Excited-state dynamics of metal nanostructures studied by ultrafast near-field spectroscopy 超高速近接場分光法による金属ナノ構造の励起ダイナミクスの研究
Excited-state dynamics of metal nanostructures studied by ultrafast near-field spectroscopy
We have developed an apparatus for ultrafast near-field microscopic measurements based on a SNOM with an apertured optical-fiber probe. To achieve very high (<20 fs) time resolution at the near-field probe tip, we tried several GVD compensator setups to compensate the dispersion arising from the fiber and compress the pulse duration. The prism pair - DFM combination setup, the grating pair - DFM combination setup, and the prism pair - grating pair - DFM combination setup were examined. We found that the former two setups could not fully remove the GVD arising from the optical elements, the optical fiber for the near-field probe in particular. This is probably because precise pre-compensation of the third-order GVD as well as the second-order one is difficult, after transmitting through 150-mm optical fiber medium, if we install only the prism pair or the grating pair.We have found that the combination of the prism pair, the grating pair, and DFM have capability to (nearly) fully remove the GVD and almost recover the original pulse width at the probe tip. The near-field pulse duration observed was ~17 fs, which is the shortest pulse duration ever obtained in an aperture-type SNOM system. Very recently, another group achieved generation of ~16-fs optical pulses with a sharp spatial confinement (~10 nm) by using a gold tip with a grating structure . Time resolution is the same order to as that in the present study. In this method, the shortest pulse duration may be limited by the dephasing time of plasmons at the tip, since the spatial confinement is based on the plasmonic excitations. Similar result was also reported for a gold-coated glass fiber tip. The present method does not have, in principle, such a limitation in the pulse width if the coating material of the probe is appropriately selected, and may have the potential to achieve near-field microscopy with an even higher time resolution (<10 fs).We applied this high time-resolution SNOM system to observe ultrafast dephasing of SPR in a gold nanoparticle, based on the time-correlated TPI-PL measurement. The dephasing time constant of the order of ~10 fs was obtained by the fitting procedure, which is in good agreement with the result previously reported based on the scattering spectral profile analysis. This is the fastest dynamics of a single-particle material ever observed directly in the time-domain by SNOM. Since the dephasing time of SPR is affected by the surface scattering, spatially resolved dephasing measurements for single nanostructures may yield fundamental information on the decay mechanism. As a next step, we are now conducting space-resolved plasmon dynamics measurements in individual metal nanoparticles.