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.
The demand for high capacity and integration in modern optical communication technology is becoming prominent. Orbital angular momentum (OAM) plays an important role in optical communication. However, there are still challenges to further expand the flexibility and capacity of optical communication in the axial direction. Here, we propose a single-layer liquid crystal device (LCD) to realize the generation of optical vortex (OV) array with arbitrary topological charge in axial multiplane, which can be applied in optical communications based on highly integrated device. The phase of the target OV array is weighted and superimposed to obtain the phase distribution of LCD. In order to obtain an OV array with uniform intensity, it is necessary to determine the optimal weight factor for each OV based on the introduced particle swarm optimization (PSO) algorithm. In the experiment, a LCD with an effective aperture of 2 mm was processed. A CCD captures the OV array image, including two OV arrays at 200*λ (156μm) in front of and behind the focal point respectively. Then, the beam passes through the 4f system of the spatial light modulator with the phase distribution of the Damman vortex grating on the spectrum plane, and the topological charge of the two OV arrays can be detected by the CCD. Our results provide an approach that based on a single liquid crystal plate, OV arrays in multiple propagation planes are realized, in which the number of propagation planes and the position of each propagation plane relative to the back focal plane can be adjusted arbitrarily, and the number, order, and position of OVs in each propagation plane can also be adjusted arbitrarily to meet the application requirements in the field of short-distance optical communication.
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