Double grating waveguide structures: 350-fold enhancement of two-photon fluorescence applying ultrashort pulses

The enhancement of ultrashort pulse excited two-photon fluorescence (TPF) based on resonant double grating waveguide structures (DGWS) was studied systematically. For a specific wavelength, polarisation and angular orientation, the transmission of the incident light through these devices vanishes while most of the light is reflected by the structure. Since these resonances are based on a guided mode phenomenon they exhibit a large evanescent field inducing enormous TPF signals of, for instance, marker or biomolecules at the waveguide surface. TPF signals for various concentrations of Rhodamine B under and out of resonance conditions were recorded. We demonstrate a resonance enhancement of the monitored TPF of more than two orders of magnitude which can be achieved by DGWS specifically designed for ultrashort pulses without beam focussing or laser amplification. Furthermore, the observed enhancement increases with decreasing dye molecule concentration at the surface, so these structures can be considered as powerful platforms for TPF detection of even very small amounts of fluorescent molecules in biotechnology and spectroscopic applications.