This was designed for the mid-IR (5.2um). I don’t recall its bandwidth but is was small on order of 100nm. The dispersion of this material in the mid-IR is relatively flat. There is active research on dispersion engineering via more complex geometry of the ‘meta-atoms’ to broaden the bandwidth for achromatic metalens optics. Other research is actively looking to design materials that could have this phase change functionality in the visible wavelength range. All stable phase change materials to my knowledge strongly absorb in the visible range.