docs: def n(self):
          '''
          np.array: The refractive index profile matrix
          of the current slab.
          '''
          try:
              n_mat = self.slabs['0'].n
              for s in range(1, self.slab_count):
                  n_mat = np.vstack((self.slabs[str(s)].n, n_mat))
          except KeyError:
              n_mat = None
          return n_mat