Fig. 7.

Validation of the spatial resolution enhancement [36]. (A) Lateral resolution (${\text{R}}_{lateral}$) measurements of OR-PAM and GR-PAM. The edge spread functions (ESFs) were obtained based on the PA amplitude across the sharp edge and the derivatives of ESFs give rise to the line spread functions (LSFs). The FWHM of ESFs for GR-PAM is smaller than OR-PAM by a factor of 1.6, which agrees with the theoretical value of$\sqrt{2}$. (B) Axial resolution was estimated for both GR-PAM and OR-PAM. ${\text{R}}_{axial\_GR}$ was assessed based on the FWHM of the fitted Gaussian curve of the differential PA signal (${PA}_{GR-PAM}$) at different axial positions of a monolayer of RBCs. ${\text{R}}_{axial\_OR}$ was obtained from the FWHM of the A-line signal envelope (dashed line). ${\text{R}}_{axial\_GR}$ is measured to be 2.3 μm, which close to the theoretical prediction 2.4 μm ($1.8\text{λ}/{NA}^2$) and more than 18 times finer than ${\text{R}}_{axial\_OR}$. Figures reproduced with publisher’s permission [36].