Abstract

The electric dipole approximation is a concept widely used to facilitate calculations and the understanding of processes involved in light-matter interactions in atomic, molecular and optical physics. In its essence, it assumes that the spatial variation of the electromagnetic wave over the target can be neglected. As a consequence of this, the influence of the magnetic field on the interaction is also not considered. The dipole approximation usually holds well for the most commonly used near-infrared laser sources around a wavelength of 800 nm and intensities of up to 10¹⁶ W/cm². However, moving from the near-infrared to the mid-infrared it was predicted that a more complete description of laser-atom interaction would have to include the magnetic field component of the ionizing radiation as well [1]. This becomes important due to higher electron velocities reached in electric fields of longer wavelength and the onset of a significant Lorentz force. We experimentally explore this limit and observe the breakdown of the electric dipole approximation in the long-wavelength limit in strong-field ionization with linearly polarized few-cycle mid-infrared laser pulses at a wavelength of 3.4 µm [2]. Our findings for linear polarization may appear surprising in the frame of the well known radiation pressure picture of Smeenk et al. [4], which we could most recently confirm for our mid-infrared wavelength with circular polarization (Fig. 1 c) upper half).

© 2015 IEEE