We are promoting advanced research on photophysical and photochemical phenomena characteristic of interactions between intense laser beam and organic materials and their analysis by using time-resolved spectroscopy and imaging methods. Also development and application of laser nano-manipulation and nano-fabrication techniques are extended in view of molecular science and engineering.

Laser induced crystallization

When an infrared laser is focused under a microscope, the proteins and organic molecules which are transparent for the laser light are trapped at the laser focal point by the photon pressure. We have succeeded for the first time to initiate crystallization by the laser trapping. The artificial control of the crystallization enables us to investigate the crystallization dynamics and mechanism under various nucleation conditions.

Fig. 1 Glycine crystal generated by optical pressure
Chem. Lett., 36, 1, 1480-1481 (2007)

When an intense femtosecond laser pulse is focused in water, on the other hand, micro explosion is induced at the laser focal point, where photons are highly condensed in the time range of 1 / 100,000,000,000,000 sec. We have named the explosion “Laser Micro/Nano Tsunami”. When the femtosecond laser is focused by highly numerical aperture objective lens, “Tsunami” is localized in the ultra small scale of ?m and raises the transient force with magnitude of 1,000,000 times larger than that of laser trapping. We have applied this “Tsunamni” to single cell manipulation and to trigger protein crystallization. We are investigating the dynamics and mechanism of “Tsunami” induced by shockwave and cavitation bubble generations in water.

Fig. 2 Laser Micro “TSUNAMI” captured by high speed camera
Appl. Phys. Lett., 91, 2, 023904 (2007)

Optical trapping and assembling dynamics of nanoparticles

We report optical trapping and assembling of colloidal particles at a glass/solution interface with a tightly focused laser beam of high intensity. It is generally believed that the particles are gathered only in an irradiated area where optical force is exerted on the particles by laser beam. Here we demonstrate that, the propagation of trapping laser from the focus to the outside of the formed assembly leads to expansion of the assembly much larger than the irradiated area with sticking out rows of linearly aligned particles like horns. The shape of the assembly, its structure, and the number of horns can be controlled by laser polarization. Optical trapping study utilizing the light propagation will open a new avenue for assembling and crystallizing quantum dots, metal nanoparticles, molecular clusters, proteins, and DNA.

Fig. 3 Optical trapping formed colloidal assembly with horns due to light propagation
Nano Lett., 16, 3058-3062 (2016)

Single nanoparticle spectroscopy

Useally nanoparticles are measured, characterized, and considered as an ensemble, where information on averaged physical and chemical properties is obtained. Each nanoparticle has its own shape, size, inner structure, and local environment which determine its spectral peak and width as well as its relaxation dynamics. It is necessary to extract the real nature of each nanoparticles from averaged peak energy and wide spectral shape, which always has some ambiguity. Thus, single nanoparticle spectroscopy is indeed necessary and indispensable to understanding the physical and chemical properties of nanoparticle by correlationg spectral data to their morphological characteristics.

Fig. 4 A schematic diagram of single particle spectroscopy.

On the basis of this strategy, we have developed several microspectroscopic techniques and investigated inorganic and organic nanoparticles at single particle level (The details are described in recent literatures. Please see "publication" in this web site). Especially, we developed light scattering microspectroscopy to visulalize and to measure electronic spectra of single non-fluorecent nanoparticles. This enables us to obtain direct correlation between size/shape of non-fluorescent nanopaticles and their spectroscopic properties. And this is also helpful for spectroscopic sensing at "nanometer dimension" because some nanoparticles can act as a stable spy in nanodimension. We are now waiting for offering us collaborative works to characterize nanomaterials and/or to demonstrate the potential of them by this methotology.

Fig. 5 Light scattering image and spectra of gold nanoparticles embedded in zeolite L cyrstals.
J. Phys. Chem. C, 112, 15089 (2008)

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