Yukie Yokota

Nanoparticle Plasmon-Assisted Two-Photon Polymerization Induced by Incoherent Excitation Source

We demonstrate the possibility to achieve optical triggering of photochemical reactions via two-photon absorption using incoherent light sources. This is accomplished by the use of arrays of gold nanoparticles, specially tailored with high precision to obtain high near-field intensity enhancement.

Highly controlled surface-enhanced Raman scattering chips using nanoengineered gold blocks.

Well defined gold nanostructures of various sizes are fabricated on glass substrates using high-resolution electron-beam lithography/lift-off techniques and detailed surface-enhanced Raman scattering (SERS) properties of crystal violet molecules are studied in order to elucidate electromagnetic (EM) field enhancement effects on the fabricated structures. SERS measurements are performed with high reproducibility using in situ Raman microspectroscopy in aqueous solution. An analysis based on EM theory is performed using field-enhancement factors obtained from finite-difference time-domain (FDTD) simulations and the analysis reproduces experimental results very well. It is noteworthy, furthermore, that the proposed analytic method of EM effects on SERS allows the estimate of the ideal local temperature of gold nanostructures by canceling out the difference in EM field factors at Stokes and anti-Stokes Raman scattering wavelengths. Thus, these experimental results demonstrate that quantitative analysis based on EM theory can be obtained using highly controlled gold nanostructures for SERS measurements with high reproducibility, a result that is promising for the construction of a SERS analysis chip. Although no SERS chip reported so far has been usable for quantitative analysis, this study opens the door for construction of a quantitative SERS chip.

Direct imaging of nanogap-mode plasmon-resonant fields

We perform direct local-field imaging of a plasmon-resonant gold nanoparticle pair separated by a gap of several nanometers using a scattering-type near-field optical microscope with a sharp silicon tip of atomic force microscope. The sharp tip allows the access for the nanogap and the high spatial resolution. Our results provide experimental evidence that the nanogap structure produces an optical spot with the size of a single nanometer (<10 nm). This is not only of fundamental importance in the field of nanophotonics, but also provide significant information for the development of plasmonic devices with the nanogap structures.

Enhancement of a Two-Photon-Induced Reaction in Solution Using Light-Harvesting Gold Nanodimer Structures

We performed a quantitative analysis of plasmon-assisted two-photon photochromic reactions on light-harvesting gold nanodimer structures. Our strategy for the quantitative analysis of two-photon-induced photochemical reactions on gold nanostructures is using not only a confined photochemical reaction chamber but also a solution system. The strong intensification of near-field light at the nanogap positions on gold nanodimer pairs promoted two-photon absorption by a closed-form diarylethene derivative, resulting in highly efficient photochromic conversion to the open-form structure.

Spectral properties and mechanism of instability of nanoengineered silver blocks

The instability of silver nanoblocks under atmospheric conditions is investigated. The localized surface plasmon resonance band of the silver nanoblocks shows a red shift, broadening, and damping with increasing storage time under atmospheric conditions. The change in spectral properties of silver nanoblocks is considered to be due to sulfidation of silver and structural breakage of silver nanoblock based on scanning electron microscope observation and numerical simulation. The effect of aspect ratio of silver nanoblocks on the change in spectral properties of the nanoengineered silver blocks is also discussed.

Spectral properties of nanoengineered Ag/Au bilayer rods fabricated by electron beam lithography

Ag/Au bimetallic nanoparticles possess the combinatory advantages of Au and Ag nanoparticles and can also be utilized to tune the properties of localized surface plasmon resonance. Ag/Au bilayer nanorods were prepared by electron beam lithography, and their spectral properties were investigated. Compared with Ag monolayer nanorods, Ag/Au bilayer nanorods show broader localized surface plasmon resonance bands, and the longitudinal mode and transverse mode localized surface plasmon bands show blueshift and redshift, respectively. The maximum near-field intensity of the longitudinal mode of the Ag/Au nanorod is less than half that of the Ag/Au nanorod without gold layer. Shape-induced modification of Ag/Au bilayer nanorods on their spectral properties was also discussed.

Optical characterization of plasmonic metallic nanostructures fabricated by high-resolution lithography

Highly homogeneous arrays of Ag, Au and Cu nanorods were fabricated on glass substrates using electron-beam lithography and lift-off techniques. Optical properties of the fabricated structures related to localized surface plasmons (LSP), and their dependencies on the nanorod size were studied experimentally by optical extinction spectroscopy. Spectral tuning of LSP resonant scattering bands in a wide spectral range, from visible to near-infrared wavelengths, can be accomplished by tailoring of the nanorod dimensions, aspect ratios, and heights. The observed results qualitatively agree with Gans theory and numerical modeling by finite-difference time-domain technique.

Polarization Dependence for Enhancement of Near-Infrared Fluorescence Intensity by Local Surface Plasmon Resonance from Arranged Gold Nanoblocks

The polarization dependence of excitation light for fluorescence enhancement of near-infrared dyes immobilized in hydrophobic DNA thin film on glass substrates with regularly arranged Au nanoblocks was investigated. The enhancement factor for S-polarized excitation was about 1.2 to 1.5 times as large as that for P-polarized excitation. P-polarized light contributed to the excitation of plasmon band both of horizontal and vertical direction of Au nanoblocks. The vertical direction corresponds to the short axis for height direction which has the absorption in visible region did not contribute the excitation of IR780. When the absorption band of Au array and the fluorescence band of IR780 were overlapped, the fluorescence enhancement did not depend on the polarization of excitation light. It is suggested that the fluorescence quenching originates from the energy transfer from the excited state of IR780 to Au nanoblocks or the increased deactivation of excited dye molecules induced by resonance with Au nanoblocks.

Modifying Plasmonic Spectral Properties of Engineered Silver Nanoblocks by Using Titanium Coating

Plasmonic spectral properties of silver nanoblocks fabricated by electron beam lithography are investigated. The extinction spectra of silver nanoblocks showed a red shift, broadening, and damping with the passage of storage time. In contrast, silver nanoblocks with titanium coating on the top did not show apparent spectral change even after storage for two months. Morphological change of the silver nanoblocks was characterized by scanning electron microscope. The present work clearly demonstrates that coating titanium on the top of the silver nanostructures is an effective way to prevent them from chemical corrosion and stabilize their plasmonic spectral properties.

Design of plasmonic metamaterials for photothermal spectroscopy and application in molecular detection(Conference Presentation)

Micro/nanofluidics have attracted much attention as ideal platforms for bioanalysis. The interest in micro/nanofluidics has resulted in a high demand of non-label detections in ultra-small volume(aL-fL). Among existing optical detections in micro/nanofluidics, photothermal spectroscopy (PTS) is an important approach, which allows detection in non-label fashion. Its principle is based on the detection of refractive index change following the thermal relaxation when molecules absorb light. Many types of PTS have been developed for microfluidics, yet the sensitivity of PTS becomes an issue in sub-micrometer spaces where thermal diffusion is dominant. On the other hand, plasmonic metamaterials, which offers unique surface condition with tailored absorption properties and strong plasmonic enhancement is widely utilized to improve the sensitivity of absorption, fluorescence, Raman spectroscopies, etc. Recently, we proposed the integration of metamaterials to improve the sensitivity of PTS. However, the strong absorption of metamaterials itself is an obstacle for detection. In this study, we propose an idea of exploiting the electromagnetically induced transparence (EIT) phenomena to suppress the absorption in metamaterials. As the EIT peak is tailored to the absorption peak of detecting molecules or the excitation light, the absorption from metamaterials is negligibly small while the strong field enhancement can be achieved. The numerical calculation of absorption and PTS signals in case of metamaterials only, and under the existence of molecules were carried out by COMSOL. The results showed the improvement of signal-to-background ratio to 3-4 orders, while the sensitivity was improved to 2 orders. The experiments are ongoing to verify the calculation.