Semiconductor nanocrystals feature multiply-excited states that display intriguing physics and significantly impact nanocrystal-based technologies. Fluorescence supplies a natural probe to investigate these states. Still, direct observation of multiexciton fluorescence has proved elusive to existing spectroscopy techniques. Heralded Spectroscopy is a new tool based on a breakthrough single-particle, single-photon, sub-nanosecond spectrometer that utilizes temporal photon correlations to isolate multiexciton emission. This proceedings paper introduces Heralded Spectroscopy and reviews some of the novel insights it uncovered into exciton–exciton interactions within single nanocrystals. These include weak exciton–exciton interactions and their correlation with quantum confinement, biexciton spectral diffusion, multiple biexciton species and biexciton emission polarization.
A temporal correlation superresolution image is based on the variance of the recorded photon time trace, while its resolution is higher than that of the complementary intensity image, it is noisier. Both images, the intensity and correlation based, are fed into a deep convolutional neural network (CNN), which produces an image that is optimized to have higher resolution than the intensity image and less noise than the correlation image. The image then passes through separate linear networks that mimic the physical blurring of the imaging setup. Preliminary experimental results show similar resolution to the experimental superresolution image with less noise.
We use SPAD array technology to study carrier dynamics and spectroscopy, and particularly those of multiply excited states in semiconductor quantum dots at room temperature and at the single particle level. This helps to reveal new information inaccessible via either ensemble spectroscopy or conventional single-particle measurements.
Since the introduction of the intensity interferometer by Hanbury Brown and Twiss in the '50s and its subsequent adaptations to quantum optic experiments, photon correlations have played a significant role in optical research. An emerging detector technology, SPAD arrays, presents new opportunities to scale these experiments and exploit photon correlations in new regimes. I will describe a new temporal-spectral photon correlation technique based on these detectors, single-particle heralded spectroscopy. I will present new insights into the photophysics of multiply-excited semiconductor quantum dots uncovered by this method, hitherto inaccessible to existing approaches.
A fluorescence correlation contrast is a straightforward approach to super-resolution imaging. Combining a SPAD array with a novel detection scheme (ISM), we obtain images with up to x4 times resolution enhancement.
Low frequency vibrations (< 200cm^(-1)) contain the structural information of molecules or the crystal lattice - making low frequency Raman imaging an ideal candidate to analyze heavy molecules, crystal formation etc. The longer integration times of spontaneous Raman spectroscopy, however, limits the study of dynamic structural changes.
In our work, we have built a stimulated Raman scattering (SRS) pump probe scheme combining impulsive excitation with a fast acousto-optics delay line. With a pixel dwell time of 25μs, we have acquired, on a shot noise limited detection system, sub second low frequency (< 200cm^(-1)) hyper-spectral SRS images of various samples.
The emerging field of quantum imaging introduces new methods to overcome classical limitations in optical microscopy. A detection apparatus capable of analyzing the quantum signature of light, is a crucial component in the heart of any such method.
We present a novel quantum imaging modality, based on a state-of-the-art single photon avalanche diode (SPAD) array in a confocal setup. This modality enables unprecedented simplicity and scalability in imaging temporal photon correlations. We demonstrate the potential of this approach by measuring temporal correlations of classical and quantum sources, as well as demonstrate a quantum super-resolution technique.
Multicore fiber bundles are attractive candidates for lensless endscopes because of their ruggedness and the relative simplicity involved in their calibration and operation. Nevertheless, the measurement of the transmission matrix of the MCF is still an interferometric process, typically requiring high stability and possibly sequential measurements. A key challenge is to replace this with a much simpler and robust technique – phase retrieval. In this talk, we will examine the major challenge in using phase retrieval in conventional MCFs. This is related to the discrete and periodic nature of the auto-correlation of an ordered MCF, resulting in the stagnation of phase retrieval algorithms in one of a multitude of local minimums. We employ phase diversity, i.e. more the one complex illumination pattern with a known phase profile can help overcome this issue. In particular, we identify that spiral phase patterns are well-suited due to their generation of complementary speckle patterns, resulting in highly non-redundant information. We experimentally demonstrate that three intensity images are sufficient to retrieve the transmission matrix with very high accuracy and success rates. Furthermore, we will also present a novel disordered MCF, which facilitates phase retrieval with a single intensity image and a priori knowledge of the core positions. This is a simple and rapidly converging method which relies on the aperiodic arrangement and the sparsity of the cores. Both these computationally inexpensive techniques highlight the potential of phase retrieval as a tool for robust phase calibration of fiber bundles in lensless imaging.
KEYWORDS: Microscopy, Sensors, Signal to noise ratio, Image enhancement, Point spread functions, Confocal microscopy, Diffraction, Image resolution, Super resolution, Single photon
Technological advancements in the creation, manipulation and detection of quantum states of light have motivated the application of such states to overcome classical limits in sensing and imaging. In particular, there has been a surge of recent interest in super-resolution imaging based on principles of quantum optics. However, the application of such schemes for practical imaging of biological samples is demanding in terms of signal-to-noise ratio, speed of acquisition and robustness with respect to sample labeling. Here, we re-introduce the concept of quantum image scanning microscopy (Q-ISM), a super-resolution method that enhances the classical image scanning microscopy (ISM) method by measuring photon correlations. Q-ISM was already utilized to achieve super-resolved images of a biological sample labeled with fluorescent nanoscrystals whose contrast is based entirely on a quantum optical phenomenon, photon antibunching. We present here an experimental demonstration of the method and discuss with further details its prospects for application in life science microscopy.
Upconversion (UC) is a nonlinear process in which two, or more, long wavelength photons are converted to a shorter wavelength photon. This process is based on sequential absorption of two or more photons, involving metastable, long lived intermediate energy states, thus is not restricted to ultrashort pulsed excitation. Hence, requirements for UC processes are long lived excited states, a ladder like arrangement of energy levels and a mechanism inhibiting cooling of the hot charge carrier. UC holds great promise for bioimaging, enabling to perform multiphoton imaging in scattering specimen at very low powers. Rare-earth-doped nanocrystals, the most commonly used ones for UC, typically require a minimal particle diameter of several tens of nanometers and have a limited action spectrum. Here, we present a novel luminescence upconversion nano-system based on colloidal semiconductor double quantum dots, consisting of a NIR-absorbing component and a visible emitting component separated by a tunneling barrier in a spherical onion-like geometry. These dual near-infrared and visible core/shell/shell PbSe/CdSe/CdS nanocrystals are shown to efficiently upconvert a broad range of NIR wavelengths up to 1.2 microns to visible emission at room temperature, covering a spectral range where there are practically no alternative upconversion systems. The particle diameter is less than ten nanometers, and the synthesis enables versatility and tunability of both the visible emission color and the NIR absorption edge. The physical mechanism for upconversion in this type of structures, as well as potential advances and extensions on this system will be discussed.
The periodic arrangement of core positions in multi-core fiber bundles introduces ‘ghost’ artifacts to endoscopic images obtained through them, whether in wide-field imaging (based on either direct imaging or speckle correlations) or in confocal scanning microscopy using wavefront shaping. Here we introduce partially disordered multi-core bundles as a means to overcome these artifacts. The benefits of their use will be discussed in the context of multiphoton scanning microscopy utilizing a spatial light modulator in the proximal end, and in the more general case of widefield imaging. We also show that both numerically and experimentally that the presence of disorder also enables to apply phase retrieval methods to characterize the phase distortion introduced due to propagation in the bundle without the need of an interferometrically stabilized reference. Thus, in addition to overcoming the challenge of ghost artifacts, disordered multi-core fibers have the potential to overcome another challenge, movement-induced phase distortions, by enabling real-time characterization of this phase distortion in reflection mode only via the proximal end.
Far-field optical microscopy beyond the Abbe diffraction limit, making use of nonlinear excitation (e.g. STED), or temporal fluctuations in fluorescence (PALM, STORM, SOFI) is already a reality. In contrast, overcoming the diffraction limit using non-classical properties of light is very difficult to achieve due to the fragility of quantum states of light. Here, we experimentally demonstrate superresolution microscopy based on quantum properties of light naturally emitted by fluorophores used as markers in fluorescence microscopy. Our approach is based on photon antibunching, the tendency of fluorophores to emit photons one by one rather than in bursts. Although a distinctively quantum phenomenon, antibunching is readily observed in most common fluorophores even at room temperature.
This nonclassical resource can be utilized directly to enhance the imaging resolution, since the non-classical far-field intensity correlations induced by antibunching carry high spatial frequency information on the spatial distribution of emitters. Detecting photon statistics simultaneously in the entire field of view, we were able to detect non-classical correlations of the second and third order, and reconstructed images with resolution significantly beyond the diffraction limit.
Alternatively, we demonstrate the utilization of antibunching for augmenting the capabilities of localization-based superresolution imaging in the presence of multiple emitters, using a novel detector comprised of an array of single photon detectors connected to a densely packed fiber bundle. These features allow us to enhance the spatial and temporal resolution with which multiple emitters can be imaged compared with other techniques that rely on CCD cameras.
Upconversion (UC) is a nonlinear process in which two, or more, long wavelength photons are converted to a shorter wavelength photon. This process is based on sequential absorption of two or more photons, involving metastable, long lived intermediate energy states, thus is not restricted to upconversion of coherent laser radiation as a non-coherent process. Hence, requirements for UC processes are long lived excited states, a ladder like arrangement of energy levels and a mechanism inhibiting cooling of the hot charge carrier. UC holds great promise for bioimaging, enabling spatially resolved imaging in a scattering specimen and for photovoltaic devices as a mean to surpass the Shockley-Queisser efficiency limit.
Here, we present a novel luminescence upconversion nano-system based on colloidal semiconductor double quantum dots, consisting of a NIR-emitting component and a visible emitting component separated by a tunneling barrier in a spherical onion-like geometry. These dual near-infrared and visible emitting core/shell/shell PbSe/CdSe/CdS nanocrystals are shown to upconvert a broad range of NIR wavelengths to visible emission at room temperature, covering a spectral range where there are practically no alternative upconversion systems. The synthesis is a three-step process, which enables versatility and tunability of both the visible emission color and the NIR absorption edge. Using this method one can achieve a range of desired upconverted emission peak positions with a suitable NIR band gap. The physical mechanism for upconversion in this structure, as well as possible extensions and improvements will be discussed. 1
(1) Teitelboim, A.; Oron, D. ACS Nano 2015, acsnano.5b05329.
The phase retrieval problem arises in various fields ranging from physics and astronomy to biology and microscopy. Computational reconstruction of the Fourier phase from a single diffraction pattern is typically achieved using iterative alternating projections algorithms imposing a non-convex computational challenge. A different approach is holography, relying on a known reference field. Here we present a conceptually new approach for the reconstruction of two (or more) sufficiently separated objects. In our approach we combine the constraint the objects are finite as well as the information in the interference between them to construct an overdetermined set of linear equations. We show that this set of equations is guaranteed to yield the correct solution almost always and that it can be solved efficiently by standard numerical algebra tools. Essentially, our method combine commonly used constraint (that the object is finite) with a holographic approach (interference information). It differs from holographic methods in the fact that a known reference field is not required, instead the unknown objects serve as reference to one another (hence blind holography). Our method can be applied in a single-shot for two (or more) separated objects or with several measurements with a single object. It can benefit phase imaging techniques such as Fourier phytography microscopy, as well as coherent diffractive X-ray imaging in which the generation of a well-characterized, high resolution reference beam imposes a major challenge. We demonstrate our method experimentally both in the optical domain and in the X-ray domain using XFEL pulses.
Super-resolution microscopy, the imaging of features below the Abbe diffraction limit, has been achieved by a number
of methods in recent years. Each of these methods relies on breaking one of the assumptions made in the derivation of
the diffraction limit. While uniform spatial illumination, linearity and time independence have been the most common
cornerstones of the Abbe limit broken in super-resolution modalities, breaking the ‘classicality of light’ assumption as a
pathway to achieve super-resolution has not been shown. Here we demonstrate a method that utilizes the antibunching
characteristic of light emitted by Quantum Dots (QDs), a purely quantum feature of light, to obtain imaging beyond the
diffraction limit.
Measuring such high order correlations in the emission of a single QD necessitates stability at saturation conditions
while avoiding damage and enhanced blinking. This ability was facilitated through new understandings that arisen from
exploring the QD ‘blinking’ phenomena. We summarize here two studies that contributed to our current understanding
of QD stability.
We investigate the quantum confined Stark effect (QCSE) of various nanoparticles (NPs) on the single molecule level at
room temperature. We tested 8 different NPs with different geometry, material composition and electronic structure, and
measured their QCSE by single molecule spectroscopy. This study reveals that suppressing the Coulomb interaction force between electron and hole by asymmetric type-II interface is critical for an enhanced QCSE. For example, ZnSe-CdS and CdSe(Te)-CdS-CdZnSe asymmetric nanorods (type-II) display respectively twice and more than three times larger QCSE than that of simple type-I nanorods (CdSe). In addition, wavelength blue-shift of QCSE and roughly linear Δλ-F (emission wavelength shift vs. the applied electric field) relation are observed for the type-II nanorods. Experimental results (Δλ-F or ΔE-F) are successfully reproduced by self-consistent quantum mechanical calculation.
Intensity reduction in blue-shifted spectrum is also accounted for. Both calculations and experiments suggest that the
magnitude of the QCSE is predominantly determined by the degree of initial charge separation in these structures.
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.
Interactions between femtosecond solitons in a nonlinear photonic-crystal fiber are of fundamental interest. But
many practical applications would abound if solitons could be arbitrarily superposed into multiples in the fiber.
Here, we numerically and experimentally demonstrate a first step towards this aim, the creation of a soliton pair
with arbitrary relative phase, delay, and frequency throughout almost the entire output parameter space with
the aid of a pre-shaped fiber input field.
Multiphoton excitation has recently found application in the fields of bioimaging, uncaging and lithography. In
order to fully exploit the advantages of nonlinear excitation, in particular the axial resolution due to nonlinearity,
most systems to date operate with point or multipoint excitation, while scanning either the laser beam or the
sample to generate the illumination pattern. Here we combine the recently introduced technique of scanningless
multiphoton excitation by temporal focusing with recent advances in digital holography to generate arbitrarily
shaped, depth resolved, two-dimensional excitation patterns completely without scanning. This is of particular
importance in applications requiring uniform excitation of large areas over short time scales, such as neuronal
activation by multiphoton uncaging of neurotransmitters. We present an experimental and theoretical analysis of
the effect of spatial patterning on the depth resolution achieved in temporal focusing microscopy. It is shown that
the depth resolution for holographic excitation is somewhat worse than that achieved for uniform illumination.
This is also accompanied by the appearance of a speckle pattern at the temporal focal plane. The origin of the
two effects, as well as means to overcome them, are discussed.
Depth resolved multiphoton microscopy is performed by collecting the fluorescent emission of two-exciton states
in colloidal quantum dots. The biexciton is formed via two sequential resonant absorption events. Due to the
large absorption cross-section and the long lifetime of the intermediate (singly excited) state, unprecedented
low excitation energy and peak powers (down to 105W/cm2) are required to generate this nonlinear response.
Depending on the quantum dot parameters, the effective two-photon cross section can be as large as 1010 GM,
orders of magnitude higher than for nonresonant excitation. The biexciton emission can be differentiated from
that of the singly excited state by utilizing its different transient dynamics. Alternate methods for discrimination
are also discussed. This system is ideal for performing
three-dimensional microscopy using low excitation power.
Moreover, it enables to perform multiphoton imaging even with
near-infrared emitting quantum dots, which
are highly compatible with imaging deep into a scattering tissue. The depth resolution of our microscope is
shown to be equivalent to a standard two-photon microscope. The system also shows slow saturation due to the
contribution of higher (triply and above) excited states to the emitted signal.
The ability to perform optical sectioning is one of the great
advantages of laser-scanning microscopy. This introduces, however,
a number of difficulties due to the scanning process, such as
lower frame rates due to the serial acquisition process. Here we
show that by introducing spatiotemporal pulse shaping techniques
to multiphoton microscopy it is possible to obtain full-frame
depth resolved imaging completely without scanning. Our method
relies on temporal focusing of the illumination pulse. The pulsed
excitation field is compressed as it propagates through the
sample, reaching its shortest duration at the focal plane, before
stretching again beyond it. Combining temporal focusing with
line-scanning microscopy results in an enhanced depth resolution,
equivalent to that achieved by point scanning. Both the
scanningless and the line-scanning techniques are applied to
obtain depth-resolved two-photon excitation fluorescence (TPEF)
images of drosophila egg-chambers.
The field of multiphoton microscopy has undergone significant advances since its beginning just over a decade ago. One of these was the development of coherent multiphoton techniques, which typically image intrinsic properties of the sample, without resorting to staining or labeling. Here we describe some recent technological developments in the two leading coherent multiphoton techniques: third-harmonic generation and coherent anti-Stokes Raman spectroscopy. These techniques are applied for visualization of a variety of biological and other samples.
Using a statistical mechanics bubble competition model, Alon et al, have shown that the 2D Rayleigh-Taylor (RT) mixing zone bubble and spike fronts evolves. The Richtmyer-Mechkov mixing zone fronts have been found to evolve also.
In order to verify the predictions of the 2D high Atwood number potential flow model for the evolution of the shock wave induced Richtmyer-Meshkov instability, shock-tube experiments were performed with a single-mode perturbation and two competing bubbles as the initial conditions. The experimental results were compared to theoretical model and to numerical simulation. In the present work the dependence of the instability on the Atwood number and the dimensionality of the instability were investigated in a shock tube apparatus. A high speed schlieren photography system were used to monitor the evolution of the unstable contact surface. Different Atwood numbers were achieved by using different gases. The results of those experiments were found to be in very good agreement with the predictions of theoretical model and numerical simulation. These results verify the key elements of the Atwood number scaling of the bubble-merger model used for the prediction of the multi-mode bubble and spike front evolution at all Atwood numbers. The dimensionality investigation of the instability evolution was done using a pyramid like initial perturbation. The results reveal the same two key elements of the bubble-merger model to describe the bubble and spike front evolution as in the 2D case except for different scaling constants.
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