We recently discovered that strong reflectivity modulations occur when a pulsed laser excites an absorption interface with an existing refractive index contrast. These modulations are observed using a low-coherence interrogation beam co-focused and co-scanned with an excitation beam to form high-resolution all-optical photoacoustic images. We call this new form of microscopy Photoacoustic Remote Sensing (PARS). To better understand the mechanism, analytical models were created of the time-evolution of these PARS signals. Shock waves propagating from the absorption interface create refractive index steps that form a time-varying multi-layer etalon. Besides an initial-pressure reflectivity change, GHz-modulations are predicted due to the propagating etalon effect. The characteristics of these modulations are related to the optical coherence length of the probe beam and the intrinsic optical properties of the sample. 1D plane-wave and 3D Mie-theory-based analytical models are compared with finite-difference time-domain simulations and experiments involving phantoms with different absorption- and refractive-index interfaces. Experimentally-observed modulations are detected with extremely high signal-to-noise ratios in phantoms and animal models. The newly predicted modulation mechanism offers a promising signature for deep all-optical absorption-contrast imaging with high fidelity.
Kevan L. Bell, Parsin H. Reza, Wei Shi, and Roger J. Zemp, "Temporal evolution of low-coherence reflectometry signals in photoacoustic remote sensing microscopy (Conference Presentation)," Proc. SPIE 10064, Photons Plus Ultrasound: Imaging and Sensing 2017, 1006422 (Presented at SPIE BiOS: January 31, 2017; Published: 24 April 2017); https://doi.org/10.1117/12.2252975.5379284509001.
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