Optical Sum-Frequency Response of Lattice Vibrations and Pulse Timing at a Free-Electron Laser [thesis]

Riko Kießling, Universitätsbibliothek Der FU Berlin
2020
The vibrational characteristics of solids and interfaces are determined by the dynamics of the underlying atomic structures. Via resonant interaction, optical fields allow access to these fundamental low-energy excitations, and hence information about the interatomic bonding and structural phases. As a vibrational and interface-sensitive probe, sum-frequency generation (SFG) spectroscopy has emerged as valuable nonlinear optical technique. Combined with the brilliance and spectral tunability of
more » ... free-electron laser-based infrared (IR) radiation, a variety of vibrational modes comes into reach. In this thesis, an experimental infrared-visible SFG setup at an accelerator-driven free-electron laser (FEL) has been implemented and applied. First, the characteristics of the coherent radiation sources and their relative pulse timing are examined. Based on balanced optical cross-correlation (BOC), the jitter between the synchronized fs table-top and mid-IR FEL pulses is determined to be about 100 fs. Arrival time drifts are correlated to the energy fluctuations of the accelerated electron bunches. The micro-/macro-pulse structure of the FEL oscillator emission is studied in detail as function of the cavity length detuning. In the limit-cycle regime, pronounced optical sub-pulses within the picosecond-short FEL micro-pulse are observed. The experimental findings are in line with theoretical calculations based on classical Maxwell-Lorentz electrodynamics. Second, zone-center optical phonons in polar dielectric crystals with broken inversion symmetry are investigated by SFG spectroscopy. Simultaneous IR and Raman activity of the vibrational modes causes a resonant enhancement of the second-order susceptibility. Linear optical effects (e.g. Reststrahlen bands) are found to strongly modify the sum-frequency response. The phase mismatch of the interacting optical waves determines the coherence length within the sample bulk, limiting SFG emission to a some micrometer thin layer. Both for silicon carbide and α-quartz, SFG spectra [...]
doi:10.17169/refubium-28042 fatcat:kmf3w76xznbypces5d2brxq7zi