Experimental Demonstration of Selective Coherent Population Transfer via a Continuum

We report the first experimental demonstration of coherent population transfer, induced by stimulated Raman adiabatic passage, via continuum states. Population is transferred from the metastable state $2s{}^{1}S_0$ to the excited state $4s{}^{1}S_0$ in helium atoms in a two-photon process mediated by coherent interaction with the ionization continuum. While incoherent techniques usually do not permit any population transfer in such a process, we show that stimulated Raman adiabatic passage allows significant population transfer to take place also via ultrafast decay channels.

Figure 1

Coupling scheme in helium atoms. The pump and the Stokes laser couple the initially populated state $2s{}^{1}S_0$ to the target state $4s{}^{1}S_0$ via the ionization continuum (solid lines). The pump laser can also ionize population transferred to the target state (dotted line). A possible Raman-type transition between the initial and the target state, mediated by off-resonant interaction to all other states in the system is also indicated (dash-dotted lines).

Figure 2

Measured electron signals versus laser tuning of the pump laser in the range of the two-photon resonance for Stokes laser preceding pump laser pulse. The circles and the triangles indicate slow and fast electrons, respectively. The solid lines show numerical simulations. The peak laser intensities are $I_P=35\mathrm{MW}/{\mathrm{cm}}^2$ and $I_S=60\mathrm{MW}/{\mathrm{cm}}^2$. The pulse delay is $-7.5\mathrm{ns}$, i.e., Stokes preceding pump pulse. The zero frequency is defined at the maximum value of the fast photoelectrons. According to the numerical simulation, this maximum should be at a static detuning of 0.1 GHz from the two-photon resonance. As the accuracy of the wavelength detection in our experiment is approximately in the same range, we cannot give a precise value of the necessary static detuning.

Figure 3

Signal of the fast photoelectrons (not calibrated to absolute transfer efficiencies) versus pump laser tuning for different pulse sequences, i.e., pump pulse following Stokes pulse (upper panel, STIRAP configuration) and pump pulse preceding Stokes pulse (lower panel). Only in the STIRAP case population is transferred to the excited state. The peak laser intensities are $I_P=40\mathrm{MW}/{\mathrm{cm}}^2$ and $I_S=35\mathrm{MW}/{\mathrm{cm}}^2$.