Daniel Wegkamp

Ultrafast changes in lattice symmetry probed by coherent phonons.

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 domain are required. Here we show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in VO(2) and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.

Instantaneous Band Gap Collapse in Photoexcited Monoclinic VO2 due to Photocarrier Doping

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 VO2 band gap to variations of the dynamically screened Coulomb interaction, supporting a fully electronically driven isostructural insulator-to-metal transition. We thus conclude that the ultrafast band structure renormalization is caused by photoexcitation of carriers from localized V 3d valence states, strongly changing the screening before significant hot-carrier relaxation or ionic motion has occurred.

Real-Time Measurement of the Vertical Binding Energy during the Birth of a Solvated Electron

Using femtosecond time-resolved two-photon photoelectron spectroscopy, we determine (i) the vertical binding energy (VBE = 0.8 eV) of electrons in the conduction band in supported amorphous solid water (ASW) layers, (ii) the time scale of ultrafast trapping at pre-existing sites (22 fs), and (iii) the initial VBE (1.4 eV) of solvated electrons before significant molecular reorganization sets in. Our results suggest that the excess electron dynamics prior to solvation are representative for bulk ASW.

Phase retrieval and compression of low-power white-light pulses

We characterize and compress sub-nJ visible white-light continuum (WLC) pulses generated by self-phase modulation in yttrium aluminium garnet. The spectral phase is retrieved by spectrally resolving the transient reflectivity from an optically excited transition metal oxide. This measured phase is compensated by applying the appropriate distortion to a deformable mirror. By comparing the response of two different materials, we show that the white-light pulses can be compressed to approximately 10 fs duration.

Photoinduced work function modifications and their effect on photoelectron spectroscopy

We investigate the effect of a spatially varying work function on photoemission experiments. It is demonstrated that a photoinduced work function change when probed by ultraviolet and two-photon photoemission spectroscopy can have pronounced effects on photoemission spectra. These effects are simulated by a simple model that reproduces the data remarkably well and allows for quantitative interpretation of the modified low energy region of the photoemission spectra. These findings are highly relevant when discussing work function determinations by photoemission spectroscopy and moreover may have substantial impact on the energy level alignment of molecule-metal or -semiconductor interfaces.

Ultrafast changes in lattice symmetry probed by coherent phonons at the onset of the photoinduced phase transition in VO2

We show that the coherent phonon spectrum, probed by whitelight spectroscopy, can be a marker for photoinduced structural transitions. In VO2, the lattice potential symmetry is completely changed on ultrafast timescales before ionic motion occurs.