A team at the IBS Center for Molecular Spectroscopy and Dynamics (CMSD), led by Director CHO Minhaeng and Professor YOON Tai Hyun of Korea University, has developed a cutting-edge ultrafast laser spectroscopy technique known as asynchronous and interferometric transient absorption (AI-TA). Developed by the center’s Optical Frequency Comb team, AI-TA enables rapid, high-resolution measurement of excited-state dynamics in perovskite nanomaterials undergoing photoinduced transformation. This method allows researchers to capture real-time changes in the structural and compositional states of light-sensitive materials, offering unprecedented insight into the photophysical behavior of next-generation optoelectronic systems.

A team at the Institute for Basic Science (IBS) Center for Molecular Spectroscopy and Dynamics (CMSD) — led by Director CHO Minhaeng (Professor of Chemistry, Korea University) and Professor YOON Tai Hyun (Department of Physics, Korea University) — has developed a powerful new spectroscopic technique that enables real-time tracking of how perovskite nanomaterials change under light.

The technique, called asynchronous and interferometric transient absorption spectroscopy (AI-TA), provides ultrafast measurements of excited-state dynamics and structural transformations in light-responsive materials. It overcomes major limitations of traditional ultrafast spectroscopy, which often requires long data acquisition times and can damage light-sensitive samples during measurement.

“We can now simultaneously observe not just how a material reacts to light but also how it transforms during the reaction itself,” explained Director CHO Minhaeng. “This makes AI-TA a powerful tool for real-time analysis of complex nanoscale processes.”

Perovskite materials are considered promising candidates for next-generation optoelectronic devices like solar cells and LEDs. Conventional femtosecond (10⁻¹⁵ second) laser-based techniques allow for ultrafast observation of their carrier dynamics, reflecting chemical and physical properties. However, such properties are highly sensitive to external factors, such as light, making it difficult to study how they behave in real-world conditions. Especially for ultrafast spectroscopy, the laser pulses used for observation can degrade or alter the sample.

The IBS team addressed this challenge by dramatically shortening measurement time using AI-TA, which relies on two precisely synchronized lasers to capture spectrally and temporally resolved snapshots of the material's transient state. With this technique, the IBS researchers could observe phenomena — including charge carrier dynamics, compositional shifts, and structural reorganizations — over broad timescales ranging from femtoseconds to several minutes.

“AI-TA is evolving into a time-resolved spectroscopic technique that leverages the precision of optical frequency comb technology to investigate molecular reactions in the femtosecond regime,” added Professor YOON Tai Hyun, co-corresponding author of the study.

The researchers applied AI-TA to two types of perovskite systems. In the first, they studied light-induced halide substitution in cesium lead halide nanocrystals, showing that increasing the chloride-to-bromide ratio led to higher bandgap energies and faster charge-carrier dynamics. In the second, they investigated light-driven transformation and agglomeration of colloidal perovskite nanoplatelets, observing how energy loss in hot carriers changed depending on agglomeration, and revealing a nuanced relationship between optical behavior and structural evolution.

“AI-TA offers a new way to study the dynamics of novel materials and various chemical substances that can be easily altered by light and other factors,” said Dr. HAN Gi Rim, first author of the study. “This experiment marks the first step in showcasing the potential of AI-TA, and we look forward to expanding its applications in future research.”

This research represents a technological leap in capturing the real-time evolution of light-sensitive materials during laser measurements. Beyond perovskites, the AI-TA method has the potential to accelerate discoveries in fields ranging from quantum materials and catalysis to next-generation optoelectronic and photonic systems.

Figure 1. Photoinduced structural and compositional changes in perovskite nanomaterials
The top panel illustrates halide substitution in cesium lead halide (CsPb(Br/Cl)₃) nanocrystals, where photoirradiation induces exchange between bromide and chloride ions, altering the Br/Cl ratio. The bottom panel shows the photoinduced aggregation of colloidally dispersed 2-monolayer CsPbBr₃ nanoplatelets, which evolve into larger agglomerates under light. The AI-TA technique was used to investigate how these transformations influence the ultrafast transient absorption spectra and charge carrier dynamics.

Figure 2. AI-TA analysis of photoinduced halide substitution in perovskite nanocrystals
Using the AI-TA technique, the Optical Frequency Comb team acquired time-resolved transient absorption spectra of CsPb (Br/Cl)₃ nanocrystals dispersed in chloroform (top left). By analyzing the data sequentially over the measurement period, the researchers detected a progressive shortening of hot carrier lifetimes and evidence of higher trap density (top right), along with a gradual blueshift in the bandgap energy (bottom left). These trends reflect a light-induced halide exchange process, where bromide-rich nanocrystals gradually convert into chloride-rich compositions during the experiment (bottom right).

Figure 3. AI-TA analysis of light-induced agglomeration in perovskite nanoplatelets
Time-resolved transient absorption spectra were collected using AI-TA over a continuous reaction period of approximately 42 minutes (left). The dataset was segmented based on reaction time to reveal the evolution of photophysical characteristics. As the colloidal nanoplatelets agglomerated under light exposure, both the spectral features (top right) and decay kinetics (bottom right) exhibited noticeable changes, indicating alterations in exciton dynamics and nanoplatelet structure during the agglomeration process.