docs: def eps_func(self):
          '''
          function: a function that when passed a `x` and `y` values,
              returns the permittivity profile of the structure,
              interpolating if necessary.
          '''
          interp_real = interpolate.interp2d(self.x, self.y, self.eps.real)
          interp_imag = interpolate.interp2d(self.x, self.y, self.eps.imag)
          interp = lambda x, y: interp_real(x, y) + 1.j*interp_imag(x, y)
          return interp