Fig. 1. Analysis of the influence of modulation parameters of Airy beam phase pattern on beam property. (a) The different phase patterns, cross-sectional beam-spot maps of xy direction at the focus position, and cross-sectional beam-spot maps of xy direction at the defocus position, respectively. (b) Left column: beam propagation property maps in the xz direction that correspond to the different modulation parameters in (a); right column: zoom-in maps of the initial beam intensity distribution that correspond to the yellow region in the left column. (c) and (d) Schematic diagram of flexibly adjustable FDIR-PAM using an SLM. NDF, neutral density filter; PD, photodetector; HWP, half-wave plate; PBS, polarization beam splitter; M, mirror; L1–L4, lenses; OL, objective lens; D, diaphragm; P, polarizer; SLM, spatial light modulator; UT, ultrasonic transducer; AMP, amplifier; DAQ, data acquisition card; PC, personal computer.

Fig. 2. Beam property characterization. (a)–(c) Phase patterns of Gaussian and Airy beams generated by SLM. (d)–(f) Measured 2D beam intensity distributions for (a) focused Gaussian, (b) Airy beam #1, and (c) Airy beam #2 at the focal plane. (g)–(h) Measured beam intensity distributions of Gaussian beam and Airy beams #1 and #2 at 1.5 mm away from the focal plane, respectively. (j)–(l) Optical intensities in the sagittal plane of Gaussian beam and Airy beams #1 and #2, respectively. The white dashed line depicts the measured FWHM. The color bar gives a linear intensity distribution. (m) Lateral resolution of the FDIR-PAM system, giving the ESFs and its first derivative extracted from the corresponding B-scan of PAM map. (n) The depth-axial distribution of lateral resolution.

Fig. 3. PAM images of a letter pattern phantom. (a)–(d) Photoacoustic imaging characterization of the letter phantom by Gaussian-beam excitation PAM (GB-PAM) and FDIR-PAM, respectively. The related line profiles at different depths are presented in (e) and (f). Scale bars, 1 mm.

Fig. 4. In vivo PAM characterization of zebrafishes, larvae and adults. (a) and (b) PAM images of two zebrafish larvae via Gaussian-beam excitation PAM (GB-PAM) and FDIR-PAM. (c) and (d) Zoom-in en face maps that correspond to the yellow boxes of (a) and (b), respectively. (e) and (f) Cross-sectional maps that correspond to the dashed lines in (c) and (d). (g) and (h) PAM images of zebrafish adult fish body using the Gaussian and Airy beams. (i) and (j) Zoom-in en face maps that correspond to yellow boxes of (g) and (h), respectively. (k) and (l) Cross-sectional maps that correspond to the dashed lines in (g) and (h). (m) Intensity profiles along the lines depicted in (I), (II), (III), and (IV).

Fig. 5. In vivo label-free PAM characterization of the mouse brain. (a) and (b) Imaging of brain vasculature via GB-PAM and FDIR-PAM. (c) and (d) Zoom-in en face maps with multiscale vessel enhancement filtering operator that correspond to the yellow boxes of (a) and (b), respectively. (e) and (f) Cross-sectional maps that correspond to the dashed lines of (c) and (d), respectively. FDIR-PAM reveals a greater number of blood vessels surrounding the right-edge areas of the mouse brain compared to conventional GB-PAM. (g) Intensity profiles along the lines depicted in (I), (II), and (III), as well as statistical analysis of vessel density, were conducted to compare GB-PAM and FDIR-PAM.