  We show, via simulations, that an optical fiber taper waveguide can be an
efficient tool for photoluminescence and resonant, extinction spectroscopy of
single emitters, such as molecules or colloidal quantum dots, deposited on the
surface of a thin dielectric membrane. Placed over a high refractive index
membrane, a tapered fiber waveguide induces the formation of hybrid mode waves,
akin to dielectric slotted waveguide modes, that provide strong field
confinement in the low index gap region. The availability of such gap-confined
waves yields potentially high spontaneous emission enhancement factors
($\approx20$), fluorescence collection efficiencies ($\approx 23%$), and
transmission extinction ($\approx 20%$) levels. A factor of two improvement in
fluorescence and extinction levels is predicted if the membrane is instead
replaced with a suspended channel waveguide. Two configurations, for operation
in the visible ($\approx 600 \nm$) and near-infrared ($\approx 1300 \nm$)
spectral ranges are evaluated, presenting similar performances.
