Listening to ultrashort laser pulses to determine their characteristics

Since the physical principle behind Camera Obscura was first described by Mozi during the 5th century BC, stating in Mo Jing (墨經, is an ancient Chinese philosophical and scientific text attributed to the Mohist school): “Light rays travel straight, like arrows; when passing through a small hole, the lower part of an object projects an image upward, and the upper part projects an image downward.” (景光之人煦若射，下者之入也高，高者之入也下。), people believed that light could only be detected by “seeing.” However, this study breaks this philosophy for hundreds of years and demonstrates that light pulse profile can also be detected by “LISTENING.” Here, we use femtosecond laser pulses to generate air plasma and sound waves via the photoacoustic effect. A nonlinear crystal-free sound autocorrelation is developed to detect these acoustic signals. The autocorrelated sound (AS) signals are measured to determine the characteristics of ultrashort laser pulses and air plasma. Potential applications for this laser-induced sound autocorrelation include a method for ultrafast characterization of laser pulses and as a diagnostic technology to determine the dynamic interaction between plasma and electromagnetic (EM) waves.