Conservation laws for electron vortices in strong-field ionisation

Abstract

We investigate twisted electrons with a well-defined orbital angular momentum, which have been ionised via a strong laser field. By formulating a new variant of the well-known strong field approximation, we are able to derive conservation laws for the angular momenta of twisted electrons in the cases of linear and circularly polarised fields. In the case of linear fields, we demonstrate that the orbital angular momentum of the twisted electron is determined by the magnetic quantum number of the initial bound state. The condition for the circular field can be related to the famous ATI peaks, and provides a new interpretation for this fundamental feature of photoelectron spectra. We find the length of the circular pulse to be a vital factor in this selection rule and, employing an effective frequency, we show that the photoelectron OAM emission spectra are sensitive to the parity of the number of laser cycles. This work provides the basic theoretical framework with which to understand the OAM of a photoelectron undergoing strong field ionisation. Graphic Abstract

Entanglement of Orbital Angular Momentum in Non-Sequential Double Ionization

“…The laser field is polarized in the z direction, in the same direction as the total OAM operator L|| , so the laser field can not change the total OAM since [ L|| , Ĥ(t)] = 0, where Ĥ(t) is the total Hamiltonian of the system, see Refs. [47,49] or Appendix B for more details.…”

“…[39][40][41][42][43]. Strong-field studies on OAM in photoelectrons include theoretically achieving high OAM values for quasi-relativistic field intensities [44] and terahertz fields [45], exploiting OAM in rescattering electrons to probe bound state structures [46] and recent work [47], providing new insight to interference vortices [48] as an interference between pairs of OAM components and [49], where conservation laws for OAM in strong-field ionization were derived. Of particular relevance, is the conservation between the initial quantum magnetic number and final OAM, which occurs for systems with rotational symmetry around the quantization axis [50].…”

Exploring symmetries in photoelectron holography with two-color linearly polarized fields

“…Besides the seminal work in [67], in recent years selection rules for HHG [68] and strong-field ionization [65] in bicircular fields have been derived. Further studies have focused on the role of the orbital angular momentum (OAM) in photoelectron vortices [40,[69][70][71], molecules [62,72], and strong-field ionization in circularly polarized fields [73,74]. This builds up on early work, which shows that HHG with two-color fields are dependent on the target [75], and that an electron's angle of return will manifest itself as dynamic shifts in structural minima in HHG from diatomic targets [49,50,76].…”

Carrier-envelope-phase and helicity control of electron vortices in photodetachment

“…During the last decade there has been an increasing interest towards the electron momentum spirals (often referred as 'vortices') in laser-induced photoionization [1][2][3][4][5][6][7][8][9][10][11][12] or photodetachment [13]. They manifest themselves, in the probability distribution of photoelectrons, as zones of large probability which follow concentric Fermat spirals with well-defined number of arms [1].…”

“…While electron vortices and spirals are formed due to the same physical phenomena, i.e., by subtile interference effects in the probability amplitude of ionization, whether one or the other are observed depends on the light field configuration and the target atom (or ion) [9,16]. Several theoretical studies have predicted the formation of momentum spirals in photoionization from a variety of atomic and molecular targets [1,3,5,12] and diverse laser field configurations [2,4]. For instance, it has been shown that a sequence of two counterrotating circularlypolarized and ultrashort laser pulses leads to momentum spirals, whereas single pulses or trains of corotating pulses lead to the formation of electron vortices [13].…”