Description
Recent advances in ultrafast optics, specifically the Tunneling Ionization with a Perturbation for the Time-domain Observation of an Electric field (TIPTOE) technique, have enabled the direct, real-time, sub-cycle measurement of few-cycle electric fields [1]. Beyond standard pulse characterization, this technique can be utilized for probing ultrafast light-matter interactions due to its attosecond temporal resolution. In this work, we utilize this technique to analyze the cascaded second-order ($\chi^{(2)}$) response in non-centrosymmetric LiNbO₃, a phenomenon that remains challenging to observe using conventional methods [2,3]. We demonstrate the mapping of phase dynamics of the nonlinear optical response in LiNbO₃, by performing spatial Z-scan with azimuthal crystal rotation. Evaluating the phase shifts obtained with the TIPTOE traces at symmetry-forbidden orientations ($2\omega$ minimum) allowed us to structurally suppress the $\omega \rightarrow 2\omega \rightarrow \omega$ cascaded energy exchange, thereby isolating the pure, intrinsic $\chi^{(3)}$ phase delay, at structurally allowed orientations ($2\omega$ maximum). By cross-correlating the TIPTOE traces across varying azimuthal orientations, we establish a phase relationship that enables the direct observation of the cascaded back-conversion. Ultimately, this framework provides a robust roadmap for mapping the cascaded second-order response and explicitly isolating the intrinsic nonlinearities of non-centrosymmetric optical materials, paving the way for advanced applications in ultrafast photonics.
Ref: [1] Park, S. et al., Optica 5, 4 (2018), 402–408. [2] DeSalvo, R.et al., Optics letters 17, 1 (1992), 28–30. [3] Vidakovi´c, P.et al., Optics letters 22, 5 (1997), 277–279.
