Twist-angle engineering of excitonic quantum interference and optical nonlinearities in stacked 2D semiconductors

Abstract

Twist-engineering of the electronic structure in van-der-Waals layered materials relies predominantly on band hybridization between layers. Band-edge states in transition-metal-dichalcogenide semiconductors are localized around the metal atoms at the center of the three-atom layer and are therefore not particularly susceptible to twisting. Here, we report that high-lying excitons in bilayer WSe2 can be tuned over 235 meV by twisting, with a twist-angle susceptibility of 8.1 meV/°, an order of magnitude larger than that of the band-edge A-exciton. This tunability arises because the electronic states associated with upper conduction bands delocalize into the chalcogenide atoms. The effect gives control over excitonic quantum interference, revealed in selective activation and deactivation of electromagnetically induced transparency (EIT) in second-harmonic generation. Such a degree of freedom does not exist in conventional dilute atomic-gas systems, where EIT was originally established, and allows us to shape the frequency dependence, i.e., the dispersion, of the optical nonlinearity.

Narrow-band high-lying excitons with negative-mass electrons in monolayer WSe2

“…Our results unveil the rich physics of complex many-body excitations in monolayer TMDCs, which emerges not only in the formation of various excitons and exciton complexes but, more importantly, in the coherent coupling and interference between excitonic transitions. In comparison to band-edge excitons, high-lying excitons appear to have larger orbital contributions from the chalcogenide atoms and are therefore much more sensitive to band hybridization with an adjacent layer in multilayer structures 46 . As we recently proved in twisted-bilayer WSe 2 , the HX energy can be tuned over a broad range by twist angle, with an average twist-angle susceptibility of 8.1 meV/°.…”

“…As we recently proved in twisted-bilayer WSe 2 , the HX energy can be tuned over a broad range by twist angle, with an average twist-angle susceptibility of 8.1 meV/°. Correspondingly, the excitonic quantum interference can be turned on and off by twisting 46 . High-lying excitons combined with band-edge excitons, therefore, form an atomic-like excitonic multilevel system, setting the basis for future exploration of quantum excitonics.…”

Excitons in mesoscopically reconstructed moiré heterostructures

“…Periodic moiré interference patterns on the length scale of a few nanometers [9] have profound effects on the electronic band structure via the formation of flat mini-bands which enhance many-body correlations and give rise to emergent magnetism [33], correlated insulating states [34][35][36][38][39][40] or Wigner crystals [38] with periodic order in spatial charge distribution [41]. Moiré effect also result in rich optical signatures of intralayer [42] and interlayer [26][27][28][29] excitons formed by Coulomb attraction among layer-locked and layer-separated electrons and holes, with angle-controlled exciton valley coherence and dynamics [21,43,44], optical nonlinearities [45] or correlated excitonic insulating states [46]. Despite extensive optical studies of moiré interference effects in TMD heterobilayers as in MoSe 2 -WSe 2 [47], a consolidated picture of the rich and partly conflicting experimental features remains elusive [31].…”

Stacking-dependent exciton multiplicity in WSe$_2$ bilayers

“…Initial studies of homobilayers were limited to natural 2H layer stacking with antiparallel alignment or 60 • twist angle [3][4][5], extracted by exfoliation from native crystals and structurally different from 3R stacking with parallel alignment or 0 • twist. Recently, variations of the twist angle in WSe 2 bilayers (BLs) away from 2H and 3R stackings revealed novel phenomena ranging from correlated electronic phases [6] to moiré exciton physics [7,8] with angle-controlled exciton valley coherence and dynamics [8][9][10], Coulomb correlations in effectively flat moiré exciton bands [11] or optical nonlinearities [12].…”