Tip-enhanced optical effects have enabled a wide palette of nanoscale investigation techniques and recently they were successfully used to modify and manipulate matter with nanoscale spatial resolution. We contribute to this latter area by reporting a viable method to modify the refractive index of polymethyl methacrylate (PMMA). Local surface changes of the refractive index are accomplished by focusing a femtosecond pulsed near-infrared laser beam on the apex of a metalized nanosized tip, traditionally used in scanning probe microscopy (SPM). The refractive index is changed over a circular area with diameter <200 nm under 790 nm illumination, representing a four-fold increase in resolution compared to the current state-of-the-art. These results are obtained at low laser pulse energies (<0.5 nJ), which represents a two orders of magnitude advantage over previously reported methods. Besides enabling modifications of the refractive index with high lateral resolution, this method can pave the way towards other important applications such the fabrication of photonic crystal lattices and surface waveguides.