Elliptically Polarized THz Generation and Manipulation from Layered Two-dimensional Materials

Elliptically polarized terahertz (THz) emitter with tunable polarization state utilized in THz time-domain spectroscopy has intriguing potential for various applications, such as data processing, medical diagnosis, chiral molecular analysis, and so on. With the rapid development of material science, two-dimensional (2D) materials have been seen as the efficient and burgeoning nanoscale THz sources due to the high carrier concentration and mobility, broad spectral response, adjustable band gap, and especially the spin-related physics. Herein, we have observed the elliptically polarized THz generation from the tin sulfide (SnS) excited by linearly polarized incident light. Interestingly, the ellipticity and the orientation angle of the reconstructed THz polarization ellipses are dependent on the pump light polarization angle, realizing the arbitrary control of the polarized THz emission. Our study could lay the foundation for the 2D materials-based compact and integrated THz emitters and modulators.

I. Introduction

Terahertz (THz) spectrum spanning from 0.1 THz to 10 THz is the key spectral regime for the upcoming sixth-generation (6G) wireless communication. In particular, the manipulation of the THz polarization state is a crucial step for the information technology and data processing[1], as well as other cutting-edge applications including chiral sensing[2], spectroscopic diagnostics[3], polarization imaging[4], and so forth. However, the development of polarized THz generation and manipulation is still in its childhood due to the long-term insufficiency of the THz emitter, modulator, and detector. Currently, the polarized THz generation and manipulation are developing towards two directions. On one hand, under circularly polarized excitations, some helicity-dependent physics such as hidden spin polarization[5], spin angular momentum transfer[6], and in-plane circular dipoles[7] are crucial ingredients for generating polarized THz waves, and the THz polarization is controlled via tuning these helicity-dependent physical processes. On the other hand, the THz emission mechanism is rich and complex in condensed-matter materials. For instance, linear optical processes including surface depletion field effect and photo Dember effect[8], and nonlinear optical processes including optical rectification, photogalvanic effect, photon drag effect, and so forth[9], [10]. Therefore, the coherent control of the different THz emission mechanisms is an efficient method to generate and manipulate the polarized THz emission even under linearly polarized excitation[11].