Fig. 1. Advantages and applications of photothermal imaging

Fig. 2. Transient photothermal properties of a PMMA bead. (a) Illustration of the transient photothermal process of a PMMA bead, the solid circle indicates the sample, and the peripheral part of sphere represents the phenomenon of sample thermal expansion caused by photothermal conversion after being excited by light; (b) center temperature of a PMMA bead varing with time under a single pulse, with pulse width τ=100 ns; (c) temperature profile of a PMMA bead and the medium at t=100 ns along the x-axis passing through the center of PMMA bead, the dotted circle indicates the boundary of the sample at xz cross-section, r represents the distance from the center of the PMMA bead

Fig. 3. Illustration of the optical path for visible wavelength photothermal imaging. (a) Scanning imaging^[52]; (b) wide-field imaging^[53]

Fig. 4. Applications of photothermal imaging at visible wavelengths. (a) Photothermal image of a single BHQ molecule submerged in glycerol^[32]; (b) scattering image (left), fluorescence image (middle), and photothermal image (right) recorded on COS7 cells, images of the first line correspond to untransfected cells and the other correspond to cells expressing a membrane protein containing a myc tag immunolabeled with anti-myc-Alexa568 and with anti-IgG 10-nm gold as the secondary antibody, details of the photothermal image (inset) revealing individual anti-IgG 10-nm gold imaging^[57]

Fig. 5. Absorption fingerprint region for nucleic acid, protein, and lipid

Fig. 6. Illustration of the optical path for mid-infrared photothermal imaging system. (a) Backpropagation scanning imaging optical path^[16]; (b) backpropagation wide field imaging optical path^[66]

Fig. 7. Applications of mid-infrared photothermal (MIP) imaging. (a) 3D MIP imaging of lipids in living cells and in vivo imaging of lipid in C. elegans^[21]; (b) MIP imaging for recording cell division of oligodendrocytes by selective excitation of the protein amide I band^[23]; (c) raw phase image and MIP images of living 3T3 cells, the arrow position indicates nucleic acids and the dashed circle is the IR illumination area^[74]; (d) local heterogeneities in cation distributions of mixed cation FA_0.1MA_0.9PbI₃ perovskite films^[26]

Fig. 8. Contrast mechanism and detection limits of photothermal imaging^[83]. (a)(b) Δσ_backscat,r and Δσ_backscat,n contributions to the overall photothermal signal for PS and PMMA; (c) relative contributions of ∆σ_backscat,n and ∆σ_backscat,r to signal as a function of the refractive index of medium(n_med) for PS; (d) SNR vs. radius (r) from photothermal imaging measurements conducted on individual PS bead

Fig. 9. Typical approaches to improve the spatial resolution of MIP imaging. (a) Resolution improved by counter-propagating configuration^[14], upper right is the MIP image of a 0.1-μm diameter PS bead and lower right is the corresponding line profile showing a full width at half-maximum (FWHM) of 0.3 μm^[14]; (b) resolution improved by deep learning convolutional neural network^[15], images of individual 0.3-μm diameter PS (left) before and (middle) after processing, and (right) corresponding cross section strength of Gaussian fitting, scale bar is 500 nm

Fig. 10. Degradation of spatial resolution due to the heat diffusion effect^[67]