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Abstract 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. 

Abstract. Information on the rate of diffusion of organic moleculeswithin secondary organic aerosol (SOA) is needed to accurately predict theeffects of SOA on climate and air quality. Diffusion can be important forpredicting the growth, evaporation, and reaction rates of SOA under certainatmospheric conditions. Often, researchers have predicted diffusion rates oforganic molecules within SOA using measurements of viscosity and theStokes–Einstein relation (D∝1/η, where D is the diffusioncoefficient and η is viscosity). However, the accuracy of thisrelation for predicting diffusion in SOA remains uncertain. Usingrectangular area fluorescence recovery after photobleaching (rFRAP), wedetermined diffusion coefficients of fluorescent organic molecules over8 orders in magnitude in proxies of SOA including citric acid, sorbitol,and a sucrose–citric acid mixture. These results were combined withliterature data to evaluate the Stokes–Einstein relation for predictingthe diffusion of organic molecules in SOA. Although almost all the data agreewith the Stokes–Einstein relation within a factor of 10, a fractionalStokes–Einstein relation (D∝1/ηξ) with ξ=0.93is a better model for predicting the diffusion of organic molecules in the SOAproxies studied. In addition, based on the output from a chemical transportmodel, the Stokes–Einstein relation can overpredict mixing times of organicmolecules within SOA by as much as 1 order of magnitude at an altitudeof ∼3 km compared to the fractional Stokes–Einstein relation with ξ=0.93. These results also have implications for other areas such as infood sciences and the preservation of biomolecules. 

Knowledge of the viscosity of particles containing secondary organic material (SOM) is useful for predicting reaction rates and diffusion in SOM particles. In this study we investigate the viscosity of SOM particles as a function of relative humidity and SOM particle mass concentration, during SOM synthesis. The SOM was generated via the ozonolysis of α-pinene at < 5 % relative humidity (RH). Experiments were carried out using the poke-and-flow technique, which measures the experimental flow time (τ_{exp, flow}) of SOM after poking the material with a needle. In the first set of experiments, we show that τ_{exp, flow} increased by a factor of 3600 as the RH increased from < 0.5 RH to 50 % RH, for SOM with a production mass concentration of 121 µg m⁻³. Based on simulations, the viscosities of the particles were between 6  ×  10⁵ and 5  ×  10⁷ Pa s at < 0.5 % RH and between 3  ×  10² and 9  ×  10³ Pa s at 50 % RH. In the second set of experiments we show that under dry conditions τ_{exp, flow} decreased by a factor of 45 as the production mass concentration increased from 121 to 14 000 µg m⁻³. From simulations of the poke-and-flow experiments, the viscosity of SOM with a production mass concentration of 14 000 µg m⁻³ was determined to be between 4  ×  10⁴ and 1.5  ×  10⁶ Pa s compared to between 6  ×  10⁵ and 5  ×  10⁷ Pa s for SOM with a production mass concentration of 121 µg m⁻³. The results can be rationalized by a dependence of the chemical composition of SOM on production conditions. These results emphasize the shifting characteristics of SOM, not just with RH and precursor type, but also with the production conditions, and suggest that production mass concentration and the RH at which the viscosity was determined should be considered both when comparing laboratory results and when extrapolating these results to the atmosphere.