Plasmon modes in single gold nanodiscs

Selective triggering in-plane and out-of-plane dipolar modes of hexagonal Au nanoplate with the polarization of excitation beam

The dipolar responses of a single hexagonal Au nanoplate are investigated under the illuminations of linearly polarized beam and tightly focused radially polarized beam (RPB). It is found from the scattering spectra that the in-plane and out-of-plane electric dipole modes can be selectively triggered with a linearly polarized beam and tightly focused RPB, respectively. The features of these two dipolar modes are further confirmed in terms of electrical field and charge maps by the finite-difference time-domain simulation. Additionally, using the multipole expansion method, the existence of the out-of-plane dipole mode is further verified by the fact that thez-component of electric dipole response has a dominant contribution to the scattered power. Moreover, by combining the back focal plane imaging technique with the simulation, the appearance of in-plane and out-of-plane dipoles in the scattering pattern are clearly discerned. Our results provide an efficient method for selectively exciting the in-plane and out-of-plane dipolar modes of the nanoplate. We envision that the ease of tuning the dipolar momentum may facilitate the enhancement of the interaction between the plasmon and emitters at single-particle level.

Differential heating of metal nanostructures at radio frequencies

Nanoparticles with strong absorption of incident radio frequency (RF) or microwave irradiation are desirable for remote hyperthermia treatments. While controversy has surrounded the absorption properties of spherical metallic nanoparticles, other geometries such as prolate and oblate spheroids have not received sufficient attention for application in hyperthermia therapies. Here, we use the electrostatic approximation to calculate the relative absorption ratio of metallic nanoparticles in various biological tissues. We consider a broad parameter space, sweeping across frequencies from 1 MHz to 10 GHz, while also tuning the nanoparticle dimensions from spheres to high-aspect-ratio spheroids approximating nanowires and nanodiscs. We find that while spherical metallic nanoparticles do not offer differential heating in tissue, large absorption cross sections can be obtained from long prolate spheroids, while thin oblate spheroids offer minor potential for absorption. Our results suggest that metallic nanowires should be considered for RF- and microwave-based wireless hyperthermia treatments in many tissues going forward.

Thermal, Quantum Antibunching and Lasing Thresholds from Single Emitters to Macroscopic Devices

Starting from a fully quantized Hamiltonian for an ensemble of identical emitters coupled to the modes of an optical cavity, we determine analytically regimes of thermal, collective anti-bunching and laser emission that depend explicitly on the number of emitters. The lasing regime is reached for a number of emitters above a critical number-which depends on the light-matter coupling, detuning, and the dissipation rates-via a universal transition from thermal emission to collective anti-bunching to lasing as the pump increases. Cases where the second order intensity correlation fails to predict laser action are also presented.

Wulff-Based Approach to Modeling the Plasmonic Response of Single Crystal, Twinned, and Core-Shell Nanoparticles

The growing interest in plasmonic nanoparticles and their increasingly diverse applications is fuelled by the ability to tune properties via shape control, promoting intense experimental and theoretical research. Such shapes are dominated by geometries that can be described by the kinetic Wulff construction such as octahedra, thin triangular platelets, bipyramids, and decahedra, to name a few. Shape is critical in dictating the optical properties of these nanoparticles, in particular their localized surface plasmon resonance behavior, which can be modeled numerically. One challenge of the various available computational techniques is the representation of the nanoparticle shape. Specifically, in the discrete dipole approximation, a particle is represented by discretizing space via an array of uniformly distributed points-dipoles; this can be difficult to construct for complex shapes including those with multiple crystallographic facets, twins, and core-shell particles. Here, we describe a standalone user-friendly graphical user interface (GUI) that uses both kinetic and thermodynamic Wulff constructions to generate a dipole array for complex shapes, as well as the necessary input files for DDSCAT-based numerical approaches. Examples of the use of this GUI are described through three case studies spanning different shapes, compositions, and shell thicknesses. Key advances offered by this approach, in addition to simplicity, are the ability to create crystallographically correct structures and the addition of a conformal shell on complex shapes.

Characterization of Overlapped Plasmon Modes in a Gold Hexagonal Plate Revealed by Three-Dimensional Near-Field Optical Microscopy

A detailed characterization of plasmon modes is important not only for a deeper understanding of plasmons but also for their practical applications. In this study, we investigated the three-dimensional near-field characteristics of high-order plasmon modes excited in a gold hexagonal nanoplate. From the near-field spectroscopic images, we found that both in-plane and out-of-plane plasmon modes observed near 900 nm were spectrally and spatially overlapped. We performed three-dimensional near-field measurement to reveal the optical characteristics of the overlapped modes in detail. We found that the steric near-field distribution near the nanoplate strongly depended on the plasmon mode, and the out-of-plane mode confines electromagnetic fields more tightly than the in-plane mode. We also found that the in-plane mode was dominantly visualized as the probe tip-sample distance increased. These findings demonstrate that the three-dimensional near-field technique enables selective visualization of a single plasmon mode even if multiple modes are spatially and spectrally overlapped.

Static and Dynamic Near-Field Measurements of High-Order Plasmon Modes Induced in a Gold Triangular Nanoplate

Precise understanding of the spatiotemporal characteristics of plasmons is essential for the development of applications of plasmonic nanoparticles. In this study, we investigated the spatiotemporal properties of high-order plasmon modes induced in a gold triangular nanoplate by static and dynamic near-field measurements. The near-field transmission measurements revealed that in-plane and out-of-plane polarized plasmon modes were simultaneously excited and these modes spectroscopically and spatially overlapped. The superposition of these modes was visualized in the near-field two-photon excitation image of the nanoplate. We performed time-resolved autocorrelation measurements on the nanoplate and found that the correlation width was broader than the excitation pulse due to the plasmon dephasing process. From the correlation width map of the nanoplate, we experimentally demonstrated that the out-of-plane plasmon mode exhibits a longer dephasing time than the in-plane plasmon mode. These findings indicate that the out-of-plane mode is desirable for improving the performance of plasmons in various applications.

Lippmann waveguide spectrometer with enhanced throughput and bandwidth for space and commercial applications

This article presents an innovative high spectral resolution waveguide spectrometer, from the concept to the prototype demonstration and the test results. The main goal is to build the smallest possible Fourier transform spectrometer (FTS) with state of the art technology. This waveguide FTS takes advantage of a customized pattern of nano-samplers fabricated on the surface of a planar waveguide that allows the increase of the measurement points necessary for increasing the spectral bandwidth of the FTS in a fully static way. The use of a planar waveguide on the other hand allows enhancing the throughput in a waveguide spectrometer compared to the conventional devices made of single-mode waveguides. A prototype is made in silicon oxynitride/silicon dioxide technology and characterized in the visible range. This waveguide spectrometer shows a nominal bandwidth of 256~nm at a central wavelength of 633~nm thanks to a custom pattern of nanodisks providing a μm sampling interval. The implementation of this innovative waveguide FTS for a real-case scenario is explored and further development of such device for the imaging FTS application is discussed.

Gap enhanced fluorescence as a road map for the detection of very weakly fluorescent emitters from visible to ultraviolet

We analyze the enhancement of the rates of both the emission and the far field radiation for dipoles placed in the gap between a metallic nanorod, or nanosphere, and a metallic substrate. For wavelengths between 150 nm and 650 nm, the response of the gapped nanostructures considered in this work is dominated by few principal modes of the nanoparticle, which include self-consistently the effect of the substrate. For wavelengths shorter than 370 nm, the far field radiative enhancements of aluminum nanostructures are significantly higher than those for gold or silver. With aluminum, bright mode resonances are tunable over tens or hundreds of nanometers by changing the size of the nanoparticle and have far field radiative enhancements of up to three orders of magnitude. These results provide a road map to label-free detection of many emitters too weakly fluorescent for present approaches.

Enhancing ultraviolet spontaneous emission with a designed quantum vacuum

We determine how to alter the properties of the quantum vacuum at ultraviolet wavelengths to simultaneously enhance the spontaneous transition rates and the far field detection rate of quantum emitters. We find the response of several complex nanostructures in the 200 - 400 nm range, where many organic molecules have fluorescent responses, using an analytic decomposition of the electromagnetic response in terms of continuous spectra of plane waves and discrete sets of modes. Coupling a nanorod with an aluminum substrate gives decay rates up to 2.7 × 10³ times larger than the decay rate in vacuum and enhancements of 824 for the far field emission into the entire upper semi-space and of 2.04 × 10³ for emission within a cone with a 60° semi-angle. This effect is due to both an enhancement of the field at the emitter's position and a reshaping of the radiation patterns near mode resonances and cannot be obtained by replacing the aluminum substrate with a second nanoparticle or with a fused silica substrate. These large decay rates and far field enhancement factors will be very useful in the detection of fluorescence signals, as these resonances can be shifted by changing the dimensions of the nanorod. Moreover, these nanostructures have potential for nano-lasing because the Q factors of these resonances can reach 107.9, higher than the Q factors observed in nano-lasers.

Surface plasmon optical antennae in the infrared region with high resonant efficiency and frequency selectivity

Infrared light has received attention for sensor applications, including fingerprint spectroscopy, in the bioengineering and security fields. Surface plasmon physics enables the operation of a light harvesting optical antenna. Gold nanochains exhibit localized surface plasmon resonance (LSPR) in the infrared region with high frequency selectivity. However, a feasible design for optical antennae with a higher resonant efficiency and frequency selectivity as a function of structural design and periodicity is still unknown. In the present study, we investigated the relationship between the resonant efficiency and frequency selectivity as a function of the structural design of gold nanochains and explored structural periodicity for obtaining highly frequency-selective optical antennae. An optical antenna design with higher resonant efficiency is proposed on the basis of its efficient interaction with non-polarized light.

Accurate Modeling of Dark-Field Scattering Spectra of Plasmonic Nanostructures

Dark-field microscopy is a widely used tool for measuring the optical resonance of plasmonic nanostructures. However, current numerical methods for simulating the dark-field scattering spectra were carried out with plane wave illumination either at normal incidence or at an oblique angle from one direction. In actual experiments, light is focused onto the sample through an annular ring within a range of glancing angles. In this paper, we present a theoretical model capable of accurately simulating the dark-field light source with an annular ring. Simulations correctly reproduce a counterintuitive blue shift in the scattering spectra from gold nanodisks with a diameter beyond 140 nm. We believe that our proposed simulation method can be potentially applied as a general tool capable of simulating the dark-field scattering spectra of plasmonic nanostructures as well as other dielectric nanostructures with sizes beyond the quasi-static limit.

Size dependence of bandgaps in a two-dimensional plasmonic crystal with a hexagonal lattice

The optical properties of surface plasmon polaritons (SPPs) are investigated at the Г point in a two-dimensional plasmonic crystal with a hexagonal lattice (Hex-PlC). The cathodoluminescence (CL) technique combined with a scanning transmission electron microscope (STEM) are used to produce spectral images of the SPP standing waves at the Г point and identify the four types of band-edge modes predicted by group theory. The systematic measurement of the band-edge energies employed here is used to determine the characteristic dependence of each band-edge mode on the structure parameters, which provides some criteria for the design of plasmonic devices with Hex-PlCs.

Local optical responses of plasmon resonances visualised by near-field optical imaging

Near-field optical imaging visualises spatial features of plasmon resonances that cause unique optical characteristics of noble metal nanostructures.