研究了超强激光和均匀及线性梯度高密度等离子体相互作用产生的三次谐波，特别研究了等离子体密度为四倍临界密度时的情形，这可能是用相对论效应产生高次谐波的最佳密度。

Third Harmonic generation by interaction of intense laser with overdense plasma is investigated for uniform plasma and linear density profile. The nonlinear resonance effect occurs at a density of four times critical density for normal incidence.Comparisons of the harmonic generation by normal and obliquely incident lasers are given.