Nanohole-Enhanced Raman Scattering

Periodic arrays of sub-wavelength apertures (nanoholes) in ultrathin Au films were used as substrates for enhanced-Raman spectroscopy in the optical range. Nanohole-enhanced (resonance) Raman scattering from oxazine 720 (oxa) adsorbed on arrays of different periodicities (distance between the center of the holes) was obtained. The overall Raman intensity of the adsorbed molecule was dependent on the periodicity of these arrays. The enhancement factor reached a maximum for the array that presented the largest transmission at the excitation wavelength of the laser. This shows that the enhancement of the Raman signal is provided by surface plasmon (SP) modes excited at the array of nanoholes. SP excitations lead to spatial localization of the electromagnetic fields in nanometric regions close to the surface. This field localization, allied to the unique vibrational signature of the Raman scattering and the simplified optical arrangement from the transmission optics, suggests that arrays of nanoholes should be useful for the fabrication of dense biochips for the detection of Raman-labeled analytes with high sensitivity and selectivity.