Fig. 2: ϕ dependence of the mode amplitudes.

a–d FFT spectra of MP1, MP2, M, and P as a function of THz polarization ϕ at low E_THz. The amplitudes are normalized to the maximal value for each mode (see colorbar at the bottom). The horizontal dashed line at 3.68 THz marks the frequency of M. The MP1, MP2, and M modes are measured at 30% of the maximal E_THz and the P mode is measured at 50% of the maximal E_THz. E_THz = 150 kV/cm. e–h Amplitudes of MP1 (e), MP2 (f), M (g), and P (h) as a function of ϕ extracted from (a–d). Solid lines are fits. MP1 is fit by the sum of Eq. (2) and Eq. (3) for a B_g mode. MP2 is fit by the sum of Eq. (2) and Eq. (3) for a A_g mode. M is fit by Eq. (3) for a A_g mode. P is fit by Eq. (2) for a A_g mode. The white and gray shadings in the polar plots indicate opposite phases. The numbers on the polar plots represent the maximal amplitudes normalized to the maximal amplitude measured at 30% of the maximal E_THz of different modes. The color bars next to the polar plots indicate the relative strengths of the nonlinear (red) and linear (blue) pathways obtained by the fitting. i–p are identical to panels (a–h) but acquired at 100% of the maximal E_THz. The error bars show experimental standard error. q–u Schematics of the interference. q shows a typical fitting (violet) of the ϕ dependence of MP1 at highest E_THz (m), along with the linear (blue) and nonlinear (red) components of the fitting. r–u shows schematics of the MP oscillations excited by the linear channel alone (L, blue curve), the nonlinear channel alone (NL, red curve), and both linear and nonlinear channels (L+NL, violet curves). The blue shaded regions highlight different phases of the oscillations. The curves are offset for clarity.