  We present a study of the highly nonlinear optical excitation of noble gases
in tapered hollow waveguides using few-femtosecond laser pulses. The local
plasmonic field enhancement induces the generation of a nanometric plasma,
resulting in incoherent extreme-ultraviolet fluorescence from optical
transitions of neutral and ionized xenon, argon, and neon. Despite sufficient
intensity in the waveguide, high-order harmonic generation is not observed. The
fluorescent emission exhibits a strong bistability manifest as an intensity
hysteresis, giving strong indications for multistep collisional excitations.
