Continuously tunable Alvarez metalenses for varifocal imaging

Compact and lightweight lenses are essential for miniaturized varifocal imaging. We designed and fabricated an Alvarez metalens composed of two cascaded, phase-complementary metalenses. Continuous focal-length tuning is achieved by the relative in-plane lateral displacement of the two metalenses, which continuously modulates the synthesized quadratic focusing phase. To suppress aberrations, Zernike basis functions were incorporated into the phase profile of the Alvarez metalens. The focusing and imaging performance of the device was then experimentally validated at the design wavelength. With a 1 mm in-plane displacement, the Alvarez metalenses (4 mm aperture, 0.62 g weight, λ=532 nm) enabled focal length modulation from 10 mm to 150 mm, all while achieving an average diffraction efficiency of approximately 40%. The integration of an in-plane-actuation-only varifocal mechanism with ultrathin metalenses enables a highly scalable approach to miniaturizing systems for imaging, optical communications, and biomedical applications.