Photoacoustic Remote Sensing

点击翻译按钮获取中文摘要

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.