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Artificial periodic structures with a pich of the order of light-wavelength can have very distinct light response compared to that of the constituent materials. In case the period is so small that the composit can be treated as an effective medium, it is called a metamaterial. If the structure is appropriately designed, it may exhibit such an extraordinary response as negative refractive index that can be never realized in the materials in nature. In case the period is larger than the wavelength of light, the composite is referred to as a photonic crystal where the diffraction is essential in the electromagnetic response.

　In our sub-group, we focus on metal-dielectric artificial compostes that can be readily tailered in such a way that we design light-matter interaction at will. We design the structure by numerical simulation, fabricate the structure using nanofabrication techniques and measure the optical response using laser systems in order to investigate novel physical phenomena such as negative index of refraction、magnetism in optical frequency, photo-induced voltage due to translational and angular momentum of light
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Left： perforated Au thin film fabricated by electron-beam lithography　Right： Periodic structure with asymmetric unit cell.

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Organic materials with conjugated p-electrons have much interests because they are model compounds of low-dimensional systems. Moreover, they make higher order structures. For example, LH2 complex in bacterial photosynthesis (Left Figure) has a beautiful ring structure. A carbon nanotube can encapsulate other molecules (Center Figure). We are investigating ultrafast dynamics in the complex systems to find new optical functions.

　We are developing femtosecond spectroscopies to measure ultrafast phenomena in the conjugated p-electron systems. Using tunable multi pulses, a proper state in the material can be selectively excited by linear and nonlinear photoexcitation. The dynamics following the excitation are measured using femtosecond absorption, fluorescence, and Raman spectroscopies.

Left: Pigment-protein complex of photosynthesis, Center: Carbon nanotube encapsulating molecule, Right: Equipment of laser spectroscopy