New Research


2014/02/01

Manipulation of photochemical reactions by laser-induced multiphoton processes.

Professor Hiroshi Miyasaka, Associate Professor Yutaka Nagasawa, Assistant Professor Syoji Ito

   Laser light has several specific properties. Pulsed oscillation with very short time durations and high directivity are representative ones. Ultrashort laser pulse can be applied to the investigation of the chemical reactions induced by the photoexcitation. Detailed information on the reaction profiles with high temporal resolution can provide the rational principle for the designing of advanced molecular systems. On the other hand, high directivity of lasers allows tight-focusing of light into a small area, where extremely high light field is created. Especially, the focusing of ultrafast laser pulse temporally and spatially confines the light energy into the tiny spot and the intense light filed can induce specific responses of molecules, which are known as nonlinear optical phenomena.

   By using these properties of lasers, we are studying the fundamental processes of molecules in the excited state, and control/ manipulation of molecules and materials through the nonlinear responses. Here, we introduce the latter topics on the laser-induced phenomena.

   Photochromic molecules can switch their colors by photochemical reactions, of which property is attracting much attention also from the viewpoints of the application into optical memories and displays. Figure shows the photograph of a photochromic solid molecular film under the microscope exposed with the invisible near-infrared femtosecond laser pulse at 1.27 mm. Colored and uncolored isomers of this molecule have no absorption at 1.27 mm. The confinement of the femtosecond laser pulse under the microscope, however, induces the simultaneous multiphoton absorption processes and controls directions of the reaction. That is, the 3-photon absorption of the uncolored-form leads to the colorization and 2-photon absorption with lower intensity induces the selective excitation of the colored-form resulting in the decolorization. This result demonstrates that the one-color reversible control of colorization and decolorization reactions can be attained only by modifying light intensities, although we usually need two light sources with different wavelengths to control the direction of the reaction. In addition, by using the switching ability of the reaction directions through the light intensity, we can form a spatial patterning in the tiny spot.

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