Acceleration of atoms in an intensive standing light wave

Laser-induced atomic strong-field processes are experimentally investigated almost without exception with progressive (continuous) electromagnetic laser fields. To explain occurring phenomena, the laser field is usually regarded as a purely classical electric field oscillating at the point of the atom only in time. In this approximation, a momentum transfer to the center of gravity of the atomic system, and thus a directional deflection, is not possible. For the first time scientists of the MBI have deflected atoms in an intense standing wave of light of extremely short duration. The investigated helium atoms, which survive the intense interaction with the stationary light wave, are accelerated extremely strongly in the laser direction due to the fixed intensity gradient. The experimental results can theoretically only be explained if both the intensity gradient and the generated magnetic fields are included in the calculations.

The basic idea for the diffraction or deflection of atomic particles in a standing light wave was already formulated in 1933 by the physicists Kapitza and Dirac in a theoretical work for electrons. Since electrons interact only weakly with a standing wave, intensive laser radiation was required to experimentally observe the Kapitza-Dirac effect about 15 years ago. On the other hand, light intensities are already sufficient for the diffraction and deflection of atoms, since the strength of the process is enhanced by resonantly increasing the interaction or the use of ultracold, slow atoms. The process is of immense importance in atomic and quantum optics.

Fig. 1 Sketched is the apparatus construction. A collimated thermal beam of helium atoms is exposed to a standing light field generated by interference of two counter-rotating laser pulses for approximately 40 fs over a space of ~20 μm. The deflection of the atom beam is measured with a location-sensitive detector, measuring the location of each deflected helium atom. Without the forces of the standing wave, the detector signal would only consist of a small spot in the middle, generated by undeflected helium atoms (dashed circle). In fact, however, a broad distribution is measured, as can be seen on the detector image. How can you imagine the forces acting on the atom? Atoms in an ongoing field of light, in which the electromagnetic fields periodically build up in place and in time, experience no accelerating forces. This is similar to a floating object on a water surface that is periodically moved up and down by a wave but does not substantially change its horizontal position. On the other hand, if one freezes wave peaks and valleys at short notice, just as the electromagnetic fields in the standing wave are stationary for a short time, then one can easily imagine that the object slips down in the direction of the wave trough. It is accelerated in this case by gravity, which is replaced in the atomic case by the ponderomotive force.

In the journal Physical Review Letters in the issue of March 21, 2014, S. Eilzer, H. Zimmermann and U. Eichmann published a paper in which they deflected atoms in an intense standing light wave for the first time, using two opposing short laser pulses Now, the Kapitza-Dirac effect for atoms has now been demonstrated in a range of laser intensities where atoms are dominated by both the dominant field strength and the extreme intensity gradient ionize high probability, ie, that an electron is released from the atomic network. The fact that this does not necessarily always happen and instead the entire undamaged atom is accelerated, is due to the recently researched at the MBI process of suppressed tunnel ionization, in which the electron oscillates during the laser pulse at a great distance from the atomic core, but ultimately not enough energy from the Laser field picks up to free themselves from the clutches of the attractive nuclear hull. During this time, the electron senses the so-called ponderomotive force, which is caused by its quasi-free periodic movement in the intensity gradient. Since the electron eventually remains bound, this force acting along the laser beam axis is transmitted to the entire atom and leads to its deflection. In continuation of earlier experiments that investigated atomic acceleration in a strong focused light field, the intensity gradient in the standing light wave is now significant over atomic dimensions and therefore also influences the atomic dynamics, see Figure 2. The theoretical description of both the electron dynamics and the Atomic center-of-gravity motion in a standing wave poses a major challenge to current strong-field theory. In further experiments the influence of the strong intensity gradient on the atomic dynamics will be investigated.

Fig. 2 The red and blue curves show velocity distributions determined from the measured deflections for standing waves generated from two elliptically polarized counter-rotating laser pulses with ellipticity ∈ = 0.85 and 0.6, respectively. In the first case, investigations on the dependence on the laser intensity have shown that the deflection of the atoms is limited by ionizing above a certain intensity gradient. Due to the changed polarization, however, in the second case, atoms can be deflected almost twice as much without ionizing them. This is obviously because dynamic processes prevent ionization. Although a qualitative understanding has been achieved, see the black curve adapted to the data, a quantitative confirmation of nonionization with simultaneous acceleration in the context of a quantum mechanical strong field theory is still pending.

Original publication

Strong-field Kapitza-Dirac scattering of neutral atoms

S. Eilzer, H. Zimmermann, U. Eichmann

Physical Review Letters 112 (2014) 113001/1-5