For analysing the 3D periodic structures at the oblique incident, a weakly conditionally stable finite-difference time-domain method is proposed by applying the Crank-Nicolson (CN) scheme to the field transformation technique. By splitting the space operator matrix in a special way, the transformed Maxwell's equations can be subdivided into two sub-steps for an efficient implementation. The proposed method does not need to introduce additional field components to handle the extra time terms and the time step size is only determined by one space discretisation.

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