Keisuke Imaeda

Optical control of plasmonic fields by phase-modulated pulse excitations

We developed an advanced near-field optical method by combining an ultrafast near-field optical microscope with a prism-based pulse shaping system. We used this apparatus to visualize plasmonic optical fields and to measure the lifetime of plasmons excited on a rough gold film. We also studied the influence of the phase-modulation of the excitation pulse on the spatial distribution of the optical fields. We found that the spatial distribution of the optical fields can be controlled by a negatively chirped pulse.

Static and Dynamic Near-Field Measurements of High-Order Plasmon Modes Induced in a Gold Triangular Nanoplate

Precise understanding of the spatiotemporal characteristics of plasmons is essential for the development of applications of plasmonic nanoparticles. In this study, we investigated the spatiotemporal properties of high-order plasmon modes induced in a gold triangular nanoplate by static and dynamic near-field measurements. The near-field transmission measurements revealed that in-plane and out-of-plane polarized plasmon modes were simultaneously excited and these modes spectroscopically and spatially overlapped. The superposition of these modes was visualized in the near-field two-photon excitation image of the nanoplate. We performed time-resolved autocorrelation measurements on the nanoplate and found that the correlation width was broader than the excitation pulse due to the plasmon dephasing process. From the correlation width map of the nanoplate, we experimentally demonstrated that the out-of-plane plasmon mode exhibits a longer dephasing time than the in-plane plasmon mode. These findings indicate that the out-of-plane mode is desirable for improving the performance of plasmons in various applications.

Near-field optical imaging of ultrafast dynamics in gold nanorods

When a metal nanostructure is locally photoexcited at a resonant wavelength, a plasmonic wave packet is generated and propagates on the surface of the nanostructure. The wave packet dynamics is closely related to plasmon mode characteristics of the material, and is regarded as one of major origins of the unique optical characteristics of metal nanostructured materials. Spatial scales of plasmon resonances are essentially smaller than the wavelengths of resonant radiation fields (or the diffraction limit), and thus it requires a method of measurement with a spatial resolution beyond the diffraction limit of light to visualize the wave packets on the materials. The time scale of plasmon dynamics in gold nanostructures is faster than 20 fs, and thus it requires a time resolution of 10 fs or higher to investigate directly the dynamic processes. In this study, we developed an ultrafast time-resolved near-field optical microscope and visualized the plasmon wave packet dynamics in gold nanostructures [1, 2].