Electrically Reconfigurable Optical Metasurfaces for Universal Wavefront Shaping

Citation

Thureja, Prachi (2025) Electrically Reconfigurable Optical Metasurfaces for Universal Wavefront Shaping. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/t5w7-xv05. https://resolver.caltech.edu/CaltechTHESIS:05022025-171646095

Abstract

The ability to control the properties of light in a compact, reconfigurable platform is essential for advancing nanophotonic technologies. Active metasurfaces --- flat optical components with tunable subwavelength elements --- enable real-time manipulation of wavefronts and thus offer a path toward versatile optical systems. This thesis furthers the development of electrically programmable metasurfaces as a step toward a universal platform for independent and comprehensive wavefront control. By integrating system-level optimization strategies, novel operation modes, and advanced material platforms, we establish a framework for next-generation, on-demand optical processing components.

First, we introduce an array-level inverse design approach for beam steering metasurfaces, that co-optimizes the spatial amplitude and phase responses to enhance target functionalities. Using the platform of a plasmonic, indium tin oxide (ITO)-based active metasurfaces, we demonstrate non-intuitive configurations that achieve high-directivity, continuous-angle beam steering up to 70°. Experimental validation confirms the effectiveness of this approach, which we further extend to advanced applications including flat-top beams, tunable beam widths, and multi-beam steering.

To expand the functional channel capacity of active metasurfaces, we then explore space-time modulation as a means of enabling multi-frequency operation. By modulating ITO-based metasurfaces operating at near-infrared wavelengths with tailored waveforms at frequencies up to 10 MHz, we experimentally generate desired frequency harmonics, which appear as sidebands offset from the incident laser frequency. Introducing phase offsets between the driving waveforms enables tunable diffraction of frequency-shifted light. Theoretical extensions of this work highlight the potential of space-time metasurfaces to realize active multitasking components capable of dynamically performing multiple independent functions.

For improved efficiency and broadband operation, we investigate electro-optically tunable metasurfaces based on the Pockels effect in barium titanate (BTO). We develop a scalable fabrication technique to obtain high-quality, thin-film BTO via stress-induced exfoliation from single-crystal substrates, preserving its bulk electro-optic properties. The experimentally measured Pockels coefficient r₃₃ exceeds that of commercially available thin-film lithium niobate, demonstrating the potential of this material platform for integration into high-speed, low-loss optical metasurfaces. Leveraging these properties, we design transmissive BTO-based metasurfaces for high efficiency beam steering at visible wavelengths.

The results presented in this thesis lay the foundation for next-generation programmable metasurfaces by addressing key challenges in materials, design methodologies, and system-level control architectures. We conclude with a discussion of future directions, including the discovery of high-performance tunable materials, the development of advanced unit cell designs for independent control over multiple optical properties, and the miniaturization of control networks for large-scale metasurfaces. Ultimately, this work advances the development of reconfigurable and intelligent optical systems capable of adapting to diverse technological demands in a broad range of imaging, communication, and computing applications.

Thesis Files