79 research outputs found
Spatiotemporal Amplitude and Phase Retrieval of Bessel-X pulses using a Hartmann-Shack Sensor
We propose a new experimental technique, which allows for a complete
characterization of ultrashort optical pulses both in space and in time.
Combining the well-known Frequency-Resolved-Optical-Gating technique for the
retrieval of the temporal profile of the pulse with a measurement of the
near-field made with an Hartmann-Shack sensor, we are able to retrieve the
spatiotemporal amplitude and phase profile of a Bessel-X pulse. By following
the pulse evolution along the propagation direction we highlight the
superluminal propagation of the pulse peak
The Valence Band Structure of Gadolinium Studied with Time-Resolved Photoemission
We have studied the response of the exchange split valence bands of
ferromagnetic gadolinium tofemtosecond laser excitation. We observe a drop of
the exchange splitting with a time constant of 0.9 ps but different response
times of minority and majority spin bands. Furthermore, even above the Curie
temperature there is a finite exchange splitting, which also decreases with
laser excitation
Ultrafast Spin Density Wave Transition in Chromium Governed by Thermalized Electron Gas
The energy and momentum selectivity of time- and angle-resolved photoemission
spectroscopy is exploited to address the ultrafast dynamics of the
antiferromagnetic spin density wave (SDW) transition photoexcited in epitaxial
thin films of chromium. We are able to quantitatively extract the evolution of
the SDW order parameter Δ through the ultrafast phase transition and show that
Δ is governed by the transient temperature of the thermalized electron gas, in
a mean field description. The complete destruction of SDW order on a sub-100
fs time scale is observed, much faster than for conventional charge density
wave materials. Our results reveal that equilibrium concepts for phase
transitions such as the order parameter may be utilized even in the strongly
nonadiabatic regime of ultrafast photoexcitation
Induced ferroelectric phases in SrTiO3 by a nanocomposite approach
Inducing new phases in thick films via vertical lattice strain is one of the critical advantages of vertically aligned nanocomposites (VANs). In SrTiO3 (STO), the ground state is ferroelastic, and the ferroelectricity in STO is suppressed by the orthorhombic transition. Here, we explore whether vertical lattice strain in three-dimensional VANs can be used to induce new ferroelectric phases in SrTiO3:MgO (STO:MgO) VAN thin films. The STO:MgO system incorporates ordered, vertically aligned MgO nanopillars into a STO film matrix. Strong lattice coupling between STO and MgO imposes a large lattice strain in the STO film. We have investigated ferroelectricity in the STO phase, existing up to room temperature, using piezoresponse force microscopy, phase field simulation and second harmonic generation. We also serendipitously discovered the formation of metastable TiO nanocores in MgO nanopillars embedded in the STO film matrix. Our results emphasize the design of new phases via vertical epitaxial strain in VAN thin films
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