Enhanced many-body interactions due to strong Coulomb interactions and quantum confinement are one of the most prominent features of two-dimensional systems. Auger process is a representative many-body interaction typically observed...

semiconducting systems. [1,2] A representative example is layered 2D semiconductors, where reduced dielectric screening leads to huge exciton binding energies up to several hundreds of meV, enabling strong excitonic light-matter interactions even at room temperature. Thus, excitons in 2D materials govern fundamental optical properties such as light absorption and emission. [3,4] Furthermore, excitons in 2D systems often couple with their unique physical properties, such as a thickness-dependent change in the bandgap nature and polarization-governed optical selection rules. These relationships provide rich physics and proper functionality for novel optoelectronic applications. [3][4][5] To fully exploit the excellent properties of excitons, it is essential to explore their behavior on ultrashort timescales. Many fundamental excitonic phenomena such as recombination, many-body interactions, and scattering mechanisms occur on femtosecond or picosecond timescales. [6][7][8][9][10][11][12][13] Also, ultrafast light-matter interactions can modulate excitons, enabling novel quantum memory and switching effects. [14][15][16][17][18][19] Therefore, understanding exciton behaviors on Strongly bound excitons are a characteristic hallmark of 2D semiconductors, enabling unique light-matter interactions and novel optical applications. Platinum diselenide (PtSe 2 ) is an emerging 2D material with outstanding optical and electrical properties and excellent air stability. Bulk PtSe 2 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 PtSe 2 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 PtSe 2 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 bandgap redshifts the exciton resonance. This study provides novel insights into exciton photophysics in platinum dichalcogenides.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.202103400.

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