Kenzo Yamaguchi

Two-dimensionally localized modes of a nanoscale gap plasmon waveguide

We report numerical analysis and experimental observation of two dimensionally localized plasmonic modes guided by a nanogap in a thin metal film. Dispersion, dissipation, and field structure of these modes are analyzed using the finite-difference time-domain algorithm. The experimental observation is conducted by the end-fire excitation of the proposed gap plasmon waveguides and detection of the generated modes using their edge scattering and charge coupled device camera imaging. Physical interpretation of the obtained results is presented and origins of the described modes are discussed.

Light detection enhanced by surface plasmon resonance in metal film

Light detection enhanced by surface plasmons was confirmed in Au film/n-type Si Schottky structures. Electrons excited directly with light in Au film overflowed into the n-type silicon, and this excitation was enhanced by surface plasmons induced by Au nanorods attached to the Au film. Excitation was clearly observed in a wavelength range corresponding to the energy of less than the band gap of silicon. The feasibility of Schottky-type photodiodes, in which electrons were never generated by absorption in semiconductors but directly excited in metal, was experimentally demonstrated.

Optical frequency signal detection through surface plasmon polaritons

We demonstrated experimentally that an optical frequency signal can be detected through surface plasmon polaritons (SPPs) using an optical heterodyne technique. We fabricated an SPP detector consisting of a Au/Si Schottky diode with seven 10-μm-long and 150-nm-wide parallel slits (a multi-slit grating). When two beams of light with slightly different wavelengths irradiated the multi-slit grating of the SPP detector, a beat signal, corresponding to the optical frequency signal, was clearly observed.

Electrically driven plasmon chip: Active plasmon filter

We have developed an electrically driven plasmon chip, i.e., an active plasmon filter, consisting of a metallic subwavelength grating modulated by a nano-electro-mechanical system (NEMS) type actuator. The device shifts the plasmon resonance wavelength and the transmittance when an electrical signal is applied. The fabricated filter shows resonance wavelength shifts of 60 nm with a bias voltage of less than 10 V. A rigorous numerical calculation confirms the origin of the surface plasmon resonance and qualitatively explains the effect. Such NEMS optical devices offer rapid voltage-controlled plasmonic tuning of 20 MHz, opening up applications in agile sensing and nanoscale object trapping using actively tailored optical hot spots.

Characteristics of light intensity enhancement of a silver nanoprism with rounded corners

We have fabricated silver nanoprisms of 100–600 nm side length by focussed ion beam lithography and measured the light intensity scattering spectra using dark‐field microscopy. Two resonance peaks due to localized surface plasmon excitation were observed in the spectra and their central frequency shown to depend on the prism size. The near‐field electromagnetic intensity distribution with TE‐polarized light at the vacuum wavelength of 632.8 nm was measured. We have obtained a much lower light intensity enhancement than previously numerically predicated. However, scattering spectra obtained numerically, taking into account roundness of the prism corners, agree well with experimental ones. At the same time, the numerically determined field distribution was different to the near‐field intensity obtained experimentally. Our results suggest the particular shape of the corner region of the prism is a key factor for obtaining a large light intensity enhancement and shaping the local field distribution.

Fabrication of transparent antifouling thin films with fractal structure by atmospheric pressure cold plasma deposition.

Antifouling surface with both superhydrophobicity and oil-repellency has been fabricated on glass substrate by forming fractal microstructure(s). The fractal microstructure was constituted by transparent silica particles of 100 nm diameter and transparent zinc-oxide columns grown on silica particles by atmospheric pressure cold plasma deposition. The sample surface was coated with a chemically adsorbed monomolecular layer. We found that one sample has the superhydrophobic ability with a water droplet contact angle of more than 150°, while another sample has a high transmittance of more than 85% in a wavelength range from 400 to 800 nm.

Nonlinear trimer resonators for compact ultra-fast switching

We propose and numerically verify a scheme for compact optical modulation which can enable complex directional switching of signals in integrated micro-optical circuits within hundreds of femtoseconds. The scheme is based on a trimer comprised of two identical silica whispering gallery mode (WGM) microresonators spaced by a central non-linear WGM resonator. The non-linear resonator is in the form of a silica cylinder with a thin coating of an ultrafast Kerr nonlinear material (a J-aggregate of cyanine dye). Using a two-dimensional finite-difference time-domain method and realistic material and structural parameters, we investigated the near-field coupling from a waveguide to the trimer and the subsequent switching process. In our scheme the sandwiched central control resonator has a resonant frequency that is mismatched to that of the input and output resonators. Therefore the optical energy is coupled from the waveguide into only the primary resonator in linear operation. However, for control light intensities of more than approximately 10(-2) W/microm the effective index and hence eigenfrequency of the central resonator can be shifted to match that of its neighbors and hence the optical energy can be redirected.

Linear and Nonlinear Optical Phenomena of Metallic Nanoparticles

We have numerically characterized localized surface plasmons (LSPs) of triangular and cubic Ag particles on a substrate using the so-called finite-difference time-domain (FDTD) method. Especially, we have evaluated the effect of roundness around the prism corners on the characteristics of LSPs and, in addition to that, have focused onto a change in the characteristics of LSPs caused by an interaction between a particle and a substrate. Introduction of roundness decreases the field enhancement and existence of the substrate has made characteristics of LSP modes change. We have also simulated nonlinear optical response of an Ag nanosphere coated with a CdS film, being a Kerr material, by using a nonlinear 3-D FDTD method for spherical coordinates. We have presented extinction spectra of a triangle Ag particle that was processed by using a focused ion beam (FIB) lithography. The experimental spectrum has been well interpreted by introducing roundness around the triangle corners. Ag particles coated with a CdS film have been synthesized by employing the reversed micelle method. We have shown the first experimental observation of the optical nonlinear response for the single Ag nanoparticle coated with a CdS material, based on the optical Kerr effect due to an electronic nonlinearity, at room temperature.

Electrically driven plasmon chip: Active plasmon lens in the visible range

We propose an active plasmon lens (APL) consisting of a nanoslit array with an electrically tunable focal profile. Since the transmission phase of a nanoslit is a function of the slit width, applying bias to the nanoslit mechanically alters the nanoslit width and hence shifts the phase front. A proof-of-concept experiment demonstrates that applying a bias voltage of 5 V at 633 nm tunes the transmission profile of the fabricated APL. Our active lens is planar and only 400 nm thick, which gives it advantages for fabrication and integration.

Self-imaging confirmed in plasmonic channel waveguides at visible wavelengths

We experimentally confirm self-imaging induced by multi-mode interference of plasmon polaritons in a channel waveguide at visible wavelengths. A designed plasmonic channel waveguide, fabricated as three structural segments at two different channel depths, operates as a single- and multi-mode waveguide. Illuminated by incident light of wavelength 635 nm, the channel plasmon polaritons propagate towards the output port if the length of the multi-mode waveguide is equivalent to twice the beat length for multi-mode interference. If the length of the multi-mode waveguide is equivalent to the beat length, only a few of these plasmon polaritons propagate to the output port as most of them are reflected at the far end of the multi-mode segment of the waveguide. Experimental results enable a clear characterization of self-imaging induced by the multi-mode interference of channel plasmon polaritons.