Arbitrarily shaped high-coherence electron bunches from cold atoms

Abstract

Ultrafast electron diffractive imaging of nanoscale objects such as biological molecules 1,2 and defects in solid-state devices 3 provides crucial information on structure and dynamic processes: for example, determination of the form and function of membrane proteins, vital for many key goals in modern biological science, including rational drug design 4 . High brightness and high coherence are required to achieve the necessary spatial and temporal resolution, but have been limited by the thermal nature of conventional electron sources and by divergence due to repulsive interactions between the electrons, known as the Coulomb explosion. It has been shown that, if the electrons are shaped into ellipsoidal bunches with uniform density 5 , the Coulomb explosion can be reversed using conventional optics, to deliver the maximum possible brightness at the target 6,7 . Here we demonstrate arbitrary and real-time control of the shape of cold electron bunches extracted from laser-cooled atoms. The ability to dynamically shape the electron source itself and to observe this shape in the propagated electron bunch provides a remarkable experimental demonstration of the intrinsically high spatial coherence of a cold-atom electron source, and the potential for alleviation of electron-source brightness limitations due to Coulomb explosion 6 . Carbon nanotube field emitters are at present the brightest available electron sources but must operate at low currents to avoid Coulomb expansion and are therefore not suitable for ultrafast imaging. Limited bunch shaping has been demonstrated with photoemission sources 7,8 , which use high-energy laser pulses to generate electrons at high current. Combined with longitudinal bunch compression, sub-100 fs pulses have been obtained with sufficient brightness for diffraction studies of gold 8,9 . However, large increases in brightness are needed for single-shot imaging of weakly scattering materials such as biological molecules, and further increases in the brightness of intrinsically hot photoemission sources will be difficult.Recent simulations 10,11 and experiments 12 show that photoionization of a cold atom cloud can produce cold electron bunches with high coherence and current. Electrons are extracted by nearthreshold photoionization of atoms that have been laser-cooled to microKelvin temperature 13 . We demonstrate here that the extracted electron bunches have extremely small transverse momentum, and show that the source has quasi-homogeneous rather than thermal coherence properties; that is, unlike conventional photocathode sources, the transverse locations of the cold electrons remain strongly correlated to their original location at the source.In addition, the internal structure of the atoms that form the underlying electron source provides a unique tool for three-dimensional control of the electron bunch shape 5 . We show that it is possible to engineer the spatial profiles of the incident excitation and photoionization laser beams to control the shape ARC Centre of Excellence in C...

High-coherence picosecond electron bunches from cold atoms

“…Typically, the pepper-pot distribution is created using a physical mask in the electron beam. The shaping capability of a CAES 11 removes the need for a mask, so that source diagnostic measurements can be performed without beamline alterations.…”

“…The theoretical emittance values were calculated from equation 6, where k B T has been replaced by DE(l) from equation 1 and the values for s r are calculated from x pp . The method of spatial light modulator phase mask production for producing arbitrary laser intensity distributions is outlined in McCulloch et al 11 . The binary transmission function of the pepper-pot was modulated to compensate for the atomic density profile of the cold atom cloud to form uniform-density electron beamlets upon ionization.…”

“…Cold atom sources have generated electron bunches with high coherence, up to 10 nm at the source 11 . The bunch charge can be a few picocoulombs, sufficient for single-shot imaging of nanocrystals of biomolecules 12,13 , but the nanosecond duration of the electron bunches has been too long for observation of useful dynamics.…”

“…With a cold atom source, electrons are typically produced by photionization of cold atoms in a two-step process, with microsecond laser excitation to an intermediate atomic state and nanosecond ionization of the excited atoms 11,13 . Timedependent extraction fields can be used to reduce the electron bunch length 14 , but picosecond switching times are not feasible.…”

Energy spread of ultracold electron bunches extracted from a laser cooled gas

“…Subsequently a small volume of rubidium atoms is ionized by a pulsed 480 nm femtosecond ionization laser beam, Figure 1d, intersecting the excitation laser beam at right angles, resulting in a cloud of cold electrons and ions. The shape and size of the ionization volume can be controlled by the overlap of the excitation and ionization laser beams [12,13]. Finally, Figure 1c, the electrons are extracted by a static electric field [14].…”