Gao Fei, Feng Xiaohua, Zhang Ruochong, Liu Siyu, Ding Ran, Kishor Rahul, Zheng Yuanjin
School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore, Singapore.
School of Information Science and Technology, ShanghaiTech University, Shanghai, China.
Sci Rep. 2017 Apr 4;7(1):626. doi: 10.1038/s41598-017-00725-4.
Photoacoustic sensing and imaging techniques have been studied widely to explore optical absorption contrast based on nanosecond laser illumination. In this paper, we report a long laser pulse induced dual photoacoustic (LDPA) nonlinear effect, which originates from unsatisfied stress and thermal confinements. Being different from conventional short laser pulse illumination, the proposed method utilizes a long square-profile laser pulse to induce dual photoacoustic signals. Without satisfying the stress confinement, the dual photoacoustic signals are generated following the positive and negative edges of the long laser pulse. More interestingly, the first expansion-induced photoacoustic signal exhibits positive waveform due to the initial sharp rising of temperature. On the contrary, the second contraction-induced photoacoustic signal exhibits exactly negative waveform due to the falling of temperature, as well as pulse-width-dependent signal amplitude. An analytical model is derived to describe the generation of the dual photoacoustic pulses, incorporating Gruneisen saturation and thermal diffusion effect, which is experimentally proved. Lastly, an alternate of LDPA technique using quasi-CW laser excitation is also introduced and demonstrated for both super-contrast in vitro and in vivo imaging. Compared with existing nonlinear PA techniques, the proposed LDPA nonlinear effect could enable a much broader range of potential applications.
基于纳秒激光照射，光声传感与成像技术已被广泛研究以探索光吸收对比度。在本文中，我们报道了一种长激光脉冲诱导的双光声（LDPA）非线性效应，其源于应力和热约束不满足的情况。与传统的短激光脉冲照射不同，所提出的方法利用长方波激光脉冲来诱导双光声信号。在不满足应力约束的情况下，双光声信号跟随长激光脉冲的正边缘和负边缘产生。更有趣的是，由于温度的初始急剧上升，第一个膨胀诱导的光声信号呈现正波形。相反，由于温度下降以及与脉冲宽度相关的信号幅度，第二个收缩诱导的光声信号呈现完全负波形。推导了一个分析模型来描述双光声脉冲的产生，该模型纳入了格林爱森饱和效应和热扩散效应，并通过实验得到了验证。最后，还介绍并展示了一种使用准连续波激光激发的LDPA技术替代方案，用于体外和体内的超对比度成像。与现有的非线性光声技术相比，所提出的LDPA非线性效应能够实现更广泛的潜在应用。
