Coherent control mechanisms in bichromatic multiphoton ionization

Abstract

Free electron vortices (FEVs) generated by multiphoton ionization (MPI) with ultrashort laser pulses have attracted significant attention due to their varied symmetries and unusual topological properties. We study two physical mechanisms of coherent control in atomic MPI with bichromatic polarization-shaped femtosecond laser pulses which give rise to the rich variety of FEVs. In the experiments, we combine pulse shaping of a carrier-envelope phase-stable supercontinuum with photoelectron tomography to generate and reconstruct three-dimensional photoelectron momentum distributions (PMDs). Simultaneous measurements of energetically separated photoelectrons from intraband and interband interference in a single PMD allow us to compare phase and polarization control of the angular distributions by both mechanisms. We investigate phase control in three scenarios: first, counterrotating circularly polarized pulses are employed to contrast the phase-insensitive angular momentum eigenstate created by intraband interference via frequency mixing with the phase-sensitive c 7 rotationally symmetric FEV from pure interband interference of two single-color ionization pathways. In the second scenario, we use orthogonal linearly polarized pulses to compare the phase control properties of a six-lobed angular momentum wave packet from intraband interference to those of a complex shaped interband PMD in the presence of phase fluctuations. Finally, we demonstrate phase control of a photoelectron hologram from mixed interband interference. In a (3 + 1) resonance enhanced MPI scheme, the red pump pulse induces a bound electron wave packet probed by the time-delayed blue pulse. The latter simultaneously creates a reference wave packet by three-photon ionization to form the photoelectron hologram. Rotation of the hologram with c 1 or c 5 rotational symmetry maps the time evolution of the bound wave packet. To analyze our results, we develop analytical expressions for the wave functions of intraband and interband interference in perturbative non-resonant MPI. The experiments are complemented with two-dimensional TDSE simulations to follow the FEV formation dynamics and to validate the physical pictures.

Molecular Free Electron Vortices in Photoionization by Polarization-Tailored Ultrashort Laser Pulses

“… with r = ( x , y ) and erf denoting the error function. Motivated by our recent experiments on potassium atoms ( Pengel et al, 2017b) and dimers ( Bayer et al, 2013) , we choose an effective nuclear charge of z = 0.9085 and a softcore parameter of a = 2.3065 Å ( Eickhoff et al, 2021c) to mimic the singly charged potassium dimer for the study of molecular FEVs. Figure 1A shows the first 15 Born-Oppenheimer (BO) potential energy curves V n ( R ) obtained by solving the time-independent Schrödinger equation with m e being the mass of the electron, for different values of R .…”

“…The temporal laser electric field is described in the spherical basis ( Wollenhaupt et al, 2009a ; Bayer et al, 2019 ; Kerbstadt et al, 2019b) . We consider a bichromatic double pulse sequence of the general form ( Eickhoff et al, 2021a ; Eickhoff et al, 2021c ). Each subpulse n = 1, 2 is characterized by an individual peak amplitude , time-delay τ n , central frequency ω n , relative phase φ n and polarization state ( q n = ±1).…”

“…In the same work, FEVs with odd rotational symmetry have been created using bichromatic fields in the limit τ → 0. In addition, we reported on crescent-shaped FEVs ( Kerbstadt et al, 2019b ; Eickhoff et al, 2020a ; Eickhoff et al, 2021c ) and a five-fold symmetric FEV ( Eickhoff et al, 2021c) created by bichromatic MPI. In the strong-field ionization regime, Mancuso et al observed crescent-shaped and three-fold symmetric FEVs using co- and counterrotating bichromatic fields, respectively ( Mancuso et al, 2015 , Mancuso et al, 2016 ).…”

“…Similarly, Eckart et al created three-fold symmetric FEVs ( Eckart et al, 2016 ; Eckart et al, 2018 ) and measured the PMD using cold target recoil-ion momentum spectroscopy ( Dörner et al, 2000 ; Ullrich et al, 2003 ). In our experiments, we applied photoelectron tomography ( Wollenhaupt et al, 2009b ; Wollenhaupt et al, 2013 ) to reconstruct the 3D PMD of the atomic FEVs ( Pengel et al, 2017a ; Pengel et al, 2017b ; Kerbstadt et al, 2019b ; Eickhoff et al, 2021c) . Recently, this method has been developed further into a holographic scheme to reconstruct the photoelectron wave function by measuring interferograms in the PMD ( Eickhoff et al, 2020a ; Eickhoff et al, 2021b ; Eickhoff et al, 2021c ).…”

Multichromatic Polarization-Controlled Pulse Sequences for Coherent Control of Multiphoton Ionization

“…The optical phases are written explicitly whenever they are used as control parameters to manipulate the resulting photoelectron wave packet. A more comprehensive theoretical description is given for example in [107][108][109]138]. Control of multiphoton processes by multichromatic pulse sequences is based on the interference of multiple pathways leading to the same final state.…”