  Investigation techniques based on tip-enhanced optical effects, capable to
yield spatial resolutions down to nanometers level, have enabled a wide palette
of important discoveries over the past twenty years. Recently, their underlying
optical setups are beginning to emerge as useful tools to modify and manipulate
matter with nanoscale spatial resolution. We try to contribute to these efforts
by reporting a method that we found viable to modify the surface refractive
index of polymethyl methacrylate (PMMA), an acrylic polymer material. The
changes in the refractive index are accomplished by focusing a femtosecond
pulsed near-infrared laser beam on the apex of a metalized nano-sized tip,
traditionally used in scanning probe microscopy (SPM) applications. The adopted
illumination strategy yields circular-shaped modifications of the refractive
index occurring at the surface of the PMMA sample, exhibiting a lateral size
<200 nm, under 790 nm illumination, representing a four-fold increase in
precision compared to the current state-of-the-art. The light intensity
enhancement effects taking place at the tip apex makes possible achieving
refractive index changes at low laser pulse energies (<0.5 nJ), which
represents two orders of magnitude advantage over the current state-of-the art.
The presented nanoimprinting method is very flexible, as it can be used with
different power levels and can potentially be operated with other materials.
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 or surface waveguides.
