  We theoretically study the spatial focusing of surface polaritons (SPs) in a
negative index metamaterial (NIMM)-atomic gas interface waveguide system, based
on cross phase modulation (XPM) in a tripod type double electromagnetically
induced transparency (EIT) scheme. In the linear region, we realize the low
loss stable propagation of SPs, and the group velocities of the probe and
signal fields are well matched via double EIT. In the nonlinear region, we show
that giant enhancement of the XPM can be obtained. Using a narrow optical
soliton in free space, we realize spatial focusing of the SPs solitons,
including bright, multi bright, and dark solitons. The full width at the
half-maximum (FWHM) of the SPs soliton can be compressed to about ten
nanometers, thus, even nanofucsing can be obtained. The results obtained here
have certain theoretical significance for micro-nano optics, and also have
application potentials in nano-scale sensing, spectral enhancement and
precision measurement.
