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.

Nonclassical Nucleation and Crystallization of LiNbO3 Nanoparticles from the Aqueous Solvothermal Alkoxide Route.

The exact molecular reaction pathway and crystallization mechanisms of LiNbO3 nanoparticles under solvothermal conditions are derived through extensive time- and temperature-resolved experiments allowing to track all the transient molecular and solid species. Starting with a simple mixing of Li/Nb ethoxides, water addition is used to promote condensation after ligand exchange with different co-solvents including alcohols and glycols of variable carbon-chain length. A nonclassical nucleation scheme is first demonstrated after the identification of new octanuclear complexes with a {Li4Nb4O10} core whose solvophobic interactions mediate their aggregation, thus, resulting in a colloidal gel at room-temperature. Upon heating, a more or less frustrated aggregation-mediated crystallization process is then evidenced leading to LiNbO3 nanocrystals of adjustable mean size between 20 and 100 nm. Such a fine control can be attributed to the variable Nb-OR (R = alkoxy/glycoxy ligand) binding interactions at the surface of crystalline intermediates. Demonstration of such a nonclassical nucleation process and crystallization mechanism for LiNbO3 not only sheds light on the entire growth process of multifunctional nanomaterials with non-perovskite crystalline structures, but also opens new avenues for the identification of novel bimetallic oxoclusters involved in the formation of several mixed oxides from the aqueous alkoxide route.

Solution-Based Synthesis Routes for the Preparation of Noncentrosymmetric 0-D Oxide Nanocrystals with Perovskite and Nonperovskite Structures.

With the miniaturization of electronic-based devices, the foreseen potential of new optical nanoprobes and the assessment of eventual size and shape effects, elaboration of multifunctional noncentrosymmetric nanocrystals with ferroelectric, pyroelectric, piezoelectric, and nonlinear optical properties are the subject of an increasing research interest. Here, the recent achievements from the solution-based methods (coprecipitation in homogeneous and nanostructured media, sol-gel processes including various chemistries and hydro/solvothermal techniques) to prepare 0-D perovskite and nonperovskite oxides in the 5-500 nm size range are critically reviewed. To cover a representative list of covalent- and ionic-type materials, BaTiO3 and its derivatives, niobate compounds (i.e., K/Na/LiNbO3 ), multiferroic BiFeO3, and crystals of lower symmetry including KTiOPO4 and some iodate compounds such as Fe(IO3 )3 and La(IO3 )3 are systematically in focus. The resulting size, morphology, and aggregation state are discussed in light of the proposed formation mechanisms. Because of a higher complexity related to their chemical composition and crystalline structures, improving the rational design of these multifunctional oxides in terms of finely-tuned compositions, crystalline hosts and structure-property relationships still need in the future a special attention of the research community to the detailed understanding of the reaction pathways and crystallization mechanisms.

Multiorder Nonlinear Mixing in Metal Oxide Nanoparticles.

Whereas most of the reports on the nonlinear properties of micro- and nanostructures address the generation of distinct signals, such as second or third harmonic, here we demonstrate that the novel generation of dual output lasers recently developed for microscopy can readily increase the accessible parameter space and enable the simultaneous excitation and detection of multiple emission orders such as several harmonics and signals stemming from various sum and difference frequency mixing processes. This rich response, which in our case features 10 distinct emissions and encompasses the whole spectral range from the deep ultraviolet to the short-wave infrared region, is demonstrated using various nonlinear oxide nanomaterials while being characterized and simulated temporally and spectrally. Notably, we show that the response is conserved when the particles are embedded in biological media opening the way to novel biolabeling and phototriggering strategies.

Microwave Synthesis and Up-Conversion Properties of SHG-Active α-(La, Er)(IO3)3 Nanocrystals.

Pure α-La(IO3)3 and α-La0.85Er0.15(IO3)3 nanocrystals were synthesized by a microwave-assisted hydrothermal method leading to a reaction yield of 87 ± 4%. Electron microscopy and dynamic light scattering characterizations provide evidence for the formation of nanocrystals with an average size of 45 ± 10 nm for α-La(IO3)3 and 55 ± 10 nm for α-La0.85Er0.15(IO3)3. When dispersed in ethylene glycol, the nanocrystal suspensions exhibit second-harmonic generation under near-infrared excitations at 800 and 980 nm whereas additional photoluminescence by up-conversion is simultaneously observed in the case of α-La0.85Er0.15(IO3)3 nanocrystals. Quantitative assessments of the second-harmonic generation efficiency from second-harmonic scattering experiments at 1064 nm result in relatively high ⟨ d⟩ coefficients measured at 8.2 ± 2.0 and 8.0 ± 2.0 pm V-1 for α-La(IO3)3 and α-La0.85Er0.15(IO3)3, respectively. The relative intensity between second-harmonic generation and photoluminescence is discussed following the excitation wavelength.

Nonlinear optical susceptibility of two-dimensional WS2 measured by hyper Rayleigh scattering: erratum.

This erratum corrects errors in the expressions for ⟨ßTMD⟩ and fitted form of IHRS and a consequent data point in Fig. 4 of a recent Letter [Opt. Lett.42, 5018 (2017)OPLEDP0146-959210.1364/OL.42.005018]. It also supplies data for the reference compound para-nitroaniline (pNA). The correction to ⟨ßTMD⟩ improves experimental agreement from 46% to within 21% of independent scissors-corrected density functional theory (DFT) calculations. Central findings from the original Letter remain intact.

Nonlinear optical susceptibility of two-dimensional WS2 measured by hyper Rayleigh scattering.

Hyper Rayleigh scattering (HRS) was used to measure the second-order nonlinear susceptibility, χ(2), for liquid exfoliated WS2 monolayers. To the best of our knowledge, it is the first reported application of the HRS technique to assess the bulk-like χ(2) of a two-dimensional (2D) material. The concentration-dependent HRS signal indicated a 4.90±0.30×10-25 esu first hyperpolarizability for 42 nm WS2 monolayers under 1064 nm laser irradiation using para-nitroaniline as an external reference. The corresponding value of χxxx(2) was calculated to be 460±28 pm V-1. This was within 46% of independent density functional theory predictions. Agreement with theory was improved over related microscopy-based approaches. These results support the use of HRS to evaluate 2D materials for nonlinear frequency mixing applications.

Cellular uptake and biocompatibility of bismuth ferrite harmonic advanced nanoparticles.

Bismuth Ferrite (BFO) nanoparticles (BFO-NP) display interesting optical (nonlinear response) and magnetic properties which make them amenable for bio-oriented diagnostic applications as intra- and extra membrane contrast agents. Due to the relatively recent availability of this material in well dispersed nanometric form, its biocompatibility was not known to date. In this study, we present a thorough assessment of the effects of in vitro exposure of human adenocarcinoma (A549), lung squamous carcinoma (NCI-H520), and acute monocytic leukemia (THP-1) cell lines to uncoated and poly(ethylene glycol)-coated BFO-NP in the form of cytotoxicity, haemolytic response and biocompatibility. Our results support the attractiveness of the functional-BFO towards biomedical applications focused on advanced diagnostic imaging. FROM THE CLINICAL EDITOR: Bismuth Ferrite nanoparticles (BFO-NP) have been recently successfully introduced as photodynamic tools and imaging probes. However, how these nanoparticles interact with various cells at the cellular level remains poorly understood. In this study, the authors performed in vitro experiments to assess the effects of uncoated and PEG-coated BFO-NP in the form of cytotoxicity, haemolytic response and biocompatibility.

Characterization of the nonlinear optical properties of nanocrystals by Hyper Rayleigh Scattering.

BACKGROUND: Harmonic Nanoparticles are a new family of exogenous markers for multiphoton imaging exerting optical contrast by second harmonic (SH) generation. In this tutorial, we present the application of Hyper-Rayleigh Scattering (HRS) for a quantitative assessment of the nonlinear optical properties of these particles and discuss the underlying theory and some crucial experimental aspects. METHODS: The second harmonic properties of BaTiO3, KNbO3, KiTiOPO4 (KTP), LiNbO3 and ZnO nanocrystals (NCs) are investigated by HRS measurements after careful preparation and characterization of colloidal suspensions. RESULTS: A detailed analysis of the experimental results is presented with emphasis on the theoretical background and on the influence of some experimental parameters including the accurate determination of the nanocrystal size and concentration. The SH generation efficiency and averaged nonlinear optical coefficients are then derived and compared for six different types of NCs. CONCLUSIONS: After preparation of colloidal NC suspensions and careful examination of their size, concentration and possible aggregation state, HRS appears as a valuable tool to quantitatively assess the SH efficiency of noncentrosymmetric NCs. All the investigated nanomaterials show high SH conversion efficiencies, demonstrating a good potential for bio-labelling applications.

Modulating the Surface Properties of Lithium Niobate Nanoparticles by Multifunctional Coatings Using Water-in-Oil Microemulsions.

Inorganic nanoparticles (NPs) have emerged as promising tools in biomedical applications, owing to their inherent physicochemical properties and their ease of functionalization. In all potential applications, the surface functionalization strategy is a key step to ensure that NPs are able to overcome the barriers encountered in physiological media, while introducing specific reactive moieties to enable post-functionalization. Silanization appears as a versatile NP-coating strategy, due to the biocompatibility and stability of silica, thus justifying the need for robust and well controlled silanization protocols. Herein, we describe a procedure for the silica coating of harmonic metal oxide NPs (LiNbO3, LNO) using a water-in-oil microemulsion (W/O ME) approach. Through optimized ME conditions, the silanization of LNO NPs was achieved by the condensation of silica precursors (TEOS, APTES derivatives) on the oxide surface, resulting in the formation of coated NPs displaying carboxyl (LNO@COOH) or azide (LNO@N3) reactive moieties. LNO@COOH NPs were further conjugated to an unnatural azido-containing small peptide to obtain silica-coated LNO NPs (LNO@Talys), displaying both azide and carboxyl moieties, which are well suited for biomedical applications due to the orthogonality of their surface functional groups, their colloidal stability in aqueous medium, and their anti-fouling properties.

High-speed tracking of murine cardiac stem cells by harmonic nanodoublers.

Potassium niobate nonlinear nanoparticles are used for the first time to monitor the evolution of embryonic stem cells (ESC) by second harmonic microscopy. These particles feature the complete absence of photo-bleaching and unlimited excitation wavelength flexibility. The potential of this approach is made evident for tissue-regeneration studies and applications, by capturing a high-speed movie of ESC-derived cardiomyocytes autonomously beating within a cluster. Time-resolved data are analyzed to retrieve 3D information of the contraction pattern at the cellular level.

On the Reaction Pathways and Growth Mechanisms of LiNbO3 Nanocrystals from the Non-Aqueous Solvothermal Alkoxide Route.

Phase-pure, highly crystalline sub-50 nm LiNbO3 nanocrystals were prepared from a non-aqueous solvothermal process for 72 h at 230 °C and a commercial precursor solution of mixed lithium niobium ethoxide in its parent alcohol. A systematic variation of the reaction medium composition with the addition of different amounts of co-solvent including butanol, 1,3-propanediol, 1,4-butanediol, and 1,5-pentanediol resulted in the formation of nanocrystals of adjustable mean size and shape anisotropy, as demonstrated from XRD measurements and TEM imaging. Colloidal stability of ethanol- and water-based suspensions was evaluated from dynamic light scattering (DLS)/zeta potential studies and correlated with FTIR data. Thanks to the evolution in the nanocrystal size and shape distribution we observed, as well as to the available literature on the alkoxide chemistry, the reaction pathways and growth mechanisms were finally discussed with a special attention on the monomer formation rate, leading to the nucleation step. The polar, non-perovskite crystalline structure of LiNbO3 was also evidenced to play a major role in the nanocrystal shape anisotropy.

Synthesis and Characterization of Novel Nanoparticles of Lithium Aluminum Iodate LiAl(IO3)4, and DFT Calculations of the Crystal Structure and Physical Properties.

Here we report on the non-hydrothermal aqueous synthesis and characterization of nanocrystalline lithium aluminum iodate, LiAl(IO3)4. Morphological and compositional analyses were carried out by using scanning electron microscopy (SEM) and energy-dispersive X-ray measurements (EDX). The optical and vibrational properties of LiAl(IO3)4 have been studied by UV-Vis and IR spectroscopy. LiAl(IO3)4 is found to crystallize in the non-centrosymmetric, monoclinic P21 space group, contrary to what was reported previously. Theoretical simulations and Rietveld refinements of crystal structure support this finding, together with the relatively high Second Harmonic Generation (SGH) response that was observed. Electronic band structure calculations show that LiAl(IO3)4 crystal has an indirect band gap Egap=3.68 eV, in agreement with the experimental optical band gap Egap=3.433 eV. The complex relative permittivity and the refraction index of LiAl(IO3)4 have also been calculated as a function of energy, as well as its elastic constants and mechanical parameters. LiAl(IO3)4 is found to be a very compressible and ductile material. Our findings imply that LiAl(IO3)4 is a promising material for optoelectronic and non -linear optical applications.

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.

Wavelength-Selective Nonlinear Imaging and Photo-Induced Cell Damage by Dielectric Harmonic Nanoparticles.

We introduce a nonlinear all-optical theranostics protocol based on the excitation wavelength decoupling between imaging and photoinduced damage of human cancer cells labeled by bismuth ferrite (BFO) harmonic nanoparticles (HNPs). To characterize the damage process, we rely on a scheme for in situ temperature monitoring based on upconversion nanoparticles: by spectrally resolving the emission of silica coated NaGdF4:Yb3+/Er3+ nanoparticles in close vicinity of a BFO HNP, we show that the photointeraction upon NIR-I excitation at high irradiance is associated with a temperature increase >100 °C. The observed laser-cell interaction implies a permanent change of the BFO nonlinear optical properties, which can be used as a proxy to read out the outcome of a theranostics procedure combining imaging at 980 nm and selective cell damage at 830 nm. The approach has potential applications to monitor and treat lesions within NIR light penetration depth in tissues.

Nanodoublers as deep imaging markers for multi-photon microscopy.

We demonstrate the possibility to excite second-harmonic (SH) active Fe(IO(3))(3) nanocrystals with two distinct laser sources at 800 and 1550 nm, and we show, by a complementary experimental and numerical study, how the wavelength flexibility inherent to non-phase-matched SH nanoparticles can be efficiently exploited to increase imaging penetration depth of markers embedded in biological samples.

Author Correction: Preparation from a revisited wet chemical route of phase-pure, monocrystalline and SHG-efficient BiFeO3 nanoparticles for harmonic bio-imaging.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Two-Photon-Triggered Photorelease of Caged Compounds from Multifunctional Harmonic Nanoparticles.

The design of stimuli-responsive nanocarriers has raised much attention to achieve higher local concentration of therapeutics and mitigate the appearance of drug resistance. The combination of imaging properties and controlled photorelease of active molecules within the same nanoconjugate has a great potential for theranostic applications. In this study, a system for NIR light-triggered release of molecular cargos induced by the second harmonic emission from bismuth ferrite harmonic nanoparticles (BFO HNPs) is presented. Silica-coated BFO HNPs were covalently conjugated to a photocaging tether based on coumarin (CM) and l-tryptophan (Trp) as a model molecular cargo. Upon femtosecond pulsed irradiation at 790 nm, Trp was efficiently released from the NP surface in response to the harmonic emission of the nanomaterial at 395 nm. The emitted signal induced the photocleavage of the CM-Trp carbamate linkage resulting in the release of Trp, which was monitored and quantified by ultrahigh performance liquid chromatography-mass spectrometry (UHPLC-MS). While a small fraction of the uncaging process could be attributed to the nonlinear absorption of CM derivatives, the main trigger responsible for Trp release was established as the second harmonic signal from BFO HNPs. This strategy may provide a new way for the application of functionalized HNPs in dual imaging delivery theranostic protocols.

Nano-FROG: Frequency resolved optical gating by a nanometric object.

We present a technique to characterize ultrashort pulses at the focal plane of a high numerical aperture objective with unprecedented spatial resolution, by performing a FROG measurement with a single nanocrystal as nonlinear medium. This approach can be extended to develop novel phase-sensitive techniques in laser scanning microscopy, probing the microscopic environment by monitoring phase and amplitude distortions of femtosecond laser pulses.

Image Correlation Spectroscopy with Second Harmonic Generating Nanoparticles in Suspension and in Cells.

The absence of photobleaching, blinking, and saturation combined with a high contrast provides unique advantages of higher-harmonic generating nanoparticles over fluorescent probes, allowing for prolonged correlation spectroscopy studies. We apply the coherent intensity fluctuation model to study the mobility of second harmonic generating nanoparticles. A concise protocol is presented for quantifying the diffusion coefficient from a single spectroscopy measurement without the need for separate point-spread-function calibrations. The technique's applicability is illustrated on nominally 56 nm LiNbO3 nanoparticles. We perform label-free raster image correlation spectroscopy imaging in aqueous suspension and spatiotemporal image correlation spectroscopy in A549 human lung carcinoma cells. In good agreement with the expected theoretical result, the measured diffusion coefficient in water at room temperature is (7.5 ± 0.3) µm2/s. The diffusion coefficient in the cells is more than 103 times lower and heterogeneous, with an average of (3.7 ± 1.5) × 10-3 µm2/s.