Pump-Probe Spectroscopy: Revealing Material Properties with Light

Spectroscopy is a technique that extracts information about materials by examining their interactions with light. Among its various forms, pump-probe spectroscopy uses short pulses of light to investigate how a material responds or changes, essentially, to observe its properties and reactions. In this method, a "pump" pulse first excites the material, and then, after a specific delay, a "probe" pulse follows to measure the spectroscopic information. Because this approach enables observation of changes in the material over time, it is also known as time-resolved spectroscopy.

Obtaining information from an excited state is no simple task. Most molecules remain in their unexcited ground state, and any signals arising from excited states can be complicated due to overlapping reactions and transitions. However, if these complex signals can be properly interpreted, they reveal key insights into photoinduced reactions, energy transfer processes, and charge carrier dynamics occurring within the material. Such information is especially crucial in semiconductors, as it provides important clues about their performance and efficiency.

What is Asynchronous Optical Sampling (ASOPS)?

Reactions triggered by light can occur in extremely short timeframes — some as brief as a few femtoseconds (10^-15 seconds). In time-resolved spectroscopy, observing such fleeting events requires precise control over the time delay between the pump and probe pulses, using the speed of light as a reference. Since light travels approximately 300,000 km per second, simply varying the optical path lengths of the pump and probe beams creates a time delay. For example, a path length difference of 0.3 mm results in a time delay of 1 picosecond (10^-12 seconds) between the two pulses. However, this method depends on mechanical devices to adjust the path length, which imposes limitations in range and consumes time for movement and stabilization.

[Figure 1] (Left) Comparison between the conventional method of creating a pump–probe time delay using optical path differences and the asynchronous optical sampling method. (Right) Comparison between wavelength-resolving methods using a prism, camera, etc., and those using an interferometer.

Asynchronous optical sampling offers a faster and more efficient way to generate these time delays. Instead of mechanically adjusting path lengths, it creates a natural time offset by setting slightly different repetition rates for the two pulse trains. For instance, if one pulse is generated every 1.1 seconds and the other every 1.0 seconds, they initially coincide, but the next pulses will be offset by 0.1 seconds, then 0.2 seconds, and so on. Using this method, one can rapidly and efficiently scan a broad range of time delays, from femtoseconds up to nanoseconds (10^-9 seconds) or more.

‘Asynchronous Interferometric Transient Absorption Spectroscopy’: Extracting Multiple Types of Information with a Single Detector

Asynchronous optical sampling alone allows us to obtain information about the time delay between pump and probe pulses, but it is difficult to acquire data based on the light's wavelength. To distinguish information that varies by wavelength, an interferometer can be used. By splitting the probe beam into two paths and recombining them to generate an interference signal, Fourier transform analysis can extract the wavelength-dependent spectral information.

This process of acquiring interference signals takes several milliseconds. However, during that time, the pump–probe delay is continuously changing at the nanosecond level due to asynchronous optical sampling, making analysis with a conventional camera difficult. Instead, if a single optical detector continuously reads the signal over time, it can capture rapid changes in the probe pulses. When analyzing this data, a technique called time division multiplexing is used: different time intervals are assigned and analyzed in segments. For example, changes in the probe pulse at nanosecond (10^-9 s) intervals reveal ultrafast dynamics; interference signals gathered over milliseconds reveal wavelength information; and over seconds to minutes, one can observe material changes over time. This is analogous to taking a photo of a forest and zooming in: depending on the level of magnification, you can observe individual leaves, compare trees, or view the entire forest.

By combining asynchronous optical sampling with interferometric detection, a new technique called Asynchronous Interferometric Transient Absorption Spectroscopy (AI-TA) was developed.

[Figure 2] Schematic illustration of light-induced reactions occurring in perovskite nanomaterials. Femtosecond laser pulses both trigger photoreactions in the nanomaterial and serve as a tool for measuring its physical properties.

[Investigating Light-Induced Property Changes in New Materials Using AI-TA]

Paper title: In situ and real-time ultrafast spectroscopy of photoinduced reactions in perovskite nanomaterials, Nature Communications, 2025

In spectroscopy capable of such rapid measurement, light serves not only as a tool for spectroscopic analysis but also as a trigger that initiates photoinduced reactions. Traditional time-resolved spectroscopy has struggled to analyze materials that are inherently sensitive to light or exist in environments or conditions vulnerable to light exposure. However, with fast measurement technology, obtaining information from such systems has become possible.

Recently, our research team used the AI-TA technique to observe light-induced changes in a new material called perovskite. Perovskite is an emerging material with potential applications in next-generation optoelectronic devices, such as solar cells and LEDs. It is known that halogen elements within perovskite can undergo exchange reactions with halogens in external solutions like chloroform when exposed to light. While traditional time-resolved spectroscopy would struggle to measure the properties of nanomaterials under such unstable conditions, AI-TA enabled us to confirm that the internal composition changes from bromine (Br) to chlorine (Cl), widening the bandgap energy and causing electrons to return more rapidly to the ground state.

[Figure 3] Using AI-TA to measure thin plate-shaped perovskite nanomaterials revealed wavelength shifts (top right) and dynamic changes (bottom right) that occur as the material aggregates under light exposure.

In addition, thin plate-shaped perovskite nanomaterials exhibit an aggregation phenomenon in colloidal form when exposed to light, where the nanoplates clump together. By analyzing how energy loss in the excited state varies depending on the degree of aggregation, the research revealed a correlation between structural changes in the material and its optical responses. Through this study, we were able to more precisely uncover how structural transformations in the material influence its optical properties, such as energy loss.

[The Future of AI-TA]

AI-TA technology enables real-time observation and analysis of dynamic changes in a material’s properties during light-induced reactions. It is expected to serve as a powerful tool for interpreting the dynamics of complex and dynamic systems. As a novel analytical technique for exploring the kinetics of new materials and various chemical substances, AI-TA is anticipated to significantly contribute not only to the development of next-generation materials but also to a wide range of applied research involving other materials.