Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F37: 2D Materials - Optics and Excitons IIIFocus
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Sponsoring Units: DMP DCOMP Chair: Feng Wang, Univ of California - Berkeley Room: LACC 411 |
Tuesday, March 6, 2018 11:15AM - 11:27AM |
F37.00001: Unifying Optical Selection Rules for Excitons in Two Dimensions: Band Topology and Winding Numbers Ting Cao, Meng Wu, Steven Louie We show that band topology can dramatically change the photophysics of two-dimensional semiconductors. For systems in which states near the band extrema are of multi-component character, the spinors describing these components (pseudospins) can pick up nonzero winding numbers around the extremal k-point. In these systems, we find that the strength and required light polarization of an excitonic optical transition are dictated by the optical matrix element winding number, a unique and heretofore unrecognized topological characteristic. We illustrate these findings in three gapped graphene systems – monolayer graphene with inequivalent sublattices and biased bi- and tri-layer graphene, where the pseudospin textures manifest into nontrivial optical matrix element winding numbers associated with different valley and photon circular polarization. This winding-number physics leads to novel exciton series and optical selection rules, with each valley hosting multiple bright excitons coupled to light of different circular polarization. This valley-exciton selective circular dichroism can be unambiguously detected using optical spectroscopy. |
Tuesday, March 6, 2018 11:27AM - 11:39AM |
F37.00002: Dark Exciton and Optical Selection Rules Revealed by In-plane Propagation of Light in Transition Metal Dichalcogenides Monolayers Cedric Robert, Gang Wang, Misha Glazov, Bernhard Urbaszek, Fabian Cadiz, Emmanuel Courtade, Thierry Amand, Delphine Lagarde, Takashi Taniguchi, Kenji Watanabe, Xavier Marie The optical selection rules for interband transitions in WSe2, WS2, and MoSe2 transition metal dichalcogenide monolayers (ML) are investigated by polarization-resolved photoluminescence (PL) experiments with a signal collection from the sample edge. We observe clear signatures of the emitted light with the electric field oriented perpendicular to the ML plane, corresponding to the optical transition of “dark” neutral excitons forbidden at normal incidence used in standard optical spectroscopy. These studies, performed at zero external magnetic field B, yield a direct determination of the bright-dark exciton splitting : we measure 40 and 55 meV in WSe2 and WS2 ML [1]. |
Tuesday, March 6, 2018 11:39AM - 11:51AM |
F37.00003: The Role of Dark Excitons in Valley Depolarisation In Monolayer Transition Metal Dichcalcogenides Alexander Pearce, Guido Burkard We present a theoretical study of the valley magneto-exciton relaxation dynamics in monolayer transition metal dichcalcogenides (TMDs) using a kinetic equation approach. The TMDs are direct band gap semiconductors with strong light-matter coupling which produce optical responses dominated by tightly bound excitons. The combination of the lattice symmetry and strong spin-orbit interaction gives rise to a rich selection rules allowing for optical control of the excitons valley polarisation. Experiments have shown that due to spin-orbit interactions there are spin forbidden transitions which lead to dark exciton states, and these states are found to possess long lifetimes due to their non-radiative decay and play a role in the depolarisation dynamics of the TMDs. Using a kinetic equation approach we investigate the interplay of the exchange interaction, a perpendicular magnetic field and the dark state scattering the time evolution of the exciton valley polarisation. We find that the influence of the dark states leads to longer valley relaxation times. We also explore the effect of an in-plane magnetic, which acts to “brighten” the dark states, which leads to an even greater increase in the valley relaxation time. |
Tuesday, March 6, 2018 11:51AM - 12:27PM |
F37.00004: Imaging Spin Dynamics in Monolayer WS2 by Time-Resolved Kerr Rotation Microscopy Invited Speaker: Elizabeth McCormick Transition metal dichalcogenide (TMD) monolayers are of great interest due to their two-dimensional (2D) nature combined with their unique band structure. The strong spin-orbit coupling due to the transition metal atom creates large spin splitting in the band structure, leading to spin/valley optical selection rules and long spin and valley lifetimes. In the tungsten compounds, WX2, optical excitation couples the upper level of the valence band to the upper level of the conduction band. This allows for the formation of “dark” excitons and trions (i.e. charged excitons), a lower energy state that cannot be optically excited or radiatively recombined. |
Tuesday, March 6, 2018 12:27PM - 12:39PM |
F37.00005: Valley-selective Optical Stark Effect Probed by Kerr Rotation Trevor LaMountain, Hadallia Bergeron, Itamar Balla, Teodor Stanev, Mark Hersam, Nathaniel Stern The valley pseudospin in transition metal dichalcogenide (TMDC) monolayers is a promising degree of freedom for coherent control. The optical Stark effect allows for valley-selective manipulation of energy levels in WS2 and WSe2 using ultrafast optical pulses. Despite these advances, understanding of valley-selective optical Stark shifts in TMDCs has been limited by reflectance-based detection methods where the signal is small and prone to background effects. We show that polarization-sensitive, time-resolved Kerr rotation is less sensitive to background effects, providing a five-fold improvement in the signal-to-noise ratio of the Stark effect optical signal and a more precise estimate of the energy shift. This increased sensitivity allows for observation of an optical Stark shift in monolayer MoS2 that exhibits both valley- and energy-selectivity, demonstrating the promise of this method for investigating this effect in other layered materials and heterostructures. |
Tuesday, March 6, 2018 12:39PM - 12:51PM |
F37.00006: Trion valley coherence in monolayer transition metal dichalcogenides Kai Hao, Lixiang Xu, Fengcheng Wu, Philipp Nagler, Kha Tran, Xin Ma, Christian Schüller, Tobias Korn, Allan MacDonald, Galan Moody, Xiaoqin (Elaine) Li Excitons and trions in monolayer transition metal dichalcogenides are formed at the boundary of the Brillouin zone at two inequivalent K and K’ valleys. Because of the valley contrasting optical selection rule, excitons and trions can be preferentially created using in a selected valley using light with a particular circular polariztion. Valley coherence — the coherent superposition between the excitonic states in the two valleys can be generated by a linearly polarized excitation. Exciton valley coherence has been detected as linearly polarized PL emission. However, the same method is not applicable to trions due to the spin-photon entanglement. With two-dimnensional Fourier transform spectroscopy, we explicitly measure the nonradiative exciton and trion valley coherence. Unlike exction, whose valley coherent time is limited by intrinsic fast population decay and electron-hole exchange interaction, our result shows that trion valley coherent time is limited by intra-valley pure dephasing processes. |
Tuesday, March 6, 2018 12:51PM - 1:03PM |
F37.00007: Observing excitons with time-resolved ARPES Alexander Kemper, Avinash Rustagi With the development and spread of time- and angle-resolved photoemission spectroscopy (tr-ARPES) comes the capability of observing electrons in the conduction bands of semiconductors after an ultrashort pump pulse. In systems with large Coulomb interactions, such as the two-dimensional dichalcogenide materials (e.g. MoS2), the possibility exists of the excited electrons binding with the holes in the valence band and forming excitons, and these may be observed through tr-ARPES. We show what the signatures of these excitations are, what the relationship is between the dispersion of the observed electrons and the underlying material properties such as conductance/valence band effective masses and exciton radii, and discuss their time dependence. |
Tuesday, March 6, 2018 1:03PM - 1:15PM |
F37.00008: Tip-enhanced photoluminescence and control of interlayer excitons in van der Waals heterostructures Molly May, Kyoung-Duck Park, Chenfeng Du, Genevieve Clark, Xiaodong Xu, Markus Raschke Two-dimensional (2D) van der Waals heterostructures have emerged as attractive candidates for applications in novel electro-optical devices due to their rich excitonic properties. Transition metal dichalcogenide (TMD) heterostructures are particularly promising candidates in which interlayer excitons were recently observed in the far field. However, the details of interlayer exciton behavior at variable interlayer distances and the mechanisms underlying the interlayer coupling have yet to be understood. We show that optical-antenna tip coupling in tip-enhanced photoluminescence (TEPL) imaging induces radiative pathways with highly enhanced PL emission through both the Purcell effect and the ability of the antenna-tip to couple to out-of-plane exciton dipole moments. Furthermore, we show precise nano-resonator gap engineering of the van der Walls heterostructure by nano-mechanical tip force control. Using this approach, we measure and control the interlayer exciton properties of a MoSe2-WSe2 heterostructure providing insight into the interlayer coupling mechanism, and paving the way for novel nano-photonic devices. |
Tuesday, March 6, 2018 1:15PM - 1:27PM |
F37.00009: Interlayer coupling and gate-tunable excitons in transition metal dichalcogenide heterostructures Catalin Spataru, Shiyuan Gao, Li Yang Bilayer van der Waals heterostructures such as MoS2/WS2 and MoSe2/WSe2 have attracted much attention recently, particularly because of their type II band alignments and the formation of interlayer exciton as the lowest-energy excitonic state. Here, we calculate the electronic and optical properties of such heterostructures with the first-principles GW+Bethe-Salpeter Equation method and reveal the important role of interlayer coupling in deciding the excited-state properties, including the band alignment and excitonic properties. Our calculation shows that due to the interlayer coupling, the low energy excitons can be widely tunable by a vertical gate field. In particular, the dipole oscillator strength and radiative lifetime of the lowest energy exciton in these bilayer heterostructures is varied by over an order of magnitude within a practical external gate field. Then we build a minimal model that captures the essential physics behind this tunability and allows the extension of the ab initio results into a larger range of electric fields. |
Tuesday, March 6, 2018 1:27PM - 1:39PM |
F37.00010: Observation of Polaronic Trions in MoS2/SrTiO3 Heterostructures Soumya Sarkar, Sinu Mathew, Maxim Trushin, Sreetosh Goswami, surajit saha, Majid Fard, Saurav Prakash, Sherman Tan, Antony George, kian ping loh, Pulickel Ajayan, Shaffique Adam, Thirumalai Venkatesan
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Tuesday, March 6, 2018 1:39PM - 1:51PM |
F37.00011: 2D semiconductors: Probing by broadband femtosecond continuum pulses Mohammad Mokim, Feruz Ganikhanov Transition metal dichalcogenides (TMDCs) are the focus of fundamental research and technological applications due to their novel electronic and optical properties. We demonstrate an effective microspectroscopy technique by tracing the dispersion of second-order nonlinear susceptibility χ(2) to characterize the monolayers of WSe2, WS2, MoS2, and MoSe2 within the photon energy range of 2.4-3.2 eV. We then estimate, with a fairly good precision, the fundamental bandgap and exciton binding energy of these semiconductors. We also provide the absolute value of χ (2). To perform the experiment, ultra-broadband continuum pulses served as the fundamental beam while its second harmonic generation (SHG) spectrum in visible and ultraviolet (UV) was detected and analyzed with better than 0.3 nm spectral resolution (<2 meV). Our results show that the experimental dispersion data provide the clear resolution of the near band-gap exciton states in comparison with traditional linear methods that target measurement of the linear dielectric function. |
Tuesday, March 6, 2018 1:51PM - 2:03PM |
F37.00012: Temperature Dependence of Photoluminescence Lifetimes of WSe2 Monolayers Mateusz Goryca, Aleksandra Lopion, Karol Nogajewski, Marek Potemski, Piotr Kossacki In photoluminescence (PL) spectra of atomically-thin layers of tungsten-based transition metal dichalcogenides (TMDs), two different types of components are typically observed: short-lived PL lines corresponding to free excitonic states, and lower-energy long-lived PL peaks related to so-called localized states. The nature of the latter features is, however, still not clear. Those lines exhibit a strong decrease of intensity with increasing temperature, which is most probably related to the opening of non-radiative recombination channels, either from relaxed excitonic states (from which the PL originates) or from excited states (acting as intermediate states during the non-resonant excitation process). In this work we analyze the properties of the WSe2 PL peaks related to the localized states as a function of temperature with the use of time-resolved spectroscopy. We determine that the intensity of those peaks is inversely proportional to their lifetime, thus the decrease of PL intensity can be explained by the opening of recombination channels from relaxed localized states. This may shed some light on the nature of those states, which is important for future optoelectronics and valleytronics applications of TMDs. |
Tuesday, March 6, 2018 2:03PM - 2:15PM |
F37.00013: Electronic and optical properties of two-dimensional III-nitrides from first principles Nocona Sanders, Dylan Bayerl, Guangsha Shi, Kelsey Mengle, Emmanouil Kioupakis While bulk III-V semiconductors enjoy broad commercial success, 2D III-nitride materials have only recently emerged, and their applications are less well understood. Extreme quantum confinement is a promising method to shift emission wavelength into the deep ultraviolet range for sterilization applications, but in 2D GaN this is counteracted by quantum confined Stark shift due to a strong inherent polarization perpendicular to the 2D plane. Additionally, increased electron-hole interaction due to quantum confinement results in exciton binding energies much larger than those of their bulk counterparts. We report the electronic and optical properties of 2D GaN, InN, and AlN using first principles calculations. We employ density functional theory and quasiparticle corrections with the GW method, as well as the Bethe-Salpeter Equation, to produce accurate band structures, exciton binding energies, and luminescence energies. Our results provide understanding on how the reduction of the thickness to the monolayer regime affects the overall electronic and optical characteristics (Nano Letters, 10.1021/acs.nanolett.7b03003). |
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