This paper reviews our recent studies on ultrafast, strong-field interactions with atoms, molecules and solid surfaces. The major interest in atomic and molecular processes is concerned with nonperturbative nonlinear processes induced with intense ultrashort laser pulses, especially, high-order harmonic generation from nonadiabatically aligned molecules and its dynamics measurements. Experimental and theoretical results demonstrate characteristic properties of the harmonic generation from coherently rotating molecules, depending on molecular structures. Concerning the interaction with solid surfaces, the periodic nanostructure formation observed in femtosecond laser ablation of thin film surfaces has been the subject for the purpose of potential applications of ultrashort laser pulses to nanoprocessing. It is found that the nano-scale ablation is initiated with laser-induced near-field on solid surfaces, and the nano-periodicity is attributed to the excitation of surface plasmon polaritons. The results obtained show that the intense ultrashort-pulse laser is a promising tool for investigating new and/or latent dynamics and functions of matter.

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  By using four-wave-mixing technique, we demonstrated the possibility of the real space mapping of exciton-exciton interaction and strain in GaN films grown on various substrates. The image obtained from the evaluation of the quantum beats indicates that these reductions correspond to the phase changes between two exciton transitions, reflecting the spatial properties of exciton-exciton interactions. The line-localization suggests a contribution of line-defect to the anomalous interactions.

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  Synchrotron radiation, electromagnetic wave emitted by relativistic electrons traveling in a strong magnetic field, is intense, highly collimated, highly polarized, pulsed and extremely broadband. Nowadays, synchrotron radiation is widely used various research fields. Although many of its applications are in the X-ray range, it is also used in the infrared and terahertz range. In these years, coherent synchrotron radiation has been studied at several synchrotron radiation sources, which is more intense than the normal one by many orders of magnitudes. New light source technologies based on coherent synchrotron radiation are being developed, which will be powerful tools for terahertz applications.

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  The optics and one of applications of the terahertz synchrotron radiation are introduced. The ideal optics for collecting bending-magnet radiation, namely three-dimensional “magic mirror”, is employed at two Japanese infrared/terahertz (IR/THz) beam lines, one is at UVSOR-II and the other at SPring-8. The brilliance at the beam line at UVSOR-II is one order and two —four orders of magnitude higher than conventional light sources in the middle-IR and THz regions, respectively. The very high brilliant THz light is useful for the THz spectroscopy at high pressures realized in a diamond anvil cell.

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  Special type of microscope has been developed of which all optical components were installed in a ultra high vacuum (UHV) chamber and separeted by CVD diamond windows from the low-vacuum part of the FT-IR interferometer. The so-call UHV microscope covers very wide wavelength regions from ultraviolet to far-infrared/Terahertz wave and make possible the spectromicroscopic measurements in the whole spectral region without any replacement of the optical windows. The details of the fundamental concepts of the development and the design of the UHV microscope, and its recent application to far-infrared spectroscopy on solids under ultra high pressures are described.

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  We introduce quantitative investigation of initial oxidation process on Si(001)-(2×1) by means of linear optical spectroscopic methods, namely, surface differential reflectance spectroscopy and reflectance difference spectroscopy. Our recent results obtained with these real-time measurement techniques revealed that the transition between two different oxide growth modes, Langmuir-type adsorption and two-dimensional island growth, could be identified. The activation energies were estimated from Arrhenius plots of the oxidation periods for the growth of an oxide monolayer. Our results suggest that a finite activation energy exists for monolayer oxide formation on Si(001)-(2×1) at high temperatures.

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  Freestanding BaTiO₃ (BTO)/Pt films were fabricated and the orientation and crack density of BTO films were investigated as a function of substrate temperature, T_sub, during BTO deposition. When T_sub is low, the BTO film exhibits preferentially (101)-orientated structure. On the other hand, (001)-orientation becomes dominant when T_sub is high. This means that controlling of orientation by changing the T_sub is possible. In addition, the density of cracks of the BTO film on the Pt foil was lower than on the Pt/Ti/SiO₂/Si substrate, as a result of the removal of clamping effect of the substrate.

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