A cold atom electron source for diffractive imaging

© 2011 Dr. Sebastian Dylan SalibaSaliba, S. D., Junker, M., Turner, L. D., & Scholten, R. E. (2009). Mode stability of external cavity diode lasers. Applied Optics, 48(35), 6692-6700. This paper was published in Applied Optics and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://dx.doi.org/10.1364/AO.48.006692. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.Saliba, S. D., & Scholten, R. E. (2009). Linewidths below 100 kHz with external cavity diode lasers. Applied Optics, 48(36), 6691-6966. This paper was published in Applied Optics and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://dx.doi.org/10.1364/AO.48.006961. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.The following paper was published in Physical Review Letters and is made available as an electronic reprint with the permission of APS: H. Schempp, G. Gunter, C. S. Hofmann, C. Giese, S. D. Saliba, B. D. DePaola, T. Amthor, M.Weidemuller, S. Sevincli and T. Pohl, Physical Review Letters, 104:173602, 2010. "Copyright (2010) by the American Physical Society." The paper can be found at the following URL on the APS website: http://dx. doi.org/10.1103/PhysRevLett.104.173602. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.McCulloch, A. J., Sheludko, D. V., Saliba, S. D., Bell, S. C., Junker, M., Nugent, K. A., & Scholten, R. E. (2011). Arbitrarily shaped high-coherence electron bunches from cold atoms. Nature Physics, 7, 785-788. This paper is not available in this version of the thesis due to copyright. It was published in Nature Physics and can be found at the following URL on the NPG website: http://dx.doi.org/10.1038/nphys2052.Cold electrons bunches can be created by near-threshold photoionisation of a cloud of cold atoms in a magneto-optical trap (MOT). The electrons can be extracted with temperatures less than 15 K and, due to their low momentum spread, an electron bunch created from cold atoms has high spatial coherence. Coupled with the potential of high brightness, a cold atom electron source (CAES) is a promising prospect for single-shot diffractive imaging of nanoscale objects. This thesis describes the construction and characterisation of a cold atom electron source. A CAES relies on laser cooling and trapping techniques which require stable, high precision external cavity diode lasers (ECDLs) with narrow linewidth, minimal frequency noise and reproducible day-to-day operation. The research presented here begins by extending the current understanding of this crucial component of the apparatus, specifically to detail a new model of laser frequency mode selection that shows the critical role played by the external cavity in determining the frequency stability. A geometric relationship is introduced, defining in a straightforward way how the mode-hop-free tuning range of an ECDL can be optimised. It is also shown that the frequency linewidth of an ECDL varies with the focus of the collimation lens used. This previously unidentified effect is described with a Gaussian optics model, and shown theoretically and experimentally to have a significant effect on the laser linewidth. A new monolithic block ECDL design based on these results was constructed for the CAES apparatus and performed favourably in comparison to previous designs, proving to be robust and reliable. A CAES is a new type of electron source whose quantitatively different properties are under intense study. A model describing the spatial coherence properties of a CAES using a statistical optics framework was developed to better understand the capabilities of the source in diffractive imaging applications. The extracted electron bunches are shown to have properties analogous to quasi-monochromatic paraxial optical wavefields. Based on the theoretical model, I have developed and implemented a novel technique to measure the transverse spatial coherence of the source using electron bunches arbitrarily shaped in two dimensions. The coherence length measurement indicates a lower limit of 10 nm for cold electron bunches, an order of magnitude larger than conventional photoemission electron sources. Such a large coherence length required the development of a diffractive imaging model for assessing the viability of current and future experiments. An analytic expression describing the electrostatic potential of a single biomolecule was derived for use in a diffraction simulation algorithm. Simulated diffraction patterns of a biomolecule were produced indicating single shot imaging of nanoscale objects using a CAES will be achievable. Compared with conventional electron sources, the CAES thus has several advantages for diffractive imaging applications: the demonstrated substantial increase in source coherence; the novel ability to create low emittance, arbitrarily shaped electron bunches; and the high brightness potential desirable for single-shot imaging. The simulations and experiments characterising the CAES source described here demonstrate the viability of diffractive imaging experiments anticipated in the near future