Intense Terahertz Sources for 2D Spectroscopy

This Ph.D. thesis covers the development of terahertz (THz) spectroscopy systems for investigating nonlinear vibrational responses in organic biomolecules such as sucrose. First, an add-on module for the DFT-based ab initio software CASTEP is developed to include a THz pulse as an external field in a molecular dynamics (MD) simulation.

With this THz induced nonlinear responses and mode couplings in CsI and sucrose are investigated for increasing field strengths, and it is found that these occur for sucrose when the field strength is in the MV/cm range. THz sources based on LiNbO3, DAST, DSTMS and 2-color air plasmas are investigated, capable of generating high pulse energies i.e. up to several µJ when pumped by fs pulses with energies on the order of ~mJ.

Beam profiles are measured with a commercial THz camera, and it is found that the crystalbased sources have Gaussian beams, while the THz radiation emitted from a 2-color air plasma has a conical beam profile. With the beam profiles measured through a focal plane, this has been reconstructed in 3D showing that the beam collapses to a single spot in focus.

Besides the off-axis THz radiation, a weak on-axis forward propagating mode has been observed having a Gaussian beam profile. In addition to the intense THz pulses focused in free-space in order to achieve the highest possible field strength, it is shown that resonant microslit arrays can be used to enhance the THz field, and with the possibility of mounting crystalline samples inside the metallic slits, this is proposed as a combined spectroscopy system for investigating high-field phenomena.

With a carefully optimized design, the slit resonance can be coupled to the lattice modes of the array structure to achieve a field enhancement of more than 35 times, which is approximately 60 % more than what is observed for a similar decoupled slit. Nonlinear spectroscopy experiments are performed on sucrose using a DSTMS THz source since this covers to the lowest lattice vibrations of sucrose from approximately 1 to 5 THz, and at the same time provided the highest field strength of almost 5 MV/cm.

In agreement with the MD II simulations, a bleaching of the 1.4 THz is observed for the highest field strengths in a 1D spectroscopy configuration. Phonon coupling in sucrose is investigated with a 2D spectroscopy experiment based on a DSTMS and a DAST source, for which a new chopper scheme is presented for a direct measurement of nonlinear signals.

With this, nonlinear pump-probe signals are measured with the highest response when the sucrose crystal is cryogenically cooled down 10 K. These signals reveal details about longlived coherence and nonlinear phonon beating in sucrose induced by the intense THz pulses.