MEMS-actuated metasurface Alvarez lens

Abstract

Miniature lenses with a tunable focus are essential components for many modern applications involving compact optical systems. While several tunable lenses have been reported with various tuning mechanisms, they often face challenges with respect to power consumption, tuning speed, fabrication cost, or production scalability. In this work, we have adapted the mechanism of an Alvarez lens - a varifocal composite lens in which lateral shifts of two optical elements with cubic phase surfaces give rise to a change in the optical power - to construct a miniature, microelectromechanical system (MEMS)-actuated metasurface Alvarez lens. Implementation based on an electrostatic MEMS generates fast and controllable actuation with low power consumption. The utilization of metasurfaces - ultrathin and subwavelength-patterned diffractive optics - as optical elements greatly reduces the device volume compared to systems using conventional freeform lenses. The entire MEMS Alvarez metalens is fully compatible with modern semiconductor fabrication technologies, granting it the potential to be mass-produced at a low unit cost. In the reported prototype operating at 1550 nm wavelength, a total uniaxial displacement of 6.3 µm was achieved in the Alvarez metalens with a direct-current (DC) voltage application up to 20 V, which modulated the focal position within a total tuning range of 68 µm, producing more than an order of magnitude change in the focal length and a 1460-diopter change in the optical power. The MEMS Alvarez metalens has a robust design that can potentially generate a much larger tuning range without substantially increasing the device volume or energy consumption, making it desirable for a wide range of imaging and display applications.

Keywords: Electrical and electronic engineering; Electronic devices; Nanophotonics and plasmonics.

Fig. 1. SEM and optical images of a MEMS-actuated Alvarez metalens.

a SEM image of a MEMS-actuated platform carrying the movable metasurface in an Alvarez metalens, with b and c being the zoomed-in SEM images portraying the electrostatic comb drive and the metasurface nanoposts, respectively. d, e Optical images of the Alvarez metasurfaces with complementary cubic phase profiles that are overlaid on top of each other in the assembled Alvarez metalens and in summation impart a quadratic phase change on the incident light

Fig. 2. Measured electrostatic actuation behavior of the MEMS platform.

a Overlaid IR image showing both the device and focal planes of a singlet metalens on the MEMS-actuated platform. b Actuated focal displacement and applied voltage over time. The general trend of the actuated displacement follows the square of the applied voltage. c Average actuated displacement as a function of applied voltage squared during both loading and unloading, showing a linear dependence. No hysteresis behavior is observed with voltage loading and unloading, verifying the reversibility of the actuation process

Fig. 3. Metasurface Alvarez lens design and simulation.

a Two 200 µm × 200 µm Alvarez metasurfaces with complementary cubic phase profiles and their summed quadratic phase when overlaid on top of each other. The phase values are normalized to 2π. b Ideal Alvarez focal tuning behavior as a function of the lateral symmetric displacement d (giving a total center-to-center offset of 2d) between the optical elements theorized with a negligible axial separation gap, an operating wavelength λ = 1550 nm and a cubic parameter A = 2.5335 × 10¹⁴ m⁻³. c Simulated focal tuning behavior when a small lateral displacement is introduced in addition to the initial center-to-center offset of 20 µm, assuming a 50 µm axial separation gap between the two metasurfaces from fabrication. The focal position is calculated from the weighted centroid of the high-intensity regions. d Simulated change in the focal location and profile when the two metasurfaces are laterally displaced at −3.5, 0, and +3.5 µm

Fig. 4. Measured focal tuning behavior of the MEMS-actuated Alvarez metalens.

IR camera screenshots at a the device plane and b the focus plane of an Alvarez metalens illuminated with a 1550 nm SLD source. The shift of focal intensity profiles along the optical axis with c decreasing and d increasing center-to-center offsets between the Alvarez metasurfaces at various actuation voltages and displacements. e, f Actuated displacement as a function of the applied voltage with the negative (positive) actuation corresponding to a decreasing (increasing) offset. Tuning in focal location g as a function of the applied voltage and h as a function of the actuated displacement with the focus size defined by various intensity thresholds, giving a focal tuning range ten times larger than the actuated Alvarez displacement. The black dash-dotted lines are provided as guides for the eye