Single Plasmonic Nanostructure Research Articles

Sub-wavelength visualization of near-field scattering mode of plasmonic nano-cavity in the far-field

Abstract Spatial visualization of mode distribution of light scattering from plasmonic nanostructures is of vital importance for understanding the scattering mechanism and applications based on these plasmonic nanostructures. A long unanswered question in how the spatial information of scattered light from a single plasmonic nanostructure can be recovered in the far-field, under the constraints of the diffraction limit of the detection or imaging optical system. In this paper, we reported a theoretical model on retrieving local spatial information of scattered light by plasmonic nanostructures in a far-field optical imaging system. In the far-field parametric sin δ images, singularity points corresponding to near-field hot spots of the edge mode and the gap mode were resolved for gold ring and split rings with subwavelength diameters and feature sizes. The experimental results were verified with Finite Difference Time Domain (FDTD) simulation in the near-field and far-field, for the edge mode and the gap mode at 566 nm and 534 nm, respectively. In sin δ image of split-ring, two singularity points associated with near-field hot spots were visualized and resolved with the characteristic size of 90 and 100 nm, which is far below the diffraction limit. The reported results indicate the feasibility of characterizing the spatial distribution of scattering light in the far-field and with sub-wavelength resolution for single plasmonic nanostructures with sub-wavelength feature sizes.

Optically Probing the Chirality of Single Plasmonic Nanostructures and of Single Molecules: Potential and Obstacles.

Circular dichroism (CD) is a standard method for the analysis of biomolecular conformation. It is very reliable when applied to molecules, but requires relatively large amounts of solution. Plasmonics offer the perspective of enhancement of CD signals, which would extend CD spectrometry to smaller amounts of molecules and to weaker chiral signals. However, plasmonic enhancement comes at the cost of additional complications: averaging over all orientations is no longer possible or reliable, linear dichroism leaks into CD signals because of experimental imperfections, scattering and its interference with the incident beam must be taken into account, and the interaction between chiral molecules and possibly chiral plasmonic structures considerably complicates the interpretation of measured signals. This Perspective aims to explore the motivations and problems of plasmonic chirality and to re-evaluate present and future solutions.

A generalized method for calculating plasmoelectric potential in non-Mie-resonant plasmonic systems

Abstract Since its first observation in 2014, plasmoelectric potential (PEP) has drawn a great deal of research interest in all-metal optoelectronics and photochemistry. As an optical thermodynamic phenomenon induced by the electron number dependent equilibrium temperature in plasmonic nanostructures, the early theoretical model developed for calculating PEP is only applicable to Mie-resonant nanostructures, such as a gold nanosphere on a conductive indium tin oxide (ITO) substrate, where the transfer efficiency of hot electrons from gold to ITO can be analytically determined. Without the presence of the substrate, the temperature increase on the gold nanosphere induced by plasmonic absorption was calculated on the basis of thermal radiation in vacuum, which probably over-estimates the actual temperature increase in comparison to realistic experimental conditions. Here, we propose an equilibrium-thermodynamics computational method to quantify the actual efficiency of plasmon-induced electron transfer between a non-Mie-resonant metallic nanostructure and a conductive substrate and hence determine the resultant plasmoelectric potential. With a less than 2.5% relative error in predicting the steady-state temperature of a Mie-resonant nanoparticle in vacuum, and a more strict evaluation of the plasmonic local heating induced temperature increase in a single plasmonic nanostructure or an array of such structures under continuous-wave illumination (CWI), our generalized method provides a robust and accurate approach for quantifying PEP in various plasmonic-particle (array)-on-film nanocavities.

Fano resonance enhanced multiphoton photoemission from single plasmonic nanostructure excited by femtosecond laser

High-flux photoelectron pulses with femto-nano spatiotemporal properties enhanced with plasmonic metallic nanostructures can provide possibilities for potential photoelectron-based devices. In this work, a large multiphoton photoemission enhancement in a single plasmonic Au-heptamer nanostructure, which supports the Fano resonance, is investigated using photoemission electron microscopy (PEEM). By exploiting the high spatial PEEM resolution and performing calculations, it is found that the photoemission electrons are mainly emitted from the junction area between the lower surface of the nanostructure and the indium tin oxide film on the substrate. It is also observed that photoemission is strongly affected by the Coulomb field. Specifically, the strong photoemission locates in the gap region with the Coulomb attraction rather than Coulomb repulsion, even though the latter exhibits a higher charge density. The measured photoelectron enhancement achieved by employing a Fano-resonant Au heptamer is supported by calculations, combining the photoemission effects of both near-field intensity in the lower surface and nanostructure absorption.

Temperature-dependent dark-field scattering of single plasmonic nanocavity

Abstract Plasmonic materials have long been exploited for enhanced spectroscopy, integrated nanophotonic circuits, sensing, light harvesting, etc. Damping is the key factor that limits their performance and restricts the development of the field. Optical characterization of single nanoparticle at low temperature is ideal for investigating the damping of plasmons but is usually technically impractical due to the sample vibration from the cryostat and the surface adsorption during the cooling process. In this work, we use a vibration-free cryostat to investigate the temperature-dependent dark-field scattering spectroscopy of a single Au nanowire on top of a Au film. This allows us to extract the contribution of electron-phonon scattering to the damping of plasmons without performing statistics over different target nanoparticles. The results show that the full width at half-maximum of the plasmon resonance increases by an amount of 5.8%, over the temperature range of 5−150 K. Electromagnetic calculations reveal that the temperature-insensitive dissipation channels into photons or surface plasmon polaritons on the Au film contribute up to 64% of the total dissipations at the plasmon resonance. This explains why the reduction of plasmon linewidth seems small at the single-particle level. This study provides a more explicit measurement on the damping process of the single plasmonic nanostructure, which serves as basic knowledge in the applications of nanoplasmonic materials.

Strong plasmon-exciton coupling between lithographically defined single metal nanoparticles and monolayer WSe2.

Strong coupling between surface plasmons and excitons leads to the formation of plexcitons with new energy states, providing a versatile platform for a range of frontier research subjects. Single plasmonic nanoparticles have recently attracted much attention for realizing strong coupling due to their small mode volume. However, the usually used chemically synthesized metal nanoparticles are unfavorable for accurately tailoring the surface plasmon resonances and adverse to the implementation of on-chip integration. Here, we report for the first time the realization of strong coupling between monolayer WSe2 and lithographically defined single metal nanoparticles. Focusing on gold nanobowties, the large Rabi splitting of 187 meV is achieved. The excitons around the nanogaps in the nanobowties contribute dominantly to the coupling strength, and the coupling strength is larger for smaller nanobowties due to the smaller mode volume. Moreover, the hybrid systems of monolayer WSe2 and gold nanoparticle monomers of nanorods, nanotriangles, and nanodisks are found to closely satisfy the criterion of strong coupling. The strong plasmon-exciton coupling realized by single plasmonic nanostructures fabricated by advanced nanofabrication techniques and monolayer semiconductors can provide new opportunities for manipulating strong light-matter interactions at the nanoscale and facilitate the development of plexciton-based nanodevices with ultrasmall footprints.

Circular Dichroism Measurement of Single Metal Nanoparticles Using Photothermal Imaging.

Circular dichroism (CD) spectroscopy is a powerful optical technique for the study of chiral materials and molecules. It gives access to an enantioselective signal based on the differential absorption of right and left circularly polarized light, usually obtained through polarization analysis of the light transmitted through a sample of interest. CD is routinely used to determine the secondary structure of proteins and their conformational state. However, CD signals are weak, limiting the use of this powerful technique to ensembles of many molecules. Here, we experimentally realize the concept of photothermal circular dichroism, a technique that combines the enantioselective signal from circular dichroism with the high sensitivity of photothermal microscopy, achieving a superior signal-to-noise ratio to detect chiral nano-objects. As a proof of principle, we studied the chiral response of single plasmonic nanostructures with CD in the visible range, demonstrating a signal-to-noise ratio better than 40 with only 30 ms integration time for these nanostructures. The high signal-to-noise ratio allows us to quantify the CD signal for individual nanoparticles. We show that we can distinguish relative absorption differences for right circularly and left circularly polarized light as small as gmin = 4 × 10–3 for a 30 ms integration time with our current experimental settings. The enhanced sensitivity of our technique extends CD studies to individual nano-objects and opens CD spectroscopy to numbers of molecules much lower than those in conventional experiments.

Nonlinear absorption and scattering of a single plasmonic nanostructure characterized by x-scan technique

Nonlinear nanoplasmonics is a largely unexplored research area that paves the way for many exciting applications, such as nanolasers, nanoantennas, and nanomodulators. In the field of nonlinear nanoplasmonics, it is highly desirable to characterize the nonlinearity of the optical absorption and scattering of single nanostructures. Currently, the common method to quantify optical nonlinearity is the z-scan technique, which yields real and imaginary parts of the permittivity by moving a thin sample with a laser beam. However, z-scan typically works with thin films, and thus acquires nonlinear responses from ensembles of nanostructures, not from single ones. In this work, we present an x-scan technique that is based on a confocal laser scanning microscope equipped with forward and backward detectors. The two-channel detection offers the simultaneous quantification for the nonlinear behavior of scattering, absorption and total attenuation by a single nanostructure. At low excitation intensities, both scattering and absorption responses are linear, thus confirming the linearity of the detection system. At high excitation intensities, we found that the nonlinear response can be derived directly from the point spread function of the x-scan images. Exceptionally large nonlinearities of both scattering and absorption are unraveled simultaneously for the first time. The present study not only provides a novel method for characterizing nonlinearity of a single nanostructure, but also reports surprisingly large plasmonic nonlinearities.

Imaging of Plasmonic Chiral Radiative Local Density of States with Cathodoluminescence Nanoscopy.

Chiral light-matter interactions as an emerging aspect of quantum optics enable exceptional physical phenomena and advanced applications in nanophotonics through the nanoscale exploitation of photon-emitter interactions. The chiral radiative properties of quantum emitters strongly depend on the photonic environment, which can be drastically altered by plasmonic nanostructures with a high local density of states (LDOS). Hence, precise knowledge of the chiral photonic environment is essential for manipulating the chirality of light-matter interactions, which requires high resolution chiral characterization techniques. In this work, chiral radiative LDOS distributions of single plasmonic nanostructures that directly govern the chiral radiative spontaneous decay of quantum emitters are imaged at the nanoscale by using cathodoluminescence nanoscopy, enabling precise and highly efficient control of chiral photon emission for chiroptical technologies. Radiative LDOS hot-spots with the chirality larger than 93% are obtained by properly designing chiral plasmonic modes of Au nanoantennas. After fabricating monolayered WSe2 nanodisks (NDs) at chiral radiative LDOS hot-spots and forming ND/Au hybrid nanostructures, the chiral radiative properties of WSe2 NDs are significantly modified, leading to chiral photoluminescence. Our experimental concept and method provide an effective way to characterize and manipulate chiral light-matter interactions at the nanoscale, facilitating future applications in chiral quantum nanophotonics such as single-photon sources and light emission devices.

Quantifying single plasmonic nanostructure far-fields with interferometric and polarimetric k-space microscopy

Optically resonant nanoantennae are key building blocks for metasurfaces, nanosensors, and nanophotonic light sources due to their ability to control the amplitude, phase, directivity, and polarization of scattered light. Here, we report an experimental technique for the full recovery of all degrees of freedom encoded in the far-field radiated by a single nanostructure using a high-NA Fourier microscope equipped with digital off-axis holography. This method enables full decomposition of antenna-physics in its multipole contributions and gives full access to the orbital and spin angular momentum properties of light scattered by single nano-objects. Our results demonstrate these capabilities through a quantitative assessment of the purity of the “selection rules” for orbital angular momentum transfer by plasmonic spiral nanostructures.

Optical properties of refractory metal based thin films

There is a growing interest in refractory metal thin films for a range of emerging nanophotonic applications including high temperature plasmonic structures and infrared superconducting single photon detectors. We present a detailed comparison of optical properties for key representative materials in this class (NbN, NbTiN, TiN and MoSi) with texture varying from crystalline to amorphous. NbN, NbTiN and MoSi have been grown in an ultra-high vacuum sputter deposition system. Two different techniques (sputtering and atomic layer deposition) have been employed to deposit TiN. We have carried out variable angle ellipsometric measurements of optical properties from ultraviolet to mid infrared wavelengths. We compare with high resolution transmission electron microscopy analysis of microstructure. Sputter deposited TiN and MoSi have shown the highest optical absorption in the infrared wavelengths relative to NbN, NbTiN or ALD deposited TiN. We have also modelled the performance of a semi-infinite metal air interface as a plasmonic structure with the above mentioned refractory metal based thin films as the plasmonic components. This study has implications in the design of next generation superconducting nanowire single photon detector or plasmonic nanostructure based devices.

Imaging Catalytic Hotspots on Single Plasmonic Nanostructures via Correlated Super-Resolution and Electron Microscopy.

Surface-plasmon (SP) enhanced catalysis on plasmonic nanostructures brings opportunities to increase catalytic efficiency and alter catalytic selectivity. Understanding the underlying mechanism requires quantitative measurements of catalytic enhancement on these nanostructures, whose intrinsic structural heterogeneity presents experimental challenges. Using correlated super-resolution fluorescence microscopy and electron microscopy, here we report a quantitative visualization of SP-enhanced catalytic activity at the nanoscale within single plasmonic nanostructures. We focus on two Au- and Ag-based linked nanostructures that present plasmonic hotspots at nanoscale gaps. Spatially localized higher reaction rates at these gaps vs nongap regions report the SP-induced catalytic enhancements, which show direct correlations with the nanostructure geometries and local electric field enhancements. Furthermore, the catalytic enhancement scales quadratically with the local actual light intensity, attributable to hot electron involvement in the catalytic enhancement mechanism. These discoveries highlight the effectiveness of correlated super-resolution and electron microscopy in interrogating nanoscale catalytic properties.

Optics and spectroscopy of a single plasmonic nanostructure

We review the interaction of laser light with a single plasmonic nanostructure. Until recently, the extremely weak optical response of a single nanostructure had only allowed spectroscopic research on ensembles of nanoparticles, where both structural and material parameters vary from one nanoparticle to another and the optical response is therefore averaged over the ensemble. Measurements at the level of a single nanostructure provide an effective tool for investigating the fundamental optical and spectroscopic properties of nanostructures, allowing insight into the mechanisms of elementary physical processes and permitting one to avoid the averaging procedure with the inevitable loss of physical information. The diverse range of linear-interaction optical phenomena occurring in a weak light field is reviewed and nonlinear interaction with high-intensity radiation is discussed.

Single plasmonic split hole resonator nanostructure as an efficient nanoscale light source

We report a brief review of the results of experimental studies on the nonlinear optical interaction of laser radiation with a hybrid plasmonic nanostructure, i.e. a split hole resonator (SHR). We demonstrate second- and third-harmonic generation and mixed-frequency generation from a single SHR nanostructure. The practical application potential of an SHR nanostructure as a nanoprobe for measuring the femtosecond laser pulse duration and designing an all-optical nanodisplay is shown.

Оптика и спектроскопия единичной плазмонной наноструктуры

Оптика и спектроскопия единичной плазмонной наноструктуры

Selective turn-on and modulation of resonant energy transfer in single plasmonic hybrid nanostructures.

Förster resonant energy transfer (FRET) is a nonradiative process by which the energy of light absorbed by a donor molecule is transferred to an acceptor molecule over a distance of several nanometers. FRET plays a crucial role in photosynthesis and nature-inspired artificial light-harvesting systems that are being explored for use in energy conversion applications. Localized plasmons of metal nanoparticles can potentially lead to a significant increase of FRET efficiency and effective donor-acceptor distance. Here, we prepare hybrid nanostructures composed of a gold nanorod and donor and acceptor molecules covalently attached to its surface, and study them on the level of a single nanoparticle by simultaneous dark-field scattering, fluorescence imaging and spectroscopy. The single-particle approach enables selective excitation of the longitudinal plasmon of the gold nanorod by polarization of the excitation light. The emission intensity of the acceptor molecules can be controllably and reversibly modulated over a wide range by the polarization angle, thus enabling a selective turn-on of the FRET process and control over the emission color of the hybrid nanostructure. Numerical simulations show that the interactions of the donor and acceptor molecules with the plasmon lead to an increase of the energy transfer efficiency by a factor of ∼65. These findings represent the concept of a novel colour switching approach and could pave the way for innovative applications in optoelectronics and nanophotonics.

Research progress of plasmonic cathodoluminesecence characterization

Surface plasmons as the collective electrons oscillation at the interface of metal and dielectric materials, have induced tremendous applications for the nanoscale light focusing, waveguiding, coupling, and photodetection. As the development of the modern technology, cathodoluminescence (CL) has been successfully applied to describe the plasmon resonance within the nanoscale. Usually, the CL detection system is combined with a high resolution scanning electron microscope (SEM). The fabricated plasmonic nanostructure is directly excited by the electron beam, and detected by an ultra-sensitive spectrometer and photodetector. Under the high energy electron stimulation, all of the plasmon resonances of the metallic nanostructure can be excited. Because of the high spatial resolution of the SEM, the detected CL can be used to analyze the details of plasmon resonance modes. In this review, we first briefly introduced the physical mechanism for the CL generation, and then discussed the CL emission of single plasmonic nanostructures such as different nanowires, nanoantennas, nanodisks and nanocavities, where the CL only describes the individual plasmon resonance modes. Second, the plasmon coupling behavior for the ensemble measurement was compared and analyzed for the CL detection. Finally, the CL detection with other advanced technologies were concluded. We believe with the development of the nanophotonics community, CL detection as a unique technique with ultra-high energy and spatial resolution has potential applications for the future plasmonic structure design and characterization.

Third harmonic generation in the short-wavelength UV range by a single plasmonic nanostructure

The nonlinear optical interaction of laser radiation with nanostructures formed in gold and aluminium nanofilms has been experimentally studied. It is shown that, despite the high susceptibility of aluminium in comparison with gold, the third-harmonic generation efficiency at a wavelength of is much higher for the nanostructures formed in a gold nanofilm because of the efficient excitation of a localised plasmon resonance at the fundamental frequency.

Circular dichroism from single plasmonic nanostructures with extrinsic chirality.

Circular dichroism (CD) studies on single nanostructures can yield novel insights into chiroptical physics that are not available from traditional ensemble-based measurements, yet they are challenging because of their weak signals. By introducing an oblique excitation beam, we demonstrate the observation and spectroscopic analysis of a prominent plasmonic chiroptical response from a single v-shaped gold nanorod dimer nanostructure. We show that circular differential scattering from the obliquely excited gold nanorod dimer yields a characteristic bisignate peak-dip spectral shape at hybridized energies of the dimer. This chiroptical response can be ascribed to extrinsic chirality which depends on the geometry configurations of the chiral arrangement. Due to strong near-field coupling, the dipole orientations of the hybridized resonance modes can be in favor of the incident circularly polarized light where a maximum g-factor of ∼0.4 is observed. Promising applications of this chiroptical arrangement as a key component can be in electronics, photonics, or metamaterials.

Elucidating the localized plasmonic enhancement effects from a single Ag nanowire in organic solar cells.

The origins of performance enhancement in hybrid plasmonic organic photovoltaic devices are often embroiled in a complex interaction of light scattering, localized surface plasmon resonances, exciton-plasmon energy transfer and even nonplasmonic effects. To clearly deconvolve the plasmonic contributions from a single nanostructure, we herein investigate the influence of a single silver nanowire (NW) on the charge carriers in bulk heterojunction polymer solar cells using spatially resolved optical spectroscopy, and correlate to electrical device characterization. Polarization-dependent photocurrent enhancements with a maximum of ∼ 36% over the reference are observed when the transverse mode of the plasmonic excitations in the Ag NW is activated. The ensuing higher absorbance and light scattering induced by the electronic motion perpendicular to the NW long axis lead to increased exciton and polaron densities instead of direct surface plasmon-exciton energy transfer. Finite-difference time-domain simulations also validate these findings. Importantly, our study at the single nanostructure level explores the fundamental limits of plasmonic enhancement achievable in organic solar cells with a single plasmonic nanostructure.