Fig. 1. (a) Schematic of the MW-OR-PAM. (b) Schematic and photograph of the custom-built transducer. (c) SRS spectrum of the MW-OR-PAM. (d) Chemical structure of tartrazine and the illustration of the 0.6 mol/L tartrazine solution inducing tissue refractive index matching. (e) Normal-incidence light transmittance as a function of the SRS spectral wavelength (adapted from Ref. [34]). BPF: band-pass filter. FC: fiber coupler. HWP: half-wave plate. NDF: neutral density filter. OL: objective lens. RI: refractive index. SMF: single-mode fiber. SP: scanning pattern.

Fig. 2. (a) Measured penetration depths with 6 dB SNR at 532 nm and (b) 604 nm. (c) Measured and fitted ESFs and LSFs at 532 nm and (d) 604 nm. (e) Axial resolutions of 532 nm and (f) 604 nm. (g) PA image of a 1951 USAF resolution target at 532 nm and (i) normalized PA amplitude profile across the line pair. (h) PA image of a 1951 USAF resolution target at 604 nm and (j) normalized PA amplitude profile across the line pair. (k) Photograph of a leaf-skeleton phantom. (l) PA image of the phantom. ESF: edge spread function. FWHM: full width at half-maximum. LSF: line spread function. PA: photoacoustic. RF: radio frequency. SNR: signal-to-noise ratio.

Fig. 3. (a) Illustration of the mouse ear. (b) PA image of hemoglobin concentration and (c) oxygen saturation acquired from mouse ear. (d) Illustration of the mouse brain. (e) PA image of hemoglobin concentration and (f) depth information acquired from mouse brain. Scale bars: 400 μm.

Fig. 4. (a) Schematic of the tartrazine-induced tissue transparency in mice and imaging timeline. (b) The photographs and PA images that were acquired during the different reversible tissue-clearing stage. (c) The photographs and PA images of the transcranial mouse brain and (d) thigh before and after tartrazine treatment. Scale bars: 400 μm.

Fig. 5. Comparison of imaging depth in the tissue phantom without (w/o) and with (w/) tartrazine treatment.

Fig. 6. Comparison of capillary network enhancement before and after tartrazine treatment. (a) Photoacoustic image before tartrazine treatment. (b) Photoacoustic image after tartrazine treatment. (c) Line profile extracted from the dashed lines L1 in (a) and L2 in (b), respectively. Scale bars: 400 µm.