Fig. 6a shows relative frequency shifts of SiN strings determined experimentally as a function of the inverse of the string width. Each measurement was repeated three times, and the average relative frequency shift and the standard deviation were calculated from these measurements. As expected from Eq. (4), the relative frequency shift increases linearly with the inverted string width. From the slope of the linear fit and Eq. (4), the absorbed power is extracted to be 1.8 nW, which is approximately 10 ppm of the readout laser power.