Fig. 4: Spectrally resolved dual-comb photoacoustic responses of polymer films.

a, b show the PA response for PDMS (a) and paraffin (b) obtained by Fourier transformation of the time-domain signal at the transducer output (obtained by dividing the measured signal by the amplifier gain) followed by scaling to optical frequencies (Eq. (2)). The averaging times are 4 min, 20 min, and 120 min for the yellow-dotted, blue-dotted, and black-solid traces, respectively. c, d show the normalized PA response from Eq. (4) (solid black) compared to the optical absorbance (base-e absorbance units) measured with a spectrophotometer (dashed red) for PDMS (c) and paraffin (d). See “Methods” section and Supplementary Note 3 for details about sample preparation. The optical excitation power is ≈24 mW for both the PDMS and paraffin measurements.