  Surface enhanced Raman scattering (SERS) in nanoscale hotspots has been
placed great hopes upon for identification of minimum chemical traces and
in-situ investigation of single molecule structures and dynamics. However,
previous work consists of either irreproducible enhancement factors (EF) from
random aggregates, or moderate EFs despite better reproducibility.
Consequently, systematic study of SERS at the single and few molecules level is
still very limited, and the promised applications are far from being realized.
Here we report EFs as high as the most intense hotspots in previous work yet
achieved in a reproducible and well controlled manner, that is, electromagnetic
EFs (EMEF) of 10^9~10 with an error down to 10^+/-0.08 from gold nanospheres on
atomically flat gold planes under radially polarized (RP) laser excitation. In
addition, our experiment reveals the EF's unexpected nonlinearity under as low
as hundreds of nanowatts of laser power.
