Epsilon-Near-Zero Mode Excitation in TCO Thin Film Coated Optical Fibers

<br/>Epsilon near zero (ENZ) materials have been extensively studied recently due to their unique and extreme optical properties [1]. Ultrathin transparent conducting oxide (TCO) ENZ—the real part of permittivity approaches zero—layer can support plasmon polariton mode with enhanced and highly confined optical field [2]. Furthermore, the ENZ properties of TCO can also be tuned dynamically by field effect dynamic [3-6]. However, excitation of ENZ resonances in optical waveguides, more specifically optical fibers, has been under investigated.<br/><br/>In this work, we demonstrate the excitation of ENZ mode in optical fiber via direct phase matching between the fundamental mode of the fiber and the ENZ mode supported by the AZO thin film. With the TCO coating near the core of a D-shaped single-mode fiber, we have shown up to 10 dB of transmission resonance in the ENZ wavelength regime. Using a supercontinuum laser (500-1750nm) we couple directly to a single mode fiber that has partially polished cladding in the center, allowing for proximity to the core. By depositing aluminum zinc oxide (AZO) film on the polished surface, this can allow for coupling to the ENZ modes of the AZO while the light is propagating through the fiber. Varying thickness of AZO coating, which changes the central ENZ wavelength, further change the coupling of the ENZ mode in the fiber by shifting the resonance peak. In comparison with uncoated fibers, there is a clear excitation of hybrid ENZ mode. By launching ultrafast high power pulse laser into the ENZ optical fiber, nonlinear ENZ mode coupling and generation of harmonic light are expected to be observed, which will be presented at this conference.<br/><br/>This work was supported by the Air Force Office of Scientific Research (Award number: FA9550-21-1-02204).<br/><b>References</b><br/>1. Liberal, I.; Engheta, N. <i>Nature Photonics</i>, 11(3), 149 (2017).<br/>2. Campione, Salvatore, Igal Brener, and Francois Marquier. <i>Physical Review B</i> 91.12, 121408 (2015).<br/>3. Alam, M. Z.; Schulz, S. A.; Upham, J.; De Leon, I.; Boyd, R. W. <i>Nature Photonics</i>, 12, (2), 79 (2018).<br/>4. Kinsey, N., DeVault, C., Kim, J., Ferrera, M., Shalaev, V. M., & Boltasseva, A. <i>Optica</i>, 2, (7), 616-622 (2015).<br/>5. Huang, Y.-W.; Lee, H. W. H.; Sokhoyan, R.; Pala, R. A.; Thyagarajan, K.; Han, S.; Tsai, D. P.; Atwater, H. A. <i>Nano Letter</i>, 16, (9), 5319-5325 (2016).<br/>6. Anopchenko, A.; Tao, L.; Arndt, C.; Lee, H. W. H. <i>ACS Photonics</i>, 5, (7), 2631-2637 (2018).