Spatial Focusing of Surface Polaritons Based on Cross-Phase Modulation

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.