Ablative Rayleigh-Taylor growth was measured with single-mode modulated planar CH foil with different ratio of Br dopant at Shenguang Ⅱ laser facility. Results show that CH (6% Br) sample first enters the nonlinear regime and has the largest perturbation amplitude of second harmonic. The reason is that the density gradient effects can suppress the generation of the second harmonic, the more the Br is doped, the smaller the density gradient scale length can be achieved. The density gradient effects also suppress the feedback of third-order harmonic to the fundamental mode, which induces the nonlinear saturation amplitude to exceed 0.1λ, as the classical prediction shows.