Osip Schwartz

Background-free third harmonic imaging of gold nanorods.

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.

Superresolution microscopy with quantum emitters.

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.

Two-color antibunching from band-gap engineered colloidal semiconductor nanocrystals.

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.

Cooperative Lamb Shift in a Mesoscopic Atomic Array

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[3–5] 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[6–10]. 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

Improved resolution in fluorescence microscopy using quantum correlations

Breaking the diffraction limit in microscopy by utilizing quantum properties of light has been the goal of intense research in the recent years. We propose a quantum superresolution technique based on non-classical emission statistics of fluorescent markers, routinely used as contrast labels for bio-imaging. The technique can be readily implemented using standard fluorescence microscopy equipment.

Vectorial Phase Retrieval for Linear Characterization of Attosecond Pulses

The waveforms of attosecond pulses produced by high-harmonic generation carry information on the electronic structure and dynamics in atomic and molecular systems. Current methods for the temporal characterization of such pulses have limited sensitivity and impose significant experimental complexity. We propose a new linear and all-optical method inspired by widely used multidimensional phase retrieval algorithms. Our new scheme is based on the spectral measurement of two attosecond sources and their interference. As an example, we focus on the case of spectral polarization measurements of attosecond pulses, relying on their most fundamental property-being well confined in time. We demonstrate this method numerically by reconstructing the temporal profiles of attosecond pulses generated from aligned CO(2) molecules.

Colloidal quantum dots as saturable fluorophores

Although colloidal quantum dots (QDs) exhibit excellent photostability under mild excitation, intense illumination makes their emission increasingly intermittent, eventually leading to photobleaching. We study fluorescence of two commonly used types of QDs under pulsed excitation with varying power and repetition rate. The photostability of QDs is found to improve dramatically at low repetition rates, allowing for prolonged optical saturation of QDs without apparent photodamage. This observation suggests that QD blinking is facilitated by absorption of light in a transient state with a microsecond decay time. Enhanced photostability of generic quantum dots under intense illumination opens up new prospects for fluorescence microscopy and spectroscopy.

Using variable pupil filters to optimize the resolution in multiphoton and saturable fluorescence confocal microscopy.

Pupil filters are widely used to improve the resolution of confocal microscopes. We analyze the possibilities of applying them to N-photon microscopy. We find that taking a linear combination of images obtained with several pupil filters can improve the resolution by a factor of N (compared to a conventional microscope). When applied to saturable fluorescence, this technique allows one to observe fluorescent objects with, in principle, unlimited spatial resolution.

Fluorescence antibunching microscopy

Utilizing quantum properties of light to break the diffraction limit has been the goal of intense research in the recent years. This paper is a progress report on a study aimed at experimentally demonstrating a superresolution microscopy technique enabled by photon antibunching, a non-classical emission statistics feature exhibited by most emitters used as fluorescent markers. We find that photon antibunching gives rise to correlations that encode high spatial frequency information on the distribution of fluorescent emitters. Detecting these correlations using photon counting instrumentation in a standard fluorescence microscope setting allows for three-dimensional superresolution imaging of fluorophore stained samples. The technique provides a quantum alternative to the established superresolution tools.

Shot noise limited characterization of ultraweak femtosecond pulse trains.

Ultrafast science is inherently, due to the lack of fast enough detectors and electronics, based on nonlinear interactions. Typically, however, nonlinear measurements require significant powers and often operate in a limited spectral range. Here we overcome the difficulties of ultraweak ultrafast measurements by precision time-domain localization of spectral components. We utilize this for linear self-referenced characterization of pulse trains having ∼ 1 photon per pulse, a regime in which nonlinear techniques are impractical, at a temporal resolution of ∼ 10 fs. This technique does not only set a new scale of sensitivity in ultrashort pulse characterization, but is also applicable in any spectral range from the near-infrared to the deep UV.