Fig. 2: Centre of intensity x_c vs. y for resonant excitation at two different angles and for two orientations of the sample with respect to the propagation direction y.

a, b The case where the principle axis \(y^\prime\) is aligned along y and corresponds to the dispersion curve in Fig. 1c. c, d The case where the principle axis \(x^\prime\) is aligned along y and corresponds to the dispersion curve in Fig. 1d. Blue points correspond to the experimental data, the red solid line gives the theoretical fit. The zero of the y-axis is defined as the point of peak intensity vs. y. The zero of the x-axis is defined such that the theoretical \(x_c\left( {y = 0} \right) = 0\). Error bars are plotted between \(\pm \sigma\) where σ is the standard deviation calculated as discussed in Supplementary Section 4. Error bars smaller than the data point symbols are not plotted