Strong-Field Kapitza-Dirac Scattering of Neutral Atoms

Laser induced strong-field phenomena in atoms and molecules on the femtosecond (fs) time scale have been almost exclusively investigated with traveling wave fields. In almost all cases, approximation of the strong electromagnetic field by an electric field purely oscillating in time suffices to describe experimental observations. Spatially dependent electromagnetic fields, as they occur in a standing light wave, allow for strong energy and momentum transfer and are expected to extend strong-field dynamics profoundly. Here we report a strong-field version of the Kapitza-Dirac effect for neutral atoms where we scatter neutral He atoms in an intense short pulse standing light wave with fs duration and intensities well in the strong-field tunneling regime. We observe substantial longitudinal momentum transfer concomitant with an unprecedented atomic photon scattering rate greater than ${10}^{16}{\mathrm{s}}^{-1}$.

Figure 1

(a) Experimental setup (for explanation see text). $\lambda /4$ and MCP denote quarter wave plates and multichannel plate, respectively. In (b) we display a detailed sketch of the time integrated intensity of the standing wave together with a raw detector image of the deflected He* atoms along the $z$ axis. The number of excited He atoms impinging on the detector is color coded. The rod electrodes remove charges from strong-field ionization.

Figure 2

Final velocity distribution of deflected He* atoms. (a) As a function of intensity $I$: c1: $2.5\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$, c2: $3.4\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$, c3: $4.2\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$, c4: $5.1\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$, and c5: $6.3\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$. The ellipticity is fixed at $\epsilon =0.85$. (b) as a function of ellipticity. Experimental curves, c2: $\epsilon =0.6$ and c3: $\epsilon =0.85$. Theoretical curve c1: $\epsilon =0.6$. The intensity is fixed at $I=6.3\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$. The experimental curves in (a) and (b) are smoothed. Visible structures on a small velocity scale are likely due to statistical effects.

Figure 3

Comparison of measured final velocity distributions with theoretical simulations. Blue dash-dotted curves in (a)–(e): same experimental velocity distributions as shown in Fig. 2, curves c1–c5, respectively. Red solid curves: Fit of the theoretical simulations to the experimental velocity distributions. Thin orange curves: Velocity distribution for fixed effective time of acceleration $S=0.5$ $\pi \tau$. Green open dots in (b): same as solid red curve, but with fit parameter $\epsilon =0.9$.