Advantages and artifacts of higher order modes in nanoparticle-enhanced backscattering Raman imaging

In order to facilitate nanoparticle-enhanced Raman imaging of complicated biological specimens, we have examined the use of higher order modes with radial and azimuthal polarizations focused onto a Au nanoparticle atomic force microscope (AFM) tip utilizing a backscattering reflection configuration. When comparing the Raman intensity profiles with the observed sample topography, the radial-polarized configuration demonstrates enhanced spatial resolution. This enhanced resolution results from the direction of the induced electron oscillation in the metal nanoparticle oriented by the electromagnetic field at the laser focus. The electric field component along the direction of laser propagation, attendant to the radial polarization, creates an enhanced field along the z-axis and normal to the sample. Substantial enhancement is observed utilizing an intermediate numerical aperture objective (NA = 0.7), necessary for backscattering measurements. The azimuthal polarization, similar to linear polarization, results in an enhanced field predominantly parallel to the sample, resulting in imaging artifacts. The Raman intensity profiles observed as the exciting laser polarization is switched between either a radially polarized or an azimuthally polarized state illustrate these imaging artifacts. Because azimuthal polarization arises readily from changes in the incident polarization onto the mode converter, the results presented here aid in identifying such artifacts when analyzing nanoparticle-enhanced Raman spectroscopic images. Due to the power law decay of the enhanced field, an enhancement orientation normal to the sample enables contrast between structures smaller than the tip dimensions as the apex of the nanoparticle tip, where the enhancement is strongest, passes over the sample. These effects are demonstrated using both carbon nanotube and fixed biological samples.