Fig. 2.

Exemplary label-free images based on third-harmonic generation (THG) signals arising from live mammalian cells in monolayer culture after (a) 5-minutes acetic acid treatment, and (b) 5-minute acetic acid treatment followed by 5-minute washout using fresh culture medium. Images labeled with (i) to (v) are taken at different acetic acid concentrations from 0% to 0.5% (pH values from 7.56 to 4.42) for treatment and washout conditions. All images are reconstructed from raw data without any processing, and image sizes are 60 μm × 60 μm.