A new study reveals how changes in refractive index can skew transient absorption data in thin-film materials, leading to misinterpretations unless properly corrected.
Rethinking Transient Absorption in Thin Films
A multinational research team has uncovered a critical flaw in the way scientists analyze ultrafast photochemical dynamics in thin films. In a study published in The Journal of Physical Chemistry C, the authors show that transient absorption spectroscopy (TAS)—a mainstay tool for studying photoinduced phenomena—is significantly influenced by refractive index changes in thin films, leading to distortions in measured optical signals (1).
Led by Hannu P. Pasanen, Ramsha Khan, Jokotadeola A. Odutola, and Nikolai V. Tkachenko, the research stems from collaborations between King Abdullah University of Science and Technology (KAUST) in Saudi Arabia and Tampere University in Finland. Their findings challenge long-held assumptions in TAS data interpretation and offer corrections that could redefine analysis protocols for a wide range of advanced materials (1,2).
TAS works by measuring changes in a sample’s light absorption as it reacts to an ultrafast laser pulse © Yuri Hoyda -chronicles-stock.adobe.com
When Light Gets Twisted
TAS works by measuring changes in a sample’s light absorption as it reacts to an ultrafast laser pulse. Traditionally, these changes are assumed to stem solely from population shifts in intermediate excited states. But the new study shows that, in solid films, light behaves differently (1,2).
“Any change in absorption is accompanied by a change in refractive index,” the authors write, pointing to electromagnetic theory. In dilute solutions, these refractive effects are minimal. But in condensed systems, such as semiconductors or perovskite films, these changes can meaningfully distort transient signals (1). The team highlights that refractive index alterations in films can mimic or obscure genuine absorption changes, potentially leading researchers to false conclusions about a material’s photoactivity (1).
Measuring Both Sides of the Beam
To isolate and correct for these distortions, the researchers propose a dual-measurement approach that combines both transmitted and reflected probe light. This method can decouple changes due to true absorption from those caused by altered refractive behavior (1).
The study presents a formula to reconstruct the corrected transient absorbance, eliminating interference artifacts and revealing a clearer picture of photophysical processes. In one example using perovskite films, the corrected spectra aligned closely with theoretical expectations, while uncorrected data misrepresented absorption features, especially in the 800 to 1500 nm range.
This correction method can also illuminate optical phenomena that don’t directly affect absorption, such as charge carrier diffusion—a process critical for understanding solar energy materials and photocatalysts (1).
Theory Meets Application
The authors employed the Drude–Lorentz (D-L) model to simulate complex changes in dielectric properties and refractive indices upon photoexcitation (1). Using this model, they demonstrated how excited-state populations modify not only the extinction coefficient k but also the real refractive index n, both of which are essential for accurate TAS interpretation (1,2).
By applying this model, they showed that even slight misinterpretations in reflectance-transmittance behavior can lead to significant discrepancies in extracted material properties like carrier mobility or recombination rates—parameters vital for solar cell development (1).
Moving Forward with More Clarity
The implications of this work extend beyond perovskites to other thin-film semiconductors, graphene, and TiO₂ nanostructures. As materials science increasingly targets nanoscale films with high optical activity, the need for accurate interpretation of ultrafast spectroscopy becomes paramount (1).
Ultimately, this study acts as a wake-up call to the photophysics community. As co-author Nikolai Tkachenko notes, “If we ignore refractive index changes in solid-state samples, we risk misunderstanding the fundamental dynamics that drive device performance” (1).
References
(1) Pasanen, H. P.; Khan, R.; Odutola, J. A.; Tkachenko, N. V. Transient Absorption Spectroscopy of Films: Impact of Refractive Index. J. Phys. Chem. C 2024, 128 (15), 6167–6179. DOI: 10.1021/acs.jpcc.4c00981
(2) Ohkita, H.; Ito, S. Transient Absorption Spectroscopy of Polymer-Based Thin-Film Solar Cells. Polymer 2011, 52 (20), 4397–4417. DOI: 10.1016/j.polymer.2011.06.061
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