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

ReS2, a layered transition metal dichalcogenide (TMD) with reduced crystal symmetry exhibiting unique anisotropic and layer-independent properties, holds great potential for optoelectronic and photonic applications. Despite a flurry of research activities in the third-order nonlinear optical response of TMDs, tuning those properties in a completely reversible manner in real time is a challenge and remains largely unexplored. Here, we experimentally demonstrate band edge carrier-induced sign reversal of the ultrafast third-order nonlinear optical response in few-layer (4–8) ReS2 nanoflakes. In particular, saturable absorption observed before hot carrier thermalization (<0.3 ps) is tuned to reverse saturable absorption (RSA) after the carrier thermalized (>0.6 ps) at the band edge and defects using a single-color pump–probe intensity scan (I-scan) technique. RSA in our experiment is due to the two-step two single-photon absorption of the long-lived (∼1000s of ps from our ultrafast transient absorption) carriers at the band edges and defects. Motivated by the results, a liquid cell-based high-performance few-layer ReS2 optical limiter is fabricated with a remarkable 0.1 GW/cm2 onset threshold and 0.64 limiting differential transmittance better than the other optical-limiting materials. These results offer a direction to manipulate the nonlinear optical response of materials which otherwise requires a large electric field, high intensity, or efficient charge transfer between donor and acceptor pairs.