Low-loss photonic-like guided mode in metal-supported optical nanofibers

Subwavelength-diameter optical nanofibers have been proved to be a versatile platform in fields from nanophotonics to cold atom physics. Practical applications of these dielectric nanofibers usually demand a substrate for either functional operation or mechanical support. However, substrate-induced leakage, which always leads to high propagation loss and a cutoff diameter larger than the vacuum wavelength (lambda), makes the nanofiber difficult to be operated as a subwavelength waveguide. Here, we demonstrate a low-loss photonic-like guided mode in a metal-supported nanofiber. Owing to its smaller-than-one real part of refractive index of the substrate and the small surface plasmon polariton fraction in the photonic-like mode, this guided mode can be propagated with a subwavelength cutoff diameter and low propagation loss. Using an Au substrate, our calculations show that the propagation length is about 400 mu m at a 1.55-mu m wavelength in a nanofiber with a diameter of 0.48 lambda, which is confirmed by our experimental results (437 mu m). We also show that, for nanofibers with higher-index materials, the cutoff diameter can go down to lambda/4. Our results may open an opportunity for circuiting and manipulating optical fields in subwavelength-diameter optical nanofibers for a variety of applications. Published under license by AIP Publishing.