High-order nonlinear dipole response characterized by extreme ultraviolet ellipsometry

Abstract

Polarization engineering and characterization of coherent high-frequency radiation are essential to investigate and control the symmetry properties of light–matter interaction phenomena at their most fundamental scales. This work demonstrates that polarization control and characterization of high-harmonic generation provides an excellent ellipsometry tool that can fully retrieve both the amplitude and phase of a strong-field-driven dipole response. The polarization control of high-harmonic generation is realized by a transient nonlinear dipole grating coherently induced by two noncollinear counterrotating laser fields. By adjusting the ellipticity of the two driving pulses simultaneously, the polarization state of every high-harmonic order can be tuned from linear to highly elliptical, and it is fully characterized through an energy-resolved extreme ultraviolet polarimeter. From the analysis of the polarization state, the ellipsometry indicated that both the amplitude and phase of the high-harmonic dipole scale rapidly with the driving laser field for higher-order harmonics, and, especially, for gases with a small ionization potential. Our experimental results were corroborated by theoretical simulations. Our findings revealed a novel high-harmonic ellipsometry technique that can be used for the next generation of high-harmonic spectroscopy and attosecond metrology studies because of its ability to provide single-digit attosecond accuracy. Our work also paves the way to precisely quantify the strong-field dynamics of fundamental processes associated with the transfer of energy and angular momentum between electron/spin systems and the symmetry-dependent properties of molecules and materials.

Electron quantum path control in high harmonic generation via chirp variation of strong laser pulses

“… are weights for each of the short and long trajectories while n is a non-linearity parameter for the HHG conversion. are the dipole phase constants 23 , 39 and is a phase offset depending on the atomic properties. In our extended model, the time dependent laser angular frequency accounts for the SPM effects caused by the laser propagation in the non-linear medium and in the plasma, as 40 , 41 : where is the propagation length of the laser beam in the Kerr medium within the Rayleigh length, excluding the HHG region (i.e.…”

Separation of photoionization and measurement-induced delays

“…Here, we solve this challenge by using circularly polarized extreme ultraviolet (XUV) and infrared (IR) fields, as used in our previous work (31), but additionally introduce the concept of mirror symmetry-broken attosecond interferometry. A left circularly polarized attosecond pulse train generated from the noncollinear highorder harmonic generation (HHG) process (32)(33)(34)(35) of argon is used to photoionize helium atoms, preparing an electronic p 1 continuum state, where p represents the orbital angular momentum quantum number and the subscript is the magnetic quantum number. Note that here the quantization axis is defined as the light propagation direction (z axis) due to the circular polarization, as illustrated in Fig.…”

Extending the degree of polarization concept to higher-order and orbital angular momentum Poincaré spheres

“…This interpretation can be also performed in the spectral regime, where the frequency components of the laser pulse might present different polarization states, giving rise to a DoP different from one. This temporal variation of the polarization state of ultrafast laser pulses is particularly interesting in the context of high-order harmonic generation, where the time-dependent polarization state of the high-frequency pulses that are emitted provides information about the ultrafast laser-driven electron dynamics that take place in the generating medium [29][30][31]. During the last decade, vector beams have been also generated in the ultrafast regime.…”