Fig. 2: System characteristics.

a Spectral fidelity demonstrated on 10-µm polystyrene beads. The NIR spectrum of PS solution collected from a UV-Vis-NIR spectrophotometer was plotted as a reference. b Spectral fidelity demonstrated on pure solvent ethylene glycol. c OPT image at 1170 nm of 200-nm PMMA beads dispersed in glycerol-d8. Individual beads and bead aggregates are observed in the image. d Cross-section profile along the marked dashed line in c to estimate the lateral resolution in OPT microscopy. e Power dependence of OPT signals as a function of SWIR excitation power. f Power dependence of OPT signals as a function of the 520 nm probe power. g OPT spectra measured from multiple concentrations of DMSO@glycerol-d8 solution. The OPT signal at 1170 nm of 1% DMSO can be clearly distinguished out of the background of glycerol-d8. h Longitudinal view of a 3D OPT imaging of a 1-µm PMMA bead at 1170 nm. i Axial OPT point spread function profile measured from h. The inset shows an image of this bead at Z = 0 µm. The axial position of the particle was precisely controlled through a piezo stage.