Photoinduced Modulation of an Excitonic Resonance Coupled to Coherent Optical Phonons and The Role of Dynamical Screening in a Layered Semiconductor

The ultrafast light-matter interactions in semiconductor materials are intensively studied due to their technological relevance in the field of, e.g., photovoltaics or data processing. The elementary photoexcitation of a semiconductor is the formation of an electron-hole pair, called exciton, as a result of the weak screening of the Coulomb interaction between the electron and the hole. The coupling of excitons with other excitations like, e.g., lattice vibrations (phonons) plays a pivotal role on the nonequilibrium optical properties of semiconductors, eventually governing their optoelectronic functionalities. However, how exciton-phonon coupling manifests in the time and energy domains has raised an active debate between experiment and theory in the last years [1]. Moreover, the effect of dynamical screening, i.e. of transient changes in the screening by the photoexcited quasi-free carriers, on the coupling of excitons to phonons has remained elusive. These open questions represent a forefront fundamental research challenge in semiconductor optics and an essential step toward the development of novel protocols for optoelectronic applications.<br/>Layered semiconductors formed by van-der-Waals stacked atomic planes offer a great quantum materials platform for the study of coupled dynamics with accessible and technologically relevant energy and time scales. The quantum space confinement of mobile carriers reduces the dielectric screening, stabilizing the excitons and favoring their coupling to lattice vibrations [2].<br/>In our work, by means of ultrafast broadband optical spectroscopy combined with ab-initio calculations of the layered semiconductor bismuth triiodide (BiI₃), we set the spectral fingerprints for the optical detection of exciton-phonon coupling in van-der-Waals layered semiconductors [2]. Our joint experimental and theoretical effort allows us to unravel the impact of exciton-phonon coupling by microscopically relating a photoinduced coherent energy modulation of the excitonic resonance to coherent optical phonons. This further enables us to track the extent of the photoinduced atomic displacement in real space with unprecedented subpicometer resolution. These findings eventually solve the debate between experiment and theory on the spectral evidences of exciton-phonon coupling, and establish the role of exciton-phonon coupling on the ultrafast exciton dynamics.<br/>Further, we address the effect of dynamical screening on the exciton-phonon coupling in BiI₃[3]. To this goal, we investigate the ultrafast broadband optical spectroscopy under excitation condition either resonant or off-resonance with the excitonic resonance in order to either exclude or enable the excitation of a population of quasi-free carriers. We find evidences that that the dynamical screening breaks the coupling of excitons to coherent optical phonons, thereby hindering the photoinduced modulation of the excitonic resonance.<br/>Finally, these complementary works move a step forward in the detection of many-body couplings in layered materials by ultrafast optical spectroscopy, paving the way to optical control of an excitonic resonance on the picosecond timescale.<br/><br/>[1] F. Paleari, Phys. Rev. Lett. 122, 187401 (2019); C. Trovatello, ACS Nano 14, 5700 (2020); D. Li, Nat. Comm. 12, 954 (2021); H.-Y. Chen, Phys. Rev. Lett. 125, 107401 (2020); J. Fu, Advanced Materials 33, 2006233 (2021)<br/>[2] A. Chernikov, et al, Phys. Rev. Lett., 113, 076802 (2014); C. Jin, Nature Nanotechnology, 13, (2018); S. Dal Conte, Trends in Chemistry, 2, 1 (2020)<br/>[3] S. Mor, et al, Phys. Rev Research, 3, 043175 (2021)<br/>[4] S. Mor, et al., in prep.