Emergence of electron coherence and two-color all-optical switching in MoS ₂ based on spatial self-phase modulation

Abstract

Generating electron coherence in quantum materials is essential in optimal control of many-body interactions and correlations. In a multidomain system this signifies nonlocal coherence and emergence of collective phenomena, particularly in layered 2D quantum materials possessing novel electronic structures and high carrier mobilities. Here we report nonlocal ac electron coherence induced in dispersed MoS 2 flake domains, using coherent spatial self-phase modulation (SSPM). The gap-dependent nonlinear dielectric susceptibility χ (3) measured is surprisingly large, where direct interband transition and two-photon SSPM are responsible for excitations above and below the bandgap, respectively. A windchime model is proposed to account for the emergence of the ac electron coherence. Furthermore, all-optical switching is achieved based on SSPM, especially with two-color intraband coherence, demonstrating that electron coherence generation is a ubiquitous property of layered quantum materials.electron coherence | transition metal dichalcogenide | self-phase modulation | optical switching | emergent phenomena R ecently 2D layered quantum materials have attracted tremendous interest since the discovery of graphene a decade ago (1). Various layered materials, ranging from boron nitride sheets to transition metal dichalcogenides and from topological insulators to high-temperature superconductors, have been intensively investigated (2-11). Strict 2D atomic crystals can now be produced at a macroscopic scale, using a variety of methods (11,12). Among them molybdenum disulfide (MoS 2 ) and related layered quantum materials are particularly interesting due to their novel optical properties and potential valleytronics applications (4,5,8,9) at a thickness of monolayer and few layers (2, 10, 13). Layered materials share common physical properties rooted in their ubiquitous 2D quantum nature, for which achieving pure coherence among electrons (lattices) is of particular interest (14-19). The presence of multiple domains is ubiquitous in many known 2D quantum materials, ranging from stripe-order cuprate superconductors to polycrystalline strongly correlated systems. For example, phase locking between different layers of stripe orders is crucial for enhancing the superconducting phase in layered hightemperature superconductors (20,21).In this work we demonstrate unambiguously that nonlocal and intraband ac electron coherence, of which the electronic wave function oscillates at an optical frequency of 10 14 Hz, can be generated in separate MoS 2 flakes, using spatial self-phase modulation (SSPM). The SSPM is a coherent third-order nonlinear optical process systematically investigated decades ago (22), where the nonlinear optical susceptibility χ (3) is uniquely determined by the laser-intensity-dependent refractive index n = n 0 + n 2 I, where n 0 and n 2 are linear and nonlinear refractive indexes, respectively. If this effect is strong enough in a material, the phenomenon of selffocusing can be directly observed. The SSPM is also f...

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“…Ignoring the type of laser, it is obviously found that the slope of the diffraction ring with light intensity decreases as the wavelength increases, which is in good agreement with previous result of Ref. 49. We can conclude that the SSPM is induced by the electronic transition to a large extent.…”

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“…17). In the SSPM investigation previously reported for MoS 2 dispersions [5], the slopes of the lines start to increase prominently when the incident photon energy exceeds the bandgap, demonstrating the SSPM effect is directly correlated with the electron excitations in the 2D material sheets [5]. Here the photon energies of the four wavelengths are larger than the bandgap of MoTe 2 (1.0 eV) [17].…”

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