Nonresonant optical driving of confined semiconductors can open up exciting opportunities for experimentally realizing strongly interacting photon-dressed (Floquet) states through the optical Stark effect (OSE) for coherent modulation of the exciton state. Here we report the first room-temperature observation of the Floquet biexciton-mediated anomalous coherent excitonic OSE in CsPbBr3 quantum dots (QDs). Remarkably, the strong exciton–biexciton interaction leads to a coherent red shift and splitting of the exciton resonance as a function of the drive photon frequency, similar to Autler–Townes splitting in atomic and molecular systems. The large biexciton binding energy of ∼71 meV and exciton–biexciton transition dipole moment of ∼25 D facilitate the hallmark observations, even at large detuning energies of >300 meV. This is accompanied by an unusual crossover from linear to nonlinear fluence dependence of the OSE as a function of the drive photon frequency. Our findings reveal crucial information on the unexplored many-body coherent interacting regime, making perovskite QDs suitable for room temperature quantum devices.

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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/cm 2 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.

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Nanosecond near resonant excitation in As50S50 thin films leads to strong nonlinear optical response, i.e. nonlinear absorption coefficient up to 4 × 10 6 cm/GW and nonlinear refractive index of 8.5 cm 2 /GW, both of which is the strongest ever reported in amorphous semiconductors. We propose a five-level energy model to explain such effect which indicates that nonlinear process is reverse saturable absorption in nature, mediated by excited state absorption from triplet-triplet transition. On the other hand, observation of negative nonlinear refractive index reveals the occurrence of self-defocusing effect. Finally, benefitting from the strong nonlinear response, we demonstrate a promising application of As50S50 thin films as an optical limiter for optoelectronic sensors.