Biological photonic systems composed of anhydrous guanine crystals evolved separately in several taxonomic groups. Here, two such systems found in fish and spiders, both of which make use of anhydrous guanine crystal plates to produce structural colors, are examined. Measurements of the photonic‐crystal structures using cryo‐SEM show that the crystal plates in both fish skin and spider integument are ∼20‐nm thick. The reflective unit in the fish comprises stacks of single plates alternating with ∼230‐nm‐thick cytoplasm layers. In the spiders the plates are formed as doublet crystals, cemented by 30‐nm layers of amorphous guanine, and are stacked with ∼200 nm of cytoplasm between crystal doublets. They achieve light reflective properties through the control of crystal morphology and stack dimensions, reaching similar efficiencies of light reflectivity in both fish skin and spider integument. The structure of guanine plates in spiders are compared with the more common situation in which guanine occurs in the form of relatively unorganized prismatic crystals, yielding a matt white coloration.

The observed intermittent light emission from colloidal semiconductor nanocrystals has long been associated with Auger recombination assisted quenching. We test this view by observing transient emission dynamics of CdSe/CdS/ZnS semiconductor nanocrystals using time-resolved photon counting. The size and intensity dependence of the observed decay dynamics seem inconsistent with those expected from Auger processes. Rather, the data suggest that in the "off" state the quantum dot cycles in a three-step process: photoexcitation, rapid trapping, and subsequent slow nonradiative decay.

The optical diffraction limit imposes a bound on imaging resolution in classical optics. Over the last twenty years, many theoretical schemes have been presented for overcoming the diffraction barrier in optical imaging using quantum properties of light. Here, we demonstrate a quantum superresolution imaging method taking advantage of nonclassical light naturally produced in fluorescence microscopy due to photon antibunching, a fundamentally quantum phenomenon inhibiting simultaneous emission of multiple photons. Using a photon counting digital camera, we detect antibunching-induced second and third order intensity correlations and perform subdiffraction limited quantum imaging in a standard wide-field fluorescence microscope.

Light is the tool of the 21 st century. New photosensitive tools offer the possibility to monitor and control neuronal activity from the sub-cellular to the integrative level. This ongoing revolution has motivated the development of new optical methods for light stimulation. Among them, it has been recently demonstrated that a promising approach is based on the use of wavefront shaping to generate optically confined extended excitation patterns. This was achieved by combining the technique of temporal focusing with different approaches for lateral light shaping including low numerical aperture Gaussian beams, holographic beams and beams created with the generalized phase contrast method. What is needed now is a precise characterization of the effect of scattering on these different methods in order to extend their use for in depth excitation.Here we present a theoretical and experimental study on the effect of scattering on the propagation of wavefront shaped beams. Results from fixed and acute cortical slices show that temporally focused spatial patterns are extremely robust against the effects of scattering and this permits their three-dimensional confinement for depths up to 550 µm.3 \body

Whenever several quantum light emitters are brought in proximity with one another, their interaction with common electromagnetic fields couples them, giving rise to cooperative shifts in their resonance frequency. Such collective line shifts are central to modern atomic physics, being closely related to superradiance[1] on one hand and the Lamb shift[2] on the other. Although collective shifts have been theoretically predicted more than fifty years ago [3], the effect has not been observed yet in a controllable system of a few isolated emitters. Here, we report a direct spectroscopic observation of the cooperative shift of an optical electric dipole transition in a system of up to eight Sr + ions suspended in a Paul trap. We study collective resonance shift in the previously unexplored regime of far-field coupling, and provide the first observation of cooperative effects in an array of quantum emitters. These results pave the way towards experimental exploration of cooperative emission phenomena in mesoscopic systems.Soon after the discovery of superradiance by Dicke[1], it was realized [3][4][5] that superradiance phenomena are accompanied by a dispersive counterpart that shifts the resonance energies of the collective excitations relative to those of isolated emitters. The superradiance effects and the resonance shift originate, respectively, from the real and imaginary parts of resonant dipole-dipole interaction between emitters. The collective shifts arise via emission and reabsorption of virtual photons, and are therefore referred to as cooperative Lamb shift [6][7][8][9][10].Although cooperative phenomena have received a great deal of scientific attention, the experimental observations of collective Lamb shift have been relatively few. Cooperative shifts have been detected in a three-photon excitation resonance in Xenon [11] and, recently, in the absorption line of Rubidium vapor confined to an ultrathin cell [7]. In both cases, the cooperative shifts, arising from statistically averaged interaction of a large ensemble of atoms, were proportional to the atomic density.In a different approach, the energy level shifts due to resonant dipole-dipole interaction in the near field were studied in a system of two fluorescent molecules embedded in a dielectric film [12]. Such near-field interactions * These authors contributed equally to this work have also played an essential role in a number of experiments with Rydberg atoms [13][14][15]. In particular, the near-field cooperative shift in a system of two atoms has been utilized to prevent the transition of more than one atom to the Rydberg state, bringing about a phenomenon known as Rydberg blockade [16][17][18].Cooperative phenomena can be amplified by placing the emitters inside a resonator. Cavity-enhanced cooperative frequency shift in a nuclear excitation has been observed in a layer of Fe atoms embedded in a planar waveguide [8]. The coupling between emitters can also be enhanced by interaction with a single mirror. Such arrangement enabled the observation...

Transmission electron microscopy (TEM) of rapidly frozen biological specimens, or cryo-EM, would benefit from the development of a phase plate for in-focus phase contrast imaging. Several types of phase plates have been investigated, but rapid electrostatic charging of all such devices has hindered these efforts. Here, we demonstrate electron phase manipulation with a high-intensity continuous-wave laser beam, and utilize it as a phase plate for TEM. We demonstrate the laser phase plate by imaging an amorphous carbon film. The laser phase plate provides a stable and tunable phase shift without charging or unwanted electron scattering. These results suggest the possibility for dose-efficient imaging of unstained biological macromolecules and cells.

Photon antibunching is ubiquitously observed in light emitted from quantum systems but is usually associated only with the lowest excited state of the emitter. Here, we devise a fluorophore that upon photoexcitation emits in either one of two distinct colors but exhibits strong antibunching between the two. This work demonstrates the possibility of creating room-temperature quantum emitters with higher complexity than effective two level systems via colloidal synthesis.

Surface plasmon resonance exhibited by noble metal nanoparticles makes them attractive agents for advanced microscopic imaging applications. In this work we study third harmonic generation in gold nanorods under conditions of resonance of the laser frequency with the longitudinal plasmon mode. Large resonant enhancement and the symmetry properties of third harmonic generation allow for background-free, orientation sensitive optical imaging of individual nanoparticles.