Fumika Nagasawa

Permanent fixing or reversible trapping and release of DNA micropatterns on a gold nanostructure using continuous-wave or femtosecond-pulsed near-infrared laser light.

The use of localized surface plasmons (LSPs) for highly sensitive biosensors has already been investigated, and they are currently being applied for the optical manipulation of small nanoparticles. The objective of this work was the optical trapping of λ-DNA on a metallic nanostructure with femtosecond-pulsed (fs) laser irradiation. Continuous-wave laser irradiation, which is generally used for plasmon excitation, not only increased the electromagnetic field intensity but also generated heat around the nanostructure, causing the DNA to become permanently fixed on the plasmonic substrate. Using fs laser irradiation, on the other hand, the reversible trapping and release of the DNA was achieved by switching the fs laser irradiation on and off. This trap-and-release behavior was clearly observed using a fluorescence microscope. This technique can also be used to manipulate other biomolecules such as nucleic acids, proteins, and polysaccharides and will prove to be a useful tool in the fabrication of biosensors.

Raman Enhancement via Polariton States Produced by Strong Coupling between a Localized Surface Plasmon and Dye Excitons at Metal Nanogaps.

Polarized Raman scattering measurement was carried out using a hybridized system of Ag nanodimer structures and organic dye molecules. Tuning of the localized surface plasmon resonance energy leads to modulation of the hybridized polariton energy. The anticrossing behavior of the polariton energy implies a strong coupling regime with maximum Rabi splitting energy of 0.39 eV. The observation proves the effective Raman enhancement via the excitation of the upper and the lower branches of the hybridized states at the gap of the metal dimer. Maximum Raman enhancement was obtained at an optimized resonant energy between the hybrid states and Raman excitation.

Highly Sensitive Detection of Organic Molecules on the Basis of a Poly(N-isopropylacrylamide) Microassembly Formed by Plasmonic Optical Trapping

We demonstrate that a poly(N-isopropylacrylamide) (PNIPAM) microassembly, formed by plasmonic optical trapping, can provide the platform for a highly sensitive detection technique for fluorescent and nonfluorescent organic molecules dissolved in aqueous solution. PNIPAM microassemblies can be easily formed by a combination with a photothermal effect and an enhanced optical force. These physical phenomena were obtained through resonant excitation of localized surface plasmon (LSP). Sparsely distributed fluorescent or nonfluorescent molecules dissolved in solution can be extracted into the PNIPAM assembly, resulting in an increase in fluorescence or Raman signals. In particular, we successfully detected quite small amounts of analytes (rhodamine B) at the 10-9 mol/L level. Using LSP is an alternative approach in analytical chemistry and can be used in addition to surface enhanced Raman scattering and surface enhanced fluorescence.

The Depolarisation Behaviour of Surface-Enhanced Raman Scattering Photons in a Metal Nanodimer Structure

The SERS signal showing highly polarised characteristics generally reflects the anisotropic electromagnetic feature of the metal nanostructures. In this chapter, characteristic response of polarised SERS from molecules strongly adsorbed on metal nanostructure was measured to discuss the electronic state at the interface. Polarised SERS measurements give information about both the incident and Raman scattering polarisations, and several interesting polarised SERS behaviour was observed. The polarisation dependence on SERS from a well-ordered Ag dimer array was measured in an aqueous solution containing the target molecule, 4,4’-bipyridine. The polarisation dependence of the scattering photons is discussed with respect to the optical properties of the metal nanostructure and adsorption structure of the target molecule.

Electronic Excitation of an Isolated Single-Walled Carbon Nanotube by Tuning Electrochemical Potential

Localised surface plasmon resonance is an effective perturbation to transfer the energy of photons to electrons in the material/molecule interface. Electronic excitation is extremely important in controlling the orientation of the molecules situated on the metal surface and the electronic state at the interface. Single-walled carbon nanotubes (SWNTs) represent a promising tool as an analyte because of their electronic structures, which are well defined by the geometrical structure characterised by chirality. In this chapter, the author measure the polarised Raman measurement performed in the electrochemical environment using a metal nanodimer array.

Simultaneous Measurement of Surface-Enhanced Raman Scattering and Conductance Using Mechanically Controllable Break Junction Technique

Ensemble measurement of the adsorbed molecules and each metal structure showed fluctuation in the depolarisation SERS behaviour and the intensity ratio in the same spectrum. For further investigation of this depolarisation behaviour to know the details on the photoexcitation process, it is important to fabricate a single molecular junction combined with spectroscopy of the single molecule. In this chapter, simultaneous measurement of conductance and polarised SERS in relation to the molecular bridged metal nanojunction is performed. The conductance measurement proves the number of molecules, while polarised Raman measurement proves the orientation and CT character.

Electrochemical Control of Strong Coupling Between Localised Surface Plasmons and Dye Excitons

An LSP can be regarded as confined light to provide a highly polarised EM field with a specific resonant energy that can effectively interact with excitons of materials. Thus, especially in the strong-coupling regime using LSPs, these properties have attracted wide research interest as an entirely new concept of molecular plasmonics. In this chapter, electrochemical potential control is adopted for tuning the LSPR energy and the definition of the electronic properties of the strong-coupling regime. Raman spectra are also acquired to explore the molecular state in the strong-coupling regime.