Optimizing Substrate-Mediated Plasmon Coupling toward High-Performance Plasmonic Nanowire Waveguides

Seeking better plasmonic waveguides is of critical importance for minimizing photonic circuits into the nanometer scale. We have made a theoretical study of the properties of surface plasmon polaritons in a metallic nanowire over substrate (NWOS) configuration. The dielectric substrate breaks the symmetry of the system and mediates the coupling of different primary wire plasmons. The lowest order hybridized mode can be used for subwavelength plasmonic waveguiding for NWOS with thin wire, for a low-permittivity substrate, and in the shorter wavelength region. For NWOS with a high-permittivity substrate, leaky radiation into the substrate raises the propagation losses so that the propagation distance is shorter in the longer wavelength region. By simply adding a high-permittivity layer onto the low-permittivity substrate, we show that leaky radiation can be blocked and high-performance plasmonic waveguiding can be extended to the near-infrared region. Importantly, the NWOS configuration is compatible with current silicon technologies and can be designed into various deep subwavelength active devices such as electro-optical or all-optical modulators

Synchronously Deriving Electron Concentration and Mobility by Temperature- and Oxygen-Dependent Conductivity of Porous ZnO Nanocrystalline Film

A simple and effective way to get electron concentration and mobility accurately is significant for the electronic and photoelectric applications of porous ZnO nanocrystalline film. On the basis of the defect ionization and the electron scattering, we proposed here a new temperature-programmed-dependent conductivity-based synchronous derivation method (TPDCBSD) to evaluate electron concentration and mobility of porous ZnO nanocrystalline film independently. The obtained results were consistent with others. Compared with the commonly used Hall-effect measurements, the TPDCBSD method is much more simple, has lower noise, and is convenient to couple external fields. More importantly, the extracted electron concentration and electron mobility are relatively independent. Besides, a series of physical parameters related to the effects of temperature and oxygen partial pressure were obtained, and the coupling effect of temperature and oxygen was discussed in this work, which are inspiring for the applications of porous ZnO nanocrystalline film

Defect Chemistry of the Metal Cation Defects in the p- and n‑Doped SnO₂ Nanocrystalline Films

Cationic interstitial and substitutional defects, which serve as a key role in shaping the material’s performance, are considered as two kinds of important defect structures in the doped SnO₂. To give a clear characterization of such metal cation defects, temperature-dependent electrical conduction measurement by the high throughput screening platform of gas-sensing materials is carried out, for the first time, to perform the defect structure studies of the p-type (Li⁺, Cd²⁺, Al³⁺), isovalent (Ti⁴⁺), and n-type (Nb⁵⁺, W⁶⁺) doped SnO₂ nanocrystalline films in the oxygen-free atmosphere. The temperature-dependent measurements indicate that subtle induced impurities are capable of evidently modifying the electrical conduction mechanism of the SnO₂. In terms of the small-polaron hopping mechanism, an improved defect chemical model is proposed in which the properties of the metal cation defects are explicitly depicted. Values for the ionization energy (Δ<i>E_D</i>) of the metal cation defects and electron hopping energy (<i>E_H</i>) in the doped SnO₂ are extracted by fitting the experimental data to the defect model. These data that reflect the nature of the metal cation defects and their effects on the electronic structure of the SnO₂ are first introduced here, and the validity of these data are confirmed. What’s more, the Δ<i>E_D</i> calculated here is of critical importance for understanding the defect structure of the metal dopants in the SnO₂

High Photoconductive Response of Gas-Sensitized Porous Nanocrystalline TiO₂ Film in Formaldehyde Ambience and Carrier Transport Kinetics

We propose a gas-sensitized porous nanocrystalline TiO₂ film with a potential application in photovoltaic devices and report about the systematic photoconductivity study of it. The quantitative results show that the gas-sensitized TiO₂ film in formaldehyde atmosphere exhibits much higher photoconductivity (3–4 orders of magnitude) and longer carrier lifetime than usual. The intriguing performance of the gas-sensitized TiO₂ film indicates the distinct charge carrier transport kinetic courses, whose contributions to the photoconductivity are shown in a designed flowchart. From the flowchart, it is clearly found that two electron loss processes, recombination and electron scavenging, are suppressed for the gas-sensitized TiO₂ film in formaldehyde gas, leading to large improvements of photoconductivity and carrier lifetime. The results provide the potential of improving efficiency of photovoltaic devices, and measuring photoconductivity under target gas appears to be a useful tool for research on photocatalytic and photoelectrical processes

Precise Sorting of Gold Nanoparticles in a Flowing System

Precise sorting of gold nanoparticles is important, but it still remains a big challenge. Traditional methods such as centrifugation can separate nanoparticles with a high throughput but at the cost of low precision. Optical tweezers enable the precise manipulation of a single nanoparticle in steady liquid environments. However, this method may become problematic when dealing with a considerable amount of nanoparticles in a flowing system due to the difficulties in balancing the additional Stokes forces by the fast velocity of streams and in controlling all dispersed nanoparticles with disorderly positions. Here, we exploit optical and hydrodynamic forces to sort gold nanoparticles in the flowing system, obtaining simultaneously high precision and considerable throughput. This is accomplished by utilizing opposite impinging streams to generate a stagnation point, near which the flow velocity becomes very small to reduce the Stokes force and to prolong the optical acting time. Nanoparticles of different sizes, confined in a narrow region by the hydrodynamic focusing, can then be separated by a laser beam of moderate power. Experimental demonstrations have been presented by sorting gold nanoparticles with diameters of 50 nm from those of 100 nm, and 100 nm from 200 nm. The sorting fidelities is ≥92% for the 50/100 nm combination and ≥86% for the 100/200 nm set, with a sorting throughput of 300 particles/min. Sorting of gold nanoparticles with smaller heterogeneity (50 and 70 nm) has also been realized with a lower throughput of <100 particles/min. Our method can also be extended to separate nanoparticles of different shapes and compositions, which shows its great promise in the fields of plasmonics and nanophotonics

How to Obtain the Correct Rabi Splitting in a Subwavelength Interacting System

We unambiguously extract the individual decay channels of a coupled plasmon-exciton system by using correlated single-particle absorption and scattering measurements. A remarkable difference in the two channels is presentclear Rabi splitting in the plasmon channel but no Rabi splitting in the exciton channel. Discordance in the absorption and scattering spectra are mainly originated from the distinct contributions of plasmon and exciton channels in the absorption and scattering process. Our findings provide insights into plasmon-exciton interaction in an open cavity and can impact the design of plexcitonic devices for ultrafast nonlinear nanophotonics

Precise Sorting of Gold Nanoparticles in a Flowing System

Precise sorting of gold nanoparticles is important, but it still remains a big challenge. Traditional methods such as centrifugation can separate nanoparticles with a high throughput but at the cost of low precision. Optical tweezers enable the precise manipulation of a single nanoparticle in steady liquid environments. However, this method may become problematic when dealing with a considerable amount of nanoparticles in a flowing system due to the difficulties in balancing the additional Stokes forces by the fast velocity of streams and in controlling all dispersed nanoparticles with disorderly positions. Here, we exploit optical and hydrodynamic forces to sort gold nanoparticles in the flowing system, obtaining simultaneously high precision and considerable throughput. This is accomplished by utilizing opposite impinging streams to generate a stagnation point, near which the flow velocity becomes very small to reduce the Stokes force and to prolong the optical acting time. Nanoparticles of different sizes, confined in a narrow region by the hydrodynamic focusing, can then be separated by a laser beam of moderate power. Experimental demonstrations have been presented by sorting gold nanoparticles with diameters of 50 nm from those of 100 nm, and 100 nm from 200 nm. The sorting fidelities is ≥92% for the 50/100 nm combination and ≥86% for the 100/200 nm set, with a sorting throughput of 300 particles/min. Sorting of gold nanoparticles with smaller heterogeneity (50 and 70 nm) has also been realized with a lower throughput of <100 particles/min. Our method can also be extended to separate nanoparticles of different shapes and compositions, which shows its great promise in the fields of plasmonics and nanophotonics

Transversely Divergent Second Harmonic Generation by Surface Plasmon Polaritons on Single Metallic Nanowires

Coherently adding up signal wave from different locations are a prerequisite for realizing efficient nonlinear optical processes in traditional optical configurations. While nonlinear optical processes in plasmonic waveguides with subwavelength light confinement are in principle desirable for enhancing nonlinear effects, so far it has been difficult to improve the efficiency due to the large momentum mismatch. Here we demonstrate, using remotely excited surface plasmon polaritons (SPPs), axial collimated but transversely divergent second harmonic (SH) generation in a single silver nanowire–monolayer molybdenum disulfide hybrid system. Fourier imaging of the generated SH signal confirms the momentum conservation conditions between the incident and reflected SPPs and reveals distinct features inherent to the 1D plasmonic waveguides: (i) the SH photons are collimated perpendicular to the nanowire axis but are divergent within the perpendicular plane; (ii) the collimation (divergence) is inversely proportional to the length of the active region (lateral confinement of the SPPs); and (iii) the SH emission pattern resembles that of an aligned dipole chain on top of the substrate with an emission peak at the critical angle. Our results pave the way to generate and manipulate SH emission around subwavelength waveguides and open up new possibilities for realizing high efficiency on-chip nonlinear optics

Manipulating Coherent Plasmon–Exciton Interaction in a Single Silver Nanorod on Monolayer WSe₂

Strong coupling between plasmons and excitons in nanocavities can result in the formation of hybrid plexcitonic states. Understanding the dispersion relation of plexcitons is important both for fundamental quantum science and for applications including optoelectronics and nonlinear optics devices. The conventional approach, based on statistics over different nanocavities, suffers from large inhomogeneities from the samples, owing to the nonuniformity of nanocavities and the lack of control over the locations and orientations of the excitons. Here we report the first measurement of the dispersion relationship of plexcitons in an individual nanocavity. Using a single silver nanorod as a Fabry-Pérot nanocavity, we realize strong coupling of plasmon in single nanocavity with excitons in a single atomic layer of tungsten diselenide. The plexciton dispersion is measured by in situ redshifting the plasmon energy via successive deposition of a dielectric layer. Room-temperature formation of plexcitons with Rabi splittings as large as 49.5 meV is observed. The realization of strong plasmon–exciton coupling by in situ tuning of the plasmon provides a novel route for the manipulation of excitons in semiconductors

Ultrasensitive Size-Selection of Plasmonic Nanoparticles by Fano Interference Optical Force

In this paper, we propose a solution for the ultrasensitive optical selection of plasmonic nanoparticles using Fano interference-induced scattering forces. Under a Gaussian beam excitation, the scattering of a plasmonic nanoparticle at its Fano resonance becomes strongly asymmetric in the lateral direction and consequently results in a net transverse scattering force, that is, Fano interference-induced force. The magnitude of this transverse scattering force is comparable with the gradient force in conventional optical manipulation experiments. More interestingly, the Fano scattering force is ultrasensitive to the particle size and excitation frequency due to the phase sensitivity of the interference between adjacent plasmon modes in the particle. Utilizing this distinct feature, we show the possibility of size-selective sorting of silver and gold nanoparticles with an accuracy of about ±10 nm and silica-gold core–shell nanoparticles with shell thickness down to several nanometers. These results would add to the toolbox of optical manipulation and fabrication