Reflectance-Only Retrieval of Refractive Index and Extinction Coefficient Dispersions in an Absorbing Thin Film via a...

Accurate knowledge of the refractive index (n) and extinction coefficient (κ) dispersions is essential for understanding the optical response of thin-film materials used in photovoltaics, optoelectronics, and related photonic applications. However, transmission-based dispersion extraction becomes unreliable for turbid, colloidal, nanostructured, or strongly absorbing thin films, where low transmission or scattering prevents accurate spectral evaluation. In this work, we introduce a reflectance-only framework based on the Paul wavelet transform applied to normal-incidence reflectance spectra. The method exploits repetition frequency analysis of interference fringes to retrieve continuous dispersions of n and κ without requiring a predefined dispersion model. The wavelet order provides explicit control over the joint spectral-Fourier resolution, allowing optimization for a given data set. The approach is validated through simulation studies and a noisy signal test with 10% additive random noise, and benchmarked against Minkov's reflectance-based envelope/extrema method. Under noisy conditions, the proposed method preserves the refractive index trend close to the reference behavior, whereas the envelope-based approach shows larger deviations. The extinction coefficient is more sensitive to noise in both methods, although the wavelet-based retrieval remains comparatively stable. Experimental validation on a CdS thin film demonstrates consistency of the refractive index dispersion with literature data, while deviations in the extinction coefficient are attributed to its sensitivity to absorption-related and microstructural variations. The method requires independent film thickness information, as in all interference-based approaches, but is otherwise nondestructive and model-free, making it suitable for a broad range of thin-film systems including doped semiconductors, colloidal nanostructures, and hybrid organic-inorganic materials where conventional transmission or ellipsometric methods are difficult to apply.