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We find that stimulated Raman excitation of an atom by a two-photon pulse can be enhanced by orders of magnitude if the photons are simultaneously frequency correlated and spatially anti-correlated. That is correlated photon pair must have an inherent time delay between its constituent photons.

A fiber-based robust, design-flexible FD-OCM technique using a Bessel-like LP02 mode is demonstrated. Extended DOF compared to Gaussian illumination of equal spot-size at focus is reported. We characterize the resolution performance of this system.

We present a robust fiber-based setup for Bessel-like beam extended depth-of-focus Fourier-domain optical coherence microscopy, where the Bessel-like beam is generated in a higher order mode fiber module. In this module a stable guided LP02 core mode is selectively excited by a long period grating written in the higher order mode fiber. Imaging performance of this system in terms of lateral resolution and depth of focus was analyzed using samples of suspended microbeads and compared to the case where illumination is provided by the fundamental LP01 mode of a single mode fiber. Illumination with the LP02 mode allowed for a lateral resolution down to 2.5 µm as compared to 4.5 µm achieved with the LP01 mode of the single mode fiber. A three-fold enhancement of the depth of focus compared to a Gaussian beam with equally tight focus is achieved with the LP02 mode. Analysis of the theoretical lateral point spread functions for the case of LP01 and LP02 illumination agrees well with the experimental data. As the design space of waveguides and long-period gratings allows for further optimization of the beam parameters of the generated Bessel-like beams in an all-fiber module, this approach offers a robust and yet flexible alternative to free-space optics approaches or the use of conical fiber tips.

We find that stimulated Raman excitation of an atom by a two-photon pulse can be enhanced by orders of magnitude if the photons are simultaneously frequency correlated and spatially anticorrelated. That is, a correlated photon pair must have an inherent time delay between its constituent photons. This counterintuitive feature is a manifestation of the uncertainty principle, which yields that frequency-correlated photons cannot be time (spatially) correlated. This is opposite to two-photon absorption by a three-level atom, for which the enhancement occurs if photons in the pulse are frequency anticorrelated and spatially correlated, that is, photons in the pair simultaneously interact with the atom. Our findings can be useful for imaging and spectroscopy of biological samples which demand low illumination intensity.

We propose to enhance the performance of localized plasmon structured illumination microscopy (LP-SIM) via intensity correlations. LP-SIM uses sub-wavelength illumination patterns to encode high spatial frequency information. It can enhance the resolution up to three-fold before gaps in the optical transfer function (OTF) support arise. For blinking fluorophores or for quantum antibunching, an intensity correlation analysis induces higher harmonics of the illumination pattern and enlarges the effective OTF. This enables ultrahigh resolutions without gaps in the OTF support, and thus a fully deterministic imaging scheme. We present simulations that include shot and external noise and demonstrate the resolution power under realistic photon budgets. The technique has potential in light microscopy where low-intensity illumination is paramount while aiming for high spatial but moderate temporal resolutions.

A classical ghost imaging experiment performed at the free-electron laser FLASH is presented. The possibility of image formation, a double bar in our case, in the beam that has never interacted with the sample is demonstrated.

Physical review / A covering atomic, molecular, and optical physics and quantum information 101(1), 013820 (2020). doi:10.1103/PhysRevA.101.013820, Here we present the results of a classical ghost imaging experiment accomplished at an XUV free-electronlaser (FEL). To perform such experiment at an FEL source each x-ray pulse was transmitted through a movingdiffuser, which created a noncorrelated speckled beam. This beam was then split in two identical branches byintroducing a beam splitter in the form of a transmission grating. In one of these branches the sample waspositioned. We demonstrate the possibility of image formation, a double bar in our case, in the beam that hasnever interacted with the sample. With this experiment we extend the quantum optics methodology to the FELcommunity., Published by Inst., Woodbury, NY

We highlight the impact of Emil Wolf's work on coherence and polarization on an ever increasing amount of applications in the 21st century. We present a brief review of how partial coherence at the level of increasing order of coherence functions is leading to evolution in the better methods for microscopy, imaging, optical coherence tomography, speckle imaging, and propagation through random media. This evolution in our capabilities is expected to have wide ramifications in Science and Engineering.

We demonstrate that by using squeezed light for two-photon absorption (TPA) of fluorescein biomarkers, a fluorescence enhancement of nearly 50 is achieved as compared to conventional TPA using coherent light.

Combining structured illumination with the evaluation of intensity auto-correlations enhances superresolution microscopy, both in widefield and confocal. We show how plasmonic illumination boosts the performance creating a huge, gapless OTF support.

The intensity correlations in incoherently scattered X-rays from a free-electron laser can be exploited to image 2D objects with a resolution close to or below the diffraction limit. The advent of accelerator-driven free-electron lasers (FEL) has opened new avenues for high-resolution structure determination via diffraction methods that go far beyond conventional X-ray crystallography methods1,2,3,4,5,6,7,8,9,10. These techniques rely on coherent scattering processes that require the maintenance of first-order coherence of the radiation field throughout the imaging procedure. Here we show that higher-order degrees of coherence, displayed in the intensity correlations of incoherently scattered X-rays from an FEL, can be used to image two-dimensional objects with a spatial resolution close to or even below the Abbe limit. This constitutes a new approach towards structure determination based on incoherent processes11,12, including fluorescence emission or wavefront distortions, generally considered detrimental for imaging applications. Our method is an extension of the landmark intensity correlation measurements of Hanbury Brown and Twiss13 to higher than second order, paving the way towards determination of structure and dynamics of matter in regimes where coherent imaging methods have intrinsic limitations14.

Two-photon absorption (TPA) fluorescence of biomarkers has been decisive in advancing the fields of biosensing and deep-tissue in vivo imaging of live specimens. However, due to the extremely small TPA cross section and the quadratic dependence on the input photon flux, extremely high peak-intensity pulsed lasers are imperative, which can result in significant photo- and thermal-damage. Previous works on entangled TPA (ETPA) with spontaneous parametric down-conversion (SPDC) light sources found a linear dependence on the input photon-pair flux, but are limited by low optical powers, along with a very broad spectrum. We report that by using a high-flux squeezed light source for TPA, a fluorescence enhancement of 47 is achieved in fluorescein biomarkers as compared to classical TPA. Moreover, a polynomial behavior of the TPA rate is observed in the DCM laser dye., 5 pages, 4 figures

Measuring the $M$th-order intensity correlation function of light emitted by two statistically independent thermal light sources may display N00N-like interferences of arbitrary order $N = M/2$. We show that via a particular choice of detector positions one can isolate $M$-photon quantum paths where either all $M$ photons are emitted from the same source or $M/2$ photons are collectively emitted by both sources. The latter superposition displays N00N-like oscillations with $N = M/2$ which may serve, e.g., in astronomy, for imaging two distant thermal sources with $M/2$-fold increased resolution. We also discuss slightly modified detection schemes improving the visibility of the N00N-like interference pattern and present measurements verifying the theoretical predictions., Comment: 9 pages, 6 figures

Physical review letters 119(5), 053401 (2017). doi:10.1103/PhysRevLett.119.053401, Established x-ray diffraction methods allow for high-resolution structure determination of crystals, crystallized protein structures, or even single molecules. While these techniques rely on coherent scattering, incoherent processes like fluorescence emission—often the predominant scattering mechanism—are generally considered detrimental for imaging applications. Here, we show that intensity correlations of incoherently scattered x-ray radiation can be used to image the full 3D arrangement of the scattering atoms with significantly higher resolution compared to conventional coherent diffraction imaging and crystallography, including additional three-dimensional information in Fourier space for a single sample orientation. We present a number of properties of incoherent diffractive imaging that are conceptually superior to those of coherent methods., Published by APS, College Park, Md.

Intensity correlation microscopy (ICM), which is prominently known through antibunching microscopy or super-resolution optical fluctuation imaging (SOFI), provides superresolution through a correlation analysis of antibunching of independent quantum emitters or temporal fluctuations of blinking fluorophores. For correlation order $m$ the PSF in the signal is effectively taken to the $m$th power, and is thus directly shrunk by the factor $\sqrt{m}$. Combined with deconvolution a close to linear resolution improvement of factor $m$ can be obtained. Yet, analysis of high correlation orders is challenging, what limits the achievable resolutions. Here we propose to use three dimensional structured illumination along with $m$th-order correlation analysis to obtain an enhanced scaling of up to $m+m=2m$. Including the stokes shift or plasmonic sub-wavelength illumination enhancements beyond $2m$ can be achieved. Hence, resolutions far below the diffraction limit in full 3D imaging can potentially be achieved already with low correlation orders. Since ICM operates in the linear regime our approach may be particularly promising for enhancing the resolution in biological imaging at low illumination levels., Comment: revised version

epl 120(1), 16003 (2017). doi:10.1209/0295-5075/120/16003, Using higher-order coherence of thermal light sources, the resolution power of standard x-ray imaging techniques can be enhanced. In this work, we applied the higher-order measurement to far-field x-ray diffraction and near-field phase contrast imaging (PCI), in order to achieve superresolution in x-ray diffraction and obtain enhanced intensity contrast in PCI. The cost of implementing such schemes is minimal compared to the methods that achieve similar effects by using entangled x-ray photon pairs., Published by IOP Publ., Les-Ulis

We propose to use intensity correlation microscopy in combination with structured illumination to image quantum emitters that exhibit antibunching with a spatial resolution reaching far beyond the Rayleigh limit. Combining intensity measurements and intensity auto correlations up to order $m$ creates an effective PSF with FWHM shrunk by the factor $\sqrt{m}$. Structured Illumination microscopy on the other hand introduces a resolution improvement of factor 2 by use of the principle of moir\'e fringes. Here, we show that for linear low-intensity excitation and linear optical detection the simultaneous use of both techniques leads to an in theory unlimited resolution power with the improvement scaling favorably as $m + \sqrt{m}$ in dependence of the correlation order $m$. Hence, yielding this technique to be of interest in microscopy for imaging a variety of samples including biological ones. We present the underlying theory and simulations demonstrating the highly increased spatial superresolution, and point out requirements for an experimental implementation., Comment: 7 pages, 6 figures

We model the full image formation process in FD-OCM for a Bessel-like LP02 illumination beam as well as Gaussian beams. The LP02 beam generated in a HOM fiber enables design flexibility and an extended DOF.