*3.5.4 RCWA approach*

The RCWA uses intermittent grating systems to diffract electromagnetic waves. The periodic permittivity and electromagnetic field are expanded as Fourier series, and material boundaries are improved with boundary conditions [43], resulting in poorly produced abrupt surfaces with a large number of Fourier harmonics. This method is highly reliable for computing far-field reflection, propagation coefficients, and diffraction orders, while calculating the near field at material boundaries remains challenging [44].
