Thin Film Allows Metasurface Lens to Change Focus

A research team headed by Christopher Dirdal from SINTEF Smart Sensors and Microsystems in Norway has created a metasurface lens that uses a piezoelectric thin film to change focal length when a small voltage is applied. Because it is extremely compact and lightweight, the lens could be useful for portable medical diagnostic instruments, drone-based 3D mapping, and other applications bolstered by the miniaturization of components.

“This type of low-power, ultracompact varifocal lens could be used in a wide range of sensor and imaging technologies where system size, weight, and cost are important. In addition, introducing precision tunability to metasurfaces opens up completely new ways to manipulate light,” Dirdal said.

To change focal length, a voltage is applied over lead zirconate titanate (PZT) membranes causing them to deform. This, in turn, shifts the distance between two metasurface lenses.

“Our novel approach offers a large displacement between the metasurface lenses at high speed and using low voltages. Compared to state-of-the-art devices, we demonstrated twice the out-of-plane displacement at a quarter of the voltage,” Dirdal said.

Metasurfaces can integrate several functionalities into a single surface, and they can also be made in large batches using standard micro- and nanofabrication techniques at potentially low cost.

“Most systems incorporating metasurfaces so far are static, meaning that the optical functionality is locked after fabrication,” Dirdal said. “However, many of the optical components that are critical for cameras, 3D-mapping lidar systems, and holographic displays require adaptivity.”

Tunability, or adaptivity, in conventional optical systems is typically accomplished using bulky and power-consuming components such as stepper motors, rotators, or magnets. To create this capability on a smaller scale, the researchers turned to MEMS technology. These chip-based electrically controlled mechanically moveable parts are fast, require little power, and are compatible with high-volume batch processing techniques — which can reduce the size, cost, and weight of optical systems.

A research team has developed a metasurface lens that uses a piezoelectric thin film to change focal length when a small voltage is applied. The work uses an architecture that features a suspended metasurface on a square silicon chip in a thin-film PZT MEMS-actuated ring (yellow). Courtesy of SINTEF Smart Sensors and Microsystems/Christopher Dirdal.

The researchers designed a device in which a metasurface is suspended on a membrane ring made of a thin-film PZT film, which allows the PZT to move the metasurface when a voltage is applied. To demonstrate how MEMS-metasurfaces functioned as a varifocal lens doublet, they placed a second metasurface lens after the MEMS-metasurface. Varying the separation distance between the lenses through MEMS displacement allowed the team to tune the focal point of the lens doublet on the fly.

The researchers showed that applying 23 V allowed the PZT membrane to move the metasurfaces 7.2 μm, producing a focal length change of around 250 μm. This range can be further improved with metalenses that have a stronger focusing power and by optimizing the MEMS design for longer stroke actuation, the group noted. Such tunability would be useful for imaging at various depths into tissue to image neurons or blood vessels.

With additional development of the MEMS architecture, the researchers said it will likely be possible to make a tunable lens device that they consider commercially relevant. They are also exploring how the MEMS-metasurface combination can be applied to various applications that have a need for small, light, and cheap optics without compromising optical quality.

The research was published in Optics Letters (www.doi.org/10.1364/OL.451750).