Bio: Kung-Hsuan Lin received a B. S. degree in electrical engineering from National Taiwan University (NTU), in 2001 and a Ph. D. degree from the Graduate Institute of Photonics and Optoelectronics, NTU, in 2006. Before he joined the Institute of Physics, Academia Sinica (IoPAS) as a lab leader, he worked at the National Taiwan Normal University (Physics Department), MIT (Chemistry Department), Industrial Technology Research Institute (Laser Center), and NTU (Center for Condensed Matter Sciences). Dr. Lin has led Laser Spectroscopy Lab in IoPAS since 2011. His research interests primarily concern femtosecond laser spectroscopy/time-resolved optical spectroscopy for material sciences and femtosecond-laser-related applications.
Abstract: In this talk, I will share our recent studies of a few van der Waals materials by using ultrafast lasers. The first part of this talk is optical nonlinearity of Group IV monochalcogenides (such as GeSe, GeS, SnSe, SnS), which have been predicted to be multiferroic materials with in-plane ferroelectricity and ferroelasticity in its monolayer form. I will report the giant second-order nonlinearity of SnS and SnSe with ferroelectric stacking. From theoretical and experimental results, the susceptibility of second harmonic generation (SHG) from SnS and SnSe with ferroelectric stacking is two to three orders of magnitude higher than the values of traditional nonlinear crystals such as BBO and KTP. The SHG anisotropy can be a tool to study ferroelastic transformation. The second part is ultrafast carrier and exciton dynamics of InSe. We utilized ultrafast optical spectroscopy to investigate the IP and OP optical features of InSe. The energy difference of IP and OP exciton in InSe (0.5 meV) was experimentally resolved for the first time. The theoretical results confirmed that the contributions of IP and OP excitons originate from different k-points and splitting states. In the presence of photocarriers, the difference of IP and OP optical gap increased up to 8 meV. Such energy difference results from the anisotropic reduction of binding energies of IP and OP excitons.