Optimal direction and propagation of mid-IR light inside rough and polished diamonds for highly-sensitive transmission measurements of nitrogen content

Optical transmission/absorption measurements in diamond provide a way for determining content of basic nitrogen defects, with their concentrations varying in different crystals by many orders of magnitude. In a traditional scheme of transmission measurement in many diamond crystals only a small fraction of incident light is transmitted and detected due to light refraction and multiple reflections. This significantly reduces signal-to-noise ratio, narrows the dynamic range of measured signal magnitudes and the corresponding range of measurable defects concentrations. In this work optimal input/output points and optical propagation paths of mid-IR light inside most characteristic forms of rough and faceted diamonds were searched and analyzed. Practical acquisition recommendations and measurement configurations are proposed, enabling to expand signal dynamic range by acquiring multiple mid-IR light reflection/transmission replica, using an integrating sphere and immersion material.