Tunable Metasurface on Multimode Optical Fiber Enabled by Wavefront Shaping

The study and design of optical metasurfaces has greatly matured over the last decade, enabling wavefront engineering with remarkable degrees of freedom via control over the intensity, phase, and polarization at a sub-wavelength scale. By integrating these nanostructures directly onto the optical fiber endface, the naturally diverging fiber output (determined by the refractive index of the core and cladding) can be modulated in a number of useful ways, including focusing¹, filtration², off-axis deflection³, and complex beam shaping (e.g. vortex beam generation⁴). However, in these nanostructured fibers, the device is limited to a single functionality and the designed output properties are fixed in during fabrication, limiting the practical photonic applications.<br/>Multimode fibers are well-poised to circumvent this issue, with the recent development of techniques to shape the output wavefront via careful control of the mode dispersion and mixing during propagation. However, the range of possible outputs is still restricted to linear combinations of the individual mode field patterns, confined to the fiber numerical aperture (NA)^5,6. In this work, we exhibit the first integration of a spatially-varied metasurface on the multimode fiber endface, which, combined with structuring of the fiber output beam via control over the input, demonstrates tunable functionality in the form of beam steering to a continuous range of angles far beyond the fiber NA. Our initial demonstrations utilize arrays of many dielectric nanopillar phase gradient metasurfaces fabricated by nanoscale 3D printing or electron beam lithography, each with their own unique combination of gradient magnitude and direction, in order to deflect light from 0 to 70° off-axis depending on the targeted location of light concentration within the fiber output. This extreme and active optical functionality is desirable for a wide range of miniaturized fiber imaging applications, where probe size can be reduced to that of a single fiber itself (~100μm).<br/><br/>References:<br/>¹Yang, J., Ghimire, I., Wu, P. C., Gurung, S., Arndt, C., Tsai, D. P. and Lee, H. W. H. "Photonic crystal fiber metalens" Nanophotonics, vol. 8, no. 3, 2019, pp. 443-449. https://doi.org/10.1515/nanoph-2018-0204<br/>² Ghimire, I., Yang, J., Gurung, S., Mishra, S. K. and Lee, H. W. H. "Polarization-dependent photonic crystal fiber optical filters enabled by asymmetric metasurfaces" Nanophotonics, vol. 11, no. 11, 2022, pp. 2711-2717. https://doi.org/10.1515/nanoph-2022-0001<br/>³ Principe, M., Consales, M., Micco, A. et al. “Optical fiber meta-tips” Light Sci Appl 6, e16226 (2017). https://doi.org/10.1038/lsa.2016.226<br/>⁴ Zhao, Y., Zhang, J., Du, J., and Wang J. “Meta-facet fiber for twisting ultra-broadband light with high phase purity” Appl. Phys. Lett. 6 August 2018; 113 (6): 061103. https://doi.org/10.1063/1.5043268<br/>⁵ Collard, L., Pisano, F., Pisanello, M., Balena, A., De Vittorio, M., Pisanello, F. “Wavefront engineering for controlled structuring of far-field intensity and phase patterns from multimodal optical fibers” APL Photonics 1 May 2021; 6 (5): 051301. https://doi.org/10.1063/5.0044666<br/>⁶ Plöschner, M., Tyc, T. and Čizmár, T. “Seeing through chaos in multimode fibres” Nature Photon 9, 529–535 (2015). https://doi.org/10.1038/nphoton.2015.112