Abstract:We report a new nonlinear optical process that occurs in a cloud of cold atoms at low-light-levels when the incident optical fields simultaneously polarize, cool, and spatially-organize the atoms. We observe an extremely large effective fifth-order nonlinear susceptibility of χ (5) = 7.6 × 10 −15 (m/V) 4 , which results in efficient Bragg scattering via six-wave mixing, slow group velocities (∼ c/10 5 ), and enhanced atomic coherence times (> 100 µs). In addition, this process is particularly sensitive to the atomic temperatures, and provides a new tool for in-situ monitoring of the atomic momentum distribution in an optical lattice. For sufficiently large light-matter couplings, we observe an optical instability for intensities as low as ∼ 1 mW/cm 2 in which new, intense beams of light are generated and result in the formation of controllable transverse optical patterns.

We demonstrate a technique for measuring the range-resolved coherent scatter form factors of different objects from a single snapshot. By illuminating the object with an x-ray pencil beam and placing a coded aperture in front of a linear array of energy-sensitive detector elements, we record the coherently scattered x-rays. This approach yields lateral, range, and momentum transfer resolutions of 1 mm, 5 mm, and 0.2 nm⁻¹, respectively, which is sufficient for the distinguishing a variety of solids and liquids. These results indicate a path toward real-time volumetric molecular imaging for non-destructive examination in a variety of applications, including medical diagnostics, quality inspection, and security detection.

We demonstrate steady-state, mirrorless superradiance in a cold vapor pumped by weak optical fields. Beyond a critical pump intensity of 1 mW/cm 2 , the vapor spontaneously transforms into a spatially self-organized state: a density grating forms. Scattering of the pump beams off this grating generates a pair of new, intense optical fields that act back on the vapor to enhance the atomic organization. We map out experimentally the superradiant phase transition boundary and show that it is well-described by our theoretical model. The resulting superradiant emission is nearly coherent, persists for several seconds, displays strong temporal correlations between the various modes, and has a coherence time of several hundred µs. This system therefore has applications in fundamental studies of many-body physics with long-range interactions as well as all-optical and quantum information processing. The study of collective light-matter interactions, where the dynamics of an individual scatterer depend on the state of the entire multi-scatterer system, has recently received much attention in the areas of fundamental research and photonic technologies [1,2]. One prominent example of collective behavior is superradiance [3], where light-induced couplings between initially incoherentlyprepared emitters cause the full ensemble to synchronize and radiate coherently [4]. While early studies of superradiance focused on collective scattering via the emitters' internal degrees of freedom, recent work demonstrates that formally identical behavior arises through the manipulation of the center-of-mass positions and momenta of cold atoms [5][6][7].In these studies, an initially uniformly-distributed gas of atoms pumped by external optical fields spontaneously undergoes a transition to a spatially-ordered state under certain circumstances [5][6][7][8]. This ordering arises from the momentum imparted to the atoms via optical scattering and can be understood as a form of atomic synchronization: instead of the atoms scattering light individually, the self-assembled density grating enables the entire ensemble to coherently scatter light as a single entity. The pump beams scatter off this grating and produce new optical fields that act back on the vapor to enhance the grating contrast. This emergent, dynamical organization can lead to reduced optical instability thresholds [9] and new phenomena [10] that are inaccessible using static, externally-imposed optical lattices [11].In order for superradiance to occur, the system must posses sufficient gain and feedback so that synchronization occurs more rapidly than dephasing. The main dephasing mechanisms are grating washout due to thermal atomic motion and the loss of photons from the interaction volume [5]. One can overcome the effects of thermal motion in free space by working at ultracold temperatures (T < 3 µK) and using optical fields detuned far from the atomic resonance in order to avoid recoilinduced heating [8,12]. Multi-mode superradiance has been observed in such systems [8], althou...

We present a method for realizing snapshot, depth-resolved material identification using only a single, energy-sensitive pixel. To achieve this result, we employ a coded aperture with subpixel features to modulate the energy spectrum of coherently scattered photons and recover the object properties using an iterative inversion algorithm based on compressed sensing theory. We demonstrate high-fidelity object estimation at x-ray wavelengths for a variety of compression ratios exceeding unity.

This document contains supplementary information to "Single-shot multispectral imaging through a thin scatterer," https://doi.org/10.1364/OPTICA.6.000864. We provide details and discussion related to our technique for performing multispectral imaging through a scatterer, including the experiment setup and components, calibration procedure and reconstruction algorithms. We also analyze the spectral resolution of the system and discuss the fundamental limitations of our system.

We use coherently scattered X-rays to measure the molecular composition of an object throughout its volume. We image a planar slice of the object in a single snapshot by illuminating it with a fan beam and placing a coded aperture between the object and the detectors. We characterize the system and demonstrate a resolution of 13 mm in range and 2 mm in cross-range and a fractional momentum transfer resolution of 15%. In addition, we show that this technique allows a 100x speedup compared to previously-studied pencil beam systems using the same components. Finally, by scanning an object through the beam, we image the full 4-dimensional data cube (3 spatial and 1 material dimension) for complete volumetric molecular imaging.

We observe a nonlinear optical process in a gas of cold atoms that simultaneously displays the largest reported fifth-order nonlinear susceptibility χ (5) = 1.9×10 −12 (m/V) 4 and high transparency.The nonlinearity results from the simultaneous cooling and crystallization of the gas, and gives rise to efficient Bragg scattering in the form of six-wave-mixing at low-light-levels. For large atom-photon coupling strengths, the back-action of the scattered fields influences the light-matter dynamics. This system may have important applications in many-body physics, quantum information processing, and multidimensional soliton formation.

X-ray diffraction tomography (XDT) records the spatially-resolved X-ray diffraction profile of an extended object. Compared to conventional transmission-based tomography, XDT displays high intrinsic contrast among materials of similar electron density and improves the accuracy in material identification thanks to the molecular structural information carried by diffracted photons. However, due to the weak diffraction signal, a tomographic scan covering the entire object typically requires a synchrotron facility to make the acquisition time more manageable. Imaging applications in medical and industrial settings usually do not require the examination of the entire object. Therefore, a diffraction tomography modality covering only the region of interest (ROI) and subsequent image reconstruction techniques with truncated projections are highly desirable. Here we propose a table-top diffraction tomography system that can resolve the spatially-variant diffraction form factor from internal regions within extended samples. We demonstrate that the interior reconstruction maintains the material contrast while reducing the imaging time by 6 folds. The presented method could accelerate the acquisition of XDT and be applied in portable imaging applications with a reduced radiation dose.