High-accuracy [R, T] optical characterization of small-area diamond samples via a custom dual-beam sample holder

We present a custom sample holder system (SHS) enabling high-fidelity reflectance and transmittance ([R, T]) measurements of small (3-7 mm) diamond samples using dual-beam spectrophotometry. Through precision alignment, standard reference material-based correction strategies, and aperture-induced distortion cancellation, the SHS achieves sub-percent absolute photometric accuracy, allowing direct inversion of [R, T] for optical constants [n, k] via a Newton-Raphson (N-R) method. This process eliminates reliance on curve-fitting, instead using branch topology-physical (p-) and mathematical (m-) branch crossings-to extract thickness, roughness, and vertical non-uniformity from fringe behavior. Applied to boron-doped diamond (BDD) homoepitaxial films, the method reduces thickness variance by over 3× compared to mass-gain measurements and reveals carrier density gradients in thin layers consistent with secondary ion mass spectroscopy (SIMS). Notably, ripple-like discontinuities in [n, k]-often dismissed as artifacts-are shown to encode real growth physics. This enables optical retrieval of effective hole mass (∼0.48 m0), carrier lifetime, and depth-dependent doping profiles non-destructively and with nanoscale sensitivity. Beyond diamond, this approach reframes spectrophotometry not as a passive measurement but as an epistemic filter: a falsification engine that tests the adequacy of optical models. Inversion-aware metrology thus enables new modes of structural verification, diagnostic clarity, and growth-process insight across small-scale and high-optical density material systems.