Richard F. Haglund

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We report the influence of uniaxial tensile mechanical strain in the range 0-2.2% on the phonon spectra and bandstructures of monolayer and bilayer molybdenum disulfide (MoS2) two-dimensional crystals. First, we employ Raman spectroscopy to observe phonon softening with increased strain, breaking the degeneracy in the E' Raman mode of MoS2, and extract a(More)
We directly trace the multi-THz conductivity of VO2 during an insulator-metal transition triggered by a 12-fs light pulse. The femtosecond dynamics of lattice and electronic degrees of freedom are spectrally discriminated. A coherent wave packet motion of V-V dimers at 6 THz modulates the lattice polarizability for approximately 1 ps. In contrast, the(More)
Free-electron-laser-based biophysical and biomedical instrumentation G. S. Edwards Physics Department and FEL Laboratory, Duke University, Durham, North Carolina 27708 R. H. Austin Department of Physics, Princeton University, Princeton, New Jersey 08544 F. E. Carroll Department of Radiology and Radiological Sciences, Vanderbilt University, Nashville,(More)
The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time(More)
Insulating spacer layers of MgO were used to identify the enhancement mechanisms of the ZnO band-edge and visible luminescence in ZnO-MgO-Ag and ZnO-MgO-Au multilayers. Purcell enhancement of the ZnO band-edge emission by both Ag and Au surface plasmon polaritons is confirmed by demonstrating that the exponential decay of this emission as a function of(More)
A. Pashkin,1 C. Kübler,1 H. Ehrke,1,* R. Lopez,2,3 A. Halabica,3 R. F. Haglund Jr.,3 R. Huber,1 and A. Leitenstorfer1 1Department of Physics and Center for Applied Photonics, University of Konstanz, D-78457 Konstanz, Germany 2Department of Physics and Astronomy and Institute of Advanced Materials, Nanoscience and Technology, University of North Carolina,(More)
Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic VO2 quasi-instantaneously into a metal. Thereby, we exclude an 80 fs structural bottleneck for the photoinduced electronic phase transition of VO2. First-principles many-body perturbation theory calculations reveal a high sensitivity of the(More)
Free-Electron Lasers (FELs) collectively operate from the terahertz through the ultraviolet range and via intracavity Compton backscattering into the X-ray and gamma-ray regimes. FELs are continuously tunable and can provide optical powers, pulse structures and polarizations that are not matched by conventional lasers. Representative research in the(More)
We observed direct desorption and ionization of angiotensin II and bovine insulin from a frozen polyacrylamide gel without the addition of an exogenous matrix, using picosecond pulses from a tunable, mid-infrared free-electron laser tuned to strong absorption bands of the gel. At 5.7, 5.9, 6.1 and 6.3 microm we were able to desorb and ionize both analyte(More)
We have measured the size dependence of a solid-solid phase transformation in discrete nanoscale volumes of vanadium dioxide (VO(2)) defined by split-ring resonators. By monitoring the in-coupling plasmonic mode while thermally cycling through the insulator-metal transition, we show that hysteresis width broadens with reduced interrogation volume (i.e.,(More)