Photoacoustic measurement technologies have demonstrated great potentials for studying physiological and pathological processes within biological tissues, including blood flow, cellular metabolism, physiochemical signature and oxygen metabolism. However, the limited bandwidth of piezoelectric transducers fails to respond broadband ultrasonic signals of complex biological particles in the conventional photoacoustic measurement systems, which possibly overlooks abundant information encoded in the frequency domain of the ultrasonic waves. By exploiting the ultrafast temporal dynamics and strongly confined evanescent field of optical surface wave, we take advantage of a high Numerical Aperture (NA) objective to develop a series of broadband and high sensitivity ultrasonic sensing technologies of optical evanescent waves. For example, Surface Plasmon Resonance (SPR) sensors could measure pressure transients caused by ultrasonically modulated light reflectivity, demonstrating a high sensitivity of approximately 675 Pa and broad bandwidth of approximately 108 MHz. By constructing a sensing layer of the multilayer film based on composite waveguide, we developed multi-layer film waveguide photoacoustic detector (MWPAD) and further improved the sensitivity (approximately 208 Pa) and bandwidth (approximately 123 MHz). Further, we have exploited extremely sensitive phase shift evanescent wave sensor to convert the ultrasonically modulated phase shift into the light interference intensity, achieving detection sensitivity at approximately 174 Pa over a broad bandwidth of approximately144 MHz. We believe that our proposed sensing technologies of optical evanescent waves potentially promote biomedical ultrasonic/photoacoustic applications.
Ultraviolet photoacoustic imaging can detect the ultrasonic signals released by the cell nucleus through the strong absorption of ultraviolet light by DNA and RNA in organisms, which thus obtains images similar to those from traditional hematoxylin-eosin staining but no need of staining or sectioning. However, the traditional Optical-Resolution Photoacoustic Microscopy (OR-PAM) working in the ultraviolet band only maintains a micrometer-scale lateral resolution in a very limited depth range. Due to short Depth of Focus (DOF), the resolving capability of photoacoustic imaging deteriorates sharply, resulting in a severe degradation of image quality and adversely affecting the reliability of histopathologic diagnosis. Based on an extended Nijboer-Zernike (ENZ) theory of optical field regulation, a millimeter-level phase plate modulator of liquid crystal is designed to engineer the wavefront of ultraviolet photoacoustic illumination, achieving an increase in the DOF of the OR-PAM system at ~210 μm while maintaining a good lateral resolution of ~1.04 μm. This surpasses the traditional OR-PAM with Gaussian-mode excitation laser. The liquid crystal modulation is potentially valuable for obtaining non-destructive and label-free histology photoacoustic images of the cell nucleus.
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