A new scheme for a double-slit experiment in the time domain is presented. Phase-stabilized few-cycle laser pulses open one to two windows (slits) of attosecond duration for photoionization. Fringes in the angle-resolved energy spectrum of varying visibility depending on the degree of which-way information are measured. A situation in which one and the same electron encounters a single and a double slit at the same time is observed. The investigation of the fringes makes possible interferometry on the attosecond time scale. From the number of visible fringes, for example, one derives that the slits are extended over about 500 as.

An efficient method is investigated for the generation of circularly polarized high-order harmonics by a bichromatic laser field whose two components with frequencies $\omega$ and $2\omega$ are circularly polarized in the same plane, but rotate in opposite directions. The generation of intense harmonics by such a driving-field configuration was already confirmed by a previous experiment. With the help of both a semiclassical three-step model as well as a saddle-point analysis, the mechanism of harmonic generation in this case is elucidated and the plateau structure of the harmonic response and their cutoffs are established. The sensitivity of the harmonic yield and the polarization of the harmonics to imperfect circular polarization of the driving fields are investigated. Optimization of both the cutoff frequency and the harmonic efficiency with respect to the intensity ratio of the two components of the driving field is discussed. The electron trajectories responsible for the emission of particular harmonics are identified. Unlike the case of a linearly polarized driving field, they have a nonzero start velocity. By comparison with the driving-field configuration where the two components rotate in the same direction, the mechanism of the intense harmonic emission is further clarified. Depending on the (unknown) saturation intensity for the bichromatic field with counter-rotating polarizations, this scheme might be of practical interest not only because of the circular polarization of the produced harmonics, but also because of their production efficiency.

Significance: The new ability to generate circularly polarized coherent (laser-like) beams of short wavelength high harmonics in a tabletop-scale setup is attracting intense interest worldwide. Although predicted in 1995, this capability was demonstrated experimentally only in 2014. However, all work to date (both theory and experiment) studied circularly polarized harmonics only in the extreme UV (EUV) region of the spectrum at wavelengths >18 nm. In this new work done in a broad international collaboration, we demonstrate the first soft X-ray high harmonics with circular polarization to wavelengths λ < 8 nm and the first tabletop soft X-ray magnetic circular dichroism (XMCD) measurements, and also uncover new X-ray light science that will inspire many more studies of circular high-harmonic generation (HHG).

We consider harmonic generation by atoms exposed to an intense laser pulse of a few femtoseconds. Our results, obtained by solving numerically the corresponding three-dimensional time-dependent Schrödinger equation, demonstrate the strong sensitivity of the harmonic spectra to the phase of the laser. These results are explained in terms of both quantum and classical dynamics. We show that this phase sensitivity may be exploited in order to probe the laser phase for ultrashort pulses. Our discussion about this new method of diagnosis takes into account propagation effects.