How to characterize the polarization of extreme-ultraviolet (XUV) high-harmonic radiation without an XUV polarizer? This question is important for two reasons. First, XUV polarizers are very lossy and bandwidth-limited. Second, high harmonic (HH) radiation fields, which are not scalar but rather vectorial, appear in many situations, such as when anisotropic media and/or a vectorial strong infra-red laser driver are used. This Optics Letters article presents a solution to the above question and demonstrates how the amplitude and phase of the two vectorial components of the HH radiation field emitted from aligned Nitrogen molecules can be retrieved, in a single shot, by analyzing the far-field fringe pattern formed in a spatially and spectrally-resolved interferometry experiment. In a nut shell, the harmonic source is first split into two sources which are spatially separated by approximately 100 μm, which in the far-field results in a spatial fringe pattern superimposed onto the spectral comb of odd-harmonics of the driver frequency. One of the two sources serves as a reference for the measurement and emits linearly-polarized HH radiation, the other is further spatially modulated into a weak sinusoidal grating of molecular excitations (rotational wavepacket) with a spatial period of approximately 10 μm, which in the far-field results in two additional Bragg peaks in the spectrogram. At specific times this source emits elliptically-polarized HH radiation due to the anisotropy induced by the molecular alignment. By measuring the fringe position and the integrated HH signal in the zero- and first-order Bragg peaks, and using a fitting algorithm, the characterization task presented at the beginning of this paragraph is then carried out.
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