Strongly bound excitons are a characteristic hallmark of 2D semiconductors, enabling unique light–matter interactions and novel optical applications. Platinum diselenide (PtSe₂) is an emerging 2D material with outstanding optical and electrical properties and excellent air stability. Bulk PtSe₂ is a semimetal, but its atomically thin form shows a semiconducting phase with the appearance of a band-gap, making one expect strongly bound 2D excitons. However, the excitons in PtSe₂ have been barely studied, either experimentally or theoretically. Here, the authors directly observe and theoretically confirm excitons and their ultrafast dynamics in mono-, bi-, and tri-layer PtSe₂ single crystals. Steady-state optical microscopy reveals exciton absorption resonances and their thickness dependence, confirmed by first-principles calculations. Ultrafast transient absorption microscopy finds that the exciton dominates the transient broadband response, resulting from strong exciton bleaching and renormalized band-gap-induced exciton shifting. The overall transient spectrum redshifts with increasing thickness as the shrinking band-gap redshifts the exciton resonance. This study provides novel insights into exciton photophysics in platinum dichalcogenides.

中文翻译：

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原子级薄 PtSe2 中激子主导的超快光学响应

强束缚激子是 2D 半导体的特征标志，可实现独特的光-物质相互作用和新颖的光学应用。二硒化铂 (PtSe ₂ ) 是一种新兴的二维材料，具有出色的光学和电学性能以及出色的空气稳定性。Bulk PtSe ₂是一种半金属，但其原子级薄的形式显示出具有带隙外观的半导体相，这使得人们期待强束缚的 2D 激子。然而，PtSe _{2 中}的激子几乎没有在实验上或理论上进行过研究。在这里，作者直接观察并从理论上证实了单层、双层和三层 PtSe _{2 中的}激子及其超快动力学单晶。稳态光学显微镜揭示了激子吸收共振及其厚度依赖性，这通过第一性原理计算得到证实。超快瞬态吸收显微镜发现激子在瞬态宽带响应中占主导地位，这是由强激子漂白和重整化带隙引起的激子位移引起的。随着带隙的缩小使激子共振红移，整个瞬态光谱随着厚度的增加而红移。这项研究为铂二硫属化物中的激子光物理提供了新的见解。