Coherent terahertz wave generation from mono- and multi-layer MoS2 through quantum interference
image: Fig. 1 THz wave generation from MoS2 through quantum interference. (a) Schematic diagram of THz wave generation from MoS2 under two-color light excitation at normal incidence. (b) THz waveforms generated from mono- and multi-layer MoS2. The back curve shows the THz signal from the bare sapphire substrate, indicating no detectable THz radiation. (c) Schematic diagram of THz wave generation from MoS2 under two-color light excitation at oblique incidence. (d) THz waveforms generated from multilayer MoS2 under two-color and single-color light excitation.
Credit: Ultrafast Science
Terahertz (THz) wave generation from advanced materials under pulsed light excitation is an effective approach to explore the native nonlinearities of materials. Recently, the two-color light excitation method has been extended to explore nonlinear properties in two-dimensional materials. Coherently injected photocurrent in semiconductor materials under two-color light excitation can be detected by monitoring the generated coherent THz radiation. The physical mechanism for the photocurrent injection can be attributed to quantum interference (QI). Previously, most of the controllable QI-induced photocurrent studies mainly focus on direct bandgap materials. QI-induced photocurrent in the indirect gap region has seldom been considered due to the weak optical response. There is a lack of investigations focusing on QI in the same kind of materials with both direct and indirect gaps. Therefore, it is important to present a clear view of QI in the same materials with both direct and indirect gaps under two-color light excitation. As one of the typical two-dimensional layered transition metal chalcogenides, multilayer MoS2 has an indirect bandgap in its band structure, while monolayer MoS2 has a direct bandgap. Thus, MoS2 material with direct and indirect bandgaps can be expected as a platform for investigating QI-induced photocurrent. While THz wave generation (i.e., photocurrent) from bulk MoS2 has been demonstrated through a second-order nonlinear process under single-color light excitation, relevant research on the generation of THz wave under two-color laser excitation is still absent.
Recently, a research team in the Center for Terahertz waves and School of Precision Instrument & Opto-electronics Engineering at Tianjin University in China experimentally investigates the THz wave generation from mono- and multi-layer MoS2 through QI under two-color field excitation, demonstrating the coherently controllable THz radiation (i.e., coherent injected photocurrent) from MoS2 through an all-optical method. This work has been published in Ultrafast Science.
Under two-color light excitation, they have successfully detected THz radiation from monolayer MoS2 and even stronger THz radiation from multilayer MoS2. They have shown that, in both direct and indirect gap materials, QI in direct gap region (Brillouin zone K-point) is the dominant process of the coherent photocurrent injection. To compare the efficiencies of the THz wave generation from MoS2 through QI and optical rectification (OR), they measure the THz radiation from multilayer MoS2 under two-color and single-color light excitation at an incident angle of 45°. Results show that QI is more efficient than OR. This work enriches the understanding of the generation and control of coherent injected photocurrent using two-color light excitation in MoS2 materials and provides deeper insight into the process of QI in materials with an indirect gap.