Giant single-step upconversion via sub-35-fs phonon dynamics in the nonlinear optical regime.

Phonon-assisted upconversion (UC) for anti-Stokes photoluminescence stands as a fundamental and widely studied process, central to both ultrafast electron-phonon coupling physics and diverse photonic applications. However, the ultrafast dynamics limit of UC has yet to be addressed, preventing its integration into the nonlinear optical regime. Here, we find a giant single-step UC of ~200 milli-electron volts via sub-35-femtosecond phonon dynamics in two-dimensional hybrid organic-inorganic perovskites in the nonlinear regime. The single-step UC approaches the phonon dynamics limit of ~23 femtoseconds and gains energy about eight times the room-temperature thermal energy (~25 milli-electron volts), enabling its synergistic integration into the nonlinear regime. Benefiting from the unique electron-phonon coupling between organic vibrations and excitons in inorganic lattices, the UC demonstrates distinctive signatures of Raman anisotropy and strong nonlinearity. This work opens new avenues for studying uncharted phonon dynamics and nonlinear optical mechanisms, offering substantial advantages in optical refrigeration, upconverting energy harvesting and optical microscopy.