The Waveguie Mode Spectroscopy is an established method for the evaluation of the layer thickness and refractive index of a waveguide or a thin ad-layer on the top. It is based on the selective excitation of waveguide modes.

Principle of Operation

An optical waveguide structure is a high refractive index medium in a lower refractive index environment. The most simple device possible is a planar waveguide consisting of a high index waveguide layer on top of allow index substrate in a low index cover medium such as air or water. If the layer thickness is sufficiently high guided optical modes exit because of total internal reflection at the high/low index interfaces. These waveguide modes show discrete wavevectors which fulfill a constructive interference condition. Solving Maxwell´s equations leads to an exponentially decaying so-called evanescent field of the guided modes in the low index medium. Because of the non-zero field distribution in the surrounding medium a thin ad-layer will interact with the guided modes resulting in a modification of their wavevector. This can be detected and used to monitor optical properties of a thin film or ad-layer deposited on top of the surface. The exponentially decaying field distribution leads to a high degree of field localization at the surface providing a very high sensitivity for the thin film characterization. To excite waveguide modes by an external laser beam momentum matching has to be achieved. This can be done by a grating or a prism coupler which allows for a mode selective excitation or detection at a certain angle of incidence. The wavevector of the mode is calculated from the measured angle of incidence at which resonant coupling occurs.
 

Waveguide Mode Spectrometer

For the experiments we are using a home built spectrometer. A grating or prism coupler can be used to couple a polarized laser beam from the external cover medium into the waveguide. The wavevector of the  waveguide modes is calculated by scanning the angle of incidence of the incoming laser beam onto the grating or prism coupler, while the incoupled power is measured by two photo-detectors situated at both ends of the waveguide.

Examples of waveguide mode measurements, presented in the winterschool of AK Knoll in Hirschegg 2003.

References

Silvia Mittler and Bernhard Menges
Frontiers in Surface Nanophotonics. Principles and Applications
David L. Andrews and Zeno Gaburro. Springer-Verlag, New York, 2007

M.T.van Os, B.Menges, R. Förch, G.J.Vancso and W.Knoll
Characterization of Plasma Polymerized Allylamine using Waveguide Mode Spectroscopy, Chemistry of Materials, 1999, Vol. 11, 3252-3257