Thinnest optical waveguide: experimental test.

A thin dielectric waveguide with a subwavelength diameter can exhibit very small transmission loss only if its diameter is greater than a threshold value, while for smaller diameters, waveguide loss grows dramatically. The threshold diameter of transition between these waveguiding and nonwaveguiding regimes is primarily determined by the wavelength of propagating light and, to a much lesser degree, by the characteristic length of the waveguide's long-range nonuniformity. For this reason, the transmission spectrum of a thin waveguide allows immediate and quite accurate determination of its thickness. An experimental test of these facts is performed for a tapered microfiber. Good agreement with the recently developed theory of adiabatic microfiber tapers is demonstrated.