Anisotropic Third-Harmonic Generation in Layered Germanium Selenide

Germanium selenide (GeSe) is a 2D layered material with an anisotropic crystal structure analogous to black phosphorus (BP). But unlike BP, GeSe is stable under ambient conditions and therefore provides more flexibility in building practical nanoscale devices. The in-plane anisotropic vibrational, electrical, and optical properties of layered GeSe originating from the low symmetry of its crystal structure are being explored mostly for building polarization-sensitive optoelectronic devices. However, the nonlinear optical properties of layered GeSe have not been investigated yet. Here, the anisotropic polarization-dependent third-harmonic generation (THG) from exfoliated thin GeSe flakes due to the low in-plane lattice symmetry is reported. Furthermore, it is also shown that the intensity and polarization state of TH emission can be controlled by the polarization state of pump beam. Moreover, it is demonstrated that the crystal's symmetry axes can be rapidly determined by characterizing the intensity profile of TH emission upon the excitation from radially or azimuthally polarized vector beam. The results of this study pave the way for realizing anisotropic nonlinear optical devices such as multiplexers, signal processors, and other prototypes for future on-chip photonic circuits and optical information processing.