Frequency difference between PM and UPM as a function of the reduced frequency (the red curve with dots, same as the bottom panel of Fig. 3). For the blue one, the value of V_g is modified in PM as shown with the blue curve in the top panel of Fig. 8. The dashed black and magenta lines are fitting curves with Eqs. (4) and (9). The resulting coefficients are (a,b) = (−1.10 × 10^-2,5.21) for Eq. (4) and (α/ 2,β) = (−1.21 × 10^-2,8.00) for Eq. (9).