Two photon fluorescence spectroscopy based on resonant grating waveguide structures

Abstract

We demonstrate an ultra sensitive method for Two Photon Fluorescence (TPF) excitation using resonant Grating Waveguide Structures (GWS). In its basic configuration, a GWS consists of a substrate, a waveguide layer and an additional grating layer. When illuminated with laser light under resonant conditions, the GWS reflects all light and leads to very high local surface intensities. This field enhancement can be exploited for TPF spectroscopy, without the need for a highly intense, focused laser light. We present the enhanced TPF signal obtained from a 23 nM drop of tetramethylrhodamine (TMR) on the top of high-finesse resonant polymeric GWS. The resonant behaviour of the GWS was tested for normal incidence with TE polarization illumination. As expected, the transmission spectral profile has a dip at resonant wavelength. The TPF spectra of TMR molecules were observed for different excitation wavelengths. Close to resonance, TPF intensity increases and the maximum signal is obtained when the excitation wavelength coincides with the resonance wavelength of the GWS. These results clearly indicate that the huge field localization at grating surface is responsible for the TPF excitation. We obtained a detection limit down to picomolar concentration of the dye molecules, offering the possibility of a highly sensitive, compact and non-destructive tool for widespread biochemical applications.