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Abstract: Detailed analysis of the commensurability oscillation (CO) has been performed on unidirectional lateral superlattices with periods ranging from $a$=92 to 184 nm. Fourier analysis reveals the second (and the third) harmonics along with the fundamental oscillation for $a$$\geq$138 nm (184 nm) at low-enough temperature, evincing the presence of corresponding harmonics in the profile of the potential modulation. The harmonics manifest themselves in CO with demagnified amplitude due to the low-pass filtering action of the thermal damping factor; with a suitable consideration of the damping effect, the harmonics of the modulation potential are found to have the amplitudes $V_2$ and $V_3$ up to roughly 30\% of that of the fundamental component $V_1$, despite the small ratio of the period $a$ to the depth $d$$\simeq$99 nm of the two-dimensional electron gas (2DEG) from the surface. The dependence of $V_n$ on $a$ indicates that the fundamental component originates at the surface, while the higher harmonics arise from the effect of the strain that penetrates down into subsurface. The manipulation of high harmonics thus provides a useful technique to introduce small length-scale modulation into high-mobility 2DEGs located deep inside the wafer.