon dephasing rate and, thereby, the strong coupling limit for the smallest investigated particles. These findings could pave the way towards ultra-strong light-matter interaction on the nanoscale and active plasmonic devices operating at room temperature.

Towards nano-scale photonics with micro-scale photons: the opportunities and challenges of mid-infrared plasmonics

Abstract Surface plasmon polaritons and their localized counterparts, surface plasmons, are widely used at visible and near-infrared (near-IR) frequencies to confine, enhance, and manipulate light on the subwavelength scale. At these frequencies, surface plasmons serve as enabling mechanisms for future on-chip communications architectures, high-performance sensors, and high-resolution imaging and lithography systems. Successful implementation of plasmonics-inspired solutions at longer wavelengths, in the mid-infrared (mid-IR) frequency range, would benefit a number of highly important technologies in health- and defense-related fields that include trace-gas detection, heat-signature sensing, mimicking, and cloaking, and source and detector development. However, the body of knowledge of visible/near-IR frequency plasmonics cannot be easily transferred to the mid-IR due to the fundamentally different material response of metals in these two frequency ranges. Therefore, mid-IR plasmonic architectures for subwavelength light manipulation require both new materials and new geometries. In this work we attempt to provide a comprehensive review of recent approaches to realize nano-scale plasmonic devices and structures operating at mid-IR wavelengths. We first discuss the motivation for the development of the field of mid-IR plasmonics and the fundamental differences between plasmonics in the mid-IR and at shorter wavelengths. We then discuss early plasmonics work in the mid-IR using traditional plasmonic metals, illuminating both the impressive results of this work, as well as the challenges arising from the very different behavior of metals in the mid-IR, when compared to shorter wavelengths. Finally, we discuss the potential of new classes of mid-IR plasmonic materials, capable of mimicking the behavior of traditional metals at shorter wavelengths, and allowing for true subwavelength, and ultimately, nano-scale confinement at long wavelengths.

Polarization properties of surface plasmon enhanced photoluminescence from a single Ag nanowire

Metallic nanowires are of great research interest due to their applications in surface plasmon polariton coupling of light. The efficiency is much dependent on the polarization of the light due to the phase matching requirement in the light-surface plasmon polariton coupling. By scanning confocal microscope, the photoluminescence from a single Ag nanowire was demonstrated strongly dependent on the excitation laser polarization, showing good consistency with the theoretical simulation. Meanwhile strong avalanche photoluminescence from a single Ag nanowire was observed when the excitation laser was polarized along the long axis of the Ag nanowire. The photoluminescence emission exhibited a polarization-sensitive spatial distribution. This may stimulate promising applications in designing polarization-controllable nanoscale plasmonic devices.

Probing the electronic structure and optical response of a graphene quantum disk supported on monolayer graphene

In this paper, we show that a graphene quantum disk (GQD) can be generated on monolayer graphene via structural modification using the electron beam. The electronic structure and local optical responses of the GQD, supported on monolayer graphene, were probed with electron energy-loss spectrum imaging on an aberration-corrected scanning transmission electron microscope. We observe that for small GQD, ∼1.3 nm in diameter, the electronic structure and optical response are governed by the dominating edge states, and are distinctly different from either monolayer graphene or double-layer graphene. Highly localized plasmon modes are generated at the GQD due to the confinement from the edge of the GQD in all directions. The highly localized optical response from GQDs could find use in designing nanoscale optoelectronic and plasmonic devices based on monolayer graphene.

Graphene-Enabled Silver Nanoantenna Sensors

Silver is the ideal material for plasmonics because of its low loss at optical frequencies but is often replaced by a more lossy metal, gold. This is because of silver's tendency to tarnish and roughen, forming Ag(2)S on its surface, dramatically diminishing optical properties and rendering it unreliable for applications. By passivating the surface of silver nanostructures with monolayer graphene, atmospheric sulfur containing compounds are unable to penetrate the graphene to degrade the surface of the silver. Preventing this sulfidation eliminates the increased material damping and scattering losses originating from the unintentional Ag(2)S layer. Because it is atomically thin, graphene does not interfere with the ability of localized surface plasmons to interact with the environment in sensing applications. Furthermore, after 30 days graphene-passivated silver (Ag-Gr) nanoantennas exhibit a 2600% higher sensitivity over that of bare Ag nanoantennas and 2 orders of magnitude improvement in peak width endurance. By employing graphene in this manner, the excellent optical properties and large spectral range of silver can be functionally utilized in a variety of nanoscale plasmonic devices and applications.

Subwavelength plasmonic waveguides based on ZnO nanowires and nanotubes: A theoretical study of thermo-optical properties

We show theoretically that plasmonic waveguide structures in ZnO nanowires and nanotubes working at optical frequencies can achieve photonic waveguiding in a subdiffraction limit. The output intensity distribution, propagation length, and thermo-optical properties with different waveguide configurations are investigated. Our results show that these waveguides have the potential to develop either high performance thermally controlled nanoscale plasmonic devices or thermally insensitive waveguides by optimizing waveguide configurations.

Nanoscale plasmonic devices for dynamically controllable beam focusing and scanning

We have performed simulations to investigate the variable focusing and scanning capability of metallic nano-slit configurations. In a symmetric nanorod configuration inside an aperture with adjustable offset of the center rod, the focal position is found to be variable in the 0.5–3.5 μm range. In a ladder configuration of the rods, the transmitted beam is found to be deflected up to 23°. Horizontal displacement of rods allows for finer control of angular scanning up to 4°. Such slit geometries offer the potential to be controlled by using nano-positioning systems for applications in dynamic beam shaping and scanning on the nanoscale.

Nanoplasmonic directional couplers and Mach–Zehnder interferometers

We present a novel design and analysis of two nano-scale plasmonic devices: a directional coupler and a Mach–Zehnder interferometer. The designs of the two devices are based on our recent work on the air-gap coupler that resulted in high coupling efficiency between a dielectric waveguide and a plasmonic waveguide. The two devices are embedded between two dielectric waveguides and operate at optical telecom wavelengths. The overall efficiency was 37% for a 2×2 directional coupler switch and above 50% for the proposed designs for a Mach–Zehnder Interferometer. The efficiency in the proposed devices can be increased using broader plasmonic waveguides.