Control of directional photocurrent through interference of one- and two-photon ionization*

Coherent control of interfering one- and two-photon processes has been the subject of extensive research to achieve phase-controlled redirection of photocurrent. The present study develops two-pathway coherent control of above-threshold photoionization(ATI) of the helium atom ground state, for final state energies up to the $N=2$ ionization threshold, described by a multichannel quantum defect(MQDT) and R-matrix calculation. Three parameters are controlled in our treatment: the optical interference phase $\Delta\Phi$, the reduced electric field strength $\chi=E_2^2/E_1$ (with $E_{2}$ and $E_1$ the respective electric fields of the fundamental and the second-order harmonic), and the final state energy $\epsilon$.  Our analysis identifies the $\Delta\Phi$ and $\chi$ parameters that can maximize the degree of control of the directional photoejection asymmetry, and we propose a new frequency-sensitive controlling scheme, by which a small energy change near resonances can flip the direction of scattering electron with high efficiency. An example of the frequency-sensitive coherent control is presented, where 90% of the photoelectrons flip their emission direction when the final state energy is changed from within the  $2p^2\ ^1S^e$ resonance to just outside it.