Attenuated Total Reflectance

Overview

Attenuated Total Reflection or ATR is a technique used together with Teng Mann to measure the R33 of electro-optic materials. A beam of IR light is directed through a prism at an angle exceeding the critical angle for internal reflection. This produces an evanescent wave at the prism surface. If a EO polymer is pressed in intimate contact with the prism they can be coupled so that he evanescent wave stimulates emission from the sample. This emission is measured with a photodiode. An electric field applied to an EO polymer changes its index of refraction which alters the critical angle.

Technique

<swf width="720" height="480">http://depts.washington.edu/cmditr/media/atr.swf</swf>

Significance

The electro-optic coefficient for a poled polymer film can be calculated as follows

<math>r_{33} = \frac {2d\Delta R} {n^3_{TM} V_m} \frac {\delta n_{eff}} {\delta \theta} \left /( \frac {\delta R \delta n_{eff} } {\delta \theta \delta n_{TM} } \right ) \,\!</math>

where

<math>R\,\!</math> is the DC reflected signal

<math>V_m\,\!</math> is the AC modulation voltage

<math>n_{eff}\,\!</math> is the ordinary index of refraction

<math>n_{TM}\,\!</math> is the film refractive index