Abstract
Electrons emitted by ultrafast interaction with an optical pulse from an electron gun have, by nature, a finite width of the energy spectra. As the electrons are accelerated and are propagating through the tube of the microscope, the dispersion leads to elongation of the originally short electron pulse. In a typical case of a narrow relative spectral width, the propagation leads to approximately linear dependence of the electron energy on time, which is called chirp.
Recent advancement in the field of electron microscopy allowed tailoring of electron quantum states by the interaction with optical fields with time independent frequency [1],[2]. Even though the chirp of electron pulse is one of its fundamental properties, it has not been utilized until now. In this contribution we propose a method for chirp correction and subsequent partial monochromatization of an electron pulse by employing the interaction with custom spatio-temporally tailored optical pulses, frequency of which varies in time. The inelastic interaction with the ponderomotive potential considered here generates a ladder of electron energy states [3]. We show that the spectral width of each of these states is coupled to the chirp of the optical fields. This principle can be generalized also to the case of the electron beam modulation by semi-infinite optical fields generated at thin membranes [4],[5].
Our approach offers the possibility of narrowing the emitted electron spectra by a factor of 10, which bring interesting opportunities for limiting the color aberrations of electron optics in ultrafast experiments and for enhancing the energy resolution in ultrafast electron spectroscopy.
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[2] Valerio Di Giulio, F. Javier García de Abajo. Free-electron shaping using quantum light. Optica 7, 1820-1830, 2020 doi: 10.1364/OPTICA.404598
[3] M. Kozák, T. Eckstein, N. Schönenberger, et al. Inelastic ponderomotive scattering of electrons at a high-intensity optical travelling wave in vacuum. Nat. Phys. 14: 121-125, 2018. doi: 10.1038/nphys4282
[4] M. Tsarev, A. Ryabov, P. Baum. Free-electron qubits and maximum-contrast attosecond pulses via temporal Talbot revivals. Physical Review Research, 3(4), 2021. doi: 10.1103/PhysRevResearch.3.043033.
[5] G.M. Vanacore, I. Madan, G. Berruto, et al. Attosecond coherent control of free-electron wave functions using semi-infinite light fields. Nat. Commun 9(2694), 2018. doi: 10.1038/s41467-018-05021-x