Our daily life is surrounded by opto-electronic devices made with wide varieties of metals and semiconductors -- photocatalysts, solar cells, light emitting diodes (LEDs), lasers, for example. The functions of these devices are based on such microscopic processes as photo-excitation of electrons and holes, energy transfer, transport, and recombinations. These processes occur typically between femtosecond and microseconds, and the competition among different processes often determines the device efficiency.
Our laboratory investigates the ultrafast optical response of metals, semiconductors and their nano-structures by using a pump-probe spectroscopy technique. Coherent optical and acoustic phonons induced by illuminating the materials with femtosecond laser pulses are of particular interest, since they can be potentially applied to all-optical nondestructive evaluation of internal structures and defects buried inside the materials.
  • Anharmonic vibrational potentials for hybrid lead halide perovskite
  • Organic-inorganic hybrid lead halide perovskites tend to show higher photovolvatic performances than their inorganic counterparts. We perform time-resolved optical Kerr effect measurements on a typical hybrido perovskite hybrid perovskite CH3NH3PbI3 and reveal the CH3NH3 (MA) molecular libration and twisting to blueshift with increasing pump fluence. Our theoretical simulations confirm the strong anharmonicity of the vibrational potentials to be the origin of the frequency blueshifts.
  • Anharmonic organic cation vibrations in the hybrid lead halide perovskite CH3NH3PbI3, Phys. Rev. Mater. 5, 105402 (2021). [pdf]
  • Interfacial charge transfer dynamics in perovskite solar cells
  • Efficient charge separation at the interfaces between the lead halide perovskite and the carrier transport layers is crucial for perovskite solar cells to achieve high power conversion efficiency. We systematically investigate the charge transfer at the interface of MAPbI3 with typical organic and inorganic hole transport materials (HTMs). The differential transient transmission signals between the two sides of the samples reveal the hole injection on sub-picosecond to tens of picosecond time scales depending on the HTM.
  • Direct Observation of Ultrafast Hole Injection from Lead Halide Perovskite by Differential Transient Transmission Spectroscopy, J. Phys. Chem. Lett. 8, 3902 (2017). [pdf]
  • Optical generation and detection of coherent acoustic phonons in indirect gap semiconductors and their heterointerfaces
  • Photoexcitation of metal and semiconductor surfaces with femtosecond laser pulses can create coherent acoustic phonons in the form of a ballistic strain pulse, but the effect is usually much weaker for semiconductors than metals. Our pump-probe reflectivity detection with high sensitivity can successfully monitor the acoustic pulse generated in Si and GaP without an additional metallic film as a opto-acoustic transducer. The same simple optical scheme can also generate and detect ultrashort acoustic pulses at lattice-matched GaP/Si(001) interface, suggesting a potential application in non-destructive evaluation of the interface quality.
  • Intrinsic coherent acoustic phonons in the indirect band gap semiconductors Si and GaP, Phys. Rev. B 95, 035207 (2017). [pdf]
  • Sub-picosecond acoustic pulses at buried GaP/Si interfaces, Appl. Phys. Lett. 111, 062105 (2017). [pdf]
    • Coherent optical phonons at lattice-matched GaP/Si(001) hetero-interfaces
    • Unlike the surfaces that can be accessed by scanning probe microscopy and photoemission spectroscopy, the electronic structures of deeply buried interfaces can be evaluated by very limited techniques. On one hand, we apply resonant coherent optical phonon spectroscopy using near ultraviolet light pulses to all-optical evaluation of the electronic structures at buried GaP/Si(001) interface, based on the knowledge that the phonon amplitudes are proportional to the built-in surface field in polar semiconductors.
    • Non-resonant coherent optical phonon spectroscopy using near infrared light pulses, on the other hand, reveals ultrafast charge injection from the Si substrate into the GaP layer. We successfully separate the interface carrier dynamics from that of the Si substrate, based on the apparently peculiar overlayer thickness-dependence of the Si optical phonon.
    • Coherent phonon spectroscopy characterization of electronic bands at buried semiconductor heterointerfaces, Appl. Phys. Lett., 108, 051607 (2016). [pdf]
    • Separation of interface and substrate carrier dynamics at a heterointerface based on coherent optical phonons, [arXiv: 2110.03899]
    • Optical evaluation of spin damping in magnetic alloy thin films
    • Low spin damping is one of the characteristics required for magnetic alloys to be potentially applied in the nanometer-scale magnetic tunnel junctions in the magnetic random access memories. We have evaluated the spin damping constant α for thin Co2FeAl Heusler alloy films with different thicknesses by means of time-resolved magneto-optical Kerr effect measurements.
    • Increased magnetic damping in ultrathin films of Co2FeAl with perpendicular anisotropy, Appl. Phys. Lett., 110, 252409 (2017). [pdf]
    • Transient breakdown of Born-Oppenheimer approximation in graphite
    • Sub-10 fs laser pulses can excite coherent oscillations at such high-frequencies as the C-C stretching (Raman G mode) of graphite. We can monitor photo-induced weakening in the phonon softening (Kohn anomaly) as a transient upshift in C-C stretching frequency on sub-picosecond time scale. This observation can be interpreted as the photoexcited carriers being unable to follow the fast atomic motion.
    • Ultrafast Electron-phonon Decoupling in Graphite, Phys. Rev. B 77, 121402(R) (2008). [pdf]