EW-CRDS with a Pellin-Broca Cavity

One way to probe condensed matter with evanescent waves.

Diagram of a low-loss optical cavity which incorporates an intra-cavity total internal reflection (TIR).

The sample contacts the back side of the Pellin-Broca and absorbs the evanescent wave emanating from the TIR site.

Light enters and exits the Pellin-Broca prism at Brewster’s angle to minimize Fresnel reflection losses.
The prism was fabricated with an accuracy of 3 arc-min from high-purity fused-silica with an estimated bulk attenuation of 50 ppm/cm.
The surfaces are superpolished to <0.05 nm RMS surface roughness.
The mirrors were commercially available, nominally 99.99% reflective, with a peak reflectivity at 620 nm.
This cavity exhibits a base ring-down time of about ~1 microsecond and yields a minimum detectable absorbance change of ~32 ppm.
Super-polishing is essential. Before this Pellin-Broca was superpolished, this standard-polished optic produced a base ring-down time of ~100 nanoseconds.

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