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Photoacoustic sensing and imaging techniques have been studied widely toexplore optical absorption contrast based on nanosecond laser illumination. Inthis paper, we report a long laser pulse induced dual photoacoustic (LDPA)nonlinear effect, which originates from unsatisfied stress and thermalconfinements. Being different from conventional short laser pulse illumination,the proposed method utilizes a long square-profile laser pulse to induce dualphotoacoustic signals. Without satisfying the stress confinement, the dualphotoacoustic signals are generated following the positive and negative edgesof the long laser pulse. More interestingly, the first expansion-inducedphotoacoustic signal exhibits positive waveform due to the initial sharp risingof temperature. On the contrary, the second contraction-induced photoacousticsignal exhibits exactly negative waveform due to the falling of temperature, aswell as pulse-width-dependent signal amplitude which is caused by theconcurrent heat accumulation and thermal diffusion during the long laserillumination. An analytical model is derived to describe the generation of thedual photoacoustic pulses, incorporating Gruneisen saturation and thermaldiffusion effect, which is experimentally proved. Lastly, an alternate of LDPAtechnique using quasi-CW laser excitation is also introduced and demonstratedfor both super-contrast in vitro and in vivo imaging. Compared with existingnonlinear PA techniques, the proposed LDPA nonlinear effect could enable a muchbroader range of potential applications.

One laser pulse generates two photoacoustic signals