We seek to apply attosecond resolution transient absorption to elucidate the early time dynamics of fundamental physical process in solid-state systems such as semiconductors, metals, and strongly correlated materials. A few femtosecond near infrared (NIR) pulse is used to perturb electronic states of the solid, and the resulting electronic changes are monitored by measuring the absorbance of an attosecond pulse train. High harmonic generation combined with amplitude gating provides attosecond pulses in the 30 to 70 eV energy range, granting access to core level transitions with attosecond time resolution. Current work focuses on determining the timescale of band gap collapse accompanying an ultrafast photoinduced phase transition in VO2. Future work will seek to apply this technique to measure the timescale of electron-electron correlations in transition metals and strongly correlated systems.
The second solid state attosecond transient absorption apparatus in D60 is devoted to studying the underlying fundamental electron dynamics in semiconductor thin films and metal nanoparticles after ultrafast excitation. In semiconductor thin films, within the first few femtoseconds during and after excitation with a 1.5 eV NIR pump, electrons are excited to a highly non-equilibrium state. By probing with an XUV pulse spanning from 28 to 42 eV, these these dynamics are examined as they occur in real time, before the electrons thermalize or interact with the lattice. Semiconductor thin films of interest include germanium, silicon germanium and gallium arsenide. An aerosol apparatus is also being designed to inject metal nanoparticles into the apparatus. By tailoring the size and composition of the metal nanoparticles, their plasmon resonance can be tuned to be resonant with a frequency-doubled version of the NIR pulse, around 3 eV. The metal nanoparticles will be coated with a thin layer of silicon, which will be sensitive to the electrons participating in the plasmon resonance. The changes in the silicon due to the presence of the plasmon will be studied via XUV pulses generated in the 90 to 120 eV energy range.
M. Schultze, K. Ramasesha, C. D. Pemmaraju, S. A. Sato, D. Whitmore, A. Gandman, J. S. Prell, L. J. Borja,
D. Prendergast, K. Yabana, D. M. Neumark, and S. R. Leone, "
Attosecond band-gap dynamics in silicon," Science 346, 1348 (2014).
2 J. S. Prell, L. J. Borja, D. M. Neumark, and S. R. Leone, " Simulation of attosecond-resolved imaging of the plasmon electric field in metallic nanoparticles," Ann. Phys. (Berlin), 525, 151 (2013).