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We report on the observation of a nonlinear intensity dependence of theterahertz radiation induced ratchet effects in bilayer graphene with asymmetricdual grating gate lateral lattices. These nonlinear ratchet currents arestudied in structures of two designs with dual grating gate fabricated on topof encapsulated bilayer graphene and beneath it. The strength and sign of thephotocurrent can be controllably varied by changing the bias voltages appliedto individual dual grating subgates and the back gate. The current consists ofcontributions insensitive to the radiation's polarization state, defined by theorientation of the radiation electric field vector with respect to the dualgrating gate metal stripes, and the circular ratchet sensitive to the radiationhelicity. We show that intense terahertz radiation results in a nonlinearintensity dependence caused by electron gas heating. At room temperature theratchet current saturates at high intensities of the order of hundreds toseveral hundreds of kWcm$^{-2}$. At $T = 4 {\rm K}$, the nonlinearity manifestsitself at intensities that are one or two orders of magnitude lower, moreover,the photoresponse exhibits a complex dependence on the intensity, including asaturation and even a change of sign with increasing intensity. This complexityis attributed to the interplay of the Seebeck ratchet and the dynamic carrierdensity redistribution, which feature different intensity dependencies and anonlinear behavior of the sample's conductivity induced by electron gasheating. Our study demonstrates that graphene-based asymmetric dual gratinggate devices can be used as terahertz detectors at room temperature over a widedynamic range, spanning many orders of magnitude of terahertz radiation power.Therefore, their integration together with current-driven read-out electronicsis attractive for the operation with high-power pulsed sources.

Nonlinear intensity dependence of ratchet currents induced by terahertz laser radiation in bilayer graphene with asymmetric periodic grating gates