We review our recent studies on ultrafast vibrational dynamics of adsorbates on metal surfaces under the influence of high electron temperature induced by femtosecond laser excitation. Energy transfer dynamics between substrate electron and adsorbate vibrations mediated by nonadiabatic coupling has been investigated by time-resolved nonlinear optical spectroscopy including infrared-visible sum frequency generation and second harmonic generation. In particular, we focus upon CO and alkali metal atoms on Pt(111) and Cu(111) surfaces.
A new microscopy, optical pump-probe scanning tunneling microscopy (OPP-STM), which enables femtosecond temporal resolution and atomic spatial resolution simultaneously, has been realized by combining STM with quantum optical method. Using circularly polarized light for excitation, the measurement of local ultrafast spin dynamics has also become possible. Detection of spin precession has enabled to provide local information of g-factor. Here, the bases for the new microscopy techniques are overviewed.
Molecular-level understanding of interfaces is of crucial importance in many fundamental and applied sciences. Vibrational sum frequency generation (VSFG) spectroscopy has intrinsic interface-selectivity and has been widely utilized to study molecular structures and dynamics at interfaces. However, conventional VSFG spectroscopy measures the intensity of the signal, providing only information about the square of second-order nonlinear susceptibility, |χ(2)|2. Recently, we developed multiplex heterodyne-detected (HD-) VSFG spectroscopy. This HD-VSFG spectroscopy enables us to measure the imaginary part of χ(2), Imχ(2), which can be directly compared to infrared absorption and Raman spectra. Moreover, we have extended the HD-VSFG spectroscopy to time-resolved (TR-) HD-VSFG spectroscopy and two-dimensional (2D) HD-VSFG spectroscopy by combining with the pump-probe technique. This review describes development of these time-resolved HD-VSFG methods and their applications to the water interfaces.
We have successfully developed plasmon-induced water splitting system that operates under irradiation by visible light. Most important feature in this system is based on the use of both sides of the same strontium titanate (SrTiO3) single-crystal substrate without an electric wire connecting the anode and the cathode. The water splitting was induced, even with a minimum chemical bias of 0.23 V due to plasmonic effects based on the efficient water oxidation. Furthermore, we succeeded in visualizing the local field distribution of the localized surface plasmon resonance (LSPR) on gold nanoparticles and elucidating the dynamics of the LSPR by a time-resolved photoemission electron microscopy.
Pump-probe reflectivity measurements on n-type GaAs with different probe light wavelengths reveal the phonon-plasmon dynamics in the bulk and at the surface depletion layer. Photoexcited hole plasma screen the coherent LO phonons within a few hundred femtoseconds at the surface, whereas impurity-doped electron plasma do so in the n-doped bulk. The coherent responses demonstrate strong interaction between the lattice and charge carriers under inhomogeneous creation and sub-picosecond transport.
Basic concept of recently-developed terahertz time-domain spectroscopy and linear and nonlinear terahertz responses of thin solid films have been reviewed. The ferroelectric soft mode driven to the anharmonic region was observed in SrTiO3 thin films using a few-cycle intense terahertz pulses. The metallic surface states of Bi ultrathin films were detected in terahertz transmittance measurements. The superconducting gaps in MgB2 and YBCO films were observed using broadband terahertz time-domain spectrometer. These results demonstrate the promising capability of terahertz time-domain spectroscopy for characterization of physical properties of thin films and the surface states.