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

Significance The new ability to generate circularly polarized coherent (laser-like) beams of short wavelength high harmonics in a tabletop-scale setup is attracting intense interest worldwide. Although predicted in 1995, this capability was demonstrated experimentally only in 2014. However, all work to date (both theory and experiment) studied circularly polarized harmonics only in the extreme UV (EUV) region of the spectrum at wavelengths >18 nm. In this new work done in a broad international collaboration, we demonstrate the first soft X-ray high harmonics with circular polarization to wavelengths λ < 8 nm and the first tabletop soft X-ray magnetic circular dichroism (XMCD) measurements, and also uncover new X-ray light science that will inspire many more studies of circular high-harmonic generation (HHG). 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 phase-matching 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 N4,5 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.

[1]  P. Oppeneer.  Chapter 3 Magneto-optical kerr spectra , 2001 .

[2]  M. Cinchetti,et al.  Engineered materials for all-optical helicity-dependent magnetic switching. , 2014, Nature materials.

[3]  M. Murnane,et al.  High flux coherent supercontinuum soft X-ray source driven by a single-stage 10 mJ, kHz, Ti:sapphire laser amplifier , 2014, 2014 Conference on Lasers and Electro-Optics (CLEO) - Laser Science to Photonic Applications.

[4]  W. Sandner,et al.  High-harmonic generation by a bichromatic bicircular laser field , 2001 .

[5]  Pavel Sidorenko,et al.  Spin angular momentum and tunable polarization in high-harmonic generation , 2014, Nature Photonics.

[6]  Farhad Salmassi,et al.  Full field tabletop EUV coherent diffractive imaging in a transmission geometry. , 2013, Optics express.

[7]  X. Tong,et al.  Generation of circularly polarized multiple high-order harmonic emission from two-color crossed laser beams , 1998 .

[8]  Long,et al.  Model calculations of polarization-dependent two-color high-harmonic generation. , 1995, Physical review. A, Atomic, molecular, and optical physics.

[9]  D. Milošević,et al.  Attosecond pulse trains with unusual nonlinear polarization , 2000 .

[10]  M. Murnane,et al.  Phase-matched generation of coherent soft X-rays , 1998, Science.

[11]  Long,et al.  Polarization-dependent high-order two-color mixing. , 1995, Physical review. A, Atomic, molecular, and optical physics.

[12]  J. A. Pérez-Hernández,et al.  High-order harmonic propagation in gases within the discrete dipole approximation , 2010 .

[13]  D. Huber,et al.  MAGNETIC LINEAR DICHROISM EFFECTS IN REFLECTION SPECTROSCOPY : A CASE STUDY AT THE FE M2,3 EDGE , 1997 .

[14]  Justin M. Shaw,et al.  Erratum: Non-collinear generation of angularly isolated circularly polarized high harmonics , 2015, Nature Photonics.

[15]  J. Okamoto,et al.  Magnetic Circular Dichroism of X-Ray Emission for Gadolinium in 4d–4f Excitation Region , 2002 .

[16]  Justin M. Shaw,et al.  Generation of bright phase-matched circularly-polarized extreme ultraviolet high harmonics , 2014, Nature Photonics.

[17]  M. Murnane,et al.  Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current , 2012, Nature Communications.

[18]  S. Bader,et al.  Enhanced interfacial magnetic coupling of Gd /Fe multilayers. , 2001, Physical review letters.

[19]  M. Murnane,et al.  Elliptically polarized high-order harmonic emission from molecules in linearly polarized laser fields. , 2009, Physical review letters.

[21]  Herbert Walther,et al.  Above-threshold ionization: From classical features to quantum effects , 2002 .

[22]  Harmonic generation beyond the Strong-Field Approximation: the physics behind the short-wave-infrared scaling laws. , 2009, Optics express.

[23]  D. Milošević.  Generation of elliptically polarized attosecond pulse trains. , 2015, Optics letters.

[24]  M. Murnane,et al.  Generation of bright isolated attosecond soft X-ray pulses driven by multicycle midinfrared lasers , 2014, Proceedings of the National Academy of Sciences.

[25]  Magneto-optics of Gd and Tb in the soft x-ray resonance regions , 2002, cond-mat/0212449.

[26]  M. Murnane,et al.  Bright Coherent Ultrahigh Harmonics in the keV X-ray Regime from Mid-Infrared Femtosecond Lasers , 2012, Science.

[27]  A. Becker,et al.  Intense-field many-body S-matrix theory , 2005 .

[28]  B. Vodungbo,et al.  Towards enabling femtosecond helicity-dependent spectroscopy with high-harmonic sources , 2015, Nature Communications.

[29]  M M Murnane,et al.  Bright, coherent, ultrafast soft X-ray harmonics spanning the water window from a tabletop light source. , 2010, Physical review letters.

[30]  X-Ray Magnetic Circular Dichroism Probed Using High Harmonics , 2014 .

[31]  M. Ivanov,et al.  Spin conservation in high-order-harmonic generation using bicircular fields , 2014, 1404.6242.

[32]  I. Christov,et al.  Phase matching of high harmonic generation in the soft and hard X-ray regions of the spectrum , 2009, Proceedings of the National Academy of Sciences.

[33]  J. Miao,et al.  Ultrahigh 22 nm resolution coherent diffractive imaging using a desktop 13 nm high harmonic source. , 2011, Optics express.

[34]  Joachim Stöhr,et al.  Magnetism From Fundamentals to Nanoscale Dynamics , 2006 .

[35]  Laurent Nahon,et al.  A table-top ultrashort light source in the extreme ultraviolet for circular dichroism experiments , 2014, Nature Photonics.

[36]  S. Witte,et al.  Direct visualization of laser-driven electron multiple scattering and tunneling distance in strong-field ionization. , 2012, Physical review letters.

[37]  D. Milošević.  High-order harmonic generation by a bichromatic elliptically polarized field: conservation of angular momentum , 2015 .

[38]  Franck Delmotte,et al.  Polarization control of high order harmonics in the EUV photon energy range. , 2011, Optics express.

[39]  Prediction of huge x-ray Faraday rotation at the GdN4,5threshold , 2002, cond-mat/0206154.

[40]  H. Mertins,et al.  Faraday rotation spectra at shallow core levels: 3p edges of Fe, Co, and Ni , 2006 .

[41]  M. Murnane,et al.  Tabletop nanometer extreme ultraviolet imaging in an extended reflection mode using coherent Fresnel ptychography , 2013, 1312.2049.

[42]  M M Murnane,et al.  Zeptosecond high harmonic keV x-ray waveforms driven by midinfrared laser pulses. , 2013, Physical review letters.

[43]  Justin M. Shaw,et al.  Probing the timescale of the exchange interaction in a ferromagnetic alloy , 2012, Proceedings of the National Academy of Sciences.

[44]  Bosheng Zhang,et al.  High contrast 3D imaging of surfaces near the wavelength limit using tabletop EUV ptychography. , 2015, Ultramicroscopy.

[45]  D. Milošević,et al.  Generation of circularly polarized high-order harmonics by two-color coplanar field mixing , 2000 .

[46]  G. Reichardt,et al.  X-ray magneto-optics in lanthanides. , 2001, Physical review letters.

[47]  Ashish Tripathi,et al.  Dichroic coherent diffractive imaging , 2011, Proceedings of the National Academy of Sciences.

[48]  J. Miao,et al.  Beyond crystallography: Diffractive imaging using coherent x-ray light sources , 2015, Science.