Studying Quasi-Parametric\nAmplifications: From Multiple PT-Symmetric Phase Transitions to Non-Hermitian\nSensing

Quasi-parametric amplification (QPA) is a nonlinear interaction\nin which the idler wave is depleted through a loss mechanism. QPA\nplays an important role in signal amplification in ultrafast photonics\nand quantum light generation. In this report, we explore the generalized\nfeatures characterized by non-Hermitian parity-time (PT) symmetry in the QPA process and uncover\nmultiple PT-symmetric phase transitions in such a QPA\nprocess in which transitions are ultrasensitive to external parameters.\nIn particular, we demonstrate the feasibility of detection of 10<sup>–11</sup> inhomogeneities of the doped absorber, which is\nseveral orders of magnitude more sensitive than similar measurements\nperformed in a linear absorption regime. In doing so, we reveal a\nfamily of PT-symmetric phase transitions appearing in\nthe QPA process and provide a distinctive nonlinear optical sensing\nmechanism for precise optical measurements.