We report observations of superradiance for atoms trapped in the near field
of a photonic crystal waveguide (PCW). By fabricating the PCW with a band edge
near the D1 transition of atomic cesium, strong interaction is achieved
between trapped atoms and guided-mode photons. Following short-pulse
excitation, we record the decay of guided-mode emission and find a superradiant
emission rate scaling as ΓˉSR∝Nˉ⋅Γ1D for average atom number 0.19≲Nˉ≲2.6 atoms, where
Γ1D/Γ0=1.1±0.1 is the peak single-atom radiative decay
rate into the PCW guided mode and Γ0 is the Einstein-A coefficient
for free space. These advances provide new tools for investigations of
photon-mediated atom-atom interactions in the many-body regime.Comment: 11 pages, 10 figure