Mechanically tunable focusing metamirror in the visible (Conference Presentation)

Unlike conventional optical components, which often present physical obstructions to the miniaturization of optoelectronic devices, the control of light using flat optics has attracted much recent attention due to unique technological opportunities presented by these devices. Optical metasurfaces, which are composed of rationally designed nanostructures, are proposed to replace some of the conventional optical elements given their compact size and more importantly, the ability to produce spatially varying phase change, amplitude modulation and polarization conversion of incident light over subwavelength dimensions. For example, a compact, flat lens with dynamically tunable focal length will be an essential component in advanced reconfigurable optical systems. Although there have been some successful demonstrations of active metalenses recently, they all work in the transmission configuration. Here, we design and realize the first reflection type, tunable lens (i.e., metamirror) operating in the visible regime (670 nm). With a designed hyperboloidal phase profile, the metamirror is fabricated on a substrate driven by external force, so its focal length can be adjusted dynamically. It is shown that the focal length can be continuously adjusted by up to 45% with a 0 to 20% lateral stretching of the substrate, while maintaining diffraction-limited focusing and high focusing efficiency. Our design as a flat optics element has strong potential in widespread applications such as wearable mixed reality electronics, biomedical instruments and integrated optics devices.