Gi Rim Han

Rod-shaped tellurium microcrystals are promising mid-infrared photonic materials and a prototypical chiral (Weyl) semiconductor, yet intrinsic band-edge relaxation at room temperature is often masked by hot-carrier and photothermal pathways under above-gap excitation. Here, we employ mid-infrared asynchronous and interferometric transient absorption (MIR AI-TA) spectroscopy with frequency-comb pulses centered near the band-edge (380 meV) to directly interrogate two band-edge resonances. The time- and frequency-resolved AI-TA data reveal a fast component characterized by a time constant of 1-2 ps and a long-lived component with a time constant of ∼50 ps, common to both resonances. A global spectro-temporal model assigns the fast component to phonon-assisted redistribution of valence holes coupled to recovery of pump-induced Peierls distortion, while the slow component corresponds to band-edge electron-hole recombination. These results demonstrate that MIR AI-TA can quantitatively disentangle coupled electronic and structural dynamics in narrow-bandgap semiconductors.

Asynchronous and interferometric transient absorption (AI-TA) spectroscopy is a powerful technique for probing ultrafast dynamics, offering both time-resolved insights and frequency-domain information with a single-point detector. In this study, we successfully measured the transient absorption signal of rod-shaped tellurium microcrystals under low pump fluence using a mid-infrared (MIR) AI-TA setup. By leveraging exceptionally stable frequency combs for both the pump and probe beams, coherent averaging in the MIR AI-TA enabled the clear observation of transient absorption signals, even in heterogeneous samples under low pump fluence. The standard deviation of the TA signal decreases with averaging time (τavg) following a 1/τavg dependence, confirming the ergodicity of the TA measurement over extended averaging periods. These results highlight the significant potential of MIR AI-TA spectroscopy for advancing our understanding of ultrafast carrier dynamics in MIR-active materials.

By employing two synchronized mode-locked femtosecond lasers and interferometric detection of the pump-probe spectra—referred to as asynchronous and interferometric transient absorption—we have developed a method for broad dynamic range and rapid data acquisition. Using asynchronous and interferometric transient absorption, we examined photochemical changes during femtosecond pump-probe experiments on all-inorganic cesium lead halide nanomaterials. The laser pulse train facilitates photoreactions while allowing real-time observation of charge carrier dynamics. In perovskite nanocrystals undergoing photo-substitution of halide anions, transient absorption spectra showed increasing bandgap energy and faster relaxation dynamics as the Cl/Br ratio increased. For colloidal perovskite nanoplatelets, continuous observation revealed both spectral and kinetic changes during the light-induced coalescence of nanoplatelets, by analyzing temporal segments. This integrated technique not only deepens understanding of exciton dynamics and environmental influences in perovskite nanomaterials but also establishes asynchronous and interferometric transient absorption as a transformative tool for real-time observation of photochemical dynamics.

Since the advent of time-resolved spectroscopy based on precision frequency technology of laser sources, it has been considered an alternative way to study dynamic processes in photochemical systems. This Perspective introduces asynchronous and interferometric nonlinear spectroscopy (AI-NS), a spectroscopic technique that combines asynchronously generated laser pulses and interferometric detection, offering an unprecedented temporal dynamic range with high spectral resolution and rapid data acquisition capabilities. By eliminating the need for mechanical delay stages, AI-NS facilitates the rapid collection of time-resolved data on dynamics ranging from femtoseconds to nanoseconds while simultaneously distinguishing frequency-dependent responses. Here, we detail the technical methodology of AI-NS and explore its applications to the studies of various systems, including semiconductors and biological systems. Additionally, we highlight prospective advancements, such as integration with multidimensional spectroscopy techniques. AI-NS not only expands the scope of spectroscopic analysis but also opens new avenues for the exploration of diverse materials and molecular systems.

Iridium(III) organometallic complexes have been a key component in commercialization of organic light-emitting diodes, but the direct relationship between their structural features and photophysical properties has not yet been fully established. Here, combined experimental and theoretical studies are carried out to elucidate the main factors governing the quantum efficiency of red phosphorescent emitters by using two heteroleptic iridium(III) complexes with high geometrical similarity. It is found that two red-emitting heteroleptic iridium complexes differing only in the steric direction of phenylquinoline (pq) and phenylisoquinoline (piq) ligands, annotated Red-pq and Red-piq, show clearly different degrees of distortion of the ligand geometry in the excited state, which leads to the higher quantum yield of Red-piq than that of Red-pq. This larger distortion of the piq ligand causes more suppressed nonradiative decay of Red-piq than that of Red-pq which is the important factor governing the higher quantum yield of Red-piq.

The excited-state proton transfer (ESPT) reaction is an important primary photochemical process because it is closely related to photophysical properties. Although ESPT research in aqueous solutions is predominant, alcoholic solvent-mediated ESPT studies are also significant in terms of photoacid-based reactions. Especially, the research for dihydroxynaphthalenes (DHNs) has been largely neglected due to the challenging data interpretation of two hydroxyl groups. A novel fluorescent dye, resveratrone, synthesized by light irradiation of resveratrol, which is famous for its antioxidant properties, can be regarded as a type of DHN, and it has distinctive optical properties, including high quantum yield, a large two-photon absorption coefficient, a large Stokes shift, and very high biocompatibility. In this study, we investigate the overall kinetics of the optical properties of resveratrone and find evidence for alcoholic solvent-mediated ESPT involvement in the radiative properties of resveratrone with a large Stokes shift. Our investigation provides an opportunity to revisit the overlooked photophysical properties of intriguing photoacid behavior and the large Stokes shift of the dihydroxynaphthalene dye.