Figure 3
Observation of resonance harmonics for thin PMMA film. (a) The correlation between the two high frequency components \({f}_{1}^{H}\), \({f}_{2}^{H}\) and the fundamental frequency (fR) shows a linear relationship. The slopes obtained by linear fittings are 3.2 and 4.7 corresponding to the 2nd and 3rd harmonics, respectively. (b) The box-whisker-plots show the measured harmonics are more spread out than the resonance frequencies, which suggests that the frequency of harmonics should provide an enhanced contrast for the imaging of thickness inhomogeneity.
