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Active metamaterials embedded with nonlinear elements are able to exhibit strong nonlinearity in microwave regime.However,existing S-parameter based parameter retrieval approaches developed for linear metamaterials do not apply in nonlinear cases.In this paper,a retrieval algorithm of high-order susceptibilities for nonlinear metamaterials is derived.Experimental demonstration shows that,by measuring the power level of each harmonic while sweeping the incident power,high-order susceptibilities of a thin-layer nonlinear metamaterial can be effectively retrieved.The proposed approach can be widely used in the research of active metamaterials.