Excitation of Epsilon-Near-Zero Mode in Optical Fiber

Experimental excitation of a highly confined epsilon-near-zero (ENZ) mode in a side-polished optical fiber coated with a deep subwavelength thick layer of aluminum-doped zinc oxide (AZO) is reported. The uniform AZO layer on the fiber is fabricated by atomic layer deposition technique and optimized to exhibit close-to-zero permittivity at the near-infrared wavelength. Highly polarization- and wavelength-dependent transmission with strong resonance strength up to 25 dB is observed in a 30-nm ENZ-coated fiber that is 17 mm long. Different from the excitation of the ENZ mode in a planar conducting oxide thin film, the hybrid ENZ mode can be excited via direct phase matching between the fundamental mode of the fiber and the ENZ mode supported by the AZO thin film. The hybrid ENZ mode in the fiber exhibits a relatively long propagation/light-matter interaction length which is a few orders of magnitude longer than those on the planar ENZ substrates. It is further shown that the hybrid resonance in the ENZ fiber can be actively tuned through the refractive index of surrounding medium and the large ENZ's nonlinearity. These ENZ-optical fibers serve as emerging in-fiber optical devices, such as advanced in-fiber ultrafast optical switches/modulators, mode-locked fiber lasers, and in-fiber optical gas/biomolecule sensors.