Nanoplasmonic Systems Research Articles

Weak plasmonic photocatalysis in unsupported colloidal gold nanorods

Metal-semiconductor plasmonic nanosystems have the ability to extract photocarriers generated on colloidal gold nanoparticles efficiently through the Schottky junction. This process leads to effective visible light photocatalysis. However, due to the short lifespan of plasmon carriers, plasmonic loss occurs before extraction by the semiconductor and transfer to the adsorbent. We aim to investigate the direct transfer of plasmonic energy to adsorbed organic pollutants in visible light photocatalysis using unsupported colloidal gold nanorods (Au NRs). Our decision to use gold nanorods is based on their exceptional ability to harness a greater amount of visible light through their longitudinal surface plasmon resonance (LSPR) at 765 nm and transverse surface plasmon resonance (TSPR) band at 521 nm. Additionally, we conducted experiments to evaluate the photocatalytic capabilities under specific wavelengths, such as blue (405 nm), green (532 nm), and red (660 nm) lasers, to gain a better understanding of the -d-sp interband transition and the role of surface plasmon in photocatalysis. Surprisingly, unsupported Au NRs showed only weak plasmonic photocatalysis under all these excitations. With the help of supporting characterization tools, we observed that without semiconductor support, the photoexcited carriers over Au nanorods are not well separated, resulting in reduced photocatalysis.

Unconventional Breathing Currents Far beyond the Quantum Tunneling Distances in Large-Gapped Nanoplasmonic Systems.

Localized surface plasmon resonance (LSPR) in plasmonic nanoparticles propels the field of plasmo-electronics, holding promise for transformative optoelectronic devices through efficient light-to-current conversion. Plasmonic excitations strongly influence the charge distribution within nanoparticles, giving rise to electromagnetic fields that can significantly impact the macroscopic charge flows within the nanoparticle housing material. In this study, we present evidence of ultralow, unconventional breathing currents resulting from dynamic irradiance interactions between widely separated nanoparticles, extending far beyond conventional electron (quantum) tunneling distances. We develop an electric analogue model and derive an empirical expression to elucidate the generation of these unconventional breathing currents in cascaded nanoplasmonic systems under irradiance modulation. This technique and theoretical model have significant potential for applications requiring a deeper understanding of current dynamics, particularly on large nanostructured surfaces relevant to photocatalysis, energy harvesting, sensing, imaging, and the development of future photonic devices.

Key Role of Asymmetric Photothermal Effect in Selectively Chiral Switching of Plasmonic Dimer Driven by Circularly Polarized Light.

Strong interaction between circularly polarized light and chiral plasmonic nanostructures can enable controllable asymmetric photophysical processes, such as selective chiral switching of a plasmonic nanorod-dimer. Here, we uncover the underlying physics that governs this chiral switching by theoretically investigating the interplay between asymmetric photothermal and optomechanical effects. We find that the photothermally induced local temperature rises could play a key role in activating the dynamic chiral configurations of a plasmonic dimer due to the temperature-sensitive molecular linkages located at the gap region. Importantly, different temperature rises caused by the opposite handedness of light could facilitate selective chiral switching of the plasmonic dimer driven by asymmetric optical torques. Our analyses on the wavelength-dependent selectively chiral switching behaviors are in good agreement with the experimental observations. This work contributes to a comprehensive understanding of the physical mechanism involved in the experimentally designed photoresponsive plasmonic nanosystems for practical applications.

Local Photochemical Nanoscopy of Hot-Carrier-Driven Catalytic Reactions Using Plasmonic Nanosystems.

Nanoscale investigation of the reactivity of photocatalytic systems is crucial for their fundamental understanding and improving their design and applicability. Here, we present a photochemical nanoscopy technique that unlocks the local spatial detection of molecular products during plasmonic hot-carrier-driven photocatalytic reactions with nanometric precision. By applying the methodology to Au/TiO2 plasmonic photocatalysts, we experimentally and theoretically determined that smaller and denser Au nanoparticle arrays present lower optical contribution with quantum efficiency in hot-hole-driven photocatalysis closely related to the population heterogeneity. As expected, the highest quantum yield from a redox probe oxidation is achieved at the plasmon peak. Investigating a single plasmonic nanodiode, we unravel the areas where oxidation and reduction products are evolved with subwavelength resolution (∼200 nm), illustrating the bipolar behavior of such nanosystems. These results open the way to quantitative investigations at the nanoscale to evaluate the photocatalytic reactivity of low-dimensional materials in a variety of chemical reactions.

Smart Design of Noble Metal–Copper Chalcogenide Dual Plasmonic Heteronanoarchitectures for Emerging Applications: Progress and Prospects

Dual plasmonic metal–semiconductor heteronanostructures have been actively investigated due to their outstanding functional characteristics arising from the merging of two materials. In this review, the synthetic approaches, various designs, enhanced properties, and prospective applications of noble metal–nonstoichiometric copper chalcogenide nanosystems are summarized. The tunable size, shape, structure type (solid or hollow), composition, and doping level of the constituents produce several configurations of heteronanocomposites reported here. The colloidal synthetic approaches for the fabrication of dual plasmonic nanomaterials with controllable plasmonic properties are presented as well. The unique features of dual plasmonic nanostructures show synergistic and plasmon resonance coupling effects and enhanced light–matter interactions, which cannot be simply assigned to the mixture of pristine materials. Dual plasmonic hybrid nanosystems are promising candidates for enhanced photocatalysis, photothermal cancer therapy, improved sensing, photovoltaic devices, and upconversion luminescence. In conclusion, a brief outlook of the identified challenges in the area of dual plasmonic noble metal–vacancy-doped copper chalcogenide nanomaterials to be addressed in the nearest future is provided.

Magneto-Optical Chirality in a Coherently Coupled Exciton-Plasmon System.

Chirality is a fundamental asymmetry phenomenon, with chiral optical elements exhibiting asymmetric response in reflection or absorption of circularly polarized light. Recent realizations of such elements include nanoplasmonic systems with broken-mirror symmetry and polarization-contrasting optical absorption known as circular dichroism. An alternative route to circular dichroism is provided by spin-valley polarized excitons in atomically thin semiconductors. In the presence of magnetic fields, they exhibit an imbalanced coupling to circularly polarized photons and thus circular dichroism. Here, we demonstrate that polarization-contrasting optical transitions associated with excitons in monolayer WSe2 can be transferred to proximal plasmonic nanodisks by coherent coupling. The coupled exciton-plasmon system exhibits magneto-induced circular dichroism in a spectrally narrow window of Fano interference, which we model in a master equation framework. Our work motivates the use of exciton-plasmon interfaces as building blocks of chiral metasurfaces for applications in information processing, nonlinear optics, and sensing.

Launching directional hypersonic surface waves in monolithic gallium phosphide nanodisks: two holes are better than one.

The emergence and rapid progress of all-dielectric nanoantennas have provided unprecedented platforms for applications in sensing, optical control of light, opto-mechanics and metrology at the nanoscale. We present a general figure-of-merit (FOM) considering both optical and vibrational responses. Detectable mechanical vibrations ranging from gigahertz to terahertz in gallium phosphide (GaP) structures on sub-wavelength scales are found to surpass their metallic counterparts in a 400-800 nm pump-probe configuration. Then, we tailored low-aspect ratio GaP disks being probed near their optical anapole resonance. We further broke the isotropy of the nanodisks and achieved pronounced directional propagation for launching surface acoustic waves (SAWs) with a double-hole structure rather than with a one-hole configuration, which could be attributed to the constructive superposition of vibration induced by the two holes in the appropriate direction. Finally, we demonstrated that the orbital angular momentum of SAWs could be generated with a spiral distribution of the two-hole nanodisks. Our work paves a new way to monolithic GaP nanoantennas towards photoacoustic applications such as hypersound routers, stirring up inverse designs of individual antennas for phononic metasurfaces, topological phononics as well as quantum phononics.

OpenSANS: A Semi-Analytical solver for Nonlocal plasmonicS

Prompted by the renewed interest the past decade in the hydrodynamic model for the description of nanoplasmonic systems, we present OpenSANS, a MATLAB toolbox for the simulation of canonical (that is, standalone spherical and cylindrical) nanoparticles and planar stratified structures whose critical dimensions are of mere nanometres. The method comprises of a generalisation of Mie theory to properly address the hydrodynamic motion of free electrons in metals and a compact S matrix formulation for the efficient treatment of multiple layers. The toolbox is built in a modular manner; each part (routine) has its own important functionality and can be used independently from the others. Our primary goal is the presentation of a highly efficient and accessible solver for the nanoplasmonics community. This is achieved by the very semi-analyticity of the chosen method and by powerful vectorisation techniques optimised in a very popular environment, MATLAB. Program summaryProgram Title: OpenSANSCPC Library link to program files:https://doi.org/10.17632/hmk224xngb.1Code Ocean capsule:https://codeocean.com/capsule/2246190Licensing provisions: GPLv3Programming language: MATLAB R2018b and newer.Supplementary material: See Appendix A.Nature of problem: Solve the scattering problem from multilayered, dielectric, local or nonlocal metallic nanoparticles, excited by a plane wave and placed in a local (dielectric or metallic) background.Solution method: Mie theory, appropriately extended to incorporate nonlocal effects (longitudinal waves) as predicted by the Hard Wall-Hydrodynamic Model, combined with S matrix theory for efficient handling of the multilayered structure.

Simultaneous Nanoscale Excitation and Emission Mapping by Cathodoluminescence.

Free-electron-based spectroscopies can reveal the nanoscale optical properties of semiconductor materials and nanophotonic devices with a spatial resolution far beyond the diffraction limit of light. However, the retrieved spatial information is constrained to the excitation space defined by the electron beam position, while information on the delocalization associated with the spatial extension of the probed optical modes in the specimen has so far been missing, despite its relevance in ruling the optical properties of nanostructures. In this study, we demonstrate a cathodoluminescence method that can access both excitation and emission spaces at the nanoscale, illustrating the power of such a simultaneous excitation and emission mapping technique by revealing a subwavelength emission position modulation as well as by visualizing electromagnetic energy transport in nanoplasmonic systems. Besides the fundamental interest of these results, our technique grants us access into previously inaccessible nanoscale optical properties.

Physics Discovery in Nanoplasmonic Systems via Autonomous Experiments in Scanning Transmission Electron Microscopy.

Physics-driven discovery in an autonomous experiment has emerged as a dream application of machine learning in physical sciences. Here, this work develops and experimentally implements a deep kernel learning (DKL) workflow combining the correlative prediction of the target functional response and its uncertainty from the structure, and physics-based selection of acquisition function, which autonomously guides the navigation of the image space. Compared to classical Bayesian optimization (BO) methods, this approach allows to capture the complex spatial features present in the images of realistic materials, and dynamically learn structure-property relationships. In combination with the flexible scalarizer function that allows to ascribe the degree of physical interest to predicted spectra, this enables physical discovery in automated experiment. Here, this approach is illustrated for nanoplasmonic studies of nanoparticles and experimentally implemented in a truly autonomous fashion for bulk- and edge plasmon discovery in MnPS3 , a lesser-known beam-sensitive layered 2D material. This approach is universal, can be directly used as-is with any specimen, and is expected to be applicable to any probe-based microscopic techniques including other STEM modalities, scanning probe microscopies, chemical, and optical imaging.

Joint Communication and Bio-Sensing With Plasmonic Nano-Systems to Prevent the Spread of Infectious Diseases in the Internet of Nano-Bio Things

With the advances in nanotechnology, novel nanosensing technologies can play a pivotal role in today’s society. Plasmonic sensing has demonstrated unprecedented detection reliability and resolution in a very compact form factor. Traditional plasmonic sensors leverage biofunctionalized metallic grating structures whose frequency response in transmission or reflection changes according to the presence of targeted biomarkers. However, these sensing setups require bulky measurement equipment to couple light to and from sensors for excitation and detection. In parallel, for over a decade, the nanoscale electromagnetic communication community has been leveraging plasmonic structures to transmit information at the nanoscale efficiently. By combining the two realms, this paper proposes the concept of joint nanoscale communication and bio-sensing systems enabled by plasmonic sensing nanoantennas. Sensing nanonodes can communicate from nanonode to nanonode for intra-body networks and from nanonode to a wearable device which, by leveraging the edge, can process and transmit the sensing information to the cloud, resulting in accurate diagnosis and reduced load on the medical testing infrastructure. First, we model the changes in the frequency response of a biofunctionalized plasmonic nanoantenna when exposed to different biomarkers. Then, we propose a chirp-spread spectrum excitation and detection system to enable simultaneous communication and sensing at the nanoscale. We present a data-driven human tissue model for communication through human tissue. We also present numerical results to demonstrate the performance of the proposed system.

Strong-Field Control of Plasmonic Properties in Core–Shell Nanoparticles

The strong-field control of plasmonic nanosystems opens up new perspectives for nonlinear plasmonic spectroscopy and petahertz electronics. Questions, however, remain regarding the nature of nonlinear light-matter interactions at sub-wavelength spatial and ultrafast temporal scales. Addressing this challenge, we investigated the strong-field control of the plasmonic response of Au nanoshells with a SiO$_2$ core to an intense laser pulse. We show that the photoelectron energy spectrum from these core-shell nanoparticles displays a striking transition between the weak and strong-field regime. This observed transition agrees with the prediction of our modified Mie-theory simulation that incorporates the nonlinear dielectric nanoshell response. The demonstrated intensity-dependent optical control of the plasmonic response in prototypical core-shell nanoparticles paves the way towards ultrafast switching and opto-electronic signal modulation with more complex nanostructures.

Emerging Methods for Controlling Hot Carrier Excitation and Emission Distributions in Nanoplasmonic Systems

Emerging Methods for Controlling Hot Carrier Excitation and Emission Distributions in Nanoplasmonic Systems

Plasmonic Nanoagents in Biophysics and Biomedicine

AbstractThe significant rise in implementation and applications of plasmonic nanosystems in biophysics, biochemistry, and medicine has culminated in the emergence of refined plasmonic enabling reagents, or “nanoagents”. These are defined as tools that allow researchers to not only investigate, but also actively manipulate biological processes and complex biosystems, such as living cells, on the nanoscale. This development is based on a combination of sensing capabilities, photothermal control, and optical force manipulation offered by metallic nanoparticles. The article reviews the trajectory that plasmonic nanoagents have taken in recent years and highlights seminal recent examples of their application, such as optical sensing both in vitro and in vivo, optical control of biomolecular interactions and protein function, the manipulation of lipid membrane properties, and the possibility of guiding cellular behavior

Regulating and Directionally Controlling Electron Emission from Gold Nanorods with Silica Coatings.

Dielectric coatings offer a versatile means of manipulating hot carrier emission from nanoplasmonic systems for emerging nanocatalysis and photocathode applications, with uniform coatings acting as regulators and nonuniform coatings providing directional photocurrent control. However, the mechanisms for electron emission through dense and mesoporous silica (SiO2) coatings require further examination. Here, we present a systematic investigation of photoemission from single gold nanorods as a function of dense versus mesoporous silica coating thicknesses. Studies with dense coatings on gold nanostructures clarify the short (∼1 nm) attenuation length responsible for severely reduced transmission through the silica conduction band. By contrast, mesoporous silica is much more transmissive, and a simple geometric model quantitatively recapitulates the electron escape probability through nanoscopic porous channels. Finally, photoelectron velocity map imaging (VMI) studies of nanorods with coating defects verify that photoemission occurs preferentially through the thinner regions, illustrating new opportunities for designing photocurrent distributions on the nanoscale.

Tuning the Optical Properties of a MoSe2 Monolayer Using Nanoscale Plasmonic Antennas.

Nanoplasmonic systems combined with optically active two-dimensional materials provide intriguing opportunities to explore and control light-matter interactions at extreme subwavelength length scales approaching the exciton Bohr radius. Here, we present room- and cryogenic-temperature investigations of a MoSe2 monolayer on individual gold dipole nanoantennas. By controlling nanoantenna size, the dipolar resonance is tuned relative to the exciton achieving a total tuning of ∼130 meV. Differential reflectance measurements performed on >100 structures reveal an apparent avoided crossing between exciton and dipolar mode and an exciton-plasmon coupling constant of g = 55 meV, representing g/(ℏωX) ≥ 3% of the transition energy. This places our hybrid system in the intermediate-coupling regime where spectra exhibit a characteristic Fano-like shape. We demonstrate active control by varying the polarization of the excitation light to programmably suppress coupling to the dipole mode. We further study the emerging optical signatures of the monolayer localized at dipole nanoantennas at 10 K.

Giant chiroptical response of twisted metal nanorods due to strong plasmon coupling

The chiroptical response of most naturally occurring chiral materials is generally very weak, which discourages further studies and limits applications. Recently, chiral plasmonic structures have received significant attention because of their large chiroptical response. Note that the interaction between the constituents is extremely important and indispensable to the chiroptical response in a chiral plasmonic system. Here, we extend the study of chiroptical response concerning the interaction between the constituents to a strong plasmon coupling regime based on the dimer of twisted metal nanorods. To this end, we realize the fabrication of twisted nanorods with a small gap size of ∼15 nm to ensure that we can observe the separated hybridized modes generated by the plasmon coupling between the twisted nanorods. The dependence of the plasmon coupling and dissymmetry factor on the gap size and twisted angle clearly demonstrates that the plasmon coupling followed by the hybridization strongly enhances the chiroptical response. The dissymmetry factor of the experimentally optimized structures reached values of up to ∼1.03, which is much larger than other nano-plasmonic systems of the same order of dimension. Furthermore, the chiroptical response, as well as the plasmon coupling, is very sensitive to the configuration of the structure, which means that they are precisely controlled by tuning the gap size and twisted angle. Our study that is based on the plasmon coupling provides a further understanding of the study of chiroptical response and will assist in the future design and improvement of chiral plasmonic structures for metamaterial applications.

Single-nanoantenna driven nanoscale control of the VO2 insulator to metal transition

Abstract The ultrafast concentration of electromagnetic energy in nanoscale volumes is one of the key features of optical nanoantennas illuminated at their surface plasmon resonances. Here, we drive the insulator to metal phase transition in vanadium dioxide (VO2) using a laser-induced pumping effect obtained by positioning a single gold nanoantenna in proximity to a VO2 thermochromic material. We explore how the geometry of the single nanoantenna affects the size and permittivity of the nanometer-scale VO2 regions featuring phase transition under different pumping conditions. The results reveal that a higher VO2 phase transition effect is obtained for pumping of the longitudinal or transversal localized surface plasmon depending on the antenna length. This characterization is of paramount importance since the single nanoantennas are the building blocks of many plasmonic nanosystems. Finally, we demonstrate the picosecond dynamics of the VO2 phase transition characterizing this system, useful for the realization of fast nano-switches. Our work shows that it is possible to miniaturize the hybrid plasmonic-VO2 system down to the single-antenna level, still maintaining a controllable behavior, fast picosecond dynamics, and the features characterizing its optical and thermal response.

Broadband Optical Activity Spectroscopy with Interferometric Fourier-Transform Balanced Detection.

Spectrally resolved measurements of optical activity, such as circular dichroism (CD) and optical rotatory dispersion (ORD), are powerful tools to study chiroptical properties of (bio)molecular and nanoplasmonic systems. The wider utilization of these techniques, however, has been impeded by the bulky and slow design of conventional spectropolarimeters, which have been limited to a narrowband scanning approach for more than 50 years. In this work, we demonstrate broadband measurements of optical activity by combining a balanced detection scheme with interferometric Fourier-transform spectroscopy. The setup utilizes a linearly polarized light field that creates an orthogonally polarized weak chiral free-induction-decay field, along with a phase-locked achiral transmitted signal, which serves as the local oscillator for heterodyne amplification. By scanning the delay between the two fields with a birefringent common-path interferometer and recording their interferogram with a balanced detector that measures polarization rotation, broadband CD and ORD spectra are retrieved simultaneously with a Fourier transform. Using an incoherent thermal light source, we achieve state-of-the-art sensitivity for CD and ORD across a broad wavelength range in a remarkably simple setup. We further demonstrate the potential of our technique for highly sensitive measurements of glucose concentration and the real-time monitoring of ground-state chemical reactions. The setup also accepts broadband pulses and will be suitable for broadband transient optical activity spectroscopy and broadband optical activity imaging.

Gain without population inversion and superluminal propagation in the metal nanoparticles-graphene nanodisks-quantum dots hybrid systems

Nanoplasmonic hybrid systems are novel materials due to their optical properties that can be manipulated by sub-wavelength structures of these materials paving the way for interesting optical phenomena such as superluminal propagation. We study the optical properties of metal nanoparticle-graphene nanodisks-quantum dots hybrid system using density matrix approach. The quantum dot is modeled as a three level lambda type configuration interacting with a weak probe field and strong control field. We realize that due to the surface plasmon effects, we can control the optical properties of the quantum dot including absorption, dispersion and group index by the geometrical features of these components in addition to the parameters of the interacting fields. A controllable switching between subliminal and superluminal propagation is observed at certain parametric conditions leading to interesting potential applications such as signal processing and optoelectronic devices.