The Plane Wave Admittance Method is essentially the same as what is called:
- Rigorous Coupled-Wave Analysis
- Fourier Modal Method
- Transfer Matrix Method with a Plane Wave Basis
I know there are codes available online for these.
The second method I do not have any experience with.
The third code, and PWEM in general, search for MIT Photonic Bands. It is free and is perhaps the most powerful photonic crystal modeling code available.