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