Many-body effects in nonlinear optical responses of 2D layered semiconductors

We performed ultrafast degenerate pump-probe spectroscopy on monolayer WSe2 near its exciton resonance. The observed differential reflectance signals exhibit signatures of strong many-body interactions including the exciton-exciton interaction and free carrier induced band gap renormalization. The exciton-exciton interaction results in a resonance blue shift which lasts for the exciton lifetime (several ps), while the band gap renormalization manifests as a resonance red shift with several tens ps lifetime. Our model based on the many-body interactions for the nonlinear optical susceptibility fits well the experimental observations. The power dependence of the spectra shows that with the increase of pump power, the exciton population increases linearly and then saturates, while the free carrier density increases superlinearly, implying that exciton Auger recombination could be the origin of these free carriers. Our model demonstrates a simple but efficient method for quantitatively analyzing the spectra, and indicates the important role of Coulomb interactions in nonlinear optical responses of such 2D materials

Electrical Control of Two-Dimensional Neutral and Charged Excitons in a Monolayer Semiconductor

Monolayer group VI transition metal dichalcogenides have recently emerged as semiconducting alternatives to graphene in which the true two-dimensionality (2D) is expected to illuminate new semiconducting physics. Here we investigate excitons and trions (their singly charged counterparts) which have thus far been challenging to generate and control in the ultimate 2D limit. Utilizing high quality monolayer molybdenum diselenide (MoSe2), we report the unambiguous observation and electrostatic tunability of charging effects in positively charged (X+), neutral (Xo), and negatively charged (X-) excitons in field effect transistors via photoluminescence. The trion charging energy is large (30 meV), enhanced by strong confinement and heavy effective masses, while the linewidth is narrow (5 meV) at temperatures below 55 K. This is greater spectral contrast than in any known quasi-2D system. We also find the charging energies for X+ and X- to be nearly identical implying the same effective mass for electrons and holes.Comment: 11 pages main text with 4 figures + 7 pages supplemental material

Electrical Tuning of Valley Magnetic Moment via Symmetry Control

Crystal symmetry governs the nature of electronic Bloch states. For example, in the presence of time reversal symmetry, the orbital magnetic moment and Berry curvature of the Bloch states must vanish unless inversion symmetry is broken. In certain 2D electron systems such as bilayer graphene, the intrinsic inversion symmetry can be broken simply by applying a perpendicular electric field. In principle, this offers the remarkable possibility of switching on/off and continuously tuning the magnetic moment and Berry curvature near the Dirac valleys by reversible electrical control. Here we demonstrate this principle for the first time using bilayer MoS2, which has the same symmetry as bilayer graphene but has a bandgap in the visible that allows direct optical probing of these Berry-phase related properties. We show that the optical circular dichroism, which reflects the orbital magnetic moment in the valleys, can be continuously tuned from -15% to 15% as a function of gate voltage in bilayer MoS2 field-effect transistors. In contrast, the dichroism is gate-independent in monolayer MoS2, which is structurally non-centrosymmetric. Our work demonstrates the ability to continuously vary orbital magnetic moments between positive and negative values via symmetry control. This represents a new approach to manipulating Berry-phase effects for applications in quantum electronics associated with 2D electronic materials.Comment: 13 pages main text + 4 pages supplementary material

Optoelectronic Properties of Two-Dimensional Materials

Thesis (Ph.D.)--University of Washington, 2015Layered materials when thinned down to their monolayer limit exhibit remarkable properties owing to their two-dimensional nature and strong electron confinement. In particular this class of materials displays strong optical properties, showing promise for applications towards future optoelectronic devices; however, due to their relative recent isolation, the optical properties of these monolayers have been largely unexplored. This thesis focuses on the interaction of these layered materials with incident optical radiation, with the focus being on monolayers of WSe$_2$ and graphene. In the first half of this thesis the strong excitonic physics of semiconducting WSe$_2$ monolayers is investigated. These excitons exhibit large interaction effects due to the strong 2D confinement which are further explored here using ultrafast pump probe techniques. Additionally these excitons possess a unique quantum degree of freedom, known as the valley pseudospin. It has been shown that this pseudospin can be optically addressed and readout using its unique circular dichroism. Here the degenerate pseudospin is controlled using an external magnetic field coupled to its valley pseudospin magnetic moment. From this work the valley pseudospin in monolayer WSe$_2$ can be further explored as a possible qubit in future quantum computing and quantum information applications. Graphene photodetectors are the focus of the second half of the thesis. Monolayer graphene is a gapless semi-metal that has been shown to display an ultrafast optoelectronic response that is dominated by hot carriers. Here these effects are investigated as a band gap is generated through the application of a perpendicular electric field in bilayer graphene and through the application of a perpendicular magnetic field in monolayer graphene inducing a Landau level quantization of the band structure. It is observed that in both cases the disruption of the continuous band structure has profound impacts on the photo-excited hot carriers. This work helps lay the foundation for future ultrafast photodetectors made out of graphene

Vapor–Solid Growth of High Optical Quality MoS₂ Monolayers with Near-Unity Valley Polarization

Monolayers of transition metal dichalcogenides (TMDCs) are atomically thin direct-gap semiconductors with potential applications in nanoelectronics, optoelectronics, and electrochemical sensing. Recent theoretical and experimental efforts suggest that they are ideal systems for exploiting the valley degrees of freedom of Bloch electrons. For example, Dirac valley polarization has been demonstrated in mechanically exfoliated monolayer MoS₂ samples by polarization-resolved photoluminescence, although polarization has rarely been seen at room temperature. Here we report a new method for synthesizing high optical quality monolayer MoS₂ single crystals up to 25 μm in size on a variety of standard insulating substrates (SiO₂, sapphire, and glass) using a catalyst-free vapor–solid growth mechanism. The technique is simple and reliable, and the optical quality of the crystals is extremely high, as demonstrated by the fact that the valley polarization approaches unity at 30 K and persists at 35% even at room temperature, suggesting a virtual absence of defects. This will allow greatly improved optoelectronic TMDC monolayer devices to be fabricated and studied routinely