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We use femtosecond Resonant Soft X-ray Diffraction to observe heterogeneous melting of antiferromagnetic order in an epitaxially grown NdNiO3 film. Cross-interface coupling launches a propagating phase front following direct lattice excitation of the LaAlO3 substrate.

We use intense broadband THz pulses to excite underdoped YBCO exhibiting competing superconducting and charge density wave ground states. We observe pronounced coherent oscillations at 1.85 and 2.65THz, attributed to renormalized low-energy phonon modes.

Using ultrafast resonant soft x ray diffraction we demonstrate photoinduced melting of antiferromagnetic order in the correlated electron insulator NdNiO3. Time dependent analysis of the resonant diffraction spectra allows us to follow the temporal evolution of the charge imbalance between adjacent Ni sites. A direct correlation between the melting of magnetic order and charge rebalancing is found. Furthermore we demonstrate that the magnetic ordering on the Ni and Nd sites which are locked together in equilibrium become decoupled during this nonthermal process. © 2013 American Physical Society.

X-ray magnetic circular dichroism spectroscopy using an X-ray free electron laser is demonstrated with spectra over the Fe L$_{3,2}$-edges. This new ultrafast time-resolved capability is then applied to a fluence-dependent study of all-optical magnetic switching dynamics of Fe and Gd magnetic sublattices in a GdFeCo thin film above its magnetization compensation temperature. At the magnetic switching fuence, we corroborate the existence of a transient ferromagnetic-like state. The timescales of the dynamics, however, are longer than previously observed below the magnetization compensation temperature. Above and below the switching fluence range, we observe secondary demagnetization with about 5 ps timescales. This indicates that the spin thermalization takes longer than 5 ps., 5 pages, 4 figures

Employing the technique of time-resolved photoemission allowed the measurement of carrier distributions in graphene. An initial separation followed by an eventual coalescence of the distributions was directly observed for the first time.

At ${T}_{0}=17.5$ K an exotic phase emerges from a heavy fermion state in URu${}_{2}$Si${}_{2}$. The nature of this hidden order (HO) phase has so far evaded explanation. Formation of an unknown quasiparticle (QP) structure is believed to be responsible for the massive removal of entropy at the HO transition, however, experiments and ab initio calculations have been unable to reveal the essential character of the QP. Here we use femtosecond pump-probe time- and angle-resolved photoemission spectroscopy (tr-ARPES) to elucidate the ultrafast dynamics of the QP. We show how the Fermi surface is renormalized by shifting states away from the Fermi level at specific locations, characterized by vector ${q}_{\ensuremath{\langle}110\ensuremath{\rangle}}=0.56\ifmmode\pm\else\textpm\fi{}0.08$ ${\text{\AA{}}}^{\ensuremath{-}1}$. Measurements of the temperature-time response reveal that, upon entering the HO, the QP lifetime in those locations increases from 42 fs to few hundred fs. The formation of the long-lived QPs is identified here as a principal actor of the HO.

Summary form only given. Previous studies have demonstrated the preponderance of intense ultrafast terahertz (THz) sources based on two-color gas plasma filaments with extremely broad bandwidths in excess of 70 THz. Applications ranging from nonlinear THz materials science to broadband hyperspectral detection drive the need for such sources to be optimized for output power and spectral content. Further, it would also be useful to have a source with a sufficiently high repetition rate frequency for sensitive lock-in coherent detection methods. Previous studies have also relied on THz field detection where propagation of the pulse through ambient conditions which are known to be deleterious to the pulse fidelity through water vapor absorption, and in turn, affects spectral fidelity. It is desirable, from the perspective of practical THz sources for spectroscopy, that these sources and their associated generation mechanisms be studied for optimal generation with peak THz fields in the MV/cm range. For instance, understanding spectral content, total energy in pulse, optimal gas concentration, focusing conditions, inter-pulse phase slippage, and tunability are all parameters for consideration that affect output in these sources.In our presentation we report on strong THz pulse generation in a variety of gases (air, Ne, Ar, Kr, and Xe) using our twocolor ultrafast pulse generation method1'2. We concentrate on kilohertz-based ultrafast generation techniques with attention to THz time domain based interferometric power spectral measurements as the gas species, pressure, and laser pulse energy are varied. We find that we can routinely produce THz pulses as short as 40 fs, μJ-level pulse energies, and having a frequency spectral bandwidth of more than 40 THz. A simple 1-D fluid model based on Maxwell's equations describing THz generation via field ionization resulting in an asymmetric transverse plasma current in the driving optical two-color field is used to simulate the results for comparison with experiment.

We report a novel ultrafast optical system capable of directly pumping low energy excitations in complex materials and probing the photoinduced changes in their properties with terahertz pulses, benchmarked through mid-infrared-pump, THz-probe measurements on InSb.

We performed ultrafast time resolved near infrared pump resonant soft x ray diffraction probe measurements to investigate the coupling between the photoexcited electronic system and the spin cycloid magnetic order in multiferroic TbMnO3 at low temperatures. We observe melting of the long range antiferromagnetic order at low excitation fluences with a decay time constant of 22.3 ± 1.1 ps which is much slower than the ~1 ps melting times previously observed in other systems. To explain the data we propose a simple model of the melting process where the pump laser pulse directly excites the electronic system which then leads to an increase in the effective temperature of the spin system via a slower relaxation mechanism. Despite this apparent increase in the effective spin temperature we do not observe changes in the wave vector q of the antiferromagnetic spin order that would typically correlate with an increase in temperature under equilibrium conditions.We suggest that this behavior results from the extremely low magnon group velocity that hinders a change in the spin spiral wave vector on these time scales. DOI: 10.1103/PhysRevB.92.184429

We investigate effects of the finite pump beam size in optical-pump/THz probe measurements experimentally and numerically. A frequency-dependent size regime in which spectral distortion is minimal is identified and a correction method is presented.

We measure the polarizability of electron-hole pairs quantum-confined in CdSe nanoparticles of 2-7 nm in diameter by the optical-pump/THz probe technique. The observed many-body effects are compared to self-consistent theoretical calculations.

Quantum confinement plays an important role in the optical properties of semiconductor quantum dots (QDs). In this work, we combine experiment and modeling to systematically investigate the size dependence of the degenerate two-photon absorption (TPA) of below-band-gap radiation in CdSe QDs. The TPA coefficient β at 800 nm of CdSe QDs of varying radii was measured using femtosecond white-light transient absorption spectroscopy by probing the pump-induced bleaching at the first exciton transition energy. β was also calculated using a model based on the multiband effective-mass approximation. Satisfactory agreement between experiment and theory was obtained. Our findings show the evolution of the TPA in the QDs from that of atom-like to bulk-like with increasing the radius R. The TPA coefficient (or the volume normalized TPA cross-section) increases with radius approximately linearly in the strong confinement regime due to the rapid increase of the joint density of states for the two-photon allowed transitions, and saturates for R > 5 nm (the exciton Bohr radius), approaching that of bulk CdSe.

We present a laser-based apparatus suitable for visible pump/extreme UV (XUV) probe time-, energy-, and angle-resolved photoemission spectroscopy utilizing high-harmonic generation from a noble gas. Tunability in a wide range of energies (currently 20-36 eV) is achieved by using a time-delay compensated monochromator, which also preserves the ultrashort duration of the XUV pulses. Using an amplified laser system at 10 kHz repetition rate, approximately 10(9)-10(10) photons/s per harmonic are made available for photoelectron spectroscopy. Parallel energy and momentum detection is carried out in a hemispherical electron analyzer coupled with an imaging detector. First applications demonstrate the capabilities of the instrument to easily select the probe wavelength of choice, to obtain angle-resolved photoemission maps (GaAs and URu(2)Si(2)), and to trace ultrafast electron dynamics in an optically excited semiconductor (Ge).

Finite pump-beam size in optical pump-terahertz (THz) probe experiments introduces distortions in both the signal amplitude and the spectral form. We show both experimentally and analytically that the pump beam must be 3-4 times larger than the THz probe beam for a one-photon excitation process to achieve

We report the measurement of terahertz (THz) radiation, generated via optical rectification in a 20 µm thick ZnTe crystal, as a function of the size of optical excitation. The result shows that, before the onset of significant higher-order nonlinear processes, the THz emission obtained with a fixed excitation power is largely size independent for excitation sizes smaller than the THz wavelength. This experimental finding is well described by a theoretical model including the generation of THz radiation through optical rectification from a subwavelength source and its propagation into the far field. The characteristic size dependence of the radiation from a subwavelength THz source is advantageous for use in apertureless near-field microscopy.

Ultrafast terahertz emission from two-color generated laser plasma gas targets is studied using air and the noble gases (neon, argon, krypton, and xenon) as the generation media. Terahertz output pulse energy and power spectra are measured as function of gas species, gas pressure, and input pulse energy up to 6 mJ per pulse using a 40-fs 1-kHz Ti:sapphire laser system as the drive source. Terahertz pulse energies approaching 1 microJ per pulse with spectral content out to 40 THz and pulse duration of 35 fs is reported. A simple one dimensional transient photocurrent ionization model is used to calculate the spectra showing good agreement with experiments.

We report optical pump-soft X-ray scattering probe studies of YBCO single crystals. The experiments reveal a picosecond relaxation dynamics of charge density waves, which is in turn strongly modified by the onset of superconductivity.

This paper describes the development of new instrumentation at the Linac Coherent Light Source for conducting THz excitation experiments in an ultra high vacuum environment probed by soft X-ray diffraction. This consists of a cantilevered, fully motorized mirror system which can provide 600 kV cm−1 electric field strengths across the sample and an X-ray detector that can span the full Ewald sphere with in-vacuum motion. The scientific applications motivated by this development, the details of the instrument, and spectra demonstrating the field strengths achieved using this newly developed system are discussed., Journal of Synchrotron Radiation, 22 (3), ISSN:0909-0495, ISSN:1600-5775

Electron-ion collisional dynamics is of fundamental importance in determining plasma transport properties, nonequilibrium plasma evolution, and electron damage in diffraction imaging applications using bright x-ray free-electron lasers (FELs). Here we describe the first experimental measurements of ultrafast electron impact collisional ionization dynamics using resonant core-hole spectroscopy in a solid-density magnesium plasma, created and diagnosed with the Linac Coherent Light Source x-ray FEL. By resonantly pumping the $1s\ensuremath{\rightarrow}2p$ transition in highly charged ions within an optically thin plasma, we have measured how off-resonance charge states are populated via collisional processes on femtosecond time scales. We present a collisional cross section model that matches our results and demonstrates how the cross sections are enhanced by dense-plasma effects including continuum lowering. Nonlocal thermodynamic equilibrium collisional radiative simulations show excellent agreement with the experimental results and provide new insight on collisional ionization and three-body-recombination processes in the dense-plasma regime.