THz Bandwidth Activation of Anharmonic Coupling in CdWO₄

Terahertz (THz) frequency light is uniquely suited to resonantly probe collective electronic, phonon, magnon, and electromagnon modes, and we are showing that intense THz pulses enable coherent control and the direct measurements of coupling between these excitations.<br/> <br/>When intense electromagnetic pulses are used in any kind of pump-probe spectroscopy, several nonlinear excitation pathways can result, leading to scenarios that required the development of multi-dimensional spectroscopies to illuminate the observed dynamics. New developments in 2D THz spectroscopy in our lab and others are enabling the direct measurement of nonlinear excitation of phonon, magnon, and electronic modes, and preliminary measurements are directly showing couplings between them. We have demonstrated clear examples where two-dimensional (2D) THz vibrational spectroscopy is needed to distinguish between nonlinear-excitation pathways, and it enables the selection of one pathway over another.<br/> <br/>With intense THz pulses, Raman-active vibrational modes can be nonlinearly excited in centrosymmetric crystalline CdWO₄. However, in single-pulse measurements, it’s unclear whether the nonlinear pathway is a photonic process (THz Raman excitation) or a phononic process (anharmonic coupling). In previous work, we showed that Raman excitation is the dominant nonlinear pathway at moderate THz field strengths. With an improved 2D THz experimental setup, we can minimize the Raman excitation and clearly isolate the anharmonic coupling signals.<br/> <br/>Interestingly, our analysis shows that coupling between IR-active modes at 2.94 THz and 3.65 THz and the Raman-active mode at 2.33 THz clearly occurs. At first glance, the coupling between three modes should only be efficient when the frequencies of the IR-active modes add up to or subtract down to the frequency of the Raman-active mode, which is not the case here. But when we consider the IR-active mode motion after resonant excitation with a broadband THz pulse, we recognize that the spectrum of the initial motion contains the resonant frequency of the mode itself, as well as a broad range of frequencies that essentially matches the spectrum of the driving THz pulse. Therefore, although the anharmonic coupling between these modes is predicted to be inefficient based on the resonant frequencies of the IR-active modes, we show that the initial THz pulse itself drives motion with more than just the resonant frequency, activating efficient coupling between vibrational modes in CdWO₄.