Abstract by Brittany Knighton
Megan F. Nielson
R. Tanner Hardy
Lauren M. Rawlings
Chemistry and Biochemistry
Picking out Nonlinear Couplings with 2D Terahertz Spectroscopy
Multi-dimensional terahertz (THz) spectroscopy is a useful tool for understanding coherent energy flow on ultrafast time scales. By exciting specific vibrational modes, and probing other modes, we can measure how anharmonic mode coupling results in coherent energy transfer, particularly in solids. This nonlinear phononic coupling provides important insights into the complex multi-dimensional potential energy surface, which represents the forces that bind atoms together and govern nearly all material properties. 2D THz spectroscopy also allows the separation of signal contributions resulting from phonon mode coupling rather than other excitation pathways.
ß-barium borate (BBO) has 15 low frequency (~1-6 THz) phonon modes, making BBO a prime sample to study phonon coupling and nonlinear THz excitation. In single-THz excitation pulse (1D) data, we observe a nonlinear vibrational response in BBO with increased THz pump power, and our 2D THz work reveals feature-rich 2D frequency spectra. We analyze these 2D spectra to understand nonlinear phononic coupling and energy flow pathways, and to disentangle the crowded nonlinear spectrum of BBO.