We are studying surface plasmon photonics from the viewpoint of chemical applications using precisely controlled metallic nanostructures. The characteristic points of nanoplasmonics is to localize electromagnetic field at a nanometer-sized spatial domain (hot site) and induce an enormous enhancement as well as a steep gradient of electromagnetic field near the hot site. Therefore, various applications can be considered such as plasmonic lithography, chemical or optical sensors, light-energy conversion systems, and so forth. In this presentation, our representative studies related to chemical applications of plasmonics are demonstrated.
A novel lithography system appropriate for the formation of various shapes of nanopatterns on a positive photoresist film at a nanometer accuracy is demonstrated. To form nano-patterns based on a photomask design, we utilized a higher-order localized surface plasmon resonance mode. This mode was used to produce homogeneous scattering light that propagates the photoresist film. The essential aspect of forming fine nano-patterns is to utilize photochemical reactions of the positive photoresist via a two-photon absorption process.
Far infrared optical sensor working on the principle of volume phase transition of polymer gel triggered by plasmon-induced radiation force is also demonstrated. Closely-spaced gold nanostructures whose plasmon resonant wavelength is far-infrared region was fabricated on a silicon substrate as an optical antenna. We explored the volume phase transition of polyacrylamide gel induced by plasmon-induced radiation force in detail and elucidated the possibility of application to optical sensor in far-infrared region.
Highly-sensitive Terahertz time-domain spectroscopy (THz-TDS) is also demonstrated. The advantage of THz-TDS can pursue not only the spectrum related to molecular fingerprints but also intermolecular interactions such as hydrogen bond and van der Waals force analogous to neutron scattering spectroscopy including information about molecular motions. However, the main drawback of the THz-TDS system is less spatial resolution and sensitivity. Recently, we have successfully elucidated the phenomenon that the signals are extraordinarily enhanced by plasmonic effect and constructed surface-enhanced THz-TDS system.
The author is very grateful to Prof. H. Misawa, Mr. K. Onishi, Mr. S. Takabatake, Mrs. H. Itoh, Mrs. W. Nakano, and Mr. S. Nozawa at Hokkaido University for their experimental contributions and fruitful discussions.
k-ueno@es.hokudai.ac.jp