Photoacoustic Remote Sensing

当我们看物体时，我们看到的是从物体表面散射回来的光线。然而，成像散射表面下的光吸收结构不仅对我们的眼睛而且对大多数成像技术都是具有挑战性的。最近一种称为光声成像的技术已经解决了这一挑战，但需要激光激发和使用超声换能器检测超声波。我们介绍了一种新的成像技术，称为光声遥感（PARS）显微镜，它使用全光学方法提供光学吸收灵敏度。这项新技术是基于一种新的检测机制。简而言之，使用脉冲激光在吸收目标中激发大的光声压信号，并在其最强的地下起源处进行光学检测。探测采用与激发激光共聚焦共扫描的探测光束，以保证最大的信号。通过使用随机相位的探针束，我们排除了与不必要的振动甚至传播超声波相关的任何相位灵敏度，并且仅对由于初始压力激发引起的强度变化敏感。由于脉冲光能转化为机械能时产生的初始压力，强度调制与折射率和散射变化有关。此外，通过记录探针光束调制的最初几纳秒，我们确保信号定位到其大的初始压力源，其尺寸为微米或更小。PARS承诺前所未有的信噪比，甚至比基于接触的成像方法更大。拟议的发展旨在进一步显著改善信噪比和扩展成像深度。我们还建议创建一种新的深度分辨方法，称为相干门控光声遥感。这有望实现与光学相干断层扫描类似的深度扫描，但提供的是吸收对比，而不是散射对比。我们还建议使用中红外光直接激发化学键与PARS读数的振动共振。这有望成为一种全新的光谱学形式，有望提供优于非线性光学或拉曼光谱的信噪比优势。生物医学，工业和军事应用将继续进行。

When we look at objects we are seeing light scattered from the surface returning to our eyes. However, imaging optically absorbing structures below the scattering surface is challenging for not only our eyes but most imaging techniques. Recently a technique called photoacoustic imaging has met this challenge but requires laser light excitation and detection of ultrasonic waves using ultrasound transducers. We introduce a new imaging technique called photoacoustic remote sensing (PARS) microscopy which provides optical absorption sensitivity using an all-optical approach. This new technology is based on a new detection mechanism. In brief, large photoacoustic pressure signals are excited in absorbing targets using a pulsed laser and detected optically at their subsurface origin where they are strongest. The detection is accomplished using a probe beam which is co-focused and co-scanned with the excitation laser to ensure maximal signal. By using a probe beam with randomized phase we preclude any phase-sensitivity associated with unwanted vibrations or even propagating ultrasound waves, and are only sensitive to intensity changes due to the initial pressure excitation. Intensity modulations are associated with refractive index and scattering changes due to the initial pressures generated when pulsed optical energy is converted to mechanical energy. Additionally, by recording only the first few nanoseconds of the probe beam modulations we ensure localization of signals to their large initial pressure origins, which are microns or smaller in size. PARS promises unprecedented signal-to-noise ratios, even larger than contact-based imaging approaches. Proposed developments aim to significantly further improve signal-to-noise and extend imaging depths. We also propose creating a new depth-resolved approach called coherence-gated photoacoustic remote sensing. This promises depth-scanning similar to optical coherence tomography but provides absorption rather than scattering contrast. We additionally propose using mid-infrared light to directly stimulate vibrational resonances of chemical bonds with PARS readout. This promises to be a fundamentally new form of optical spectroscopy which is hoped may provide signal-to-noise advantages over nonlinear optical or Raman spectroscopies. Biomedical, industrial, & military applications will be pursued.