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Photothermal heating of nanoparticles has applications in nanomedicine, photocatalysis, photoelectrochemistry, and data storage, but accurate measurements of temperature at the nanoparticle surface are lacking. Here we demonstrate progress toward a super-resolution DNA nanothermometry technique capable of reporting the surface temperature on single plasmonic nanoparticles. Gold nanoparticles are functionalized with double-stranded DNA, and the extent of DNA denaturation under heating conditions serves as a reporter of temperature. Fluorescently labeled DNA oligomers are used to probe the denatured DNA through transient binding interactions. By counting the number of fluorescent binding events as a function of temperature, we reconstruct DNA melting curves that reproduce trends seen for solution-phase DNA. In addition, we demonstrate our ability to control the temperature of denaturation by changing the Na+ concentration and the base pair length of the double-stranded DNA on the nanoparticle surface. This degree of control allows us to select narrow temperature windows to probe, providing quantitative measurements of temperature at nanoscale surfaces.

Free-electron-based measurements in scanning transmission electron microscopes (STEMs) reveal valuable information on the broadband spectral responses of nanoscale systems with deeply subdiffraction limited spatial resolution. Leveraging recent advances in manipulating the spatial phase profile of the transverse electron wavefront, we theoretically describe interactions between the electron probe and optically stimulated nanophotonic targets in which the probe gains energy while simultaneously transitioning between transverse states with distinct phase profiles. Exploiting the selection rules governing such transitions, we propose phase-shaped electron energy gain nanospectroscopy for probing the 3D polarization-resolved response field of an optically excited target with nanoscale spatial resolution. Considering ongoing instrumental developments, polarized generalizations of STEM electron energy loss and gain measurements hold the potential to become powerful tools for fundamental studies of quantum materials and their interaction with nearby nanostructures supporting localized surface plasmon or phonon polaritons as well as for noninvasive imaging and nanoscale 3D field tomography.

Noble metal nanoparticles host surface plasmon resonances that confine electromagnetic waves below the diffraction limit of light, making them effective building blocks for near-field optical anten...

To better understand how hydrogen bonding influences the excited-state landscapes of aza-aromatic materials, we studied hydrogen-bonded complexes of 2,5,8-tris (4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a molecular photocatalyst related to graphitic carbon nitride, with a variety of phenol derivatives (R-PhOH). By varying the electron-withdrawing character of the para-substituent on the phenol, we can modulate the strength of the hydrogen bond. Using time-resolved photoluminescence, we extract a spectral component associated with the R-PhOH-TAHz hydrogen-bonded complex. Surprisingly, we noticed a striking change in the relative amplitude of vibronic peaks in the TAHz-centered emission as a function of R-group on phenol. To gain a physical understanding of these spectral changes, we employed a displaced-oscillator model of molecular emission to fit these spectra. This fit assumes that two vibrational modes are dominantly coupled to the emissive electronic transition and extracts their frequencies and relative nuclear displacements (related to the Huang-Rhys factor). With the aid of quantum chemical calculations, we found that heptazine ring-breathing and ring-puckering modes are likely responsible for the observed vibronic progression, and both modes indicate decreasing molecular distortion in the excited state with increasing hydrogen bond strength. This finding offers new insights into intermolecular excited-state hydrogen bonding, which is a crucial step toward controlling excited-state proton-coupled electron transfer and proton transfer reactions.

Exciton polaritons (EPs) are ubiquitous light-matter excitations under intense investigation as test beds of fundamental physics and as components for all-optical computing. Owing to their unique attributes and facile experimental tunability, EPs potentially enable strong nonlinearities, condensation, and superfluidity at room temperature. However, the diffraction limit of light and the momentum content of fast electron probes preclude the characterization of EPs in nanoscale structured cavities exhibiting energy-momentum dispersion. Here we present fully relativistic analytical theory and companion numerical simulations showing that these limitations can be overcome to measure EPs in periodic nanophotonic cavities on their natural energy, momentum, and length scales via lattice electron energy gain spectroscopy. With the combined high momentum resolution of light and nanoscale spatial resolution of focused electron beams, lattice electron energy gain spectroscopy can expose deeply subwavelength EP features using currently available monochromated, aberration-corrected scanning transmission electron microscopes.

Understanding resonant coupling in plasmonic nanoassemblies is a challenging scientific endeavor, especially for particles with complex nanoarchitectures. Our ability to both model and measure this...

Nanophotonics research has focused recently on the ability of non-linear optical processes to mediate and transform optical signals in a myriad of novel devices, including optical modulators, transducers, color filters, photodetectors, photon sources, and ultrafast optical switches. The inherent weakness of optical nonlinearities at smaller scales has, however, hindered the realization of efficient miniaturized devices, and strategies for enhancing both device efficiencies and synthesis throughput via nanoengineering remain limited. Here, we demonstrate a novel mechanism by which second harmonic generation, a prototypical non-linear optical phenomenon, from individual lithium niobate particles can be significantly enhanced through nonradiative coupling to the localized surface plasmon resonances of embedded gold nanoparticles. A joint experimental and theoretical investigation of single mesoporous lithium niobate particles coated with aispersed layer of $\sim$10-nm diameter gold nanoparticles shows that a $\sim$32-fold enhancement of second harmonic generation can be achieved without introducing finely tailored radiative nanoantennas to mediate photon transfer to or from the non-linear material. This work highlights the limitations of current strategies for enhancing non-linear optical phenomena and proposes a route through which a new class of subwavelength nonlinear optical platforms can be designed to maximize non-linear efficiencies through near-field energy exchange., Comment: Main text: 12 pages, 4 figures. Supplementary information: 47 pages, 18 figures, 4 tables. Edits, v2: Corrected typos in metadata

Although photothermal imaging was originally designed to detect individual molecules that do not emit or small nanoparticles that do not scatter, the technique is now being applied to image and spectroscopically characterize larger and more sophisticated nanoparticle structures that scatter light strongly. Extending photothermal measurements into this regime, however, requires revisiting fundamental assumptions made in the interpretation of the signal. Herein, we present a theoretical analysis of the wavelength-resolved photothermal image and its extension to the large particle scattering regime, where we find the photothermal signal to inherit a nonlinear dependence upon pump intensity, together with a contraction of the full-width-at-half-maximum of its point spread function. We further analyze theoretically the extent to which photothermal spectra can be interpreted as an absorption spectrum measure, with deviations between the two becoming more prominent with increasing pump intensities. Companion experiments on individual 10, 20, and 100 nm radius gold nanoparticles evidence the predicted nonlinear pump power dependence and image contraction, verifying the theory and demonstrating new aspects of photothermal imaging relevant to a broader class of targets.

The virtual photons that are exchanged when a free-electron vortex beam interacts with a nanoscopic target unlock an explicit connection between polarized optical spectroscopy and the inelastic scattering of scalar electron waves.

In this talk I will discuss recent collaborative work between the University of Washington, Notre Dame, and Oak Ridge National Laboratory in developing a new electron microscopy technique to perform nano-ellipsometry measurements on individual nanoparticles. Focus will be made on inverting the low-loss electron energy-loss spectrum to retrieve the complex-valued and frequency-dependent dielectric functions of a series of individual tin-doped indium oxide nanocrystals with tin doping concentration ranging from 1−10 atomic percent. This method, devoid from ensemble averaging, illustrates the potential for nanoscale electron-beam ellipsometry measurements on materials that cannot be prepared in bulk form or as thin films.

Author(s): Liberko, JJ; Busche, JA; Seils, R; Bechtel, HA; Rack, PD; Masiello, DJ; Camden, JP | Abstract: Spectroscopic interrogation of materials in the midinfrared with nanometer spatial resolution is inherently difficult due to the long wavelengths involved, reduced detector efficiencies, and limited availability of spectrally bright, coherent light sources. Technological advances are driving techniques that overcome these challenges, enabling material characterization in this relatively unexplored spectral regime. Synchrotron infrared nanospectroscopy (SINS) is an imaging technique that provides local sample information of nanoscale target specimens in an experimental energy window between 330 and 5000 cm-1. Using SINS, we analyzed a series of individual gold nanorods patterned on a SiO2 substrate and on a flake of hexagonal boron nitride. The SINS spectra reveal interactions between the nanorod photonic Fabry-Perot resonances and the surface phonon polaritons of each substrate, which are characterized as avoided crossings. A coupled oscillator model of the hybrid system provides a deeper understanding of the coupling and provides a theoretical framework for future exploration.

Analog quantum simulation - the technique of using one experimentally well-controlled physical system to mimic the behavior of another - has quickly emerged as one of the most promising near term strategies for studying strongly correlated quantum many-body systems. In particular, systems of interacting photons, realizable in solid-state cavity and circuit QED frameworks, for example, hold tremendous promise for the study of nonequilibrium many-body phenomena due to the capability to locally create and destroy photons. These systems are typically modeled using a Jaynes-Cummings-Hubbard (JCH) Hamiltonian, named due to similarities with the Bose-Hubbard (BH) model. Here, we present a non-perturbative procedure for transforming the JC Hamiltonian into a dressed operator representation that, in its most general form, admits an infinite sum of bosonic $k$-body terms where $k$ is bound only by the number of excitations in the system. We closely examine this result in both the dispersive and resonant coupling regimes, finding rapid convergence in the former and contributions from $k\gg1$ in the latter. Through extension to a two-site JCH system, we demonstrate that this approach facilitates close inspection of the analogy between the JCH and BH models and its breakdown for resonant light-matter coupling. Finally, we use this framework to survey the many-body character of a two-site JCH for general system parameters, identifying four unique quantum phases and the parameter regimes in which they are realized, thus highlighting phenomena realizable with finite JCH-based quantum simulators beyond the BH model. More broadly, this work is intended to serve as a clear mathematical exposition of bosonic many-body interactions underlying JC-type systems, often postulated through analogy to Kerr-like nonlinear susceptibilities or by matching coefficients to obtain the appropriate eigenvalue spectrum., Comment: 25 pages, 6 figures

While many plasmonic phenomena have been realized by using standard nanoscale synthesis in a single 2-dimensional plane, enhanced functionality should be possible by extending into the third dimens...

The unique optical properties of surface plasmon resonances in nanostructured materials have attracted considerable attention, broadly impacting both fundamental research and applied technologies r...

Control of light-matter interactions is central to numerous advances in quantum communication, information, and sensing. The relative ease with which interactions can be tailored in coupled plasmonic-photonic systems makes them ideal candidates for investigation. To exert control over the interaction between photons and plasmons, it is essential to identify the underlying energy pathways which influence the system's dynamics and determine the critical system parameters, such as the coupling strength and dissipation rates. However, in coupled systems which dissipate energy through multiple competing pathways, simultaneously resolving all parameters from a single experiment is challenging as typical observables such as absorption and scattering each probe only a particular path. In this work, we simultaneously measure both photothermal absorption and two-sided optical transmission in a coupled plasmonic-photonic resonator consisting of plasmonic gold nanorods deposited on a toroidal whispering-gallery-mode optical microresonator. We then present an analytical model which predicts and explains the distinct line shapes observed and quantifies the contribution of each system parameter. By combining this model with experiment, we extract all system parameters with a dynamic range spanning 9 orders of magnitude. Our combined approach provides a full description of plasmonic-photonic energy dynamics in a weakly coupled optical system, a necessary step for future applications that rely on tunability of dissipation and coupling.

The ability to control and manipulate temperature at nanoscale dimensions has the potential to impact applications including heat-assisted magnetic recording, photothermal therapies, and temperature-driven reactivity. One challenge with controlling temperature at nanometer dimensions is the need to mitigate heat diffusion, such that the temperature only changes in well-defined nanoscopic regions of the sample. Here we demonstrate the ability to use far-field laser excitation to actively shape the thermal near-field in individual gold nanorod heterodimers by resonantly pumping either the in-phase or out-of-phase hybridized dipole plasmon modes. Using single-particle photothermal heterodyne imaging, we demonstrate localization bias in the photothermal intensity due to preferential heating of one of the nanorods within the pair. Theoretical modeling and numerical simulation make explicit how the resulting photothermal images encode wavelength-dependent temperature biases between each nanorod within a heterodimer, demonstrating the ability to actively manage the thermal near-field by simply tuning the color of incident light.

Refinements in energy monochromation and aberration correction in state-of-the-art scanning transmission electron microscopes has opened access to the far infrared regime for spectroscopic characterization at the nanoscale. At these low energies, the dielectric environment, such as a dielectric slab adjacent to the target specimen, may no longer play a passive role in the spectrum. Instead, the environment may itself host resonances that mix with those of the target and complicate interpretation of its spectral responses. This paper explores a theoretical description of the coupling between the collective vibrational surface modes of a dielectric particle and dielectric slab of varying thickness for the purpose of elucidating the interacting phononic excitations in dielectric materials typical of inelastic electron scattering measurements in the infrared. Dynamical coordinates and a governing Hamiltonian are rigorously defined in the quasistatic limit to account for phonon mode mixing and forcing by an aloof electron probe, which travels along a grazing trajectory, parallel to the dielectric slab. As the spectral window of interrogation by fast electron probes has been extended down to thermal energies with unprecedented meV energy resolution, theoretical models like that presented herein are crucial for accurate interpretation of experimental data.

While the thermal and electromagnetic properties of plasmonic nanostructures are well understood, nanoscale thermometry still presents an experimental and theoretical challenge. Plasmonic structures can confine electromagnetic energy at the nanoscale, resulting in local, inhomogeneous, controllable heating. But reading out the temperature with nanoscale precision using optical techniques poses a difficult challenge. Here we report on the optical thermometry of individual gold nanorod trimers that exhibit multiple wavelength-dependent plasmon modes resulting in measurably different local temperature distributions. Specifically, we demonstrate how photothermal microscopy encodes different wavelength-dependent temperature profiles in the asymmetry of the photothermal image point spread function. These point spread function asymmetries are interpreted through companion numerical simulations of the photothermal images to reveal how differing thermal gradients within the nanorod trimer can be controlled by exciting its hybridized plasmonic modes. We also find that hybrid plasmon modes that are optically dark can be excited by our focused laser beam illumination geometry at certain beam positions, thereby providing an additional route to modify thermal profiles at the nanoscale beyond wide-field illumination. Taken together these findings demonstrate an all-optical thermometry technique to actively create and measure thermal gradients at the nanoscale below the diffraction limit.

Leveraging recent advances in electron energy monochromation and aberration correction, we record the spatially resolved infrared plasmon spectrum of individual tin-doped indium oxide nanocrystals using electron energy-loss spectroscopy (EELS). Both surface and bulk plasmon responses are measured as a function of tin doping concentration from 1-10 atomic percent. These results are compared to theoretical models, which elucidate the spectral detuning of the same surface plasmon resonance feature when measured from aloof and penetrating probe geometries. We additionally demonstrate a unique approach to retrieving the fundamental dielectric parameters of individual semiconductor nanocrystals via EELS. This method, devoid from ensemble averaging, illustrates the potential for electron-beam ellipsometry measurements on materials that cannot be prepared in bulk form or as thin films.

Continuous wave (cw) photon stimulated electron energy loss and gain spectroscopy (sEELS and sEEGS) is used to image the near field of optically stimulated localized surface plasmon resonance (LSPR) modes in nanorod antennas. An optical delivery system equipped with a nanomanipulator and a fiber-coupled laser diode is used to simultaneously irradiate plasmonic nanostructures in a (scanning) transmission electron microscope. The nanorod length is varied such that the m = 1, 2, and 3 LSPR modes are resonant with the laser energy and the optically stimulated near field spectra and images of these modes are measured. Various nanorod orientations are also investigated to explore retardation effects. Optical and electron beam simulations are used to rationalize the observed patterns. As expected, the odd modes are optically bright and result in observed sEEG responses. The m = 2 dark mode does not produce a sEEG response, however, when tilted such that retardation effects are operative, the sEEG signal emerges. Thus, we demonstrate that cw sEEGS is an effective tool in imaging the near field of the full set of nanorod plasmon modes of either parity.

Materials possessing strong midinfrared responses are of current interest because of their potential application to long-wavelength metamaterials, photonic devices, molecular detection, and catalysis. Here, we utilize high-energy resolution (80 ${\mathrm{cm}}^{\ensuremath{-}1}$, 10 meV) electron-energy-loss spectroscopy (EELS) in a monochromated and aberration-corrected scanning transmission electron microscope (STEM) to resolve multipolar surface plasmon resonances (SPRs), sometimes called Fabry-P\'erot (FP) resonances, in gold nanowires with mode energies spanning from $\ensuremath{\sim}1000$ to $8000\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. STEM-EELS provides access to these mid- to near-IR responses in a single acquisition, avoiding the difficulties inherent in obtaining the same data using near-field optical techniques. The experimentally measured FP resonance energies and linewidths, together with analytical modeling and full-wave numerical electrodynamics simulations, provide a comprehensive picture of the radiative and intrinsic contributions to the total damping rates. We find some FP modes with dephasing times $g60\phantom{\rule{0.16em}{0ex}}\mathrm{fs}$, which is almost twice the longest previously reported plasmon dephasing time for individual Au nanoparticles in the infrared. The long dephasing times and the broad tunability of the FP resonance energies throughout the infrared region suggest additional opportunities for harnessing infrared plasmonic energy before dephasing occurs.

Limited approaches exist for imaging and recording spectra of individual nanostructures in the midinfrared region. Here we use infrared photothermal heterodyne imaging (IR-PHI) to interrogate single, high aspect ratio Au nanowires (NWs). Spectra recorded between 2,800 and 4,000 cm(−1) for 2.5–3.9-μm-long NWs reveal a series of resonances due to the Fabry–Pérot modes of the NWs. Crucially, IR-PHI images show structure that reflects the spatial distribution of the NW absorption, and allow the resonances to be assigned to the m = 3 and m = 4 Fabry–Pérot modes. This far-field optical measurement has been used to image the mode structure of plasmon resonances in metal nanostructures, and is made possible by the superresolution capabilities of IR-PHI. The linewidths in the NW spectra range from 35 to 75 meV and, in several cases, are significantly below the limiting values predicted by the bulk Au Drude damping parameter. These linewidths imply long dephasing times, and are attributed to reduction in both radiation damping and resistive heating effects in the NWs. Compared to previous imaging studies of NW Fabry–Pérot modes using electron microscopy or near-field optical scanning techniques, IR-PHI experiments are performed under ambient conditions, enabling detailed studies of how the environment affects mid-IR plasmons.

Experimental characterization of the optical and magnetic properties of noble metal nanoclusters has exposed a size regime in between single-ring and extended two-dimensional nanocluster networks where the behavior of magnetic plasmon resonances is not well understood. In this intermediate size regime, individual electric dipole plasmons on each nanoparticle within the cluster can hybridize into delocalized magnetic modes that couple to either the electric or magnetic field of light with seemingly arbitrary energy ordering. Here, using a coupled-dipole model that includes fully retarded interactions between electric dipole plasmons, we show that magnetic plasmon resonance energies can be controllably tuned and even made to cross as a function of nanocluster size. Experimental confirmation of this prediction is challenging because optical selection rules dictate the simultaneous excitation of many spectrally overlapping magnetic plasmon modes. However, based on analytic modeling and numerical simulation, w...

Mislocalization is a quantitative measure of the inability to locate the positions of individual molecular emitters in plasmon-enhanced super-resolution fluorescence microscopy. It is due to an unfortunate side-effect that scrambles the spatial profile of a molecule’s fluorescence signal when plasmonic nanoantennas are introduced to boost that signal. In this article, we present an understanding of the mislocalization problem in plasmon-enhanced super-resolution fluorescence microscopy based upon a simple and intuitive theoretical model. In particular, we derive an analytic expression for mislocalization and demonstrate explicitly how it depends upon both the macroscopic interference of the coherent emission from molecular and plasmonic emitters and the microscopic dynamics of the coupled system. To derive this expression, we draw upon an analogy to the Fano interference problem and show that the spatial asymmetry in the intensity profile can be encapsulated into a single effective parameter that depends ...

Facile control of the radiative and nonradiative properties of plasmonic nanostructures is of practical importance to a wide range of applications in the biological, chemical, optical, information, and energy sciences. For example, the ability to easily tune not only the plasmon spectrum but also the degree of coupling to light and/or heat, quality factor, and optical mode volume would aid the performance and function of nanophotonic devices and molecular sensors that rely upon plasmonic elements to confine and manipulate light at nanoscopic dimensions. While many routes exist to tune these properties, identifying new approaches-especially when they are simple to apply experimentally-is an important task. Here, we demonstrate the significant and underappreciated effects that substrate thickness and dielectric composition can have upon plasmon hybridization as well as downstream properties that depend upon this hybridization. We find that even substrates as thin as ∼10 nm can nontrivially mix free-space plasmon modes, imparting bright character to those that are dark (and vice versa) and, thereby, modifying the plasmonic density of states as well as the system's near- and far-field optical properties. A combination of electron energy-loss spectroscopy (EELS) experiment, numerical simulation, and analytical modeling is used to elucidate this behavior in the finite substrate-induced mixing of dipole, quadrupole, and octupole corner-localized plasmon resonances of individual silver nanocubes.

Hybrid photonic-plasmonic systems have tremendous potential as versatile platforms for the study and control of nanoscale light-matter interactions since their respective components have either high-quality factors or low mode volumes. Individual metallic nanoparticles deposited on optical microresonators provide an excellent example where ultrahigh-quality optical whispering-gallery modes can be combined with nanoscopic plasmonic mode volumes to maximize the system's photonic performance. Such optimization, however, is difficult in practice because of the inability to easily measure and tune critical system parameters. In this Letter, we present a general and practical method to determine the coupling strength and tailor the degree of hybridization in composite optical microresonator-plasmonic nanoparticle systems based on experimentally measured absorption spectra. Specifically, we use thermal annealing to control the detuning between a metal nanoparticle's localized surface plasmon resonance and the whispering-gallery modes of an optical microresonator cavity. We demonstrate the ability to sculpt Fano resonance lineshapes in the absorption spectrum and infer system parameters critical to elucidating the underlying photonic-plasmonic hybridization. We show that including decoherence processes is necessary to capture the evolution of the lineshapes. As a result, thermal annealing allows us to directly tune the degree of hybridization and various hybrid mode quantities such as the quality factor and mode volume and ultimately maximize the Purcell factor to be 10

In this Letter, we exploit recent breakthroughs in monochromated aberration-corrected scanning transmission electron microscopy (STEM) to resolve infrared plasmonic Fano antiresonances in individual nanofabricated disk-rod dimers. Using a combination of electron energy-loss spectroscopy (EELS) and theoretical modeling, we investigate and characterize a subspace of the weak coupling regime between quasi-discrete and quasi-continuum localized surface plasmon resonances where infrared plasmonic Fano antiresonances appear. This work illustrates the capability of STEM instrumentation to experimentally observe nanoscale plasmonic responses that were previously the domain only of higher resolution infrared spectroscopies.

The strong light-matter interactions between dyes and plasmonic nanoantennas enable the study of fundamental molecular-optical processes. Here, we overcome conventional limitations with high-throughput single-molecule polarization-resolved microscopy to measure dye emission polarization modifications upon near-field coupling to a gold nanorod. We determine that the emission polarization distribution is not only rotated toward the nanorod's dominant localized surface plasmon mode as expected, but it is also unintuitively broadened. With a reduced-order analytical model, we elucidate how this distribution broadening depends upon both far-field interference and off-resonant coupling between the molecular dipole and the nanorod transverse plasmon mode. Experiments and modeling reveal that a nearby plasmonic nanoantenna affects dye emission polarization through a multicolor process, even when the orthogonal plasmon modes are separated by approximately 3 times the dye emission line width. Beyond advancing our understanding of plasmon-coupled emission modifications, this work promises to improve high-sensitivity single-molecule fluorescence imaging, biosensing, and spectral engineering.

The ability to control and manipulate temperature at nanoscale dimensions has the potential to impact applications including heat-assisted magnetic recording, photothermal therapies, and temperature-driven reactivity. One challenge with controlling temperature at nanometer dimensions is the need to mitigate heat diffusion, such that the temperature only changes in well-defined nanoscopic regions of the sample. Here we demonstrate the ability to use far-field laser excitation to actively shape the thermal near-field in individual gold nanorod heterodimers by resonantly pumping either the in-phase or out-of-phase hybridized dipole plasmon modes. Using single-particle photothermal heterodyne imaging, we demonstrate localization bias in the photothermal intensity due to preferential heating of one of the nanorods within the pair. Theoretical modeling and numerical simulation make explicit how the resulting photothermal images encode wavelength-dependent temperature biases between each nanorod within a heterodimer, demonstrating the ability to actively manage the thermal near-field by simply tuning the color of incident light.

Engineering dielectric nanoparticles to select and manipulate light at specific resonance energies has great potential in fields such as solar energy conversion and optical communication. To accomplish this challenging task, it is necessary to develop tools to probe the optical resonance behavior of nanostructure locally. This work shows that focused electron beams and monochromated electron energy-loss spectroscopy provide a unique way for studying the optical resonance properties of dielectric structures with nanometer-scale spatial resolution. Resonant optical-frequency geometric modes excited by fast electrons show up as a series of pronounced peaks within the band gap of the spectra, where absorption is negligible.

Single-molecule fluorescence microscopy is used to follow dynamic ligand reorganization on the surface of single plasmonic gold nanorods. Fluorescently labeled DNA is attached to gold nanorods via a gold-thiol bond using a low-pH loading method. No fluorescence activity is initially observed from the fluorescent labels on the nanorod surface, which we attribute to a collapsed geometry of DNA on the metal. Upon several minutes of laser illumination, a marked increase in fluorescence activity is observed, suggesting that the ligand shell reorganizes from a collapsed, quenched geometry to an upright, ordered geometry. The ligand reorganization is facilitated by plasmon-mediated photothermal heating, as verified by controls using an external heat source and simulated by coupled optical and heat diffusion modeling. Using super-resolution image reconstruction, we observe spatial variations in which ligand reorganization occurs at the single-particle level. The results suggest the possibility of nonuniform plasmonic heating, which would be hidden with traditional ensemble-averaged measurements.

Macroscopic, many-body self-trapped and quantum superposition states of the gaseous double-well Bose-Einstein condensate (BEC) are investigated within the context of a multiconfigurational bosonic self-consistent field theory based upon underlying spatially symmetry-broken one-body wave functions. To aid in the interpretation of our results, an approximate model is constructed in the extreme Fock state limit, in which self-trapped and superposition states emerge in the many-body spectrum, striking a delicate balance between the degree of symmetry breaking, the effects of the condensate's mean field, and that of atomic correlation. It is found, in both the model and full theory, that the superposition state lies energetically below its related self-trapped counterpart even when many configurations are involved. Noticeably different spatial density profiles are associated with each type of excited state, thus providing a rigorous justification for approximate descriptions of high-lying excited states of the BEC.

From fundamental discovery to practical application, advances in the optical and quantum sciences rely upon precise control of light-matter interactions. Systems of coupled optical cavities are ubiquitous in these efforts, yet the design and active modification of the hybridized mode properties remains challenging. Here, we demonstrate the design, fabrication, and analysis of a tunable heterogeneous photonic molecule consisting of a ring resonator strongly coupled to a nanobeam photonic crystal cavity. Leveraging the disparity in mode volume between these two strongly coupled cavities, we combine theory and experiment to establish the ability to actively tune the mode volume of the resulting supermodes over a full order of magnitude. As the mode volume determines the strength of light-matter interactions, this work illustrates the potential for strongly coupled cavities with dissimilar mode volumes in applications requiring designer photonic properties and tunable light-matter coupling, such as photonics-based quantum simulation.

We leverage the high spatial and energy resolution of monochromated aberration-corrected scanning transmission electron microscopy to study the hybridization of cyclic assemblies of plasmonic gold nanorods. Detailed experiments and simulations elucidate the hybridization of the coupled long-axis dipole modes into collective magnetic and electric dipole plasmon resonances. We resolve the magnetic dipole mode in these closed loop oligomers with electron energy loss spectroscopy and confirm the mode assignment with its characteristic spectrum image. The energy splitting of the magnetic mode and antibonding modes increases with the number of polygon edges (n). For the n=3-6 oligomers studied, optical simulations using normal incidence and s-polarized oblique incidence show the respective electric and magnetic modes’ extinction efficiencies are maximized in the n=4 arrangement.

Negative-index metamaterials composed of magnetic plasmon oligomers are actively being investigated for their potential role in optical cloaking, superlensing, and nanolithography applications. A significant improvement to their practicality lies in the ability to function at multiple distinct wavelengths in the visible part of spectrum. Here we utilize the nanometer spatial-resolving power of electron energy-loss spectroscopy to conclusively demonstrate hybridization of magnetic plasmons in oligomer dimers that can achieve this goal. We also show that breaking the dimer's symmetry can induce all-magnetic Fano interferences based solely on the interplay of bright and dark magnetic modes, allowing us to further tailor the system's optical responses. These features are engineered through the design of the oligomer's underlying nanoparticle elements as elongated Ag nanodisks with spectrally isolated long-axis plasmon resonances. The resulting magnetic plasmon oligomers and their hybridized assemblies establish a new design paradigm for optical metamaterials with rich functionality.

Electron energy-loss spectroscopy (EELS) offers a window to view nanoscale properties and processes. When performed in a scanning transmission electron microscope, EELS can simultaneously render images of nanoscale objects with sub-nanometer spatial resolution and correlate them with spectroscopic information of $\sim10 - 100$ meV spectral resolution. Consequently, EELS is a near-perfect tool for understanding the optical and electronic properties of individual and few-particle plasmonic metal nanoparticles assemblies, which are significant in a wide range of fields. This review presents an overview of basic plasmonics and EELS theory and highlights several recent noteworthy experiments involving the electron-beam interrogation of plasmonic metal nanoparticle systems., When citing this paper, please use the following: Cherqui C, Thakkar N, Li G, Camden JP, Masiello DJ. 2015. Characterizing localized surface plasmons using electron energy-loss spectroscopy. Annu. Rev. Phys. Chem. 67: Submitted. Doi: 10.1146/annurev-physchem-040214-121612

Electron energy-loss spectroscopy (EELS) provides detailed nanoscopic spatial and spectral information on plasmonic nanoparticles that cannot be discerned with far-field optical techniques. Here we demonstrate that EELS is capable of mapping the relative phases of individual localized surface plasmons that are hybridized within nanoparticle assemblies. Within the context of an effective plasmon oscillator model, we demonstrate the relationship between the self-induced back-force on the electron due to the plasmon and the EEL probability and use this to present a rubric for determining the relative phases of hybridized localized surface plasmons in EELS. Comparison between the analytical oscillator model, experiment, and numerical electrodynamics simulation is made across a variety of nanoparticle monomer, dimer, and trimer systems.

We report the self-assembly of ultrasmooth AuxAg1–x nanoparticles with homogeneous composition via pulsed laser-induced dewetting (PLiD). The nanoparticles are truncated nanospheres that sustain unique plasmonic features. For the first time an electron energy loss spectroscopy (EELS) study elucidating the size and composition effects on the plasmonic modes of truncated AuxAg1–x nanospheres is carried out. EELS characterization captures a linear red-shift in both bright and dark modes as a function of the atomic fraction of Au and a progressive red-shift of all modes as the size increases. The results are interpreted in the context of Mie theory and electron beam simulations. Armed with the full plasmonic spectrum of the AuxAg1–x system, the truncated spheres and their ordered arrays synthesized via PLiD have promise as elements in advanced photonic devices.

Optical manipulation of charge on the nanoscale is of fundamental importance to an array of proposed technologies from selective photocatalysis to nanophotonics. Open plasmonic systems where collective electron oscillations release energy and charge to their environments offer a potential means to this end as plasmons can rapidly decay into energetic electron-hole pairs; however, isolating this decay from other plasmon-environment interactions remains a challenge. Here we present an analytic theory of noble-metal nanoparticles that quantitatively models plasmon decay into electron-hole pairs, demonstrates that this decay depends significantly on the nanoparticle's dielectric environment, and disentangles this effect from competing decay pathways. Using our approach to incorporate embedding material and substrate effects on plasmon-electron interaction, we show that predictions from the model agree with four separate experiments. Finally, examination of coupled nanoparticle-emitter systems further shows that the hybridized in-phase mode more efficiently decays to photons whereas the out-of-phase mode more efficiently decays to electron-hole pairs, offering a strategy to tailor open plasmonic systems for charge manipulation.