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Model 2010/M Applications

Measuring Thin Film Electro-optic Coefficients

By measuring index change due to a voltage (electrical field) applied across a sample, electro-optic coefficients can be easily measured with the Model 2010/M system ¹,². A schematic for E-O coefficient measurement using the Model 2010/M is shown below:

To avoid the need for the user to deposit a top conducting coating on the sample, Metricon supplies coupling prisms coated with a thin semi-transparent metallic coating on the coupling face so the coating functions as the top electrode. The coated prism is mounted in the usual metallic prism mounting clamp with conductive epoxy so that the entire clamp is in electrical contact with the coupling face of the prism which is in turn in contact with the top surface of the sample. Voltage is then applied via the mounting clamp between the conductive prism coating and a bottom electrode such as a thin ITO or metallic layer provided by the user on the back (substrate) side of the sample. A DC voltage is shown in the schematic but use of an AC voltage can reduce the voltage needed to observe a meaningful shift in the mode location for samples with smaller E-O coefficients².

The metallic coating does cause some distortion in the mode shape, but normally the shape is less important than the shift in the location of the mode or critical angle under bias and this shift can still be very accurately measured.

Coated 200-P-1 and 200-P-4 prisms (mounted with conductive epoxy in a standard prism clamp) are available from Metricon allowing electrical bias measurements on samples with modes with effective index up to ~2.0. For higher index samples (above ~2.1) where the fundamental mode has effective index above 2.0 the angular shift under bias of higher order modes (for example the second or third modes) can be used.

Note: Many materials must be electrically poled to enhance their E-O properties. If the electrode required for poling is transparent or semi-transparent and remains on the sample after poling, it can serve as the top electrode and E-O measurements can be made with a standard (uncoated) prism.

The only other requirement is that the fixturing which supports the prism clamp be electrically insulated from the rest of the system. For new systems this can be easily done at the factory before shipment (please order option 2010-EO). For existing systems, this insulation can also be done by the customer (please consult Metricon for details).

Important note: Because the thin metallic coating is fragile and easily worn, coated prisms may provide only a few hundred measurements before the coating no longer serves as an effective electrode for the top surface of the sample and should only be used for measurements where a voltage must be applied to the sample.

¹ D.Y. Wang, et al, "Electro-Optic Characterization of Epitaxial Ba0.7Sr0.3TiO3 Thin Films Using Prism Coupling Technique", Current Applied Physics (2010) doi:10.1016/j.cap.2010.09.014.

² D. H. Park, "Characterization of Linear Electro-Optic Effect of Poled Organic Thin Films", PhD dissertation, University of Maryland, 2008.