Nonlinear Optical Absorption of ReS₂ Driven by Stacking Order

Abstract

The stacking order of 2D materials can result in fascinating physical properties. Two different stacking orders (AA and AB) were recently identified in ReS2, a rising star in the transition metal dichalcogenides family with a unique anisotropic property. Their optical and vibrational properties show drastically different features. With intensity-scan spectrometry, the nonlinear optical absorption of both AA and AB stackings in ReS2 was investigated. Saturable absorption (SA) only occurs in AB stacking at certain polarization angles, whereas it is absent in AA stacking. This saturation behavior is attributed to strong exciton–exciton annihilation (EEA). Excited-state absorption (ESA) is prominent in both stacking orders, but more anisotropic with polarization in AB stacking. Our results provide new insights in using ReS2 as a nonlinear optical material in forefront optoelectronics.

Band Edge Carrier-Induced Sign Reversal of an Ultrafast Nonlinear Optical Response in Few-Layer ReS2 Nanoflakes

“…The two excitons X 1 and X 2 centered at 1.485 ± 0.005 eV and 1.516 ± 0.003 eV, respectively, are separated from the continuum absorption centered at 1.560 ± 0.005 eV, using the Elliott equation for a hydrogenic Wannier–Mott exciton model, modified for two excitons. − Details are provided in SI Section S.4. The position of X 1 and X 2 is consistent with the two lowest excitons reported by Zhou et al in AB stacking, originating from the transition of nonbonding Re 5d t 2g to nonbonding Re 5d t 2g * . Noticeably, the third high-energy exciton ∼1.57 eV reported in their PL measurements is not exclusively observed in the optical absorption in the present study because it gets buried in the continuum absorption.…”

“…The in-plane vibrational A 1 g mode is centered at 151 cm –1 , and out-of-plane vibrational modes A 4 g , A 5 g , A 6 g , and A 7 g are centered at 132, 141, 161, and 211 cm –1 , respectively . The separation between the modes A 1 g and A 4 g is ∼19 cm –1 which indicates that the number of layers is ∼4–8 with AB stacking. , From the previous studies and our result, we conclude that AB stacking is a common structure in liquid-phase exfoliated ReS 2 . , The ground-state optical absorption of few-layer ReS 2 nanoflakes demonstrating multiple excitons, well in agreement with previous reports, is shown in Figure d. The two excitons X 1 and X 2 centered at 1.485 ± 0.005 eV and 1.516 ± 0.003 eV, respectively, are separated from the continuum absorption centered at 1.560 ± 0.005 eV, using the Elliott equation for a hydrogenic Wannier–Mott exciton model, modified for two excitons. − Details are provided in SI Section S.4.…”

“…Transition metal dichalcogenides (TMDs) of the general formula MX 2 (M = transition metal and X = chalcogen) allow the study of the quantum-confined materials in the thickness limit of one or a few unit cells. , Among them, the most studied materials are high-symmetry MoS­(Se) 2 and WS­(Se) 2 because of their very distinct optical properties such as crossover from indirect bandgaps in bulk to direct bandgaps in the monolayer, − large exciton binding energy, spin-valley coupling, harmonic generation, and saturable absorption (SA). , Recently, low-symmetry TMDs, such as ReS­(Se) 2 and W­( Mo )­Te 2 , have attracted much attention because of their intrinsic anisotropic properties of fundamental and technological importance. − For instance, they show a polarization-dependent optical stark effect and quantum beats which hold great potential for quantum information processing. , …”

“…By simultaneously measuring the polarization-dependent transmission and reflection, Meng et al have demonstrated anisotropy in the optical and nonlinear optical properties of mechanically exfoliated ReS 2 . Recently, Zhou et al have shown that RSA in ReS 2 originates from the third exciton level, satisfying the necessary condition that the excited-state absorption cross section is more than the ground-state cross section . Despite a flurry of research activities in the third-order nonlinear optical response, manipulating those properties in a completely reversible and real-time manner is a challenge though it is of great importance for applications like all-optical deterministic quantum logic and optical limiting.…”

“…24 originates from the third exciton level, satisfying the necessary condition that the excited-state absorption cross section is more than the ground-state cross section. 12 Despite a flurry of research activities in the third-order nonlinear optical response, manipulating those properties in a completely reversible and real-time manner is a challenge though it is of great importance for applications like all-optical deterministic quantum logic and optical limiting. Nonetheless, the nonlinear optical response can be tuned by utilizing intralevel charge transfer between singlet and triplet states or the donor−acceptor moieties; 30−32 however, the design criteria for the TMDs have not been well developed yet.…”