Masamitsu Fujii

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.

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.

The plasmonic Raman sensor using periodic nanofocusing arrays

A novel plasmonic Raman sensor using periodic nano-hole and, potentially, nanofocusing arrays is investigated numerically and experimentally. The effect of structural parameters (such as periodicity of the structure, hole dimensions, etc.) is determined and investigated. The analysed structures are fabricated in thin gold films by means of focused ion beam lithography. Optical characteristics of the fabricated arrays are determined experimentally and compared with the theoretical predictions. Experimental field enhancements are determined and also compared with the theoretical predictions.

Optical Amplifier Using Nonlinear Nanodefect Cavity in Photonic Crystal

We have proposed a new optical amplifier in a photonic crystal. The amplifier consists of a nonlinear nanodefect cavity and a cross waveguide. The input–output characteristics have been simulated for the amplifier with three- or four-port cross waveguide in a two-dimensional photonic crystal by the finite-difference time-domain method. In the amplifier, a signal light can be amplified by an energy transfer from a control light to the signal light at the nonlinear nano-defect cavity. The calculated amplification factor of the amplifier was 2.33×104. When optical transistor was used with an amplifier with a three-port cross waveguide in a two-dimensional photonic crystal, the calculated switching rate, the on–off ratio and the amplification factor of the transistor were 12 Gbit/s, 1.95×105 and 2.29×104, respectively. The transistor can work well even under a low power input in the signal port and can be utilized as a small amplifier and an optical sensor with a fast response.

Characteristics of Whispering Gallery Modes in Single Dielectric Spheroid Excited by Gaussian Beam

We have numerically analyzed a coupling between a Gaussian light and spheroids by using a finite-difference time-domain method. In a configuration where the polarization of light is directed along the short axis of the spheroid and light is incident along the long axis, there exists a complex whispering gallery mode (WGM), which cannot be observed for any spheres. The characteristics of such a complex WGM are similar to those produced from a combination of several WGMs. In a configuration where the polarization of light is directed along the long axis of the spheroid and light is incident along the short axis, the Q-factor of a WGM excited there decreases compared with that of the WGM of a sphere because the curvature around the edges of the long axis is considerably large and thus the confinement of light must be weak.

Finite-Difference Time-Domain Analysis on Nonlinear Fabry-Perot Resonator in Optical Waveguide Geometry

The finite-difference time-domain (FDTD) method is applied to investigate the characteristics of nonlinear Fabry-Perot (FP) resonators in optical waveguide geometries. Our treatment, which is the extension of Trans method developed to analyse the characteristics of nonlinear photonic crystals, correctly reproduces analytical results on optical bistabilities of the FP resonator provided by Chen and Mills in 1987. It is found that the minimum power required for optical bistabilities is significantry decreased by adopting the optical waveguide geometry, compared with the case of Chen and Mills, i.e., a conventional nonlinear FP interferometer.

Optical Properties of Low-Loss Ag Films and Nanostructures on Transparent Substrates

We demonstrate the fabrication of a low-loss single-crystalline Ag nanostructure deposited on transparent substrates. Our approach is based on an epitaxial growth technique in which a NaCl(001) substrate is used. The NaCl substrate is dissolved in water to allow the Ag film to be transferred onto the desired substrates. Focused ion beam milling is subsequently employed to pattern a nanoarray structure consisting of 200 nanorods. The epitaxial Ag films with nanoarray structures grown in the study exhibited very flat and smooth surfaces having excellent crystallinity and local misorientation of less than 1°. Further, spectroscopic ellipsometry measurements indicated that the imaginary part of the dielectric constant of the single-crystalline film was smaller than that of a conventional polycrystalline film. Moreover, we used the three-dimensional finite-difference time-domain method to analyze the plasmonic properties of the nanoarray structure by considering the actual processed structure. Characteristically, when the SiO2 substrate was etched by ion beam milling to a depth of 30 nm, the spectrum showed a spectral shape 20% sharper than that of the substrate with no etching (depth: 0 nm). The plasmonic performance of the single-crystalline Ag nanostructure was largely determined by its structural precision and the dielectric properties of the metal.

Active plasmon devices

We have developed an electrically controlled active plasmon device that consists of a metallic subwavelength grating modulated by a NEMS actuator. The device shifts the plasmon resonance wavelength, and the effect is explained by calculation.