Measurement of the polarization of high-order harmonics from aligned N_2 molecules by spatial interferometry

Abstract

The polarization of high-harmonics from aligned N(2) molecules was measured by observing the visibility of spatial interference between two high-harmonics generated separately. The minimum visibility was observed at an angle of 60 degrees between the polarization of the harmonic generation laser field and the molecular orientation. In this case, the angular shift of harmonic polarization is 15 degrees from the molecular orientation. Our measurement of the visibility variation matches the theoretical prediction based on the harmonic field calculation for aligned N(2) molecules.

Origin and implication of ellipticity in high-order harmonic generation from aligned molecules

“…Hence, the condition |M − M | = 1 is violated in these three geometries but is satisfied for other orientations of the pump field polarization with respect to that of the probe field. These results explain analytically the observations of the several recent polarization experiments [9][10][11]14,18], and generalize the measured data to an arbitrary molecule. From a physical perspective, these results are readily understood: in all three geometries,ˆ pu ˆ pr , pu ⊥ˆ pr , and pu = 0, the cylindrical symmetry of space is conserved, and hence emission cannot be observed in a direction that breaks this symmetry.…”

“…Reference [13] argued that in order for ellipticity to be observed it is necessary that the molecule be prepared in a state that carries an electric ring current, in contradiction to both the reasoning of [9,10] and the observations of [11]. Measurements of the angle of rotation of emitted, linearly polarized, light with respect to the ionization field polarization, reported in [14], were found in variance with the results of [9]. Several other observations reported in Refs.…”

Probing the origin of elliptical high-order harmonic generation from aligned molecules in linearly polarized laser fields

“…Traditionally, the power (or radiation energy) spectra have been the main topic of HHG studies both experimentally and theoretically. However, recent developments in molecular alignment technology [8] open a possibility of investigating other HHG properties including the phase [9-11] and polarization [12][13][14] of the harmonic radiation.For ensembles of atoms or unaligned molecules, the polarization of the harmonics is expected to be the same as the polarization of the driving laser field. For aligned molecules, however, the harmonic radiation has two components: one parallel and another perpendicular to the laser polarization direction.…”

“…Traditionally, the power (or radiation energy) spectra have been the main topic of HHG studies both experimentally and theoretically. However, recent developments in molecular alignment technology [8] open a possibility of investigating other HHG properties including the phase [9-11] and polarization [12][13][14] of the harmonic radiation.…”

“…For aligned molecules, however, the harmonic radiation has two components: one parallel and another perpendicular to the laser polarization direction. Experiments [12,13] showed that the linear polarization state of HHG driven by linearly polarized laser fields is tilted due to the nonvanishing perpendicular component of HHG. Recently, the elliptical polarization state of HHG driven by linearly polarized laser fields has been measured [14]; speculations on its possible origins include, for example, multielectron effects and the influence of the Coulomb potential.…”