Electronic states modulation by coherent optical phonons in 2D halide perovskites

Excitonic effects underpin the fascinating optoelectronic properties of 2D perovskites that are highly favorable for photovoltaics and light-emitting devices. Analogous to switching in transistors, manipulating these excitonic properties in 2D perovskites using coherent phonons could unlock new applications. Presently, a detailed understanding of this underlying mechanism remains modest. Herein, the origins of the carrier-phonon coupling in 2D perovskites using transient absorption (TA) spectroscopy are explicated. The exciton fine structure is modulated by coherent optical phonons dominated by the vibrational motion of the PbI6 octahedra via deformation potential. Originating from impulsive stimulated Raman scattering, these coherent vibrations manifest as oscillations in the TA spectrum comprising of the generation and detection processes of coherent phonons. This two-step process leads to a unique pump- and probe-energy dependence of the phonon modulation determined by the imaginary part of the refractive index and its derivative, respectively. The phonon frequency and lattice displacement of the inorganic octahedra are highly dependent on the organic cation. This study injects fresh insights into the exciton-phonon coupling of 2D perovskites relevant for emergent optoelectronics development. Ministry of Education (MOE) National Research Foundation (NRF) This research was supported by Nanyang Technological University under its start-up grant (M4080514); the Ministry of Education under its AcRF Tier 1 grant (RG91/19) and Tier 2 grants (MOE2017-T2-2-002, MOE2019-T2-1-006 and MOE2019-T2-1-097); and the National Research Foundation (NRF) Singapore under its NRF Investigatorship (NRF-NRFI-2018-04). Z.S. gratefully acknowledges Ministry of Education Singapore for the funding of this research through the following grants: AcRF Tier 1 (RG195/17 and RG156/19) and AcRF Tier 3 (MOE2016-T3-1-006 (S)).