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arXiv:1806.10504 · Physical Review Accelerators and Beams

Compact ultracold electron source based on a grating magneto-optical trap

J. G. H. Franssen, T. C. H. de Raadt, M. A. W. van Ninhuijs, O.J. Luiten

Published Feb 8, 2019• 40 citations

Abstract

The ultrafast and ultracold electron source, based on near-threshold photoionization of a laser-cooled and trapped atomic gas, offers a unique combination of low transverse beam emittance and high bunch charge.Its use is however still limited because of the required cold-atom laser-cooling techniques.Here we present a compact ultracold electron source based on a grating magneto-optical trap (GMOT), which only requires one trapping laser beam that passes through a transparent accelerator module.This makes the technique more widely accessible and increases its applicability.We show the GMOT can be operated with a hole in the center of the grating and with large electric fields applied across the trapping region, which is required for extracting electron bunches.The calculated values of the applied electric field were found to agree well with measured Stark shifts of the laser cooling transition.The electron beams extracted from the GMOT have been characterized.Beam energies up to 10 keV were measured using a time-of-flight method.The normalized root-mean-squared transverse beam emittance was determined using a waist scan method, resulting in ϵ ¼ 1.9 nm rad.The root-mean-squared transverse size of the ionization volume is 30 μm or larger, implying an electron source temperature in the few-10 K range, 2-3 orders of magnitude lower than conventional electron sources, based on photoemission or thermionic emission from solid state surfaces.

DOI: 10.1103/physrevaccelbeams.22.023401

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The ultrafast and ultracold electron source, based on near-threshold photoionization of a laser-cooled and trapped atomic gas, offers a unique combination of low transverse beam emittance and high bunch charge.Its use is however still limited because of the required cold-atom laser-cooling techniques.Here we present a compact ultracold electron source based on a grating magneto-optical trap (GMOT), which only requires one trapping laser beam that passes through a transparent accelerator module.This makes the technique more widely accessible and increases its applicability.We show the GMOT can be operated with a hole in the center of the grating and with large electric fields applied across the trapping region, which is required for extracting electron bunches.The calculated values of the applied electric field were found to agree well with measured Stark shifts of the laser cooling transition.The electron beams extracted from the GMOT have been characterized.Beam energies up to 10 keV were measured using a time-of-flight method.The normalized root-mean-squared transverse beam emittance was determined using a waist scan method, resulting in ϵ ¼ 1.9 nm rad.The root-mean-squared transverse size of the ionization volume is 30 μm or larger, implying an electron source temperature in the few-10 K range, 2-3 orders of magnitude lower than conventional electron sources, based on photoemission or thermionic emission from solid state surfaces.

We present the first observation of subpicosecond electron bunches from an ultracold electron source. This source is based on near-threshold, two-step, femtosecond photoionization of laser-cooled rubidium gas in a grating magneto-optical trap. Bunch lengths as short as 735±7 fs (rms) have been measured in the self-compression point of the source by means of ponderomotive scattering of the electrons by a 25 fs, 800 nm laser pulse. The observed temporal structure of the electron bunch depends on the central wavelength of the ionization laser pulse, in agreement with detailed simulations of the atomic photoionization process. This shows that the bunch length limit imposed by the atomic photoionization process has been reached.

This thesis presents details of a new type of cold atom electron source (CAES) that uses an alternating current magneto-optical trap (AC-MOT). Cold atom electron sources produce electron bunches by threshold photoionisation from a population of laser-cooled atoms. The electrons extracted from the atoms are made into a beam which has an ultra-low transverse velocity distribution. The low velocity distribution increases the brightness and reduces the energy spread of the beam, making CAESs potential sources for future electron diffraction and electron spectroscopy studies. The newly commissioned source detailed here produces focused electron bunches with characteristic temperatures as low as 12.6 ± 0.8 K, at a rate of up to 10 Hz, and with a bunch charge of 2.2±0.2fC, comparable to other state-of-the-art cold atom electron sources. The AC-MOT CAES can also produce cold bunches with energies <200 eV, which is lower than has previously been reported for this type of source. The electrons originate from a population of laser-cooled 85Rb atoms held in an AC- MOT. The AC-MOT CAES design allows the magnetic trapping fields to be switched to zero in 32 μs, more than ��175 times faster than is possible using a comparable DC-MOT source. This means that electron bunches can be created and extracted in a magnetic field-free environment without any reduction in trap density and hence bunch charge, as occurs in conventional DC-MOT sources. The design, development, characterisation, and operation of the systems needed for the new AC-MOT CAES are detailed in this thesis. This includes details of the atom trapping system, the photoionisation system, the electron extraction system and the various timing systems for creation and characterisation of the resultant cold electron bunches. Finally, the characterisation of the AC-MOT CAES itself is presented, and the measured beam metrics are used to compare the new AC-MOT CAES with other CAES designs which are under development.

Abstract Laser-cooled atoms are increasingly being used to realise practical quantum devices, motivating the development of compact and robust atom sources. Grating magneto-optical traps (gMOTs) simplify the cold-atom source architecture but are typically vapour-loaded and provide limited atomic flux. Here we explore the loading of gMOTs from cold-atom beams. We numerically simulate loading to show that unbalanced diffracted beams deflect incoming atoms away from the trap centre, thereby strongly constraining radial loading. In contrast, axial loading injects atoms directly into the trapping volume and largely avoids these effects. We experimentally demonstrate rapid axial loading of a gMOT, achieving loading rates of

2.1 \times 10^{9}

2.1 × 10 9 atoms s −1 using a moving optical molasses to transfer atoms from a 2D MOT into the gMOT. These results establish axial loading as a robust route to high-flux gMOT operation for portable cold-atom systems.

Laser-cooled atoms are increasingly being used to realise practical quantum devices, motivating the development of compact and robust atom sources. Grating magneto-optical traps (gMOTs) simplify the cold-atom source architecture but are typically vapour-loaded and provide limited atomic flux. Here we explore the loading of gMOTs from cold-atom beams. We numerically simulate loading to show that unbalanced diffracted beams deflect incoming atoms away from the trap centre, thereby strongly constraining radial loading. In contrast, axial loading injects atoms directly into the trapping volume and largely avoids these effects. We experimentally demonstrate rapid axial loading of a gMOT, achieving loading rates of

2.1 \times 10^9

atoms~s

^{-1}

using a moving optical molasses to transfer atoms from a 2D MOT into the gMOT. These results establish axial loading as a robust route to high-flux gMOT operation for portable cold-atom systems.

Compared with the conventional three-dimensional magneto-optical trap (3D MOT), the grating magneto-optical trap (GMOT) enables the cooling and trapping of atoms with only a single beam of light, thereby significantly reducing the overall volume of the device. Here, a grating chip with high diffraction efficiency is designed, and its parameters are optimized through simulations, which effectively suppresses the power of the zero-order diffracted light. Subsequently, the grating chip is fabricated and characterized, and using this chip, we successfully trap 1.7×106 atoms. The temperatures of the atomic cloud in the axial and radial directions are 5.6 mK and 17.0 mK, respectively. The successful implementation of the atom cooling and trapping experiment based on the diffraction grating provides important reference for the miniaturization of cold atom-based quantum sensors.

We present a two-dimensional grating magneto-optical trap (2D gMOT) for strontium, which produces a slow atomic beam in a compact form factor. The strontium 2D gMOT, using permanent magnets with an atomic dispenser, cools and traps atoms along two dimensions in the diffraction plane of a grating chip using the 1 S 0 → 1 P 1 (blue) transition. The system is configured in such a way that the atoms emitted from the dispenser source do not degrade the diffraction efficiency of the grating for long-term stable use. The performance of the 2D gMOT is analyzed with plug and probe beams, where the atomic source achieves a flux of 2.1 × 10 8 atoms/s and mean velocity of 6.4 m/s for an input laser power of 75 mW. The velocity measurement allows for the precise characterization of atom-photon scattering properties, showing that the compact 2D gMOT is suitable for portable quantum devices such as atomic clocks, quantum sensors, and precision spectroscopy.