Evanescent field sensors

Evanescent field sensor technology deals with the detection of biochemical processes on the surface of optical waveguides where the excitation of particular target molecules is provided by the evanescent field of the guided light. E.g., waveguide based optical biosensors enable a highly efficient and selective excitation of fluorescent molecules in close proximity to the waveguide surface with a penetration depth into the adjacent medium in the range of 100 nm. The high intensity of the evanescent field leads to an enhanced fluorescence excitation compared to direct irradiation and permits the detection of extremely small analyte quantities.

In our design studies we use FEM simulations to improve the light coupling to the waveguides and fabricate the corresponding high-frequency gratings by direct laser ablation. For example, a large spectral acceptance can preserve the pulse duration of ultrashort-pulse sources inside the waveguide. The optimized couplers allow a use of the waveguide sensors in advanced fluorescence analysis techniques like two-photon excitation or fluorescence correlation spectroscopy.

Using evanescent excitation with its close proximity of the fluorescent molecules to the inter-face of the waveguide layer, a substantial part of the fluorescence light is coupled back into and collected by the waveguide. The coupling efficiency depends on position, environment and orientation of the molecules. The utilization of this signal for fluorescence detection and analysis can allow a significant simplification of the optical instrumentation. An analysis of the measured power distribution provides a validation of a theoretical model on the dipole emis-sion near interfaces and yields information about molecule orientation and position at the waveguide surface.

Inorganic non-centrosymmetric nano-crystals are attracting increasing attention as second harmonic (SH) imaging probes in bioimaging applications and are not affected by bleaching or blinking. Parallel excitation by the evanescent field of a planar waveguide allows to generate a simultaneous SH response of such nanoparticles over a large area. Polarization analysis and defocused imaging reveal information on the orientation of the crystal axis of individual particles, which might therefore be used as optical probes of the local field. An new and fascinating detail is the formation of interference patterns generated from the nonlinear emis-sion of distinct nanoparticles. The observed patterns can be explained and simulated on the basis of a dipole radiation model.

Further information:

A. Selle, C. Kappel, M.A. Bader, G. Marowsky, K. Winkler, U. Alexiev
Picosecond-pulse-induced two-photon fluorescence enhancement in biological material by application of grating waveguide structures
Optics Letters 30, 1683-1685 (2005)

R. Bäumner, L. Bonacina, J. Enderlein, J. Extermann, T. Fricke-Begemann, G. Marowsky, J.-P. Wolf
Evanescent-field-induced second harmonic generation by noncentrosymmetric nanoparticles
Optics Express 22, 23218 (2010)