Controlled Second Harmonic Generation with Optically Trapped Lithium Niobate Nanoparticles.

We report the second harmonic generation (SHG) response from a single 34 nm diameter lithium niobate nanoparticle. The experimental setup involves a first beam devoted to the optical trapping of single nanoparticles, whereas a second arm involves a femtosecond laser source leading to the SHG emission from the trapped nanoparticles. SHG operation where one to three nanoparticles are present in the optical trap is first demonstrated, highlighting the transition between coherent and incoherent SHG, the latter known as hyper-Rayleigh scattering (HRS). With a spatial light modulator moving the optical trap in and out of the focus of the femtosecond beam, the SHG intensity is switched back and forth between a low and a high level. This controlled operation opens new avenues for nanoparticle characterization and applications in sensing or communication and information technologies and constitutes the first step in the design of active substrateless metasurfaces.

Solvent-Induced Aggregation of Self-Assembled Copper-Cysteine Nanoparticles Reacted with Glutathione: Enhancing Linear and Nonlinear Optical Properties.

Copper-thiolate self-assembly nanostructures are a unique class of nanomaterials because of their interesting properties such as hierarchical structures, luminescence, and large nonlinear optical efficiency. Herein, we synthesized biomolecule cysteine (Cys) and glutathione (GSH) capped sub-100 nm self-assembly nanoparticles (Cu-Cys-GSH NPs) with red fluorescence. The as-synthesized NPs show high emission enhancement in the presence of ethanol, caused by the aggregation-induced emission. We correlated the structure and optical properties of Cu-Cys-GSH NPs by measuring the mass, morphology, and surface charge as well as their two-photon excited fluorescence cross-section (σ2PEPL), two-photon absorption cross-section (σTPA) and first hyperpolarizability (ß) of Cu-Cys-GSH NPs in water and water-ethanol using near-infrared wavelength. We found a high ß value as (77 ± 10) × 10-28 esu (in water) compared to the reference medium water. The estimated values of σ2PEPL and σTPA are found to be (13 ± 2) GM and (1.4 ± 0.2) × 104 GM, respectively. We hope our investigations of linear and nonlinear optical properties of copper-thiolate self-assemblies in water and its solvent-induced aggregates will open up new possibilities in designing self-assembled systems for many applications including sensing, drug delivery, and catalysis.

Hyper Rayleigh scattering from DNA nucleotides in aqueous solution.

Nucleotides are organic compounds consisting of a phosphate group, a nitrogenous base, namely adenine (A), thymine (T), cytosine (C), or guanine (G), and a sugar, here deoxyribose. The magnitude of the first hyperpolarizability ß of these four DNA nucleotides was determined in aqueous solution with the nonlinear optical technique of hyper rayleigh scattering under non resonant conditions at a fundamental wavelength of 800 nm. The smallest value is found to be 1.67 ± 0.15 × 10-30 esu for thymidine-5'-monophosphate and the highest is 1.76 ± 0.16 × 10-30 esu for 2'-guanosine-5'-monophosphate. Polarization resolved studies were also performed to question the symmetry of the first hyperpolarizability tensor and access the ratio of some elements of the first hyperpolarizability tensor. These experimental results were then compared to the theoretical values of these first hyperpolarizabilities obtained with the density functional theory at the level of the PCM-B3LYP/6-31G+(d) basis and taking into account the solvent.

Combining second harmonic generation and multiphoton excited photo-luminescence to investigate TiO2 nanoparticle powders.

Disentangling Second Harmonic Generation (SHG) and Multiphoton Excited Photoluminescence (MEPL) signals in microscopy experiments is not an easy task. Two methods have been so far proposed based either on a time domain or a spectral domain analysis of the collected signals. In this report, a new method based on polarization discrimination is proposed to separate these SHG and MEPL contributions. In order to demonstrate this operation, intensity depth profiles are recorded for an anatase titanium dioxide powder consisting of 22 nm diameter nanoparticles using ultrafast femtosecond laser excitation. Polarization analysis of these intensity depth profiles is therefore performed and demonstrates a polarization angle shift for the SHG intensity contribution as compared to the MEPL one, allowing for the discrimination of the two SHG and MEPL contributions. The fundamental beam is set at two different wavelengths in order to provide a SHG photon energy above and below the anatase TiO2 band-gap of 3.2 eV, leading to a change in the relative intensity weight and a spectral shift between the SHG and MEPL contributions. This operation further demonstrates the potential of the method when the spectral domain disentangling cannot be performed. SHG profiles are by far narrower than those of MEPL. This study where both SHG and MEPL contributions are observed offers perspectives in photonics of powder materials as the different origin and properties of the two processes can be separated.

Ligand impact on reactive oxygen species generation of Au10 and Au25 nanoclusters upon one- and two-photon excitation.

In photodynamic therapy (PDT), light-sensitive photosensitizers produce reactive oxygen species (ROS) after irradiation in the presence of oxygen. Atomically-precise thiolate-protected gold nanoclusters are molecule-like nanostructures with discrete energy levels presenting long lifetimes, surface biofunctionality, and strong near-infrared excitation ideal for ROS generation in PDT. We directly compare thiolate-gold macromolecular complexes (Au10) and atomically-precise gold nanoclusters (Au25), and investigate the influence of ligands on their photoexcitation. With the ability of atomically-precise nanochemistry, we produce Au10SG10, Au10AcCys10, Au25SG18, and Au25AcCys18 (SG: glutathione; AcCys: N-acetyl-cysteine) fully characterized by high-resolution mass spectrometry. Our theoretical investigation reveals key factors (energetics of excited states and structural influence of surface ligands) and their relative importance in singlet oxygen formation upon one- and two-photon excitation. Finally, we explore ROS generation by gold nanoclusters in living cells with one- and two-photon excitation. Our study presents in-depth analyses of events within gold nanoclusters when photo-excited both in the linear and nonlinear optical regimes, and possible biological consequences in cells.

Hyper-Rayleigh scattering of adenine, thymine, and cytosine in neat water.

The first hyperpolarizabilities of the DNA bases thymine and cytosine were determined by hyper-Rayleigh scattering in neat water despite their low solubility. Due to the low intensity levels collected, count statistics were performed instead of the standard dilution procedure. The first hyperpolarizabilities were found to be ßThymine = (2.99 ± 0.44) × 10-30 esu for thymine and ßCytosine = (3.35 ± 0.21) × 10-30 esu for cytosine. Due to its weak solubility, only an upper limit ßAdenine < (1.82 ± 0.10) × 10-30 esu could be set for adenine. The first hyperpolarizability of guanine could not be measured because of its very weak solubility. Theoretical static and 800 nm dynamic first hyperpolarizability tensor elements were also computed with Gaussian 09 for comparison.

First hyperpolarizability of water at the air-vapor interface: a QM/MM study questions standard experimental approximations.

Surface Second-Harmonic Generation (S-SHG) experiments provide a unique approach to probe interfaces. One important issue for S-SHG is how to interpret the S-SHG intensities at the molecular level. Established frameworks commonly assume that each molecule emits light according to an average molecular hyperpolarizability tensor ß(-2ω,ω,ω). However, for water molecules, this first hyperpolarizability is known to be extremely sensitive to their environment. We have investigated the molecular first hyperpolarizability of water molecules within the liquid-vapor interface, using a quantum description with explicit, inhomogeneous electrostatic embedding. The resulting average molecular first hyperpolarizability tensor depends on the distance relative to the interface, and it practically respects the Kleinman symmetry everywhere in the liquid. Within this numerical approach, based on the dipolar approximation, the water layer contributing to the Surface Second Harmonic Generation (S-SHG) intensity is less than a nanometer. The results reported here question standard interpretations based on a single, averaged hyperpolarizability for all molecules at the interface. Not only the molecular first hyperpolarizability tensor significantly depends on the distance relative to the interface, but it is also correlated to the molecular orientation. Such hyperpolarizability fluctuations may impact the S-SHG intensity emitted by an aqueous interface.

Gold-seeded Lithium Niobate Nanoparticles: Influence of Gold Surface Coverage on Second Harmonic Properties.

Hybrid nanoparticles composed of an efficient nonlinear optical core and a gold shell can enhance and tune the nonlinear optical emission thanks to the plasmonic effect. However the influence of an incomplete gold shell, i.e., isolated gold nano-islands, is still not well studied. Here LiNbO3 (LN) core nanoparticles of 45 nm were coated with various densities of gold nano-seeds (AuSeeds). As both LN and AuSeeds bear negative surface charge, a positively-charged polymer was first coated onto LN. The number of polymer chains per LN was evaluated at 1210 by XPS and confirmed by fluorescence titration. Then, the surface coverage percentage of AuSeeds onto LN was estimated to a maximum of 30% using ICP-AES. The addition of AuSeeds was also accompanied with surface charge reversal, the negative charge increasing with the higher amount of AuSeeds. Finally, the first hyperpolarizability decreased with the increase of AuSeeds density while depolarization values for Au-seeded LN were close to the one of bare LN, showing a predominance of the second harmonic volumic contribution.

Four orders-of-magnitude enhancement in the two-photon excited photoluminescence of homoleptic gold thiolate nanoclusters following zinc ion-induced aggregation.

Herein we report unprecedented determination of the molar mass of zinc mediated assemblies of homoleptic gold nanoclusters, based on charge detection mass spectrometry measurements. The accurate determination of the molar mass of zinc coordinated assemblies of gold clusters has further allowed unambiguous determination of the two-photon excited photoluminescence cross section of the same. Furthermore, in line with one-photon excited photoluminescence measurements, four orders-of-magnitude enhancement in two-photon excited photoluminescence of gold nanoclusters has been observed following complexation with zinc ions. The study reported herein is envisioned to not only deepen the fundamental understanding of the multiphoton excitation properties of atomic clusters but also widen their application potential as luminescence markers.

Functionalized Au15 nanoclusters as luminescent probes for protein carbonylation detection.

Atomically precise, ligand-protected gold nanoclusters (AuNCs) attract considerable attention as contrast agents in the biosensing field. However, the control of their optical properties and functionalization of surface ligands remain challenging. Here we report a strategy to tailor AuNCs for the precise detection of protein carbonylation-a causal biomarker of ageing. We produce Au15SG13 (SG for glutathione) with atomic precision and functionalize it with a thiolated aminooxy moiety to impart protein carbonyl-binding properties. Mass spectrometry and molecular modelling reveal the key structural features of Au15SG12-Aminooxy and its reactivity towards carbonyls. Finally, we demonstrate that Au15SG12-Aminooxy detects protein carbonylation in gel-based 1D electrophoresis by one- and two-photon excited fluorescence. Importantly, to our knowledge, this is the first application of an AuNC that detects a post-translational modification as a nonlinear optical probe. The significance of post-translational modifications in life sciences may open avenues for the use of Au15SG13 and other nanoclusters as contrast agents with tailored surface functionalization and optical properties.

Long-Range Orientational Organization of Dipolar and Steric Liquids.

Long-range orientational correlations in liquids have received recent renewed interest, in particular for the neat water case. These long-range orientational correlations, exceeding several tens of nanometers, originate from the presence of the strong permanent water dipolar moment. However, the exact dependence with the dipolar moment and the role of other local forces like steric hindrance has never been addressed. In this work, we experimentally measure long-range correlations for a set of liquids differing by their molecular weight and dipolar moment, in order to reveal the origin of their long-range organization. Hence, we show that the dipolar moment of a solvent molecule is not the unique feature determining the orientational correlation. Steric hindrance significantly helps to structure the liquids as well. In order to quantify these long-range correlations, we also derive theoretically the polarization resolved second harmonic scattering intensity as a function of the rotational invariants describing the dipolar and octupolar interaction.

Longitudinal position dependence of the second-harmonic generation of optically trapped silica microspheres.

We report the design of a setup combining the simultaneous and independent optical trapping and second-harmonic generation (SHG) of 1 µm diameter silica microspheres with two independent laser beams. Optical trapping is achieved with a tightly focused continuous wave infrared laser beam whereas the SHG intensity from the trapped microparticles is obtained with a 810 nm femtosecond pulsed laser. The silica microparticles are dispersed in an aqueous solution, and a microfluidic channel flow is used to remove untrapped microparticles. We show by the perpendicular displacement of the optical trap from the microfluidic channel wall that it is possible to control the contribution of the channel wall/solution interface to the overall SHG intensity. Stable trapping and SHG detection of two microparticles is also demonstrated. Combining the independent trapping of centrosymmetrical silica microparticles with SHG offers new avenues for analytical studies like surface sensing or all-optical devices where the SHG intensity is controlled by the trapping beam.

Ordering and Nonideality of Air-Ionic Liquid Interfaces in Surface Second Harmonic Generation.

The air-ionic liquid interface for a series of ionic liquids involving imidazolium cations [Cnmim] with different alkyl chain lengths (n = 2, 4, 6, 8, 10, and 12) and the same [NTf2] imide anion has been studied by polarization-resolved second harmonic generation (SHG). An increase as a function of the alkyl chain length of the orientational parameter reveals the increasing ordering of the air-pure ionic liquid interfaces although it is not possible to disentangle the change in mean tilt angle from a change in the tilt angle probability distribution width. Besides, the study of the air-mixed ([C12mim])x([C2mim])1-x[NTf2] ionic liquid interface clearly demonstrates the interfacial nonideality of the mixed ionic liquids. The long alkyl chain cation perturbs the interface as seen from the orientational parameter and displaces the short alkyl chain one for bulk mixture contents as low as 10%. At higher long alkyl chain cation bulk mixture contents, the interface behaves close to a pure long alkyl chain ionic liquid.

Second harmonic scattering from mass characterized 2D graphene oxide sheets.

In this communication, we report the second harmonic scattering from mass characterized 2D graphene oxide sheets dispersed in an aqueous suspension, in the femtosecond regime at 800 nm laser excitation. Charge-detection mass-spectrometry, performing at the single sheet level, allows for an exhaustive molar mass distribution and thus concentration for these 2D nanomaterials samples. The orientation-averaged hyperpolarizability value is (1.36 ± 0.15) × 10-25 esu as determined by the concentration-dependent harmonic scattering signal. In addition, the multi-photon excited fluorescence spectrum is characterized by a broad band in the visible range between 350 and 700 nm centered at about 500 nm.

Machine learning-based prediction of glioma margin from 5-ALA induced PpIX fluorescence spectroscopy.

Gliomas are infiltrative brain tumors with a margin difficult to identify. 5-ALA induced PpIX fluorescence measurements are a clinical standard, but expert-based classification models still lack sensitivity and specificity. Here a fully automatic clustering method is proposed to discriminate glioma margin. This is obtained from spectroscopic fluorescent measurements acquired with a recently introduced intraoperative set up. We describe a data-driven selection of best spectral features and show how this improves results of margin prediction from healthy tissue by comparison with the standard biomarker-based prediction. This pilot study based on 10 patients and 50 samples shows promising results with a best performance of 77% of accuracy in healthy tissue prediction from margin tissue.

Two dimensional diffusion-controlled triplet-triplet annihilation kinetics.

Diffusion controlled chemical reactions are usually observed in three dimensional media. In contrast, planar bimolecular reactions taking place between reagents adsorbed at a soft interface are two-dimensional and therefore cannot be studied within the same formalism. Indeed, soft interfaces allow the adsorbed species to freely diffuse in a liquid-like manner. Here, we present the first experimental observation of a diffusion-controlled reaction in an environment that is planar at the ångström scale. By means of time-resolved surface second harmonic generation, an inherently surface sensitive technique, we observed that the kinetics of the diffusion of the reagents in the plane decreases as the surface concentration of adsorbed species increases. This is of course not the case for bulk reactions where the rates always increase with the reactant concentration. Such changes in the kinetics regime were rationalised as the evolution from a regular 2D free diffusion regime to a geometry-controlled scheme.

Ligand shell size effects on one- and two-photon excitation fluorescence of zwitterion functionalized gold nanoclusters.

Gold nanoclusters (Au NCs) are an emerging class of luminescent nanomaterials but still suffer from moderate photoluminescence quantum yields. Recent efforts have focused on tailoring their emission properties. Introducing zwitterionic ligands to cap the metallic kernel is an efficient approach to enhance their one-photon excitation fluorescence intensity. In this work, we extend this concept to the nonlinear optical regime, i.e., two-photon excitation fluorescence. For a proper comparison between theory and experiment, both ligand and solvent effects should be considered. The effects of ligand shell size and of aqueous solvent on the optical properties of zwitterion functionalized gold nanoclusters have been studied by performing quantum mechanics/molecular mechanics (QM/MM) simulations.

Sub-100 nanometer silver doped gold-cysteine supramolecular assemblies with enhanced nonlinear optical properties.

The ability of gold(i) thiolates to self-assemble into supramolecular architectures opens the route for a new class of nanomaterials with a unique structure-optical property relationship. However, for a confirmed structure-optical property relationship, a control of the supramolecular architectures is required. In this work, we report a simple synthesis of sub-100 nanometer gold-cysteine and silver doped gold-cysteine supramolecular assemblies. We explore in particular silver-doping as a strategy to enhance the optical properties of these supramolecular assemblies. By an accurate characterization of as-synthesized supramolecular nanoparticles, we have been able to measure for the first time, their absolute two-photon absorption cross-section, two-photon excited fluorescence cross-section and first hyperpolarizabilities at different near-IR wavelengths. Huge values are obtained for silver doped gold-cysteine supramolecular assemblies, as compared to their corresponding undoped counterpart. In addition, we employ DFT and TD-DFT methods to study the geometric and electronic structures of model gold-cysteine and silver doped gold-cysteine compounds in order to address the structure-linear/nonlinear optical property relationship. The aim is to gain insights into the origin of the nonlinear optical enhancement of silver-doped gold supramolecular assemblies.

On the molecular optical nonlinearity of halogen-bond-forming azobenzenes.

We study hyper-Rayleigh scattering and computed molecular hyperpolarizability in a series of azobenzene chromophores in chloroform and dimethylformamide as solvents. The chromophores form halogen or hydrogen bonds of varying strength with dimethylformamide molecules, differently from what is expected for chloroform. We show that hyperpolarizability is unaffected or sligthly lower with the azobenzene forming the strongest halogen bond. Solid supramolecular polymers with the same chromophores have previously demonstrated clearly higher second-order nonlinear responses when a halogen-bond-accepting polymer is used, the larger increase being associated with the stronger halogen bond. The present study proves that the higher optical nonlinearity in polymers lies in the better ordering of the chromophores instead of changes in molecular hyperpolarizability, highlighting the unique properties of halogen bonding in supramolecular chemistry.

Salt-induced Long-to-Short Range Orientational Transition in Water.

We report the long-range orientational organization of water using polarization-resolved second harmonic scattering operated in a right-angle configuration. A transition is observed between the neat water orientational organization involving an azimuthal molecular orientation distribution towards a radial molecular orientation distribution when salt is added. These two orientational phases are quantitatively described using a molecular model of the second harmonic scattering response. It is observed that the long-range correlation present in the neat water phase abruptly disappears and is replaced by a shorter range correlation centered around the ions as the salt concentration is increased.