Mechanism of color change of flexible metafilm with structural parameters and stretching methods

Color change metafilm is promising for the color printing of displays and imaging. A metallic array on an elastic film can realize dynamic color change by mechanical extensions, such as uniaxial or biaxial stretching. In this study, an electromagnetic model of a flexible metafilm composed of a microscale Al cylindrical array on a polydimethylsiloxane elastic film was constructed to study the underlying optical mechanism of color change, especially brightness and saturation changes, of the flexible metafilm with structural parameters (diameter and height of the Al cylinders) and stretching methods (uniaxial or biaxial stretching). The 3D finite time domain difference method was used to simulate the propagation behavior of electromagnetic waves through metafilm. With increasing diameter, the lightness increases while the saturation decreases, which is due to the change of surface plasmon resonance from local surface plasmon resonance (LSPR) to propagation surface plasmon (PSP) and Wood Anomaly (WA) and finally to magnetic polariton excitation. With increasing height, lightness first decreases and then increases, while the change in saturation is opposite, which is due to the first increasing and then decreasing of the intensities of PSP and WA. By comparing the dynamic color changes of metafilm under uniaxial stretching and biaxial stretching, it is found that uniaxial stretching achieves lower lightness and saturation under small strain and higher lightness and saturation under large strain, which is caused by the enhanced WA and PSP on the Al-air interface at short wavelengths and the weakened LSPR on the Al-PDMS interface at long wavelengths. This discovery paves the way for practical applications of structural color display with high saturation and brightness.