Session H15: 2D Materials (Semiconductors) -- Optical Properties II

First-Principles Theory of Nonlinear Optical Responses in 2D Semiconductors and Topological Materials

Giant nonlinear optical responses were recently observed in 2D semiconductors and topological materials. Understanding their microscopic origin thus becomes important for both fundamentals of nonlinear optics and development of technological applications. Here I will report our recent first-principles theoretical development towards unveiling the electronic structure origin of their nonlinear optical responses, including (a) second harmonic generation [1] and nonlinear photocurrent [2] in 2D noncentrosymmetric materials and (b) nonlinear ferroelectric photocurrent in topological materials [3]. I will also discuss microscopic relations between nonlinear responses and topological quantities such as shift vector, Berry curvature dipole, etc. Our findings suggest the possibility to achieve deep understanding of nonlinear optical responses in 2D and topological materials using first-principles approaches. References: [1] H Wang and X Qian. Nano Letters 17, 5027-5034 (2017). [2] H Wang and X Qian. Quantum Nonlinear Ferroic Optical Hall Effect. submitted (2018). [3] H Wang and X Qian. Nonlinear Ferroelectric Photocurrent in Topological Materials. in preparation (2018).   

Exciton-dynamics of molybdenum disulfide on gallium nitride

Layered transition metal dichalcogenides (TMD) have generated significant research interest because of interesting optical properties that are strongly dependent on the substrate. The integration of two dimensional (2D) materials with the bulk semiconductors provides an attractive platform to enhance the device functionality. Of all the 2D/3D heterostructures, MoS₂/GaN structures are at the top because of strong lattice matching and direct gap structures of both semiconductors. The properties of the heterostructures are governed by excitons, which are weakly bound in nitrides but are strongly bound in MoS₂. Here we present the exciton dynamics of single layer MoS₂ on GaN substrate. We studied the non-equilibrium exciton-dynamics with the pump-probe spectroscopy using a tunable laser as a pump and a supercontinuum white light probe. The transient absorption spectrum shows different excitonic states. We will discuss the evolution of these states at different delay times starting as early as 200 fs and the corresponding decay kinetics. Our study will be useful to understand the energy transfer as well as the charge transport across the junction in the heterostructure which is crucial to enhance the performance of the device.
  

Influence of the electric field on the optical properties of bilayer graphene nanoribbons

Increasing attention has been paid to multilayer graphene, especially to bilayer graphene nanoribbons (BLGNRs)[1]. The interlayer interactions in BLGNRs result in a deformation of the electronic band structure with a change in the slope of the subbands. Recent studies show that the bandgap of BLG is widely tunable by external electric fields (EF) [2,3]. Here, the optical properties of BLGNRs under effects of EF are investigated by using a tight-binding model and the gradient approximation. The EF can induce the subband (anti)crossing, change the subband spacing, cause the oscillating bands, and distort the band-edge states as well. Our results demonstrate that the optical absorption spectra exhibit rich prominent peaks that mainly stem from the subbands. In addition, the number, spectral intensity, and energy of the absorption peaks are strongly dependent on the interlayer atomic interactions and the ribbon width. The dependence of the optical excitations on both the magnitude and direction of the EF is studied as well. This study could be validated by optical spectroscopy measurements. These results provide a view of the possibilities for applying future optoelectronic applications base on BLGNRs.   

Excitation Density-Dependent Exciton Transport in a h-BN Encapsulated WSe2 Monolayer

We report an excitation density-dependent visualization of excitonic energy diffusion in a h-BN encapsulated WSe₂monolayer. At low excitation densities, we observe a linear evolution of the mean-squared displacement of the exciton density with a diffusivity of 0.5 cm²/s and a mono-exponential decay (300ps) of time-resolved photoluminescence (TRPL). At high excitation densities, however, the TRPL splits into two regimes: an excitation density-dependent, short-lived regime (150ps to 80ps), and an excitation-independent, long-lived regime (300ps). This observation and the fact that the exciton density preserves its initially Gaussian profile suggest that the results are not due to exciton-exciton annihilation but due to the remaining density of unfilled trap states. We also observe an excitation density-dependent increase in exciton diffusivity in the short-lived regime that saturates at 3 cm²/s and eventually transitions into the long-lived regime with a diffusivity of 0.5 cm²/s. At the transition point, the exciton density corresponds to the trap density in the WSe₂monolayer which was measured to be 5*10¹¹/cm².

Z.L, D.C-L, S.J contributed equally to this work.

  

Ultrafast carrier and structural dynamics of supported monolayer MoS2

Two-dimensional materials, such as graphene and transition metal dichalcogenides, have been considered promising for novel (opto)electronic and energy applications due to their unique properties at the mono- to few-layer limit. A thorough understanding of their carrier dynamics and energy transport behavior is therefore needed. Here, we present ultrafast carrier and energy-transport dynamics observed in sapphire-supported monolayer MoS₂ following photoexcitation, using femtosecond transient reflectivity and ultrafast electron diffraction (UED). In particular, both monolayer MoS₂ and the sapphire substrate were probed by reflection UED, which allows direct monitoring of structural motions at the interface. It is determined that dissipation of the photoexcitation energy undergoes a few steps with their characteristic time constants: ultrafast carrier relaxation and recombination as well as carrier-phonon coupling in MoS₂, energy transport from the monolayer to the substrate on a 10-ps time scale, and slower thermal diffusion in bulk sapphire. Temperature-dependent observations will also be discussed.   

Optical absorption in 2D MoS2 monolayers conformally grown on 3D Si and SiO2 nanocone arrays

We prepared 2D MoS₂ monolayers conformally coated on Si and SiO₂ nanocone (NC) arrays using metal organic chemical vapor deposition technique, and investigated the influences of the refractive indices of 3D NCs on the optical properties of 2D MoS₂ monolayers. The height, bottom diameter, and the period of the hexagonal Si NC array were 460, 250, and 300 nm, respectively. The SiO₂ NC array was prepared by thermal oxidation of the Si NC array. The photoluminescence and Raman intensities of the MoS₂ monolayer on the SiO₂ NC were higher than those on the Si NC, although the Si NC exhibited much lower optical reflectivity in the visible wavelength range compared with the SiO₂ NC. Numerical calculations showed that the strongly confined light in the high refractive index Si NC prevented a large electric field formation at the NC surface. In contrast, the weak light confinement in the low refractive index SiO₂ NC resulted in a large electric field intensity and enhanced absorption in the MoS₂ monolayer on the SiO₂ NC. This work demonstrates that the 2D MoS₂ and 3D Si hybrid nanostructures can provide a useful means to realize high-performance optoelectronic devices.   

Photocurrent Measurement of Monolayer Transition Metal Dichalcogenides

Monolayer transition metal dichalcogenides (TMDs) exhibit unique excitonic physics due to the reduced screening and enhanced Coulomb interactions. The strong light matter interaction in monolayer TMDCs gives rise to large response of photocurrent which involves carrier extraction after the excitation of the exciton. Photocurrent measurement thus provides a unique tool to investigate the combined effects of optical absorption and gain mechanism, revealing the spatial electric field distribution and photocurrent generation mechanism. The monolayer TMDs based photodetector exhibits large signal to noise ratio and can be exploited for sensitive photon sensor. The photocurrent measurement can also read out the valley degree of freedom of monolayer TMDs, excited by light of certain helicity.   

Exciton Up-conversion in Transition Metal Dichalcogenide Monolayers

The optical properties of Transition Metal Dichalcogenide (TMD) monolayers are governed by robust excitons. We have systematically investigated the excited exciton states by photoluminescence up-conversion spectroscopy in high quality MoS2, MoSe2, MoTe2 and WSe2 monolayers (MLs) encapsulated in hBN [1]. The excitation laser is tuned into resonance with the A:1s exciton ground state transition, and we observe for all the investigated MLs clear emission of excited exciton states (A:2s, A:3s…) up to 450 meV above the laser energy. The optical transitions are further investigated by reflectivity, photoluminescence excitation, and resonant Raman scattering, confirming their origin as excited excitonic states. We interpret the efficient up-conversion process as the consequence of exciton-exciton interactions where one of the excitons is annihilated while the second exciton acquires large extra energy. This mechanism is expected to be quite weak because it should satisfy both energy and momentum conservation laws. However our model calculations suggest an efficient exciton-exciton (Auger) scattering mechanism specific to TMD monolayers involving an excited conduction band, thus generating high-energy excitons with small wave vectors.
[1] B. Han et al, PRX 8, 031073 (2018)   

Valley Polarized Excitonic Complexes in Monolayer TMD alloy

Monolayer transition metal dichalcogenides (TMDs) have superior optical properties. Particularly, Tungsten based TMDs are known for their excellent valley polarization due to the strong spin-orbit coupling. However, the optical bandgap is limited by these two materials, which hinders the valleytronics applications that rely on the resonant excitation. We explore the possibility to overcome this limitation through the monolayer alloy, WS_2xSe_2(1-x), which promises an atomically thin semiconductor with tunable bandgap. We find that the high-quality BN encapsulated monolayer alloy WS_0.6Se_1.4 inherits the superior optical properties of tungsten-based TMDs, including a trion splitting of ~ 6 meV and valley polarization as high as ~60%. In particular, we demonstrate the emerging interlayer electron-phonon coupling in the BN/ WS_0.6Se_1.4 /BN van der Waals heterostructure. This coupling can be sensitively tuned by electrostatic gating and renders the otherwise optically silent Raman modes visible.   

Ultrafast charge dynamics and photoluminescence in bilayer MoS2

In a recent experiment it was shown that despite having an indirect band gap, the dominating peak in the emission spectrum of bilayer MoS₂ corresponds to direct transitions. [1] To understand this phenomenon, we have applied density-matrix based time-dependent density-functional theory to examine ultrafast charge dynamics and emissive properties of bilayer MoS₂. In particular, we demonstrate that despite initial accumulation of excited charge at the valleys between the K and Γ points of the two-dimensional Brillion zone, photoemission takes place through direct charge recombination at the K and K′ points. Analysis of the phonon spectrum suggests that the main reason for the direct emission is phonon-assisted transfer of the excited electrons to the K and K′ valleys. We also analyze the role of the spatial structure of the electron and hole excitations in the ultrafast charge dynamics and hence photoemission to find that d(Mo)-p(S) hybridized character of the holes facilitates inter-layer charge transfer. Our results thus reveal the importance of ultrafast charge dynamics in photoemissive properties of a few-layer transition-metal dichalcogenides.
[1] K. Xiao et al., private communication   

Polarisation switching and electrical control of interlayer excitons in two-dimensional van der Waals heterostructures

Interlayer excitons in van der Waals heterostructures of transition metal dichalcogenides are of great interest due to their fascinating spin-valley and moiré physics. These aspects could be implemented to realize next-generation photonic and valleytronic devices, as well as exploring new physical phenomena. However, the efficient manipulation of the exciton valley-state, a necessary requirement for valley information encoding, is still lacking. In this talk, we will demonstrate electrical control of interlayer excitons in a MoSe₂/WSe₂ heterostructure. Encapsulation of aligned monolayers with boron nitride allows to resolve two separate interlayer transitions with opposite helicities under circularly polarized excitation, either preserving or reversing the polarization of incoming light. By electrically modulating these resonances, we realize a polarization switch with tuneable emission intensity and wavelength in the near-infrared. By applying magnetic fields, we assign the origin of the effect to the moiré-induced brightening of forbidden optical transitions, as also predicted by recent theoretical works. The ability to control the polarization of interlayer excitons is a step forward towards the practical manipulation of the valley degree-of-freedom in device applications.   

Intrinsic multi-particle bound-state luminescence emission from high-quality monolayer tungsten diselenide

Monolayer WSe₂ is an interesting semiconductor that hosts versatile light emitting manybody states. While the bright exciton photoluminescence (PL) is ubiquitous, PL at higher energies are challenging to observe due to Kasha’s rule, and PL at lower energies are often plagued by extrinsic peaks from trapped excitons. In this talk, I’ll show that with our high-quality samples, excited Rydberg exciton PL up to the 4s exciton become visible, and in the lower energy window PL due to four-particle biexcitons and five-particle exciton-trions are resolved. In magnetic fields up to 31 Tesla, the Rydberg exciton energy displays increasing curvature against magnetic field strength from 1s to 4s, reflecting their increasingly larger diameter. The biexciton and the exciton-trion are found to be intervalley complexes composed of a dark exciton in one valley and a bright exciton / trion in the other valley. This spin-valley configuration gives rise to counter-intuitive inverted PL valley polarization as compared to the two-particle bright and dark excitons.
[1] S.-Y. Chen et al. Nature Commun. 9, 3717 (2018).
[2] S.-Y. Chen et al. Phys. Rev. Lett. 120, 046402 (2018).   

Room-Temperature Valley Coherence in a Polaritonic System

In the flourishing field of valleytronics, demand for coherently manipulating valley information at elevated temperature continues to escalate. Monolayer transition metal dichalcogenide, due to its strongly bonded excitons and degenerated valleys, nominates itself as a promising candidate for room temperature operation of valley degree of freedom (DOF). Through the hybridization
of valley-resolved exciton and helicity-resolved photon mode, the valley DOF will be inherited by half-light, half-matter polaritons. Here, we demonstrate non-vanishing valley coherence of exciton-polaritons at room temperature in a cavity-embedded monolayer Tungsten Diselenide. The extra decay path through the exciton-cavity coupling, which is free of decoherence, is the key for intervalley phase correlation. These observations pave the way for room temperature valleytronic devices.   

Exciton behaviors at the metal-semiconductor interface

The application of transition metal dichalcogenides in optoelectronics technology requires an understanding of how metal-semiconductor contacts impact exciton photophysics. In this study we explore changes in excitons at the interface of monolayer tungsten diselenide (1L-WSe₂) and a series of conventional metals. Low-temperature hyperspectral PL measurements reveal intriguing changes in exciton linewidth and energy on approach to the junction that persist over multiple microns. In particular, the neutral exciton experiences a blueshift as the interface between the metal and WSe₂ is approached, a phenomenon that persists at higher temperatures. These results are difficult to rectify as a charge doping effect since electrostatic gating of WSe₂ leads to insignificant shifts of the exciton.¹ Lattice strain from the contact is also not responsible since a redshift would be expected.² The precise mechanisms governing this effect therefore remain an open question but a critical one for the 2D community.

1. Li, Z. et al. Nat. Commun. 9, 3719 (2018).
2. Aas, S. et al. Opt. Express 26, 28672 (2018).   

Valley-selective exciton bistability in a suspended monolayer semiconductor

Monolayer transition metal dichalcogenide (TMD) semiconductors such as WSe₂, are direct band gap materials, which exhibit over 80% reflectance contrast at the fundamental exciton resonance. This extremely strong light-matter interaction can lead to optical nonlinearity or even bistability at high optical pump intensity. Furthermore, the valley degree of freedom (DOF) carries valley contrasting orbital magnetic moments, which enables K and K’ valleys of the Brillouin zone exclusively coupled to the incident light with opposite helicity. Valley-selective exciton bistability can be achieved, in principle, by combining the above two properties of monolayer TMD semiconductors. In this work, we demonstrate robust bistable exciton resonance by continuous-wave (cw) optical excitation in a suspended WSe₂ sample. The detailed excitation wavelength and power dependence studies of the sample reflectance, as well as numerical simulation, quantitatively support a photothermal mechanism with internal feedback contributing to the observed excitonic bistability. The presence of external magnetic field further allows control of the sample reflectivity purely by varying the polarization of incident light.