Pixelated Control of Tunable VO₂ Metasurfaces

Next-generation integrated photonics will include on-chip scale devices with unique capabilities to sense, process, and transmit information and energy. These devices offer significant SWaP-C benefits compared to traditional integrated circuits or conventional optical components [1]. A core requirement of many of these devices is active manipulation of light. As such, active, tunable, dynamic, or reconfigurable optical devices at small scales (metasurfaces) will be critical to achieve this vision. Reconfigurable metasurfaces are devices with on-demand active optical functionalities, unlike passive metasurfaces that lack post-fabrication tunability. The manipulation of light in these devices is within a thin quasi-two-dimensional layer rather than via gradual accumulation across a volume [2]. Such dynamic integrated photonic circuits will be applicable in telecommunications, quantum computing, biomedical sensing/imaging, energy, defense, environmental monitoring, and more [3].

Dynamic tuning in active metasurfaces can be achieved by utilizing phase change materials and external stimuli such as optical, electrical, thermal, or chemical inputs [4]. A continuously tunable metasurface based on dielectric resonances in vanadium dioxide (VO₂) nanoantennas, yielding independent active amplitude and phase control, has been demonstrated in previous work [5]. Our current work focuses on the design and experimental demonstration of pixelated control in reconfigurable and dynamically integrated VO₂-based photonic devices. The VO₂-based metasurface design with reprogrammable functionalities involves precisely engineered nanoantenna arrays coupled with microheater arrays of transparent conductive oxide. Through the electrical bias of the microheaters, localized heating of nanoantenna pixels is achieved (11 nanoantennas per pixel), resulting in a change in the optical response as light passes through each pixel.

The array of VO₂ nanoantennas is patterned using reactive ion etching on a fused quartz substrate. A spacer layer of amorphous SiO₂ that is 2.3 μm thick is deposited on top of the VO₂ nanoantennas. A transparent conductive oxide (indium tin oxide or cadmium-doped zinc oxide) is next deposited and patterned into electrode strips on the spacer material. By modulating the applied voltage, resulting in joule heating, individual pixels of VO₂ nanoantennas can be tuned across their insulator-to-metal transition, providing a continuously tunable amplitude modulation or discrete switching between OFF (transparent) and ON (opaque) states. Simulations show pixelated control of the VO₂-based optical surface under optimized bias, with a pixel size of 10 μm and optical modulation depth of 17.4 dB. Further design and fabrication optimization is underway toward implementation in a prototype chip-scale spatial light modulator. We have improved our VO₂ thin film fabrication process, and temperature-dependent transmittance data confirms the high-quality VO₂ film (RF sputtered with in-situ heating at 600-650^oC). Further fabrication steps are underway. This prototype spatial light modulator will demonstrate the ability to provide fine-tuned, dynamic amplitude modulation, with spatial resolution similar to existing large-scale spatial light modulators.

References
[1] M. A. Butt, X. Mateos, and R. Piramidowicz, Physics Letters A, vol. 516, p. 129633, 2024.
[2] N. Yu and F. Capasso, Nature Mater., vol. 13, no. 2, pp. 139-150, 2014.
[3] N. L. Kazanskiy, S. N. Khonina, and M. A. Butt, Photonics, vol. 9, no. 5, p. 331, 2022.
[4] Q. He, S. Sun, and L. Zhou, Research, vol. 2019, 2019.
[5] I. O. Oguntoye, S. Padmanabha, M. Hinkle, T. Koutsougeras, A. J. Ollanik, and M. D. Escarra, ACS Applied Materials & Interfaces, vol. 15, no. 34, pp. 41141–41150, 2023.