Waveguide modes are the technique of choice for the characterization of films with a thickness greater than 500 nm. A very high precision in the layer characterization is achieved that cannot be matched by any other technique! In particular, waveguide modes are ideally suited for anisotropic samples. Measurements with different polarizations specifically question selected element of the anisotropic refractive index and the combined data lead to their independent determination. This is a major difference to ellipsometric measurements. Here, all refractive index elements contribute simultaneously. As a consequence, the data analysis is complex and bears an inherent ambiguity.

Waveguide modes are in a way related to surface plasmons and can only be excited in very special arrangements. The following section guides you through the basic concepts.#

Basics:

What is a waveguide?
A waveguide is region with an elevated refractive index as compared to the adjacent media. The layer must have a minimum thickness in order to support waveguide modes.

Ray optics: Light is trapped by the Total Internal Reflection Field picture: Light is propagating in well defined modes

Excitation of waveguide modes requires a special experimental geometry. One common arrangement uses a prism coated on its base with a thin metal layer. The experiment consists in the measurement of the intensity of the reflected light as a function of the angle of incidence. At certain distinct angles, wave guide modes are excited. The energy of incident light is then partly transferred to the wave guide mode and as a result a minimum in the reflected intensity is observed. The measurement may contain several dips in the reflectivity. Each minimum refers to a to a specific mode with a characteristic field distribution characterized by the number of nodal planes. The different field distributions are schematically sketched in the figure on the left hand side.

Waveguide modes: experimental arrangement: Measure the reflected light as a function of the angle of incidence. The formation of a waveguide mode shows off as a dip in the reflectivity

TE modes are determined by ny and thickness t TM modes are determined by nx , nz, and t System is over determined n and t are decoupled Ideally suited for anisotropic system!

TE modes are only affected by the thickness and the refractive index ny in the direction of the electric field. TM modes are affected only by the thickness, nx and nz, hence polarization dependent measurements are ideally suited for determining the properties of anisotropic samples. This method provides a high accuracy in the determination of the optical parameters of suitable samples.

Further reading

Literature
Hunsperger
Integrated optics: Theory and Technology
Springer Verlag, Berlin (1991)