Dr. Tahara has developed innovative and original spectroscopic methods in each of the three major fields of advanced spectroscopy: ultrafast spectroscopy, interfacial nonlinear spectroscopy, and single-molecule spectroscopy. By enabling the observation of molecular phenomena that could not previously be observed, he has pioneered new areas in the research of complex molecular systems. Below, his main achievements are described.

1. Study of Ultrafast Reactions through the Establishment and Advancement of Femtosecond Time-Resolved Raman Spectroscopy
In ultrafast spectroscopy, light pulses with durations shorter than the vibrational period of molecules induce coherent nuclear wavepacket motion, allowing molecular vibrations to be observed in real-time. Around 2000, Dr. Tahara was among the first to recognize the importance of observing nuclear wavepacket motion for reaction studies and began researching this phenomenon in electronically excited states using ultrashort pulses.
Notably, he developed time-resolved impulsive stimulated Raman spectroscopy (TR-ISRS), which induces nuclear wavepacket motion in reacting molecules through impulsive stimulated Raman processes. By Fourier transforming the oscillatory components of the observed transient signals, TR-ISRS provides femtosecond time-resolved Raman spectra. Moreover, by employing stable sub-7-femtosecond light pulses, he made it possible to measure femtosecond time-resolved Raman spectra across the entire range of the vibrational frequency (0-3000 cm^⁻¹). This breakthrough elucidated the dynamics of complex molecular systems such as photo-responsive proteins and metal complex aggregates. As a result, time-domain Raman spectroscopy became a powerful tool for studying ultrafast structural dynamics.
Furthermore, Dr. Tahara extended femtosecond stimulated Raman spectroscopy (FSRS), which operates in the frequency domain, to the ultraviolet wavelength region. This enabled the observation of ultrafast changes in the protein part of photoreceptor proteins and led to the first observation of the Raman spectrum of an intermediate state (the fantom state) with a twisted CC double bond (~90°) that was long believed to form during the cis-trans photoisomerization reaction. By identifying its structure, he resolved a longstanding question about one of the most fundamental photochemical reactions.

2. Study of Liquid Interface Dynamics Using Phase-Sensitive Interface-Selective Nonlinear Spectroscopy
Interfaces are the regions where many important molecular phenomena occur, yet their molecular-level understanding has lagged behind that of bulk solutions. Even-order nonlinear optical phenomena occur only in regions lacking inversion symmetry, such as interfaces, enabling selective observation of interfacial molecules through even-order nonlinear spectroscopy. Dr. Tahara introduced ultrashort pulse laser technology into interface research and developed a series of novel interface-selective even-order nonlinear spectroscopy.
In particular, he advanced interface-selective nonlinear spectroscopy by developing heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. This method uses the interference of femtosecond pulse light to determine both the phase and amplitude of sum frequency signals generated at interfaces. It allows for the measurement of interface-selective vibrational spectra that can be directly compared to infrared and Raman spectra and can provide information on the absolute orientation of interfacial molecules (e.g., upward/downward orientation) as well.
Furthermore, by combining HD-VSFG spectroscopy with femtosecond photoexcitation, he extended the technique to femtosecond time-resolved measurements, achieving ultrafast spectroscopy at liquid interfaces on par with solution-based studies. By combining HD-VSFG with femtosecond infrared photoexcitation, he realized infrared-excited time-resolved HD-VSFG spectroscopy, which propelled the study of vibrational dynamics at liquid interfaces. Notably, he clarified hydrogen-bond dynamics and vibrational relaxation processes of water molecules at interfaces occurring on a femtosecond time scale. He also combined HD-VSFG with ultraviolet femtosecond pulse excitation to develop UV-excited time-resolved HD-VSFG spectroscopy. This enabled the observation of hydrated electrons at the water surface. Moreover, he succeeded in tracking ultrafast reactions occurring at the water surface, discovering that the photochemical reaction of phenol proceeds drastically faster at the interface than in bulk water. This finding can open a new research field, i.e., reaction dynamics at liquid interfaces.

3. Study of Structural Dynamics in Biomacromolecules through the Development of Single-Molecule Spectroscopy with Sub-Microsecond Resolution
Dr. Tahara has developed unique spectroscopic methods, as mentioned above, enabling novel dynamics studies in solutions and at interfaces. These methods are all based on the "pump-probe" method using light pulses. On the other hand, biomolecules such as proteins, DNA, and RNA continuously fluctuate in their structure at room temperature and undergo structural changes through interactions with other molecules. These structural changes are essential for their biological functions but are thermally induced, making them inaccessible to conventional time-resolved spectroscopy based on the pump-probe method.
To study such unsynchronized structural dynamics of biomolecules, Dr. Tahara developed a novel method called two-dimensional fluorescence lifetime correlation spectroscopy (2D FLCS), which significantly surpasses the limitations of traditional single-molecule spectroscopy. In 2D FLCS, the combination of high-repetition-rate short-pulse laser excitation and fluorescence correlation analysis enables tracking changes in the fluorescence lifetime of single molecules with sub-microsecond time resolution and providing quantitative information.
Using this 2D FLCS, Dr. Tahara investigated the folding dynamics of proteins, DNA, and RNA, revealing structural dynamics on a microsecond time scale. Notably, he discovered that the structural dynamics of the preQ1 riboswitch, an mRNA segment involved in bacterial transcription regulation, undergo significant changes upon interaction with small molecules. Based on these findings, he proposed a molecular mechanism for transcriptional control through such interactions.

In summary, Dr. Tahara has applied his advanced spectroscopic methodologies to important fundamental problems and conducted groundbreaking interdisciplinary research. The novel spectroscopic methods he developed are now employed by leading research groups worldwide and have earned him high international acclaim. These achievements are recognized as highly deserving of the CSJ Award.