Fig. 1. Schematics of non-contact photoacoustic imaging. (a) Air-coupled type; (b) homodyne Michelson interferometer type; (c) speckle pattern type; (d) homodyne Mach-Zehnder interferometer type; (e) heterodyne Mach-Zehnder interferometer type; (f) non-interferometric type

Fig. 2. Air-coupled non-contact photoacoustic (PA) imaging. (a) Imaging of ink in acoustic-resolution mode^[22]; (b) in vivo PA imaging of vessels in rabbit’s ear before and after burn^[23]; (c) PA image of finger skin and spectra of different spots^[32]

Fig. 3. Non-contact PA imaging with homodyne Michelson interferometer. (a) PA imaging of a mouse ear^[36]; (b) dual-modal imaging of a mouse ear^[45]; (c) blood flow velocity imaging of a mouse ear^[46]; (d) PA imaging of a mouse ear with a long coherence length laser^[50]; (e) PA imaging of a zebrafish with Gaussian beam(above) and dual non-diffracting Bessel beam (below)^[51]

Fig. 4. Non-contact PA imaging with homodyne Mach-Zehnder interferometer. (a) PA imaging of ink tube^[52]; (b) dual-modal imaging of a skeleton leaf^[53]; (c) dual-modal imaging of black/white bristle^[53]; (d) PA imaging of a mouse ear^[55]

Fig. 5. Non-contact PA imaging with heterodyne Mach-Zehnder interferometer. (a) Reconstructed images of black polyethylene terephthalate fibers and chicken chorioallantoic membrane^[56]; (b) cross section of the chicken gizzards and reconstructed image with 360° scan^[58]

Fig. 6. Non-contact PA imaging using laser speckle pattern. (a) Three-dimensional reconstructed image of ink tube^[38]; (b) PA image of an ink tube based on speckle contrast^[62]; (c) speckle patterns at different time^[67]; (d) correlation coefficients of the speckle patterns as a function of time and its exponential fitted curve^[67]

Fig. 7. Non-interferometric non-contact PA imaging. (a) PA imaging of a mouse ear^[68];(b) hyperspectral PA imaging acquired on an unstained formalin-fixed paraffin-embedded human breast tissue slide^[80]; (c) imaging of oxyhemoglobin saturation with dual-wavelength^[72]; (d) dual-modal imaging of the zebrafish larva microinjected with a rhodamine B solution^[81]

Fig. 8. PARS imaging performed on tissue blocks of human breast^[82]. (a)(b) PARS large field of view imaging with 266 nm excitation and higher resolution imaging of dashed box; (c)--(f) PARS imaging of tissue blocks with 250 nm and 420 nm excitation, bottom left inserts are H&E-stained images

Fig. 9. In-vivo imaging of the ocular tissue with dual-modal PARS-OCT^[91]. (a) Three-dimensional volume OCT imaging of the eye; (b) OCT cross-section imaging of the iris vasculature (yellow arrows); (c)--(e) OCT and PARS images of the iris vasculature and their overlay image

Fig. 10. Non-contact dual-modal imaging of melanoma progression^[48]. (a)--(d) Non-contact PA images; (e)--(h) blood flow images; (i)--(l) OCT images corresponding to the white dotted lines 1--7 in (a)--(d), H&E is hematoxylin and eosin-stained histologic image corresponding to OCT image in (l)