τin -1 = (τd + τs) The working principle of asynchronous optical sampling (ASOPS) is similar, however, a detuning repetition frequency Δf of the pump laser compared to the probe laser is adopted to form a pump-probe delay. With the repetition frequency difference Δf, when the first pulses of the pump laser and the probe laser coincide, a successive offset for the subsequent pulses as a delay time will be generated. Delay times range from femtoseconds to several ns (5 ns for 100 MHz repetition rate). 4 a-d) Adapted with permission from ref. Copyright 2022 American Chemical Society.