Electrically Tunable Single Polaritonic Quantum Dot at Room Temperature.

Exciton-polaritons confined in plasmonic cavities are hybridized light-matter quasiparticles, with distinct optical characteristics compared to plasmons and excitons alone. Here, we demonstrate the electric tunability of a single polaritonic quantum dot operating at room temperature in electric-field tip-enhanced strong coupling spectroscopy. For a single quantum dot in the nanoplasmonic tip cavity with variable dc local electric field, we dynamically control the Rabi frequency with the corresponding polariton emission, crossing weak to strong coupling. We model the observed behaviors based on the quantum confined Stark effect in the strong coupling regime.

Geometry-Independent Ultrafast Energy Transfer in Bioinspired Arrays Containing Electronically Coupled BODIPY Dimers as Energy Donors.

In photosynthetic light-harvesting complexes, strong interaction between chromophores enables efficient absorption of solar radiation and has been suggested to enable ultrafast energy funneling to the reaction center. To examine whether similar effects can be realized in synthetic systems, and to determine the mechanisms of energy transfer, we synthesized and characterized a series of bioinspired arrays containing strongly-coupled BODIPY dimers as energy donors and chlorin derivatives as energy acceptors. The BODIPY dimers feature broad absorption in the range of 500-600 nm, complementing the chlorin absorption to provide absorption across the entire visible spectrum. Ultrafast (~10 ps) energy transfer was observed from photoexcited BODIPY dyads to chlorin subunits. Surprisingly, the energy-transfer rate is nearly independent of the position where the BODIPY dimer is attached to the chlorin and of the type of connecting linker. In addition, the energy-transfer rate from BODIPY dimers to chlorin is slower than the corresponding rate in arrays containing BODIPY monomers. The lower rate, corresponding to less efficient through-bond transfer, is most likely due to weaker electronic coupling between the ground state of the chlorin acceptor and the delocalized electronic state of the BODIPY dimer, compared to the localized state of a BODIPY monomer.

Associations of small vessel disease and acute symptomatic seizures in ischemic stroke patients.

BACKGROUND AND PURPOSE: Cerebral microbleeds (CMBs), markers of small vessel disease are frequent in ischemic stroke, yet the association with acute symptomatic seizures (ASS) has not been well characterized. METHODS: A retrospective cohort of hospitalized patients with anterior circulation ischemic stroke. The association of CMBs with acute symptomatic seizures was assessed using a logistic regression model and causal mediation analysis. RESULTS: Of 381 patients, 17 developed seizures. Compared with patients without CMBs, those with CMBs had a three-fold higher unadjusted odds of seizures (unadjusted OR: 3.84, 95% 1.16-12.71, p = 0.027). After adjusting for confounders such as stroke severity, cortical infarct location, and hemorrhagic transformation, the association between CMBs and ASS was attenuated (adjusted OR: 3.11, 95%CI: 0.74-11.03, p = 0.09). The association was not mediated by stroke severity. CONCLUSION: In this cohort of hospitalized patients with anterior circulation ischemic stroke, CMBs were more likely to be found in patients with ASS than those without ASS, an association that was attenuated when accounting for stroke severity, cortical infarct location, and hemorrhagic transformation. Evaluation of the long-term risk of seizures associated with CMBs and other markers of small vessel disease is warranted.

Engineering giant excitonic coupling in bioinspired, covalently bridged BODIPY dyads.

Strong excitonic coupling in photosynthetic systems is believed to enable efficient light absorption and quantitative charge separation, motivating the development of artificial multi-chromophore arrays with equally strong or even stronger excitonic coupling. However, large excitonic coupling strengths have typically been accompanied by fast non-radiative recombination, limiting the potential of the arrays for solar energy conversion as well as other applications such as fluorescent labeling. Here, we report giant excitonic coupling leading to broad optical absorption in bioinspired BODIPY dyads that have high photostability, excited-state lifetimes at the nanosecond scale, and fluorescence quantum yields of nearly 50%. Through the synthesis, spectroscopic characterization, and computational modeling of a series of dyads with different linking moieties, we show that the strongest coupling is obtained with diethynylmaleimide linkers, for which the coupling occurs through space between BODIPY units with small separations and slipped co-facial orientations. Other linkers allow for broad tuning of both the relative through-bond and through-space coupling contributions and the overall strength of interpigment coupling, with a tradeoff observed in general between the strength of the two coupling mechanisms. These findings open the door to the synthesis of molecular systems that function effectively as light-harvesting antennas and as electron donors or acceptors for solar energy conversion.

Low-Frequency Oscillations in Optical Measurements of Metal-Nanoparticle Vibrations.

Time-resolved optical measurements of vibrating metal nanoparticles have been used extensively to probe the ultrafast mechanical properties of the nanoparticles and of the surrounding liquid, but nearly all investigations so far have been limited to the linear regime. Here, we report the observation of a low-frequency oscillating signal in transient-absorption measurements of nanoparticles with octahedral gold cores and cubic silver shells; the signal appears at the difference of two mechanical vibrational frequencies in the particles, suggesting a nonlinear mixing process. We tentatively attribute this proposed mixing to a nonlinear coupling between a vibrational mode of the nanoparticle and its optical-frequency plasmon resonance. The optimization of this nonlinear transduction may enable high-efficiency opto-mechanical frequency mixing in the GHz-THz frequency regime.

Optical measurement of the picosecond fluid mechanics in simple liquids generated by vibrating nanoparticles: a review.

Standard continuum assumptions commonly used to describe the fluid mechanics of simple liquids have the potential to break down when considering flows at the nanometer scale. Two common assumptions for simple molecular liquids are that (1) they exhibit a Newtonian response, where the viscosity uniquely specifies the linear relationship between the stress and strain rate, and (2) the liquid moves in tandem with the solid at any solid-liquid interface, known as the no-slip condition. However, even simple molecular liquids can exhibit a non-Newtonian, viscoelastic response at the picosecond time scales that are characteristic of the motion of many nanoscale objects; this viscoelasticity arises because these time scales can be comparable to those of molecular relaxation in the liquid. In addition, even liquids that wet solid surfaces can exhibit nanometer-scale slip at those surfaces. It has recently become possible to interrogate the viscoelastic response of simple liquids and associated nanoscale slip using optical measurements of the mechanical vibrations of metal nanoparticles. Plasmon resonances in metal nanoparticles provide strong optical signals that can be accessed by several spectroscopies, most notably ultrafast transient-absorption spectroscopy. These spectroscopies have been used to measure the frequency and damping rate of acoustic oscillations in the nanoparticles, providing quantitative information about mechanical coupling and exchange of mechanical energy between the solid particle and its surrounding liquid. This information, in turn, has been used to elucidate the rheology of viscoelastic simple liquids at the nanoscale in terms of their constitutive relations, taking into account separate viscoelastic responses for both shear and compressible flows. The nanoparticle vibrations have also been used to provide quantitative measurements of slip lengths on the single-nanometer scale. Viscoelasticity has been shown to amplify nanoscale slip, illustrating the interplay between different aspects of the unconventional fluid dynamics of simple liquids at nanometer length scales and picosecond time scales.

COVID-19 Vaccine Hesitancy in Patients with Inflammatory Bowel Disease.

BACKGROUND AND AIMS: COVID-19 vaccine hesitancy varies across the USA. Data on COVID-19 vaccine hesitancy in patients with inflammatory bowel disease (IBD) are lacking. We assessed COVID-19 vaccine hesitancy and its associated variables in patients with IBD. METHODS: We evaluated voluntary patient survey responses during routine clinical visits to our IBD center. Data collected included demographic and clinical characteristics. Descriptive statistics, univariate and multivariate analyses were performed to evaluate significant associations with COVID-19 vaccine hesitancy. RESULTS: A total of 239 individuals completed the survey. Over a third of respondents (35.6%) expressed hesitancy toward receiving the COVID-19 vaccine due to vaccine safety concerns (49.4%) and efficacy (23.5%), while others reported non-specific concerns (34.1%). On univariate analysis, Crohn's disease (OR 2.33 CI 1.28-4.25 p = 0.0056), use of biologic medications (OR 1.93 CI 1.16-3.23, p = 0.012), previous self-reported vaccine refusal (OR 8.13 CI 2.90-22.82 p = 0.0001), earlier date of survey administration (OR 2.01 CI 1.17-3.44 p = 0.011), and self-reported COVID infection (OR 2.55 CI 1.16-5.61 p = 0.0056) were more likely to be associated with COVID-19 vaccine hesitancy. On multivariate analysis, patient age, previous vaccine refusal and date of survey administration were more likely to be associated with COVID-19 vaccine hesitancy. CONCLUSIONS: Over one-third of patients with IBD expressed COVID-19 vaccine hesitancy. Vaccine safety and efficacy were the most common reasons. Younger age, previous vaccine refusal and earlier date of survey were more likely to be associated with hesitancy. Our findings suggest that there is room for targeted education to improve COVID-19 vaccine uptake in patients with IBD.

Three-dimensional optical trapping and orientation of microparticles for coherent X-ray diffraction imaging.

Optical trapping has been implemented in many areas of physics and biology as a noncontact sample manipulation technique to study the structure and dynamics of nano- and mesoscale objects. It provides a unique approach for manipulating microscopic objects without inducing undesired changes in structure. Combining optical trapping with hard X-ray microscopy techniques, such as coherent diffraction imaging and crystallography, provides a nonperturbing environment where electronic and structural dynamics of an individual particle in solution can be followed in situ. It was previously shown that optical trapping allows the manipulation of micrometer-sized objects for X-ray fluorescence imaging. However, questions remain over the ability of optical trapping to position objects for X-ray diffraction measurements, which have stringent requirements for angular stability. Our work demonstrates that dynamic holographic optical tweezers are capable of manipulating single micrometer-scale anisotropic particles in a microfluidic environment with the precision and stability required for X-ray Bragg diffraction experiments-thus functioning as an "optical goniometer." The methodology can be extended to a variety of X-ray experiments and the Bragg coherent diffractive imaging of individual particles in solution, as demonstrated here, will be markedly enhanced with the advent of brighter, coherent X-ray sources.

Second Harmonic Generation from a Single Plasmonic Nanorod Strongly Coupled to a WSe2 Monolayer.

Monolayer transition metal dichalcogenides, coupled to metal plasmonic nanocavities, have recently emerged as new platforms for strong light-matter interactions. These systems are expected to have nonlinear-optical properties that will enable them to be used as entangled photon sources, compact wave-mixing devices, and other elements for classical and quantum photonic technologies. Here, we report the first experimental investigation of the nonlinear properties of these strongly coupled systems, by observing second harmonic generation from a WSe2 monolayer strongly coupled to a single gold nanorod. The pump-frequency dependence of the second-harmonic signal displays a pronounced splitting that can be explained by a coupled-oscillator model with second-order nonlinearities. Rigorous numerical simulations utilizing a nonperturbative nonlinear hydrodynamic model of conduction electrons support this interpretation and reproduce experimental results. Our study thus lays the groundwork for understanding the nonlinear properties of strongly coupled nanoscale systems.

Angle-independent plasmonic substrates for multi-mode vibrational strong coupling with molecular thin films.

Vibrational strong coupling of molecules to optical cavities based on plasmonic resonances has been explored recently because plasmonic near-fields can provide strong coupling in sub-diffraction limited volumes. Such field localization maximizes coupling strength, which is crucial for modifying the vibrational response of molecules and, thereby, manipulating chemical reactions. Here, we demonstrate an angle-independent plasmonic nanodisk substrate that overcomes limitations of traditional Fabry-Pérot optical cavities because the design can strongly couple with all molecules on the surface of the substrate regardless of molecular orientation. We demonstrate that the plasmonic substrate provides strong coupling with the C=O vibrational stretch of deposited films of PMMA. We also show that the large linewidths of the plasmon resonance allow for simultaneous strong coupling to two, orthogonal water symmetric and asymmetric vibrational modes in a thin film of copper sulfate monohydrate deposited on the substrate surface. A three-coupled-oscillator model is developed to analyze the coupling strength of the plasmon resonance with these two water modes. With precise control over the nanodisk diameter, the plasmon resonance is tuned systematically through the modes, with the Rabi splitting from both modes varying as a function of the plasmon frequency and with strong coupling to both modes achieved simultaneously for a range of diameters. This work may aid further studies into manipulation of the ground-state chemical landscape of molecules by perturbing multiple vibrational modes simultaneously and increasing the coupling strength in sub-diffraction limited volumes.

Contemporary relationship between medical expenditures and quality of life among adults with epilepsy in the United States.

AIMS: Epilepsy exacts substantial adverse economic and quality of life (QoL) costs. Clarifying the quantitative and qualitative relationships between total and out-of-pocket (OOP) healthcare expenditures and QoL could shed insights into how they influence each other, and have done so over recent times. METHODS: We used the Medical Expenditure Household Components 2003-2014 to identify a total of 2450 adults with epilepsy, representing a weighted population of 1,942,413. Quality of life was assessed using the Physical Component Summary (PCS) and the Mental Component Summary (MCS) derived from the Short-form 12 Version 2 (SF-12 V2), converted into quartiles of equal distribution, with higher quartiles indicating a better QoL. We computed unadjusted mean and adjusted (through a generalized linear model (GLM)) total and OOP healthcare expenditures by QoL categories among adults with epilepsy (reported as dollars in 2016). RESULTS: The pooled estimates of total healthcare expenditures decreased as PCS and MCS quartiles of QoL increased [PCS: costs for quartile 1 = $21,792 (95% confidence interval (CI): $18,416-$25,168 vs. costs for quartile 4 = $6057 (95% CI: $4648-$7466) and MCS: costs for quartile 1 = $19,040 (95% CI: $15,544-$22,535) vs. quartile 4 = $12,939 (95% CI: $8450-$17,429)]. Similarly, the pooled estimates of OOP healthcare expenditures and QoL were inversely related [PCS: costs for quartile 1 = $1849 (95% CI: $1583-$2114) vs. costs for quartile 4 = $948 ($709-$1187) and MCS: costs for quartile 1 = 1812 (95% CI: $1483-2141) vs. quartile 4 = $1317 (95% CI: $982-$1652)]. The association between QoL and total and OOP healthcare expenditures was unchanged after adjusting for socioeconomic and healthcare system related confounders in the GLM. Overall, healthcare expenditures were stable across years independently of the QoL; only OOP expenditures decreased between 2003-2006 and 2011-2014 for quartile 1 of PCS and MCS. CONCLUSION: Quality of life and OOP health expenditures are independently and inversely related to each other among adults with epilepsy. Over the decade studied in the United States, there was a decrease in OOP health expenditures among those patients with epilepsy with the lowest QoL, possibly reflecting a rise in insurance coverage after the Affordable Care Act.

Solvent-dependent energy and charge transfer dynamics in hydroporphyrin-BODIPY arrays.

Arrays of hydroporphyrins with boron complexes of dipyrromethene (BODIPY) are a promising platform for biomedical imaging or solar energy conversion, but their photophysical properties have been relatively unexplored. In this paper, we use time-resolved fluorescence, femtosecond transient absorption spectroscopy, and density-functional-theory calculations to elucidate solvent-dependent energy and electron-transfer processes in a series of chlorin- and bacteriochlorin-BODIPY arrays. Excitation of the BODIPY moiety results in ultrafast energy transfer to the hydroporphyrin moiety, regardless of the solvent. In toluene, energy is most likely transferred via the through-space Förster mechanism from the S1 state of BODIPY to the S2 state of hydroporphyrin. In DMF, substantially faster energy transfer is observed, which implies a contribution of the through-bond Dexter mechanism. In toluene, excited hydroporphyrin components show bright fluorescence, with quantum yield and fluorescence lifetime comparable to those of the benchmark monomer, whereas in DMF, moderate to significant reduction of both quantum yield and fluorescence lifetime are observed. We attribute this quenching to photoinduced charge transfer from hydroporphyrin to BODIPY. No direct spectral signature of the charge-separated state is observed, which suggests that either (1) the charge-separated state decays very quickly to the ground state or (2) virtual charge-separated states, close in energy to S1 of hydroporphyrin, promote ultrafast internal conversion.

Giant Modal Gain Coefficients in Colloidal II-VI Nanoplatelets.

Modal gain coefficient is a key figure of merit for a laser material. Previously, net modal gain coefficients larger than a few thousand cm-1 were achieved in II-VI and III-V semiconductor gain media, but this required operation at cryogenic temperatures. In this work, using pump-fluence-dependent variable-stripe-length measurements, we show that colloidal CdSe nanoplatelets enable giant modal gain coefficients at room temperature up to 6600 cm-1 under pulsed optical excitation. Furthermore, we show that exceptional gain performance is common to the family of CdSe nanoplatelets, as shown by examining samples having different vertical thicknesses and lateral areas. Overall, colloidal II-VI nanoplatelets with superior optical gain properties are promising for a broad range of applications, including high-speed light amplification and loss compensation in plasmonic photonic circuits.

Nonmonotonic Dependence of Auger Recombination Rate on Shell Thickness for CdSe/CdS Core/Shell Nanoplatelets.

Nonradiative Auger recombination limits the efficiency with which colloidal semiconductor nanocrystals can emit light when they are subjected to strong excitation, with important implications for the application of the nanocrystals in light-emitting diodes and lasers. This has motivated attempts to engineer the structure of the nanocrystals to minimize Auger rates. Here, we study Auger recombination rates in CdSe/CdS core/shell nanoplatelets, or colloidal quantum wells. Using time-resolved photoluminescence measurements, we show that the rate of biexcitonic Auger recombination has a nonmonotonic dependence on the shell thickness, initially decreasing, reaching a minimum for shells with thickness of 2-4 monolayers, and then increasing with further increases in the shell thickness. This nonmonotonic behavior has not been observed previously for biexcitonic recombination in quantum dots, most likely due to inhomogeneous broadening that is not present for the nanoplatelets.

pH-Induced Surface Modification of Atomically Precise Silver Nanoclusters: An Approach for Tunable Optical and Electronic Properties.

Noble metal nanoclusters (NCs) play a pivotal role in bridging the gap between molecules and quantum dots. Fundamental understanding of the evolution of the structural, optical, and electronic properties of these materials in various environments is of paramount importance for many applications. Using state-of-the-art spectroscopy, we provide the first decisive experimental evidence that the structural, electronic, and optical properties of Ag44(MNBA)30 NCs can now be tailored by controlling the chemical environment. Infrared and photoelectron spectroscopies clearly indicate that there is a dimerization between two adjacent ligands capping the NCs that takes place upon lowering the pH from 13 to 7.

Low-threshold stimulated emission using colloidal quantum wells.

The use of colloidal semiconductor nanocrystals for optical amplification and lasing has been limited by the need for high input power densities. Here we show that colloidal nanoplatelets produce amplified spontaneous emission with thresholds as low as 6 µJ/cm(2) and gain as high as 600 cm(-1), both a significant improvement over colloidal nanocrystals; in addition, gain saturation occurs at pump fluences 2 orders of magnitude higher than the threshold. We attribute this exceptional performance to large optical cross-sections, slow Auger recombination rates, and narrow ensemble emission line widths.

Viscoelastic flows in simple liquids generated by vibrating nanostructures.

Newtonian fluid mechanics, in which the shear stress is proportional to the strain rate, is synonymous with the flow of simple liquids such as water. We report the measurement and theoretical verification of non-Newtonian, viscoelastic flow phenomena produced by the high-frequency (20 GHz) vibration of gold nanoparticles immersed in water-glycerol mixtures. The observed viscoelasticity is not due to molecular confinement, but is a bulk continuum effect arising from the short time scale of vibration. This represents the first direct mechanical measurement of the intrinsic viscoelastic properties of simple bulk liquids, and opens a new paradigm for understanding extremely high frequency fluid mechanics, nanoscale sensing technologies, and biophysical processes.

Carrier cooling in colloidal quantum wells.

It has recently become possible to chemically synthesize atomically flat semiconductor nanoplatelets with monolayer-precision control over the platelet thickness. It has been suggested that these platelets are quantum wells; that is, carriers in these platelets are confined in one dimension but are free to move in the other two dimensions. Here, we report time-resolved photoluminescence and transient-absorption measurements of carrier relaxation that confirm the quantum-well nature of these nanomaterials. Excitation of the nanoplatelets by an intense laser pulse results in the formation of a high-temperature carrier population that cools back down to ambient temperature on the time scale of several picoseconds. The rapid carrier cooling indicates that the platelets are well-suited for optoelectronic applications such as lasers and modulators.

Three-dimensional optical trapping and manipulation of single silver nanowires.

We report the first experimental realization of all-optical trapping and manipulation of plasmonic nanowires in three dimensions. The optical beam used for trapping is the Fourier transform of a linearly polarized Bessel beam (termed FT-Bessel). The extended depth of focus of this beam enables the use of a retroreflection geometry to cancel radiation pressure in the beam propagation direction, making it possible to trap highly scattering and absorbing silver nanowires. Individual silver nanowires with lengths of several micrometers can be positioned by the trapping beam with a precision better than 100 nm and are oriented by the polarization of the trapping light with a precision of approximately 1°. Multiple nanowires can be trapped simultaneously in spatially separated maxima of the trapping field. Since trapping in the interferometric FT-Bessel potential is robust in bulk solution and near surfaces, it will enable the controlled assembly of metal nanowires into plasmonic nanostructures.

Long-lived charge-separated states in ligand-stabilized silver clusters.

Recently developed synthesis methods allow for the production of atomically monodisperse clusters of silver atoms stabilized in solution by aromatic thiol ligands, which exhibit intense absorption peaks throughout the visible and near-IR spectral regions. Here we investigated the time-dependent optical properties of these clusters. We observed two kinetic processes following ultrafast laser excitation of any of the absorption peaks: a rapid decay, with a time constant of 1 ps or less, and a slow decay, with a time constant that can be longer than 300 ns. Both time constants decrease as the polarity of the solvent increases, indicating that the two processes correspond to the formation and recombination, respectively, of a charge-separated state. The long lifetime of this state and the broad optical absorption spectrum mean that the ligand-stabilized silver clusters are promising materials for solar energy harvesting.