Large In-Plane Anisotropic Terahertz Emission Induced by Asymmetric Polarization in Low-Symmetric PdSe₂

Abstract

Palladium diselenide (PdSe 2 ) exhibits air stability, low symmetry, and high carrier mobility, resulting in unique inplane anisotropy for polarized optoelectronic devices. However, the relationship of the symmetry and the terahertz (THz) radiation remains elusive yet significant for both the THz source in technology and nonlinear optical physics in science. Herein, we observed large in-plane anisotropic THz radiation from multilayer PdSe 2 under femtosecond laser excitation. The THz emission demonstrates 2α dependence on the optical polarization angle from the resonant optical rectification combined with a background from the photocarrier acceleration under the surface depletion field. Interestingly, the in-plane THz emission along and perpendicular to the puckered direction demonstrates large anisotropy. Furthermore, the THz time-domain signals exhibit reversed polarities along the positive and negative puckered directions. This asymmetric polarization could relate to the bonding of Pd−Se, resulting in the unidirectional photon-induced current. Our results bridge the gap between the low-symmetry two-dimensional materials and the THz technology, which could promote the development of THz-polarized devices based on low-symmetry layered materials.

Interplay between Ultrafast Shift Current and Ultrafast Photon Drag Current in Tellurium Nanotubes

“…On the one hand, this result validates the dominant mechanism based on the second-order nonlinear optical effects as the THz amplitude is quadratically proportional to the incident optical electric field for E THz ∝ I ∝ χ (2) •E in •E in . 47 On the other hand, the electrostatic screening of the photocarriers under either surface depletion field 51 or photo-Dember field 26 can result in a saturable THz amplitude with the increscent pump fluence, which is not observed in the Te nanotubes. Concerning the characteristic properties of the Te nanotubes, the surface state density is too low to form a strong surface depletion field 25 due to the narrow band gap (∼0.8 eV).…”

Interplay between transient interfacial photocurrent and surface nonlinear photocurrent in graphene/MoS2 heterostructure

“…This phenomenon is similar to the THz radiation dominated by second-order nonlinear effects in Te nanotube film [9] and Bi2S3 films [11] . These findings demonstrate that the THz radiation of Gr/M heterostructure strongly depends on the polarization state of the pump beam, which is closely related to nonlinear effects [13] , suggesting the dominant mechanism of nonlinear effects to the THz radiation under 0.42 mJ/cm 2 excitation. On the contrary, when the pump fluence increases to 1.41 mJ/cm 2 , the THz electric field of the Gr/M heterostructure exhibits the same amplitude and polarity as shown in Figure 1b.…”