Detailed observation of space-charge dynamics using ultracold ion bunches.

Control of Coulomb expansion in charged particle beams is of critical importance for applications including electron and ion microscopy, injectors for particle accelerators and in ultrafast electron diffraction, where space-charge effects constrain the temporal and spatial imaging resolution. The development of techniques to reverse space-charge-driven expansion, or to observe shock waves and other striking phenomena, have been limited by the masking effect of thermal diffusion. Here we show that ultracold ion bunches extracted from laser-cooled atoms can be used to observe the effects of self-interactions with unprecedented detail. We generate arrays of small closely spaced ion bunches that interact to form complex and surprising patterns. We also show that nanosecond cold ion bunches provide data for analogous ultrafast electron systems, where the dynamics occur on timescales too short for detailed observation. In a surprising twist, slow atoms may underpin progress in high-energy and ultrafast physics.

High-coherence picosecond electron bunches from cold atoms.

Ultrafast electron diffraction enables the study of molecular structural dynamics with atomic resolution at subpicosecond timescales, with applications in solid-state physics and rational drug design. Progress with ultrafast electron diffraction has been constrained by the limited transverse coherence of high-current electron sources. Photoionization of laser-cooled atoms can produce electrons of intrinsically high coherence, but has been too slow for ultrafast electron diffraction. Ionization with femtosecond lasers should in principle reduce the electron pulse duration, but the high bandwidth inherent to short laser pulses is expected to destroy the transverse coherence. Here we demonstrate that a two-colour process with femtosecond excitation followed by nanosecond photoionization can produce picosecond electron bunches with high transverse coherence. Ultimately, the unique combination of ultrafast ionization, high coherence and three-dimensional bunch shaping capabilities of cold atom electron sources have the potential for realising the brightness and coherence requirements for single-shot electron diffraction from crystalline biological samples.

Non-iterative imaging of inhomogeneous cold atom clouds using phase retrieval from a single diffraction measurement.

We demonstrate a new imaging technique for cold atom clouds based on phase retrieval from a single diffraction measurement. Most single-shot diffractive imaging methods for cold atoms assume a monomorphic object to extract the column density. The method described here allows quantitative imaging of an inhomogeneous cloud, enabling recovery of either the atomic density or the refractive index, provided the other is known. Using ideas borrowed from density functional theory, we calculate the approximate paraxial diffracted intensity derivative from the measured diffracted intensity distribution and use it to solve the Transport of Intensity Equation (TIE) for the phase of the wave at the detector plane. Back-propagation to the object plane yields the object exit surface wave and then provides a quantitative measurement of either the atomic column density or refractive index. Images of homogeneous clouds showed good quantitative agreement with conventional techniques. An inhomogeneous cloud was created using a cascade electromagnetically induced transparency scheme and images of both phase and amplitude parts of refractive index across the cloud were separately retrieved, showing good agreement with theoretical results.