Sensitive Detection of Ultrafast Photoacoustics Using Modulated Asynchronous Optical Sampling

Ultrafast photoacoustics can be used to “look through” opaque thin layers, by both exciting an acoustic pulse and detecting its echo optically at the top surface [1]. Because the acoustically induced reflectivity changes are typically very small, rapid scanning with high signal-to-noise ratio is challenging. A common alternative to using a mechanical delay stage to scan the pump-probe delay is asynchronous optical sampling (ASOPS) [2]. This scheme consists of two separate pulsed lasers with a fixed repetition rate offset for fast scanning. Here, we show that adding frequency modulation to ASOPS can significantly improve the measurement noise floor.