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We study spontaneous emission from a rubidium atom into the fundamental andhigher-order modes of a vacuum-clad ultrathin optical fiber. We show that thespontaneous emission rate depends on the magnetic sublevel, the type of modes,the orientation of the quantization axis, and the fiber radius. We find thatthe rate of spontaneous emission into the TE modes is always symmetric withrespect to the propagation directions. Directional asymmetry of spontaneousemission into other modes may appear when the quantization axis does not lie inthe meridional plane containing the position of the atom. When the fiber radiusis in the range from 330 nm to 450 nm, the spontaneous emission into theHE$_{21}$ modes is stronger than into the HE$_{11}$, TE$_{01}$, and TM$_{01}$modes. At the cutoff for higher-order modes, the rates of spontaneous emissioninto guided and radiation modes undergo steep variations, which are caused bythe changes in the mode structure. We show that the spontaneous emission fromthe upper level of the cyclic transition into the TM modes is unidirectionalwhen the quantization axis lies at an appropriate azimuthal angle in the fibertransverse plane.

Channeling of spontaneous emission from an atom into the fundamental and higher-order modes of a vacuum-clad ultrathin optical fiber