Design principles of tunable optical filters using phase change materials

When designing static filters such as edge, line, bandpass etc.. the conventional approach relies on exploiting the resonances and bandgaps (stop-bands) that occur inside a multilayer thin film structure. When designing for tunability, a different approach is needed, arising primarily from the fact that the refractive index changes that occur in a phase change material (PCM) are generally complex-valued. In this paper, we will review the principles and strategies for designing tunable optical filters.