A research group led by Prof. WU Kaifeng and ZHU Jingyi from the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences recently reported the observation of an excitonic Bloch-Siegert shift in CsPbI₃ perovskite quantum dots (QDs), which advances current fundamental understanding for coherent light-matter interaction in low-dimensional solid-state materials.

The study was published in Nature Communications on Sept. 22.

Coherent interaction between a two-level system and a periodic light field contains both co- and counter-rotating wave parts, which correspond to the so-called optical Stark effect and Bloch-Siegert shift, respectively. Observing the latter has always been challenging, not only because it is weak but it is often accompanied by a much stronger Stark shift.

In this study, the researchers reported a strong excitonic Bloch-Siegert shift in CsPbI₃ perovskite QDs at room temperature. The band structure and spin-orbit coupling of this material led to spin-selective quasi-particle transition rules, analogous to the valley-selectivity in transition metal dichalcogenides (TMDs), thus offering a new playground to test excitonic effects on the optical Stark effect and Bloch-Siegert shift.

Importantly, unlike TMDs whose excitonic interaction depended sensitively on their underlying substrates, potentially responsible for the disparity in the above-mentioned studies of TMDs, the excitonic interaction in these colloidal QDs was deterministically strong, as they were uniformly surrounded by low-refractive-index organic ligands and solvents.

The researchers found that by controlling the light helicity, they could largely restrict the optical Stark effect and Bloch-Siegert shift to different spin-transitions, especially when the pump light was tuned from visible to infrared, achieving a Bloch-Siegert shift as strong as four meV.

The ratio between the Bloch-Siegert and optical Stark shifts was found to be systematically higher than the one predicted by the quasi-particle picture at 12 different pump wavelengths. By accounting for the co- and counter-rotating Floquet states of ground, exciton, and biexciton states, they quantitatively reproduced the experimental observations with a biexciton binding energy of 65 meV.

"Our new model depicts a unified physical picture of the interplay between the optical Stark effect, biexcitonic optical Stark effect and Bloch-Siegert shift in low-dimensional materials displaying strong many-body interactions," said Prof. WU.