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Low-order harmonic generation is usually considered to be a nonlinear process that can be fully explained within perturbation theory.The first nonperturbative analysis of the below-threshold harmonics (BTHs) within the strong field approximation based quautum-orbit theory by neglecting the Coulomb field effect of the atomic core [1] has shown that the major contribution to BTHs comes from a specific trajectory which does not belong to the usual "short-long" classification scheme and corresponds to the electron moving straight back to the atomic core from the tunneling exit,providing significantly larger harmonic generation probability than the usual trajectories.