Metasurface-based focus-tunable mirror.

Abstract

Varifocal mirrors, which have various applications in optical coherent tomography and three-dimensional displays, are traditionally based on the fluid pressure or mechanical pusher to deform the mirror. The limitations of conventional varifocal mirrors are obvious, such as the heavy size of the device and constraints of tunability, due to their mechanical pressure control elements. The reprogrammable metasurface, a new flat photonic device with multifunction in an ultrathin dimension, paves the way towards an ultrathin and lightweight mirror with precise phase profile. Here, an active reconfigurable metasurface is proposed to achieve the manipulation of the wavefront. The meta-atom in the metasurface is integrated with one varactor diode to manipulate the electromagnetic response. As the bias voltage increases from 0 to 20 V, the resonant frequency shifts from 5.5 to 6.0 GHz, which generates a broad tunable phase region, leading to 5 diopters (about 50%) change without any mechanical element and a broad tunable frequency band. In addition, the focus point can not only be steered in the axial line above the metasurface but also in the whole working plane. The proposed focus-tunable metasurface mirror may be a key in enabling future ultrathin reconfigurable optical devices with applications such as multiphoton microscopy, high speed imaging and confocal microscopy.