Structural and Magnetic Characterization of Large Area, Free-Standing Thin Films of Magnetic Ion Intercalated Dichalcogenides Mn0.25TaS2 and Fe0.25TaS2

Free-standing thin films of magnetic ion intercalated transition metal dichalcogenides are produced using ultramicrotoming techniques. Films of thicknesses ranging from 30nm to 250nm were achieved and characterized using transmission electron diffraction and X-ray magnetic circular dichroism. Diffraction measurements visualize the long range crystallographic ordering of the intercalated ions, while the dichroism measurements directly assess the orbital contributions to the total magnetic moment. We thus verify the unquenched orbital moment in Fe0.25TaS2 and measure the fully quenched orbital contribution in Mn0.25TaS2. Such films can be used in a wide variety of ultrafast X-ray and electron techniques that benefit from transmission geometries, and allow measurements of ultrafast structural, electronic, and magnetization dynamics in space and time

Parametric frequency mixing in the magneto-elastically driven FMR-oscillator

We demonstrate the nonlinear frequency conversion of ferromagnetic resonance (FMR) frequency by optically excited elastic waves in a thin metallic film on dielectric substrates. Time-resolved probing of the magnetization directly witnesses magneto-elastically driven second harmonic generation, sum- and difference frequency mixing from two distinct frequencies, as well as parametric downconversion of each individual drive frequency. Starting from the Landau-Lifshitz-Gilbert equations, we derive an analytical equation of an elastically driven nonlinear parametric oscillator and show that frequency mixing is dominated by the parametric modulation of FMR frequency

Quasi-phase-matched generation of coherent extreme-ultraviolet light

Includes bibliographical references (page 54).High-harmonic generation is a well-known method of producing coherent extreme-ultraviolet (EUV) light, with photon energies up to about 0.5 keV. This is achieved by focusing a femtosecond laser into a gas, and high harmonics of the fundamental laser frequency are radiated in the forward direction. However, although this process can generate high-energy photons, efficient high-harmonic generation has been demonstrated only for photon energies of the order 50-100 eV. Ionization of the gas prevents the laser and the EUV light from propagating at the same speed, which severely limits the conversion efficiency. Here we report a technique to overcome this problem, and demonstrate quasi-phase-matched frequency conversion of laser light into EUV. Using a modulated hollow-core waveguide to periodically vary the intensity of the laser light driving the conversion, we efficiently generate EUV light even in the presence of substantial ionization. The use of a modulated fibre shifts the energy spectrum of the high-harmonic light to significantly higher photon energies than would otherwise be possible. We expect that this technique could form the basis of coherent EUV sources for advanced lithography and high-resolution imaging applications. In future work, it might also be possible to generate isolated attosecond pulses

XUV digital in-line holography using high-order harmonics

A step towards a successful implementation of timeresolved digital in-line holography with extreme ultraviolet radiation is presented. Ultrashort XUV pulses are produced as high-order harmonics of a femtosecond laser and a Schwarzschild objective is used to focus harmonic radiation at 38 nm and to produce a strongly divergent reference beam for holographic recording. Experimental holograms of thin wires are recorded and the objects reconstructed. Descriptions of the simulation and reconstruction theory and algorithms are also given. Spatial resolution of few hundreds of nm is potentially achievable, and micrometer resolution range is demonstrated.Comment: 8 pages, 8 figure

Probing photo-induced melting of antiferromagnetic order in La0.5Sr1.5MnO4 by ultrafast resonant soft X-ray diffraction

Photo-excitation in complex oxides1 transfers charge across semicovalent bonds, drastically perturbing spin and orbital orders2. Light may then be used in compounds like magnetoresistive manganites to control magnetism on nanometre lengthscales and ultrafast timescales. Here, we show how ultrafast resonant soft x-ray diffraction can separately probe the photo-induced dynamics of spin and orbital orders in La0.5Sr1.5MnO4. Ultrafast melting of CE antiferromagnetic spin order is evidenced by the disappearance of a (1/4,1/4,1/2) diffraction peak. On the other hand the (1/4,1/4,0) peak, reflecting orbital order, is only partially reduced. Cluster calculations aid our interpretation by considering different magnetically ordered states accessible after photo-excitation. Nonthermal coupling between light and magnetism emerges as a primary aspect of photo-induced phase transitions in manganites.Comment: 7 pages manuscript, 4 figure

Driving Magnetization Dynamics in an On-Demand Magnonic Crystal via the Magnetoelastic Interactions

Using spatial light interference of ultrafast laser pulses, we generate a lateral modulation in the magnetization profile of an otherwise uniformly magnetized film, whose magnetic excitation spectrum is monitored via the coherent and resonant interaction with elastic waves. We find an unusual dependence of the magnetoelastic coupling as the externally applied magnetic field is angle- and field-tuned relative to the wave vector of the magnetization modulation, which can be explained by the emergence of spatially inhomogeneous spin-wave modes. In this regard, the spatial light interference methodology can be seen as a user-configurable, temporally windowed, on-demand magnonic crystal, potentially of arbitrary two-dimensional shape, which allows control and selectivity of the spatial distribution of spin waves. Calculations of spin waves using a variety of methods, demonstrated here using the plane-wave method and micromagnetic simulation, can identify the spatial distribution and associated energy scales of each excitation, which opens the door to a number of excitation methodologies beyond our chosen elastic wave excitation

Deconstructing the Hubbard Hamiltonian by Ultrafast Quantum Modulation Spectroscopy in Solid-state Mott Insulators

Optical pulses at THz and mid-infrared frequencies tuned to specific vibrational resonances modulate the lattice along chosen normal mode coordinates. In this way, solids can be switched between competing electronic phases and new states are created. Here, we use vibrational modulation to make electronic interactions (Hubbard-U) in Mott-insulator time dependent. Mid-infrared optical pulses excite localized molecular vibrations in ET-F₂TCNQ, a prototypical one-dimensional Mott-insulator. A broadband ultrafast probe interrogates the resulting optical spectrum between THz and visible frequencies. A red-shifted charge-transfer resonance is observed, consistent with a time-averaged reduction of the electronic correlation strength U. Secondly, a sideband manifold inside of the Mott-gap appears, resulting from a periodically modulated U. The response is compared to computations based on a quantum-modulated dynamic Hubbard model. Heuristic fitting suggests asymmetric holon-doublon coupling to the molecules and that electron double-occupancies strongly squeeze the vibrational mode

Transient Grating Spectroscopy in Magnetic Thin Films:Simultaneous Detection of Elastic and Magnetic Dynamics

Surface magnetoelastic waves are coupled elastic and magnetic excitations that propagate along the surface of a magnetic material. Ultrafast optical techniques allow for a non-contact excitation and detection scheme while providing the ability to measure both elastic and magnetic components individually. Here we describe a simple setup suitable for excitation and time resolved measurements of high frequency magnetoelastic waves, which is based on the transient grating technique. The elastic dynamics are measured by diffracting a probe laser pulse from the long-wavelength spatially periodic structural deformation. Simultaneously, a magnetooptical measurement, either Faraday or Kerr effect, is sensitive to the out-of-plane magnetization component. The correspondence in the response of the two channels probes the resonant interaction between the two degrees of freedom and reveals their intimate coupling. Unraveling the observed dynamics requires a detailed understanding of the spatio-temporal evolution of temperature, magnetization and thermo-elastic strain in the ferromagnet. Numerical solution of thermal diffusion in two dimensions provides the basis on which to understand the sensitivity in the magnetooptic detection