No abstract

No abstract

Circularly-polarized extreme UV and X-ray radiation provides valuable access to the structural, electronic and magnetic properties of materials. To date, this capability was available only at largescale X-ray facilities such as synchrotrons. Here we demonstrate the first bright, phase-matched, extreme UV circularly-polarized high harmonics and use this new light source for magnetic circular dichroism measurements at the M-shell absorption edges of Co. We show that phase matching of circularly-polarized harmonics is unique and robust, producing a photon flux comparable to the linearly polarized high harmonic sources that have been used very successfully for ultrafast element-selective magneto-optic experiments. This work thus represents a critical advance that makes possible element-specific imaging and spectroscopy of multiple elements simultaneously in magnetic and other chiral media with very high spatial and temporal resolution, using tabletop-scale setups. IntroductionCircularly polarized radiation in the extreme ultraviolet (EUV) and soft X-ray spectral regions has proven to be extremely useful for investigating chirality-sensitive light-matter interactions. It enables studies of chiral molecules using photoelectron circular dichroism 1 , ultrafast molecular decay dynamics 2 , the direct measurement of quantum phases (e.g. Berry's phase and pseudo-spin) in graphene and topological insulators 3-5 and reconstruction of band structure and modal phases in solids 6 . For magnetic materials, circularly polarized soft x-rays are particularly useful for X-ray Magnetic Circular Dichroism (XMCD) spectroscopy 7 . XMCD enables element-selective probing as well as coherent imaging and holography of magnetic structures with nanometer resolution [8][9][10] . Moreover, it can also be used to extract detailed information about the magnetic state by distinguishing between the spin and orbital magnetic moments of each element. Thus, time-resolved XMCD can probe the element-specific dynamics of the spin and orbital moments when interacting with the electronic and phononic degrees of freedom in a material [11][12][13][14] . However, the time resolution available to date for XMCD has been > 100 fs, limited by the pulse duration and timing jitter of synchrotron pulses [15][16][17] . To date it has not been possible to probe spin dynamics of multiple elements simultaneously within the same sample, because the photon energy must be tuned across the various absorption edges at the large-scale facilities where these experiments are currently performed.2 Table-top soft x-ray sources based on high harmonic upconversion of femtosecond laser pulses represent a viable alternative to large-scale sources for many applications, due to their unique ability to generate bright, broadband, ultrashort and coherent light with an energy spectrum reaching into the keV region 18 . High harmonic generation (HHG) not only enables coherent imaging of nanometer structures with a spatial resolution approaching the diffraction limit 19 , but also accesses...

We present an experimental study on wave propagation in highly nonlocal optically nonlinear media, for which far-away boundary conditions significantly affect the evolution of localized beams. As an example, we set the boundary conditions to be anisotropic and demonstrate the first experimental observation of coherent elliptic solitons. Furthermore, exploiting the natural ability of such nonlinearities to eliminate azimuthal instabilities, we perform the first observation of stable vortex-ring solitons. These features of highly nonlocal nonlinearities affected by far-away boundary conditions open new directions in nonlinear science by facilitating remote control over soliton propagation. DOI: 10.1103/PhysRevLett.95.213904 PACS numbers: 42.65.Tg, 42.65.Jx, 47.27.Te Nonlocality plays an important role in many areas of nonlinear physics. Nonlocality typically arises from an underlying transport mechanism (heat [1], atoms in a gas [2], charge carriers [3,4], etc.) or from long-range forces (e.g., electrostatic interactions in liquid crystals [5]) and many-body interactions as with matter waves in BoseEinstein condensates [6] or plasma waves [7]. In nonlinear optics specifically, nonlocality was found in photorefractives [3,[8][9][10], in thermal nonlinear media [11][12][13][14], in atomic vapors [2], and in liquid crystals [5,15]. In principle, nonlocality acts to spread out the effects of localized excitations, and as such it can suppress modulation instabilities of homogeneous states [16]. However, in spite of the natural ''averaging'' tendency inherent to nonlocality, even highly nonlocal nonlinear media can support solitons [2,5,[17][18][19][20]. Moreover, it was suggested that nonlocality can prevent the catastrophic collapse of self-focused beams, allowing 2 1 D solitons in Kerr-type media [2,17,21]. In a similar vein, it was proposed that nonlocality can suppress azimuthal instabilities of vortex-ring beams [22,23], but such an experiment has thus far never been reported. Finally, nonlocality can considerably alter soliton interactions, e.g., giving rise to attraction between out of phase solitons [19,20,24] and between dark solitons [25], which without nonlocality always repel, and causing attraction between well separated solitons [26].Here we present an experimental study on solitons in a nonlinear medium with an extremely large range of nonlocality, such that far-away boundary conditions directly affect the soliton beam. We use the thermal optical nonlinearity in lead glass, which is of the self-focusing type. The nonlocal nature of this thermal nonlinearity is manifested in the heat-transfer (Poisson-type) equation, for which boundary conditions greatly influence the temperature distribution. The nonlinear index change is proportional to the temperature change; hence, the boundary conditions, even from afar, significantly affect the refractive index structure supporting solitons. We show that setting transversely anisotropic boundary conditions (e.g., rectangular boundaries in the transverse plane) f...

No abstract