Laser has high potential in advancing chemical research by realizing new spectroscopy, analysis, reaction, and fabrication, while its spatial resolution was limited to light wavelength. Dr. Hiroshi Masuhara has utilized various lasers and microscopes, developed new spectroscopy and imaging methods with nm resolution, explored novel nm chemical phenomena, elucidated their mechanism and dynamics, and extended the studies to material and bio applications. His achievement is summarized as follows.
1. Laser Nano Spectroscopy and Nano Photochemistry
Dr. Masuhara and his colleagues developed various time-resolved reflection spectroscopies and demonstrated how very essential they are for solid state photochemistry. They initiated and contributed by a series of seminal papers to the development of ps- and fs-diffuse reflectance, ps- and fs-regular reflectance, ps-total internal reflectance (evanescent wave absorption and fluorescence), and fs-transient grating spectroscopies. Photophysical and photochemical dynamics of nano crystals, nm-thin films and nm-surface/interface layers of solids were measured and analyzed with high energy and temporal resolutions similar to those in solution. Ultrafast intersystem crossing, charge separation, exciplex formation, and photothermal heating processes characteristic of the "solid" state were elucidated and first reported by the Masuhara group for aromatic molecules, dyes, EDA complexes, TiO2, polymers, and resists.
The single particle approach has been applied to understand spectroscopic properties and photophysics and photochemistry of nanoparticles. Spectroscopy of individual nanoparticles is examined as functions of their shape, size, morphology, internal structure, and environment. By combining an inverted optical microscope and AFM, the Masuhara group developed novel detection techniques such as Rayleigh light scattering spectroscopy of individual nanoparticles in addition to single particle fluorescence spectroscopy. Novel nm size effects on aromatic/polymer crystals and polymer nanospheres were investigated and their origins were confirmed to be structural confinement, namely, size-dependence of crystal lattice softening, molecular packing, and polymer conformation. These novel effects are characteristic of molecular material, and completely different from well-known nm size effects of semiconductors and metals which are due to electron confinement. This single nanoparticle spectroscopy is complementary to single molecule spectroscopy and can bridge the gap in understanding the relation between properties of molecules and materials.
2. Laser Nano Manipulation and Chemistry of Photon Pressure
A focused near-infrared laser beam exerts photon force on nano materials, enabling their manipulation. The optical trapping studies have been conducted by physicists, while its extension to chemistry was started by the Masuhara group. The minimum size of the trapped particles in solution at room temperature was confirmed to be a few nm. The force is strong enough to suppress electrostatic repulsion between electrolytes and to break hydrogen-bonding network around polymers in water. Larger photon force is exerted on molecules with higher polarizability. Such molecular structure-photon force relation opened a new field that could be coined "chemistry of photon force".
This is further being extended to trapping dynamics of single nano particles in solution at room temperature. For 100 nm-sized particles, successive trapping was followed one by one, while photon-force assisted-aggregation was clearly confirmed. Furthermore, absolute potential shape of an optical trapping-well was determined by measuring directly fluctuation of a single probing microparticle in solution, while spectroscopic identification of assembled structures in the trapping potential was done for model gold nanoparticles. By the 3-dimensional trapping and manipulation of nanoparticles and their fixation onto substrate, the nanoparticles were patterned with resolution of a few tens nm. Although the spatial resolution is less than that of the 2-dimensional surface manipulation by a STM tip at low temperature under vacuum, the much broader applicability is very important for biological material, cells, and protein crystals.
3. Laser Nano Ablation: Dynamics and Bio Application
Intense pulsed laser excitation of nano droplets, aggregates, crystals, powders, and films generates high density excited states and intermediates. Their mutual interactions and their successive absorption of excitation photons lead to ablation, expansion/contraction, surface protrusion, and so on. The Masuhara group developed time-resolved imaging methods to probe laser-induced morphological dynamics and combined them with the time-resolved spectroscopy. Then they extended systematic studies on ns-, ps-, and fs-laser ablation and related phenomena and determined rates of expansion, surface roughening, ablation, and contraction. Namely, the Masuhara group demonstrated how electronic excitation of molecules in solids evolves leading to morphological changes. On the basis of these results, molecular mechanisms of laser ablation could be proposed: a cyclic multiphotonic absorption mechanism in the case of ns-excitation and for fs-excitation a transient pressure mechanism due to rapid photothermal conversion. All the processes are within the framework of a classic Jablonski diagram without additional states and intermediates such as plasma. Hence laser ablation and related dynamics are now understood as typical nonlinear photochemical phenomena.
By utilizing the transient pressure induced by fs-excitation, Masuhara and his colleagues unraveled novel laser ablation phenomena and developed new methodologies. Femtosecond multiphoton excitation of molecular films gives discrete and multistep etching, fs ablation of microcrystals in solution gives the smallest nanocrystal of a dye with a size of 13 nm as a stable nanocolloid, and a shockwave induced by the transient pressure can remove a living cell from a substrate without damage. Recently the Masuhara group has succeeded in the preparation of high quality crystals of proteins and their model compounds by introducing fs-pulse into supersaturated protein solution. In addition, they succeeded in controlling the crystal growth with fs-multiphoton excitation. These results have important implications in the area of bioscience.
As described above, Dr. Masuhara has proposed and demonstrated new methodologies and concepts of nano chemistry which can be realized only by laser. His scientific achievement has a great impact not only on chemistry but also on physics, nano material engineering, and life science, and is opening frontier in photoscience. It has been recognized widely and internationally, and is enough eligible for the Chemical Society of Japan Award.