Probing the dark side of the exciton

doi:10.1126/science.abf0371

GSTDTAP > 气候变化

DOI	10.1126/science.abf0371
	Probing the dark side of the exciton
	Meng Xing Na; Ziliang Ye
	2020-12-04
发表期刊	Science
出版年	2020
英文摘要	Two-dimensional (2D) semiconductors, such as transition-metal dichalcogenides, may enable new optoelectronic technologies ([ 1 ][1]). The optical excitation in these atomically thin materials creates tightly bound excitons composed of an excited electron and a valence-state hole ([ 2 ][2]), as well as a plethora of exciton complexes due to the reduced screening in Coulomb attraction ([ 3 ][3]–[ 5 ][4]). So far, excitons with large momenta have not been directly probed because photons only carry very small momenta and cannot directly interact with large-momentum excitons, but these dark excitons are predicted to exist in certain 2D semiconductors ([ 6 ][5], [ 7 ][6]). On page 1199 of this issue, Madéo et al. ([ 8 ][7]) used time- and angle-resolved photoemission spectroscopy (TR-ARPES) to directly probe dark excitons in monolayer tungsten diselenide (WSe2). By tracking the dynamics of electrons that constitute both bright and dark excitons, the authors reveal how both are formed and show that the latter outnumber the former at steady state. ![Figure][8] Bright and dark excitons Bright excitons have electrons in the K valley conduction bands and have zero momentum. Dark excitons have electrons in the Q valley conduction bands and carry momentum. GRAPHIC: C. BICKEL/ SCIENCE ![Figure][8] Movies of dark-exciton formation Dark excitons cannot be probed directly with photons. Madéo et al. used photoemission microscopy to reveal them in monolayer tungsten diselenide (WSe2). GRAPHIC: C. BICKEL/ SCIENCE There are two families of excitons in 2D semiconductors, with electrons at different valley-like local minima of the conduction band. Each valley is named after its location in the Brillouin zone (momentum-space). Bright excitons consist of electrons and holes both located in K valleys that form through the direct transition of the electron to an excited state with the same momentum (see the first figure). As the momentum of the exciton is defined by the momentum difference between its constituting electron and hole, these so-called K-K excitons have zero momentum and can directly interact with light. Dark Q-K excitons are composed of an electron excited indirectly into a Q valley with a different momentum than that of the hole left in a K valley. Valleys with opposite momenta are equally populated in this study. Madéo et al. performed their TR-ARPES experiment by combining a photoemission electron microscope (PEEM) with an extreme ultraviolet (XUV) light source (see the second figure). They first shine an ultrafast visible-light pulse at the WSe2 monolayer. By tuning the wavelength of this pulse, they can excite either excitons or free carriers. After a variable delay time, an ultrafast XUV pulse is shone onto the sample. The high photon energy of the XUV light then ejects electrons from the monolayer. With no surface state present, the authors directly accessed the electrons' energy and momentum inside the material by analyzing the kinetic energy and emission angle of the ejected electron (measured here by an angle-resolved time-of-flight analyzer). Both the visible and XUV light are ultrafast pulses. The first visible pulse excites the system, and the second XUV pulse acts effectively as a camera shutter. By fine-tuning the delay between the excitation and the shutter, the authors obtain a “movie” of electrons in momentum space in which free carriers, bright excitons, and dark excitons all have distinct signatures. The authors find that both bright and dark excitons exist in the monolayer WSe2. Notably, the bright K-K excitons preferentially convert into dark Q-K excitons through phonon scattering and lead to a density of dark excitons double that of the bright excitons. The dark excitons also have a longer lifetime, so they become a reservoir for the bright excitons at steady state. Tuning the excitation light off the exciton resonance excites free electrons and holes, and after about 500 fs, a quasi-equilibrium exciton ensemble forms. By comparing the photoemission signal from excitons with the free-electron energy at the band edge, Madéo et al. deduce the exciton binding energy for both bright and dark excitons. These results, which constitute a direct probe of dark-exciton formation and its binding energy and dynamics, were made possible by a convergence of improvements in the spatial resolution of the ARPES instrument as well as the high photon energy of the ultrafast high-harmonic XUV source that could probe the entire Brillouin zone of WSe2 ([ 9 ][9]). Although dark excitons do not interact directly with light, the electrons that constitute such dark excitons are accessible by photoemission ([ 10 ][10], [ 11 ][11]). Besides the Q-K exciton, other dark excitons in 2D semiconductors that are accessible by the technique of Madéo et al. include intervalley excitons, spin-triplet excitons, Rydberg states with finite angular momentum, and higher-order exciton complexes ([ 12 ][12]–[ 14 ][13]). Another equally rewarding direction would be to study the wave function of a single exciton that was obscured in the current study by phonon scattering and other scattering channels ([ 10 ][10], [ 11 ][11]). The rapid progress in van der Waals heterostructures also calls for more direct techniques to resolve their emerging electronic structures ([ 15 ][14]). It is expected that there will be many exciting opportunities to apply these powerful electronic probes in studying the excited states of quantum materials. 1. [↵][15]1. K. F. Mak, 2. J. 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领域	气候变化 ; 资源环境
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文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/305823
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Meng Xing Na,Ziliang Ye. Probing the dark side of the exciton[J]. Science,2020.
APA	Meng Xing Na,&Ziliang Ye.(2020).Probing the dark side of the exciton.Science.
MLA	Meng Xing Na,et al."Probing the dark side of the exciton".Science (2020).