Yuki Kobayashi

Research Outline - Ultrafast Spectroscopy of Quantum Materials

The Kobayashi Research Group / Quantum Materials Chemistry Lab at the University of Michigan-Ann Arbor is dedicated to exploring the quantum nature of chemical and materials dynamics by using advanced laser spectroscopy. Our research interests range from electronic-vibrational coherences in small molecules to quasi-particle many-body states in low-dimensional materials. We are committed to developing new tools in spectroscopy, such as attosecond x-ray light sources, and using them to solve complex chemical problems. Our overarching goal is to unlock the potential of quantum effects in the development of sustainable energy and information technologies.

Attosecond Spectroscopy

Electrons drive chemical reactions, changing their shapes upon interactions with external stimuli. Capturing such fleeting motion of electrons is fundamental to advance our knowledge of reaction mechanisms, from light-to-energy conversion to chemcal-bond rearrangement. However, electrons dynamics can occur in the sub-femtosecond regime, and the conventional femtosecond spectroscopy lacks the matching temporal resolution. To tackle this challenge, we develop table-top x-ray/xuv light sources, which can achieve the ultimate attosecond temporal resolution. The attosecond x-ray/xuv supercontinuum covers multiple elemental edges of transition metals, halogens, and chalcogens, thereby giving us unique capabilities to perform composition-specific probing of electron dynamics. We apply this technique to heterostructure layered device, metal-ligand complexes, and element-tagged polyatomic molecules toward more efficient energy and information materials.

Quantum Materials

Quantum nature of electrons become prominent when they are confined in nanometer-scale spaces. Reduced dielectric screening and spatial confinement enhance many-body effects in nanomaterials, and the results are manifested as, for example, excitonic quasiparticle states with enormous optical intensity. Questions remain as to whether and how we can harness such quantum effects for optoelectronic applications while circumventing decoherence processes that can compete with the desired outcome. We apply ultrafast spectroscopy to novel two-dimensional materials and investigate the evolution of quantum states therein. Our spectroscopic tools can resonantly access band-to-band and intra-excitonic transitions in two-dimensional materials in the non-perturbative regime with a few-femtosecond resolution. The experimental results will provide new insights into realization of coherence-based optoelectronics with unmatched compactness and efficiency.

Lightwave Nanophotonics

Extreme light-matter interactions induced by strong-laser fields can create exotic non-equilibrium quantum states in materials. The example phenomena include the dynamic Franz-Keldysh effects, formation of Floquet replicas, and the frequency up-conversion process of high-harmonic generation. The novel nanomaterials such as monolayer transition-metal dichalcogenides offer a particularly attractive platform for these photonic experiments as they exhibit significantly enhanced optical properties compared to the conventional bulk materials.  We combine the frontiers of laser technology and materials science to explore the limit of photonic applications of nanomaterials. We develop phase-stabilize intense light sources that can efficiently control the energy landscapes of materials while avoiding the sample damage. We also implement solid-state high-harmonic generation as a way to realize ultrafast supercontinuum in the ultraviolet regime.

Physical Chemistry Education & Lab Safety

Our group is dedicated to promoting the diversity and equity in physical-chemistry education. We believe that research experience at the university is essential for students to succeed in academic environments and ultimately boost socioeconomic mobility. Students in our group will have hands-on experience on femtosecond laser,  x-ray optics, high-vacuum instrumentation, and quantum-mechanical simulations. As part of our commitment to diversity, we work with neighboring communities in Michigan to improve access to research opportunities and promote inclusivity. In addition to our research objectives, we prioritize lab safety and implement stringent protocols to prevent potential hazards associated with toxic and explosive chemicals, high voltage components, pressurized systems, cryogens, and intense lasers. We hold regular group meetings to review and maintain the highest standards of lab safety.

Highlighted Publications

[1] Yuki Kobayashi,† Christian Heide,† Amalya C. Johnson, Vishal Tiwari, Fang Liu, David A. Reis, Tony F. Heinz, and Shambhu Ghimire, "Floquet engineering of strongly-driven excitons in monolayer tungsten disulfide," Nat. Phys. 19, 171 (2023)

[2] Yuki Kobayashi and Stephen R. Leone, "Perspective: characterizing coherences in chemical dynamics with attosecond time-resolved x-ray absorption spectroscopy," J. Chem. Phys. 157, 180901 (2022)

[3] Yuki Kobayashi,† Christian Heide,† Hamed K. Kelardeh, Amalya Johnson, Fang Liu, Tony F. Heinz, David A. Reis, and Shambhu Ghimire, "Polarization flipping of even-order harmonics in monolayer transition-metal dichalcogenides," Ultrafast Science 2021, 9820716 (2021)

[4] Yuki Kobayashi, Kristina F. Chang, Sonia Marggi Poullain, Valeriu Scutelnic, Tao Zeng, Daniel M. Neumark, and Stephen R. Leone, “Coherent electronic-vibrational dynamics in deuterium bromide probed via attosecond transient absorption spectroscopy,” Phys. Rev. A 101, 063414 (2020)

[5] Yuki Kobayashi, Kristina F. Chang, Tao Zeng, Daniel M. Neumark, and Stephen R. Leone, “Direct mapping of curve-crossing dynamics in IBr by attosecond transient absorption spectroscopy,” Science 365, 79-83 (2019)

[6] Yuki Kobayashi, Maurizio Reduzzi, Kristina F. Chang, Henry Timmers, Daniel M. Neumark, and Stephen R. Leone, “Selectivity of electronic coherence and attosecond ionization delays in strong-field double ionization,” Phys. Rev. Lett. 120, 233201 (2018)

[7] Yuki Kobayashi, Henry Timmers, Mazyar Sabbar, Stephen R. Leone, and Daniel M. Neumark, “Attosecond transient-absorption dynamics of xenon core-excited states in a strong driving field,” Phys. Rev. A 95, 031401(R) (2017)