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Near-Field Optical Microscopy, Plasmons, Excited States of Nanomaterials
Development of Advanced Near-Field Spectroscopic Imaging and Application to Nanomaterials
There is much demand for the studies of local optical properties of molecular assemblies and materials, to understand nanoscale physical and chemical phenomena and/or to construct nanoscale optoelectronic devices. Scanning nearfield optical microscopy (SNOM) is an imaging method that enables spatial resolution beyond the diffraction limit of light. Combination of this technique with various advanced spectroscopic methods may provide direct methods to probe dynamics in nanomaterials and nanoscale functionalities. It may yield essential and basic knowledge to analyze origins of characteristic features of the nanomaterial systems. We have constructed apparatuses of near-field spectroscopy and microscopy for excited-state studies of nanomaterials, with the feasibilities of nonlinear and time-resolved measurements. The developed apparatuses enable near-field measurements of twophoton induced emission, femtosecond time-resolved signals,
and circular dichroism, in addition to conventional transmission, emission, and Raman-scattering. Based on these methods, we are investigating the characteristic spatiotemporal behavior of various metal-nanostructure systems and molecular assemblies. Typical examples are shown in Figure 1. We succeeded in visualizing wave functions of resonant plasmon modes in single noble metal nanoparticles, confined optical fields in noble metal nanoparticle assemblies, and so forth.
Figure 1. (Left four panels) Near-field transmission images of gold nanorod (20 nmD × 510 nmL). The wavelengths of observation were 647, 679, 730, and 830 nm from left to right. The spatial oscillating features were attributed to the square amplitudes of the resonant plasmonic wave functions. (Right) Near-field two-photon excitation image of dimers of spheric gold nanoparticles (diameter 100 nm) observed at 785 nm. The arrows indicates the incident light polarization. Dotted circles represent approximate positions of the particles.
- H. Okamoto, T. Narushima, Y. Nishiyama and K. Imura, “Local Optical Responses of Plasmon Resonance Visualized by Near-Field Optical Imaging,” Phys. Chem. Chem. Phys. 17, 6192–6206 (2015).
- H. Okamoto and K. Imura, “Visualizing the Optical Field Structures in Metal Nanostructures,” J. Phys. Chem. Lett. 4, 2230–2241 (2013).
- H. Okamoto and K. Imura, “Near-Field Optical Imaging of Enhanced Electric Fields and Plasmon Waves in Metal Nanostructures,” Prog. Surf. Sci. 84, 199–229 (2009).