Optical polarization analogs in inelastic free-electron scattering.

Advances in the ability to manipulate free-electron phase profiles within the electron microscope have spurred development of quantum-mechanical descriptions of electron energy loss (EEL) processes involving transitions between phase-shaped transverse states. Here, we elucidate an underlying connection between two ostensibly distinct optical polarization analogs identified in EEL experiments as manifestations of the same conserved scattering flux. Our work introduces a procedure for probing general tensorial target characteristics including global mode symmetries and local polarization.

A composite electrodynamic mechanism to reconcile spatiotemporally resolved exciton transport in quantum dot superlattices.

Quantum dot (QD) solids are promising optoelectronic materials; further advancing their device functionality requires understanding their energy transport mechanisms. The commonly invoked near-field Förster resonance energy transfer (FRET) theory often underestimates the exciton hopping rate in QD solids, yet no consensus exists on the underlying cause. In response, we use time-resolved ultrafast stimulated emission depletion (STED) microscopy, an ultrafast transformation of STED to spatiotemporally resolve exciton diffusion in tellurium-doped cadmium selenide-core/cadmium sulfide-shell QD superlattices. We measure the concomitant time-resolved exciton energy decay due to excitons sampling a heterogeneous energetic landscape within the superlattice. The heterogeneity is quantified by single-particle emission spectroscopy. This powerful multimodal set of observables provides sufficient constraints on a kinetic Monte Carlo simulation of exciton transport to elucidate a composite transport mechanism that includes both near-field FRET and previously neglected far-field emission/reabsorption contributions. Uncovering this mechanism offers a much-needed unified framework in which to characterize transport in QD solids and additional principles for device design.

Near-field enhancement of optical second harmonic generation in hybrid gold-lithium niobate nanostructures.

Nanophotonics research has focused recently on the ability of nonlinear 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 nonlinear 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 a dispersed layer of ~10 nm diameter gold nanoparticles shows that a ~32-fold enhancement of second harmonic generation can be achieved without introducing finely tailored radiative nanoantennas to mediate photon transfer to or from the nonlinear material. This work highlights the limitations of current strategies for enhancing nonlinear optical phenomena and proposes a route through which a new class of subwavelength nonlinear optical platforms can be designed to maximize nonlinear efficiencies through near-field energy exchange.

Nonlinear effects in single-particle photothermal imaging.

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.

Polarization-Resolved Electron Energy Gain Nanospectroscopy With Phase-Structured Electron Beams.

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.

Nanometer-Scale Spatial and Spectral Mapping of Exciton Polaritons in Structured Plasmonic Cavities.

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.

Active Control of Plasmonic-Photonic Interactions in a Microbubble Cavity.

Active control of light-matter interactions using nanophotonic structures is critical for new modalities for solar energy production, cavity quantum electrodynamics (QED), and sensing, particularly at the single-particle level, where it underpins the creation of tunable nanophotonic networks. Coupled plasmonic-photonic systems show great promise toward these goals because of their subwavelength spatial confinement and ultrahigh-quality factors inherited from their respective components. Here, we present a microfluidic approach using microbubble whispering-gallery mode cavities to actively control plasmonic-photonic interactions at the single-particle level. By changing the solvent in the interior of the microbubble, control can be exerted on the interior dielectric constant and, thus, on the spatial overlap between the photonic and plasmonic modes. Qualitative agreement between experiment and simulation reveals the competing roles mode overlap and mode volume play in altering coupling strengths.

Toward Quantitative Nanothermometry Using Single-Molecule Counting.

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.

Imaging Infrared Plasmon Hybridization in Doped Semiconductor Nanocrystal Dimers.

Carrier-doped semiconductor nanocrystals (NCs) offer strong plasmonic responses at frequencies beyond those accessible by conventional plasmonic nanoparticles. Like their noble metal analogues, these emerging materials can harness free space radiation and confine it to the nanoscale but at resonance frequencies that are natively infrared and spectrally tunable by carrier concentration. In this work we combine monochromated STEM-EELS and theoretical modeling to investigate the capability of colloidal indium tin oxide (ITO) NC pairs to form hybridized plasmon modes, providing an additional route to influence the IR plasmon spectrum. These results demonstrate that ITO NCs may have greater coupling strength than expected, emphasizing their potential for near-field enhancement and resonant energy transfer in the IR region.

Wavelength-Dependent Photothermal Imaging Probes Nanoscale Temperature Differences among Subdiffraction Coupled Plasmonic Nanorods.

Plasmonic structures confine electromagnetic energy at the nanoscale, resulting in local, inhomogeneous, controllable heating, but reading out the temperature 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 asymmetries are interpreted through companion numerical simulations to reveal how thermal gradients within the trimer can be controlled by exciting its hybridized plasmon modes. We also find that plasmon modes that are optically dark can be excited by focused laser beam illumination, providing another route to modify thermal profiles beyond wide-field illumination. Taken together these findings demonstrate an all-optical thermometry technique to actively create and measure nanoscale thermal gradients below the diffraction limit.

High spatial and energy resolution electron energy loss spectroscopy of the magnetic and electric excitations in plasmonic nanorod oligomers.

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.

Intermolecular Hydrogen Bonding Tunes Vibronic Coupling in Heptazine Complexes.

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.

Electron Beam Infrared Nano-Ellipsometry of Individual Indium Tin Oxide Nanocrystals.

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.

Near field excited state imaging via stimulated electron energy gain spectroscopy of localized surface plasmon resonances in plasmonic nanorod antennas.

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.

Far-field midinfrared superresolution imaging and spectroscopy of single high aspect ratio gold nanowires.

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.

Elucidating Energy Pathways through Simultaneous Measurement of Absorption and Transmission in a Coupled Plasmonic-Photonic Cavity.

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.

Direct Observation of Infrared Plasmonic Fano Antiresonances by a Nanoscale Electron Probe.

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 and theoretical modeling, we investigate and characterize a subspace of the weak coupling regime between quasidiscrete and quasicontinuum 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.

Rotation of Single-Molecule Emission Polarization by Plasmonic Nanorods.

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.