The second-order optical nonlinearity σ(2)(ω) has been a focus of basic research and
technological development for decades as it is both a probe of inversion symmetry
breaking in media and the basis for generating coherent light from far-infrared to ultraviolet
wavelengths. Here, we focus on the relation between band geometry and nonlinear optics.
We measured second harmonic generation (SHG) with incident photon energy from 0.4 eV
- 1.6 eV on a polar semimetal TaAs with a sharp resonant peak detected at 0.7 eV, that is
larger than previously measured in any crystal. Our discovery of a giant anisotropic σ(2)(ω)
in TaAs raises the following questions: what is special about TaAs and/or polar metals that
accounts for large resonant optical nonlinearity, and, is there a fundamental upper bound
on σ(2)(ω) in such inversion breaking crystals? I will describe in the Rice-Mele model based
on the band-geometric theory of nonlinear optical response that addresses these questions.
Liang Wu got his B.S. in Physics from Nanjing University in 2010 and Ph.D in Physics
at Johns Hopkins University in 2015. He was a postdoc fellow in Physics at University of
California, Berkeley from 2016 to 2018 before joining the department of Physics and
Astronomy at the University of Pennsylvania in July 2018. He has been using terahertz
spectroscopy and ultrafast optics to study topological insulators, Weyl semimetals
and quantum spin liquids. He received the Richard L. Greene Dissertation Award in
Experimental Condensed Matte Physics by the American Physical Society (2017) and the
Michelson Postdoctoral Prize Lectureship by Case Western Reserve University (2017). He
was selected as “30 under 30” in Science by Forbes Magazine.