Millimeter-scale focal length tuning with MEMS-integrated meta-optics employing high-throughput fabrication

Abstract

Miniature varifocal lenses are crucial for many applications requiring compact optical systems. Here, utilizing electro-mechanically actuated 0.5-mm aperture infrared Alvarez meta-optics, we demonstrate 3.1 mm (200 diopters) focal length tuning with an actuation voltage below 40 V. This constitutes the largest focal length tuning in any low-power electro-mechanically actuated meta-optic, enabled by the high energy density in comb-drive actuators producing large displacements at relatively low voltage. The demonstrated device is produced by a novel nanofabrication process that accommodates meta-optics with a larger aperture and has improved alignment between meta-optics via flip-chip bonding. The whole fabrication process is CMOS compatible and amenable to high-throughput manufacturing.

Figure 1

Design of the MEMS-integrated Alvarez meta-optic. (a) The designed cubic phase profiles for the two complementary Alvarez meta-optics and the total quadratic phase profile when they overlay. Colorbar indicates a 2π phase span in radian. (b) Schematic of the scatterer made of a cylindrical silicon nitride nanopost on a silicon substrate. (c) The simulated transmission coefficients for the nanoposts as a function of the duty cycle. (d) Schematic of Alvarez meta-optics integrated with an electrostatic MEMS actuator.

Figure 2

Device fabrication. (a) Summarized fabrication process flow. (b) Scanning electron microscopy (SEM) image of an Alvarez meta-optic integrated with a MEMS tuning platform. (c) Close-up view of the silicon nitride nanoposts sitting on the central silicon platform. (d) Comb-drive details showing the part of the mobile flexure backbone and interdigitated finger array. (e) Fabricated chip carrying mobile Alvarez meta-optics on electrostatic actuators. (f) Final assembled stack with another chip carrying static Alvarez meta-optics aligned and bonded on top.

Figure 3

Experimental characterization of the Alvarez lens. (a) Experimental setup for electrostatic actuation and focal profile acquisition. (b) Exemplary device and focal planes of the Alvarez meta-optic lens actuated at 40 V towards the right. (c) Measured actuated displacement and actuating voltage of the MEMS platform follow the same trend closely, showing negligible hysteresis. (d) Actuated displacement for both directions follows the comb-drive quadratic characteristics closely. The light shade bands indicate the associated error bars.

Figure 4

Focal tuning measurements of the MEMS Alvarez meta-optical lens. (a) Normalized focal profiles along the optical axis (z-axis) capturing the intensities at 5 µm increments across a 0.8 mm span around the main intensity lobes. Square plots on the right show the intensity distributions in the found focal planes. Inset: normalized intensities across the focal spots at various actuation voltages. (b) Actuated displacement and the corresponding focal length modulated by actuation voltage. Dashed lines show the fitting to the theoretical behaviors. (c) Tunable focal length as a function of actuated displacement closely follows the theoretical reciprocal Alvarez tuning behavior.