Coherent light-matter interaction offers a promising avenue for quantum state engineering and ultrafast optical modulation. The optical Stark effect (OSE), a light-driven coherent modification of energy level spectra, has been extensively studied in excitonic two-level systems. However, limitations in controllability arise from the restriction of driving fields to large detuning energies and weak intensities to minimize real excitation effects. Multilevel systems offer enhanced controllability through phenomena such as quantum interference and the Autler-Townes effect (ATE). However, practical implementations of these multilevel systems have been hampered by limitations in control-light wavelengths and the requirement of cryogenic temperatures. This study focuses on lead halide perovskites (LHPs), promising materials for optoelectronic devices, due to their multiband structure induced by strong spin-orbit coupling. The strong spin-orbit coupling in LHPs, significantly larger than in benchmark materials like GaAs and monolayer transition metal dichalcogenides, results in a large energy splitting of the conduction band. The researchers hypothesize that coherent excitation of inter-conduction band transitions will enable efficient modulation of band-edge transitions using near-infrared control light at room temperature, overcoming previous limitations. This work aims to investigate this hypothesis by exploring the coherent optical manipulation of excitons utilizing the energy states with large spin-orbit splitting in CsPbBr3 NCs.

The optical Stark effect (OSE) has been a subject of extensive research, particularly in excitonic two-level systems. Equation (1) in the paper describes the Stark shift in these systems, demonstrating the dependence on detuning energy and Rabi frequency. However, minimizing real excitation effects requires large detuning and weak intensities, limiting controllability. Multilevel systems offer enhanced control through quantum interference and the Autler-Townes effect (ATE). Previous studies have explored multilevel systems in semiconductor nanostructures utilizing fine-structure splitting, exciton-biexciton transitions, or intra/intersubband transitions. However, these approaches are often limited to low temperatures due to the small energy differences involved. Furthermore, the control-light wavelengths are restricted to near-infrared (shorter than 1 μm) or mid-infrared regions, depending on the specific transition used. Lead halide perovskites (LHPs) have emerged as promising materials for strong light-matter interaction studies, exhibiting a large OSE at room temperature. Their strong spin-orbit coupling creates a multiband structure, offering the potential for novel coherent control schemes. While large OSE has been observed in LHPs, the potential of using the spin-orbit split states for coherent manipulation remained unexplored.

The researchers synthesized CsPbBr3 nanocrystals (NCs) with an average size of 6.9 nm using a modified hot-injection method. The cubic crystal structure of these NCs is crucial for the study. Circularly polarized pump-probe spectroscopy was employed to investigate the coherent modulation of band-edge exciton transitions. A regenerative amplifier (1028 nm central wavelength, 10 kHz repetition rate, 300 fs pulse duration) provided the source for pump and probe pulses. An optical parametric amplifier generated the pump pulses, while a white-light probe pulse was produced by focusing a beam into water. Achromatic quarter-waveplates were used to generate σ+ and σ− polarized pulses. The pump-probe delay time was controlled with a mechanical delay stage. The CsPbBr3 NCs were dispersed in hexane, and the solution was stirred during measurements to prevent photo-charging. The background signal from hexane was subtracted from the data. The energy shift of the band-edge exciton was determined using a spectral weight transfer method. Local field correction was incorporated into the analysis to account for local field effects. A three-level system model utilizing the dressed state picture and rotating-wave approximation was employed to interpret the results. The model considered the hybridization of the photon-dressed states and the lowest conduction band states. The transition dipole moment of the inter-CB transitions was extracted from the experimental data by fitting the model to the observed energy shifts.

The researchers observed an anomalous enhancement of the exciton energy shift in CsPbBr3 NCs at room temperature as the pump energy was varied from the visible to the near-infrared region. This non-monotonic behavior deviates from the typical two-level OSE. The observed enhancement is attributed to a crossover from the two-level OSE to the three-level ATE due to the strong spin-orbit coupling in the material. The inter-conduction band (CB) transitions between spin-orbit split states were found to possess a large transition dipole moment (25 D), significantly larger than that of the band-edge transitions (19 D). This large dipole moment is attributed to the involvement of wavefunctions beyond the R point. The three-level ATE model accurately reproduces the observed energy shift, particularly in the near-infrared region, highlighting the significant contribution of the inter-CB transitions. Importantly, the pump-induced exciton population, which often hampers ultrafast optical responses, is suppressed in the three-level ATE region, suggesting a path towards efficient ultrafast optical switching. The study demonstrates that the enhancement of the energy shift becomes prominent for excitation energies below half of the bandgap energy, falling within the telecommunication wavelength region. Further analysis revealed that the incoherent component, associated with real excitation of excitons, is suppressed in the near-infrared region, where the ATE dominates. This suppression of incoherent effects opens possibilities for efficient ultrafast optical switching under high-intensity excitation conditions without material damage.

The findings demonstrate that strong spin-orbit coupling in CsPbBr3 NCs enables a novel coherent optical modulation scheme. The crossover from the two-level OSE to the three-level ATE, driven by the large transition dipole moment of the inter-CB transitions, allows for efficient exciton manipulation in the near-infrared region at room temperature. The suppression of incoherent effects in this regime signifies a significant advancement in ultrafast optical switching applications. The utilization of near-infrared light extends the control-light wavelength range beyond previous limitations, opening up possibilities for integration with existing telecommunication technologies. The achievement of room-temperature operation eliminates the need for cryogenic cooling, making the approach more practical and cost-effective. This work significantly advances the understanding of coherent light-matter interaction in LHPs and provides a new platform for developing advanced optoelectronic devices.

This study demonstrates a novel coherent optical modulation scheme enabled by the strong spin-orbit coupling in CsPbBr3 perovskite nanocrystals. The researchers observed a crossover from the two-level optical Stark effect to the three-level Autler-Townes effect, facilitated by large dipole moments in interconduction band transitions. This leads to efficient exciton manipulation at room temperature and extended control-light wavelengths into the near-infrared region. The suppression of incoherent effects in this regime suggests potential for efficient ultrafast optical switching applications. Future work could explore other LHP materials with varying spin-orbit coupling strengths and investigate the potential of this approach for creating more complex quantum devices.

The study primarily focuses on CsPbBr3 nanocrystals. The generalizability of the findings to other LHP compositions and nanostructures needs further investigation. The three-level model used is a simplification of the complex energy level structure in LHPs. More sophisticated models that incorporate additional energy levels and interactions might be needed for a more complete understanding. While the suppression of incoherent effects is observed, the possibility of other unforeseen incoherent processes at higher intensities warrants further exploration.