Bright circularly polarized soft X-ray high harmonics for X-ray magnetic circular dichroism

Abstract

We demonstrate, to our knowledge, the first bright circularly polarized high-harmonic beams in the soft X-ray region of the electromagnetic spectrum, and use them to implement X-ray magnetic circular dichroism measurements in a tabletop-scale setup. Using counterrotating circularly polarized laser fields at 1.3 and 0.79 μm, we generate circularly polarized harmonics with photon energies exceeding 160 eV. The harmonic spectra emerge as a sequence of closely spaced pairs of left and right circularly polarized peaks, with energies determined by conservation of energy and spin angular momentum. We explain the single-atom and macroscopic physics by identifying the dominant electron quantum trajectories and optimal phasematching conditions. The first advanced phase-matched propagation simulations for circularly polarized harmonics reveal the influence of the finite phase-matching temporal window on the spectrum, as well as the unique polarization-shaped attosecond pulse train. Finally, we use, to our knowledge, the first tabletop X-ray magnetic circular dichroism measurements at the N 4,5 absorption edges of Gd to validate the high degree of circularity, brightness, and stability of this light source. These results demonstrate the feasibility of manipulating the polarization, spectrum, and temporal shape of high harmonics in the soft X-ray region by manipulating the driving laser waveform.X-rays | high harmonics generation | magnetic material | ultrafast light science | phase matching H igh-harmonic generation (HHG) results from an extreme nonlinear quantum response of atoms to intense laser fields. When implemented in a phase-matched geometry, bright, coherent HHG beams can extend to photon energies beyond 1.6 keV (1, 2). For many years, however, bright HHG was limited to linear polarization, precluding many applications in probing and characterizing magnetic materials and nanostructures, as well as chiral phenomena in general. Although X-ray optics can in principle be used to convert extreme UV (EUV) and X-ray light from linear to circular polarization, in practice such optics are challenging to fabricate and have poor throughput and limited bandwidth (3). A more appealing option is the direct generation of elliptically polarized (4-6) and circularly polarized (7-9) high harmonics. In recent work we showed that by using a combination of 0.8 and 0.4 μm counterrotating driving fields, bright (i.e., phase-matched) EUV HHG with circular polarization can be generated at wavelengths λ > 18 nm and used for EUV magnetic dichroism measurements (10-13).Here we make, to our knowledge, the first experimental demonstration of circularly polarized harmonics in the soft X-ray region to wavelengths λ < 8 nm, and use them to implement soft X-ray magnetic circular dichroism (XMCD) measurements using a tabletop-scale setup. By using counterrotating driving lasers at 0.79 μm (1.57 eV) and 1.3 μm (0.95 eV), we generate bright circularly polarized soft X-ray HHG beams with photon energies greater than 160 eV (14) and with flux comparable...

Observation of ionization enhancement in two-color circularly polarized laser fields

“…In the case of HHG, the use of a bicircular driving laser field provides a bright (i.e. phase matched) source of circularly polarized EUV [23][24][25][26][27][28][29] and soft X-ray [30] beams with sufficient flux for applications in magnetic spectroscopies of materials [28,30]. In the case of SFI, bicircular fields allow electrons to be driven in 2D trajectories prior to rescattering from the parent ion [31][32][33][34][35][36][37][38].…”

“…In the case of SFI, bicircular fields allow electrons to be driven in 2D trajectories prior to rescattering from the parent ion [31][32][33][34][35][36][37][38]. Interestingly, the shape of the bicircular waveform -and therefore the physics of the HHG and SFI processes -can be modified by changing the relative wavelengths, intensities, and ellipticities of the two driving fields [29][30][31]36]. Recent studies have explored the acceleration and rescattering/recombination steps of SFI and HHG driven by a bicircular field [30,36,[39][40][41].…”

Retrieval of target structure information from laser-induced photoelectrons by few-cycle bicircular laser fields

“…These techniques have been demonstrated to be capable of imaging molecular structural changes with unprecedented subangstrom spatial and few-femtosecond temporal resolutions [10,11,14,19]. Both these techniques are based on the rescattering physics [20,21] with linearly polarized laser pulses.Quite recently, efficient high-order harmonic generation with bicircular pulses has been achieved [22,23], which provides a new tabletop circularly polarized coherent extreme ultraviolet to soft x-ray source with photon energies up to 160 eV [24]. Subsequently, these harmonics have been demonstrated to be very sensitive to electronic structure [25][26][27][28][29][30][31].…”

“…Quite recently, efficient high-order harmonic generation with bicircular pulses has been achieved [22,23], which provides a new tabletop circularly polarized coherent extreme ultraviolet to soft x-ray source with photon energies up to 160 eV [24]. Subsequently, these harmonics have been demonstrated to be very sensitive to electronic structure [25][26][27][28][29][30][31].…”

Controlling Nonsequential Double Ionization in Two-Color Circularly Polarized Femtosecond Laser Fields

“…ray beams [35] bright enough for applications, such as performing the first tabletop X--ray circular dichroism measurements [33,35], using high--harmonic spectroscopy to measure the orbital angular momentum symmetry of atomic orbitals [36], and three--dimensional attosecond meteorology [37]. Recently, photoelectron spectroscopy was used to experimentally observe electron--ion rescattering in these fields [38,39].…”