Probing excitonic mechanics in suspended and strained transition metal dichalcogenides monolayers
Open Access
- Author:
- Xie, Hongchao
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 31, 2019
- Committee Members:
- Chaoxing Liu, Dissertation Advisor/Co-Advisor
Chaoxing Liu, Committee Chair/Co-Chair
Kin Fai Mak, Committee Member
Mikael Caleb Rechtsman, Committee Member
Venkatraman Gopalan, Outside Member - Keywords:
- 2D semiconductors
Transition metal dichalcogenides
optomechanics
Valley
Exciton
strain
suspended monolayers - Abstract:
- Over the past decade, the interest in two-dimensional (2D) materials, especially for atomically thin transition metal dichalcogenide (TMD) semiconductors, had dramatically thrived for both fundamental science and practical applications. The reduced dielectric screening in 2D mainly attributes to the strong excitonic effect in atomically thin TMD semiconductors. This pronounced exciton feature can maintain at room temperature, which indicates strong light-matter interaction and possible optoelectronic application using monolayer semiconductors. Meanwhile, the absence of inversion symmetry and out-of-plane mirror symmetry jointly endows carriers in monolayer TMDs with a new valley degree of freedom (DOF). Namely, in hexagonally-arranged lattice of 2D materials, electrons that residing at band edges of K and K’ valleys can carry opposite valley magnetic moments and Berry curvatures, which allows the further control of valley-indexed carriers with polarized light, electrical and magnetic fields. Besides, the large strain sustainability of monolayer TMDs gives rise to mechanically tunable band gap with 70 meV redshift per 1% strain up to recorded 10% applied strain. Thus, the interaction of macroscopic mechanical means with valley electrons makes monolayer TMD semiconductor a promising platform to implement novel valley-controlled mechanical devices. This motivates the experimental studies demonstrated in this dissertation. In this dissertation, we investigate the valley contrasting coupling between optoelectronic carriers (exciton & flowing electrons) and mechanics in a monolayer TMD semiconductor. In the first parts (Chapter 1&2), I will present emerging properties of TMD monolayers and discuss interesting physics that can study after suspending or straining these atomically thin materials. The fabrication and measurement of typical TMD suspended devices will also be demonstrated in details. In the secondary part (Chapter 3), we demonstrate robust exciton bistability by continuous-wave optical excitation in a suspended monolayer WSe2 at a much lower intensity level of 103 W/cm2. The observed bistability is originated from a photothermal mechanism, which can provide both optical nonlinearity and internal passive feedback in a simple cavity-less structure. This is supported by detailed excitation wavelength and power dependence studies of the sample reflectance, as well as by numerical simulation including the temperature-dependent optical response of monolayer WSe2. Furthermore, under a finite magnetic field, the bistability becomes valley dependent and controllable not only by light intensity but also by light helicity due to the exciton valley Zeeman effect, which open up an exciting opportunity in controlling light with light using monolayer materials. In the following part (Chapter 4), we report the observation of exciton-optomechanical coupling in a suspended monolayer MoSe2 mechanical resonator. In particular, we have observed light-induced damping and anti-damping of mechanical vibrations and modulation of the mechanical spring constant by moderate optical pumping near the exciton resonance with variable detuning. The observed exciton-optomechanical coupling strength is also highly gate-tunable. Our observations can be fully explained by a model based on photothermal backaction and gate-induced mirror symmetry breaking in the device structure. The observation of gate-tunable exciton-optomechanical coupling in a monolayer semiconductor may find novel applications in nanoelectromechanical systems (NEMS) and in exciton-optomechanics. In the last part of this dissertation (Chapter 5), we present the study of magnetization purely originated from the valley DOF in strained MoS2 monolayers. By breaking the three-fold rotational symmetry in single-layer MoS2 via a uniaxial stress, we have demonstrated the pure electrical generation of valley magnetization in this material, and its direct imaging by Kerr rotation microscopy. The observed out-of-plane magnetization is independent of in-plane magnetic field, linearly proportional to the in-plane current density, and optimized when the current is orthogonal to the strain-induced piezoelectric field. These results are fully consistent with a theoretical model of valley magnetoelectricity driven by Berry curvature effects. Furthermore, the effect persists at room temperature, opening possibilities for practical valleytronic devices.