International audienceWe demonstrate theoretically and experimentally that the three-dimensional orientation of a single fluorescent nanoemitter can be determined by polarization analysis of the emitted light (while excitation polarization analysis provides only the in-plane orientation). The determination of the emitter orientation by polarimetry requires a theoretical description, including the objective numerical aperture, the 1D or 2D nature of the emitting dipole, and the environment close to the dipole. We develop a model covering most experimentally relevant microscopy configurations and provide analytical relations that are useful for orientation measurements. We perform polarimetric measurements on high-quality core-shell CdSe/CdS nanocrystals and demonstrate that they can be approximated by two orthogonal degenerated dipoles. Finally, we show that the orientation of a dipole can be inferred by polarimetric measurement, even for a dipole in the vicinity of a gold film, while in this case, the well-established defocused microscopy is not appropriate

Plasmonic near fields, wherein light is magnified and focused within nanoscale volumes, are utilized in a broad array of technologies including optoelectronics, catalysis, and sensing. Within these nanoscale cavities, increases in temperature are expected and indeed have been demonstrated. Heat generation can be beneficial or detrimental for a given system or technique, but in either case it is useful to have knowledge of local temperatures. Surface-enhanced Raman spectroscopy (SERS), potentially down to the limit of single-molecule (SM) detection, has been suggested as a viable route for measuring nanoscale temperatures through simultaneous collection of Stokes and anti-Stokes SER scattering, as the ratio of their intensities is related to the Boltzmann distribution. We have rigorously verified SM detection in anti-Stokes SERS of rhodamine 6G on aggregated Ag nanoparticles using the isotopologue method. We observe a broad distribution in the ratio of anti-Stokes and Stokes signal intensities among SM events. An equivalent distribution in high-coverage, single-aggregate SERS suggests that the observed variance is not a SM phenomenon. We find that the variance is instead caused by a combination of local heating differences among hot spots as well as variations in the near-field strength as a function of frequency, effectively causing nonequivalent enhancement factors (EFs) for anti-Stokes and Stokes scattering. Additionally, we demonstrate that dark-field scattering cannot account for the frequency dependence of the optical near field. Finite-difference time-domain simulations for nanoparticle aggregates predict a significant wavelength dependence to the ratio of anti-Stokes/Stokes EFs, confirming that the observed variation in this ratio has strong nonthermal contributions. Finally, we outline the considerations that must be addressed in order to accurately evaluate local temperatures using SERS.

Ultrafast optical pump, X-ray diffraction probe experiments were performed on CdSe nanocrystal (NC) colloidal dispersions as functions of particle size, polytype, and pump fluence. Bragg peak shifts related to heating and peak amplitude reduction associated with lattice disordering are observed. For smaller NCs, melting initiates upon absorption of as few as ∼15 electron-hole pair excitations per NC on average (0.89 excitations/nm for a 1.5 nm radius) with roughly the same excitation density inducing melting for all examined NCs. Diffraction intensity recovery kinetics, attributable to recrystallization, occur over hundreds of picoseconds with slower recoveries for larger particles. Zincblende and wurtzite NCs revert to initial structures following intense photoexcitation suggesting melting occurs primarily at the surface, as supported by simulations. Electronic structure calculations relate significant band gap narrowing with decreased crystallinity. These findings reflect the need to consider the physical stability of nanomaterials and related electronic impacts in high intensity excitation applications such as lasing and solid-state lighting.

We use arrays of liquid crystal defects, linear smectic dislocations, to trap semi-conductor CdSe/CdS dot-in-rods which behave as single photon emitters. We combine measurements of the emission diagram together with measurements of the emitted polarization of the single emitters. We show that the dot-inrods are confined parallel to the linear defects to allow for a minimization of the disorder energy associated with the dislocation cores. We demonstrate that the electric dipoles associated with the dotin-rods, tilted with respect to the rods, remain oriented in the plane including the smectic linear defects and being perpendicular to the substrate, most likely due to the dipole/dipole interactions between the dipoles of the liquid crystal molecules and the dot-in-rods ones. Using smectic dislocations, we can consequently orient nanorods along a unique direction for a given substrate, independently of the ligands' nature, without any induced aggregation, leading as well to a fixed azimuthal orientation for the associated dot-in-rods' dipoles. These results open the way for a fine control of nanoparticle anisotropic optical properties, in particular a fine control of single photon emission polarization.Control of single photon emitters is a major objective in the field of nanophotonics.[1] The synthesis of colloidal semiconductor inorganic nanocrystals having specific light-emission properties has been providing important advances in this field. In particular, recent developments in synthesis methodologies, fully compatible with standard nanofabrication technologies have enabled a superior 3 control on nanocrystals composition and morphology.Rod-shaped nanocrystals showing pronounced polarization, behaving as emitting linear dipoles, have been obtained. [2][3][4] The encapsulation of a spherical core into a rod-like shell [5] resulted in non-blinking inorganic single photon emitters, [6] hereafter referred to as dot-in-rods (DRs). Moreover it has been recently shown that, by increasing the thickness of the shell, it is possible to greatly suppress photoluminescence blinking and to improve DRs overall photo-stability, while keeping a low probability of multi-photon emission. [7] Such features are of primary importance when nanocrystalsare used in applications demanding a control of photons'polarization, such as coupling with complex photonic cavities [8][9] or quantum cryptography.[10] The control of the polarization of the emitted light also requires the capacity to control the particle orientation. Howevertechnologies aimed at guiding nanocrystal orientation at the single particle level are still poorly discussed in literature.Alignednanoparticleshave been obtained through mechanical rubbing, [11] short-range interactions [12][13] or patterned substrates. [14] Liquid crystal-like structures, composed of alarge number of elongated nanocrystalsassembled in multi-layers have also been evidenced on both solid substrates [15][16][17][18] and water films. [18][19][20] Orientation and positional ordering of CdS and CdSe...

We use polarization resolved micro-photoluminescence to analyze the dipolar nature of single core/shell cadmium selenide/cadmium sulfide (CdSe/CdS) dotin-rods. Polarization analysis, anisotropy measurements on more than 400 nanoparticles, and defocused imaging suggest that these nanoparticles behave as linear dipoles. The same methods were also used to determine the threedimensional orientation of the emission dipole, which proved to be consistent with the hypothesis of a linear dipole tilted with respect to the rod axis. Moreover, we observe that for high-energy pumping, the excitation transition of the dot-in-rod cannot be approximated by a single linear dipole, contrary to the emission transition.

Three-dimensional plasmonic superlattice microcavities, made from programmable atom equivalents comprising gold nanoparticles functionalized with DNA, are used as a testbed to study directional light emission. DNA-guided nanoparticle colloidal crystallization allows for the formation of micrometer-scale single-crystal bodycentered cubic gold nanoparticle superlattices, with dye molecules coupled to the DNA strands that link the particles together, in the form of a rhombic dodecahedron. Encapsulation in silica allows one to create robust architectures with the plasmonically active particles and dye molecules fixed in space. At the micrometer scale, the anisotropic rhombic dodecahedron crystal habit couples with photonic modes to give directional light emission. At the nanoscale, the interaction between the dye dipoles and surface plasmons can be finely tuned by coupling the dye molecules to specific sites of the DNA particle-linker strands, thereby modulating dye-nanoparticle distance (three different positions are studied). The ability to control dye position with subnanometer precision allows one to systematically tune plasmon-excition interaction strength and decay lifetime, the results of which have been supported by electrodynamics calculations that span length scales from nanometers to micrometers. The unique ability to control surface plasmon/ exciton interactions within such superlattice microcavities will catalyze studies involving quantum optics, plasmon laser physics, strong coupling, and nonlinear phenomena.DNA programmable assembly | directional emission | anisotropic 3D microcavity | nanoparticle surface plasmon | fluorescence enhancement M icrocavities are important photonic architectures that can be used to couple dipole emitters with optical modes and enhance light-matter interactions typically with long cavity lifetimes (high Q factors). They have been used to understand phenomena in cavity quantum electrodynamics (1, 2), and to enable molecular sensing (3) and lasing (4) technologies. In recent years, researchers have developed strategies for incorporating plasmonic nanostructures into microcavity structures to enhance light-matter interactions via strong light confinement (small mode volume, V), resulting in materials that are capable of plasmon lasing (5-7). These plasmonic microcavities exploit two different aspects of the architectures simultaneously: guided optical modes that resonate or scatter light (via the microcavity geometry) and near-field-driven optical confinement (via the metallic nanostructure). Such hybrid photonic structures are desirable because they can lead to high Q factors and strong light confinement within a single architecture, significantly enhancing light-matter interactions by a large cavity figure of merit, Q/V. However, most of the plasmonic microcavities have been limited to 1D or 2D microcavity geometries and plasmonic mode confinement (7) (metallic nanostructures) due to limitations in fabrication methods (6,8). Importantly, 3D counterparts of these 1D and 2D a...

Localized surface plasmon resonances (LSPRs) arising from metallic nanoparticles offer an array of prospective applications that range from chemical sensing to biotherapies. Bipyramidal particles exhibit particularly narrow ensemble LSPR resonances that reflect small dispersity of size and shape but until recently were only synthetically accessible over a limited range of sizes with corresponding aspect ratios. Narrow size dispersion offers the opportunity to examine ensemble dynamical phenomena such as coherent phonons that induce periodic oscillations of the LSPR energy. Here, we characterize transient optical behavior of a large range of gold bipyramid sizes, as well as higher aspect ratio nanojavelin ensembles with specific attention to the lowest-order acoustic phonon mode of these nanoparticles. We report coherent phonon-driven oscillations of the LSPR position for particles with resonances spanning 670 to 1330 nm. Nanojavelins were shown to behave similarly to bipyramids but offer the prospect of separate control over LSPR energy and coherent phonon oscillation period. We develop a new methodology for quantitatively measuring mechanical expansion caused by photogenerated coherent phonons. Using this method, we find an elongation of approximately 1% per photon absorbed per unit cell and that particle expansion along the lowest frequency acoustic phonon mode is linearly proportional to excitation fluence for the fluence range studied. These characterizations provide insight regarding means to manipulate phonon period and transient mechanical deformation.

The vibrational and optical properties of Ag20, Ag84, and Ag120 closed-shell clusters are investigated through a combination of continuum mechanics and density functional theory approaches. The acoustic vibrational frequencies associated with these tetrahedral silver clusters are found to be in close correspondence for the two theories, demonstrating the ability of finite-element calculations to reproduce first-principles computational results, even down to few-atom structures. TDDFT calculations of the absorption spectra of these clusters indicate a strong plasmon-like mode both for equilibrium structures of the clusters and for structures where the acoustic breathing mode is excited by amounts that are accessible to ultrafast experiments. The plasmon-like mode energy is found to vary linearly with the acoustic mode displacement (for small displacements), with a slope that increases with increasing cluster size. For larger clusters, the TDDFT slope is larger than the FDTD slope, which indicates that there are systematic errors in the continuum theory result for small particles. We also examine the ground and plasmon excited potential energy curves and show that the displacement in equilibrium geometry between these curves is too small to give breathing mode excitation that is consistent with observations based on vertical excitation alone. This suggests that breathing mode excitation arises during internal conversion after the initial photoexcitation.