David M. Villeneuve

High-harmonic generation in solids allows probing and controlling electron dynamics in crystals on few femtosecond timescales, paving the way to lightwave electronics. In the spatial domain, recent advances in the real-space interpretation of high-harmonic emission in solids allows imaging the field-free, static, potential of the valence electrons with picometer resolution. The combination of such extreme spatial and temporal resolutions to measure and control strong-field dynamics in solids at the atomic scale is poised to unlock a new frontier of lightwave electronics. Here, we report a strong intensity-dependent anisotropy in the high-harmonic generation from ReS₂ that we attribute to angle-dependent interference of currents from the different atoms in the unit cell. Furthermore, we demonstrate how the laser parameters control the relative contribution of these atoms to the high-harmonic emission. Our findings provide an unprecedented atomic perspective on strong-field dynamics in crystals, revealing key factors to consider in the route towards developing efficient harmonic emitters.

The gas-phase technique of noncollinear two-color excitation is adapted to study extreme-ultraviolet high-order harmonic generation (HHG) in magnesium oxide. We observe high-order wave-mixing pathways with brightness comparable to that of the single-color HHG. Simulations reveal that the efficiencies are dominated by the microscopic physics as opposed to macroscopic effects. Building on previous work, we develop a general analytic theory of high-order wave mixing that explains our observations and demonstrates how a perturbative nonlinear optical framework emerges form the interaction between the weak field and the laser-dressed state of the crystal in the strong-field approximation. The theory outlines a photon-mixing picture of high-order wave mixing and connects the strong asymmetry between sum- and difference-frequency pathways to the cutoff in the HHG spectrum. Our work establishes the potential of high-order wave mixing as a probe of the microscopic attosecond physics in solids and points to new opportunities in solid-state strong-field photonics.

High-harmonic generation in solids allows probing and controlling electron dynamics in crystals on few femtosecond timescales, paving the way to lightwave electronics. In the spatial domain, recent advances in the real-space interpretation of high-harmonic emission in solids allows imaging the field-free, static, potential of the valence electrons with picometer resolution. The combination of such extreme spatial and temporal resolutions to measure and control strong-field dynamics in solids at the atomic scale is poised to unlock a new frontier of lightwave electronics. Here, we report a strong intensity-dependent anisotropy in the high-harmonic generation from ReS₂ that we attribute to angle-dependent interference of currents from the different atoms in the unit cell. Furthermore, we demonstrate how the laser parameters control the relative contribution of these atoms to the high-harmonic emission. Our findings provide an unprecedented atomic perspective on strong-field dynamics in crystals and suggest that crystals with a large number of atoms in the unit cell are not necessarily more efficient harmonic emitters than those with fewer atoms.

An electron wave function is characterized by the phase and amplitude distributions over position or momentum space. Recent attosecond technologies allow us to obtain the phase information of photoelectrons using interference between several optical transition pathways. We demonstrate that by employing a two-path photoionization interference process, the complex wave function of the photoelectron is fully mapped in two-dimensional momentum space. We ionize neon gas by an extreme ultraviolet (XUV) attosecond pulse train consisting of both odd and even harmonics in the presence of an infrared (IR) laser field. By controlling the generation process of the attosecond pulse train, we isolate two ionization pathways for interfering with the photoelectrons: one is the two-photon ionization process due to the odd harmonic excitation with one IR photon absorption, and the other is the one-photon ionization by an even harmonic. We record the photoelectron momentum distributions via velocity map imaging as a function of the XUV and IR delay. Using three different experimental conditions, we show that the detailed structure of the amplitude and phase distributions of photoelectrons can be resolved in the two-dimensional momentum space within the bandwidth determined by the attosecond XUV pulse. We separate the measured photoelectron wave function into those produced by each ionization pathway. Our method will be applicable to more complex molecules.

Using the semiclassical two-step model for strong-field ionization, we theoretically investigate subcycle interference structures in the photoelectron momentum distribution. Specifically, we focus on the low-momentum fanlike interference structure. Employing a time-variable soft-core Coulomb potential, we demonstrate that the low-momentum interference arises from the interference between drifted and undrifted electrons from opposite direct quarter cycles. We also find that the main scattering in the nuclear Coulomb potential occurs just after ionization. Our findings suggest that the low-momentum region of the photoelectron spectrum is particularly sensitive to the ion potential and thereby offers another path to probe ultrafast electronic structure dynamics.

We demonstrated coherent control of molecular absorption line shape and optical gain in XUV. The control is achieved by creating a quantum coherence in the ground electronic state of hydrogen molecules.

Development of attosecond laser technology allows us to measure electron dynamics such as photoionization delays between different species or electronic states. In general, the measured photoionization phase is a mixture of the spectral phase of the extreme ultraviolet (XUV) pulse and the atomic phases inherent to the optical transitions. Hence, it is difficult to disentangle these phases independently. Here we separate these phases by using an XUV attosecond pulse train containing both even and odd harmonic orders, generated by an 800- and 400 nm laser pulse, in the presence of the infrared 800-nm pulse. We measure the photoelectron angular distributions as a function of two independently controlled delays, the XUV-IR and the 800–400 nm delays, with attosecond time resolution. We analyze the photoelectron angular distributions to determine the relative amplitudes and phases of each angular momentum component. Using an in situ technique, we determine the phases of the harmonic orders and thereby completely determine the atomic phases. Using the obtained atomic phases and amplitudes, we reconstruct the real and imaginary parts of the continuum wave functions associated with three individual photoionization pathways.