Voltage tunable mid-wave infrared reflective varifocal metalens via an optomechanic cavity

Metalenses enable the multifunctional control of light beams with an optically thin layer of nanoantennas. Efficient on-chip voltage tuning of the focal length is the crucial step toward the integration of metalenses into dynamically tunable optical systems. We propose and numerically investigate the on-chip electrical tuning of a reflective metalens via an optomechanic cavity. Light is focused by an array of silicon nanopillar antennas separated from a deformable metallic reflector by a small air gap. A transparent electrode is inserted into the optomechanic cavity to electrostatically deform the reflector and rearrange the reflection phase profile, resulting in a shift in the focal point. Two modes of voltage tuning via the relative curvature change of the reflector are analyzed. In mode 1, the size of the air gap is modified through the nearly parallel shift of the reflector, whereas in mode 2, the distribution of the air-gap size is tailored by the curvature change of the reflector. With the designed working wavelength of 3.8 µm and the initial focal length of 80.35 µm, the focal length is shifted by 20.3 µm in mode 1 and 7.25 µm in mode 2. Such a device can be used as a free space coupler between quantum cascade lasers and mid-infrared fibers with variable coupling efficiency.