Tunable focusing is a desired property in a wide range of optical
imaging and sensing technologies but has tended to require bulky
components that cannot be integrated on-chip and have slow actuation
speeds. Recently, integration of metasurfaces into electrostatic
micro-electromechanical system (MEMS) architectures has shown
potential to overcome these challenges but has offered limited
out-of-plane displacement range while requiring large voltages. We
demonstrate for the first time, to the best of our knowledge, a
movable metasurface lens actuated by integrated thin-film PZT MEMS,
which has the advantage of offering large displacements at low
voltages. An out-of-plane displacement of a metasurface in the range
of 7.2
μ
m is demonstrated under a voltage
application of 23 V. This is roughly twice the displacement at a
quarter of the voltage of state of the art electrostatic out-of-plane
actuation of metasurfaces. Using this tunability, we demonstrate a
varifocal lens doublet with a focal shift of the order of 250
μ
m at the wavelength 1.55 μm. The
thin-film PZT MEMS-metasurface is a promising platform for
miniaturized varifocal components.
As metasurfaces begin to find industrial applications there is a need to develop scalable and cost-effective fabrication techniques which offer sub-100nm resolution while providing high throughput and large area patterning. Here we demonstrate the use of UV-Nanoimprint Lithography and Deep Reactive Ion Etching (Bosch and Cryogenic) towards this goal. Robust processes are described for the fabrication of silicon rectangular pillars of high pattern fidelity. To demonstrate the quality of the structures, metasurface lenses, which demonstrate diffraction limited focusing and close to theoretical efficiency for NIR wavelengths λ ∈ (1.3 μm, 1.6 μm), are fabricated. We demonstrate a process which removes the characteristic sidewall surface roughness of the Bosch process, allowing for smooth 90-degree vertical sidewalls. We also demonstrate that the optical performance of the metasurface lenses is not affected adversely in the case of Bosch sidewall surface roughness with 45 nm indentations (or scallops). Next steps of development are defined for achieving full wafer coverage.
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