Yuya Ishii

Highly polarized luminescence from aligned conjugated polymer electrospun nanofibers

In this contribution we show highly polarized photoluminescence (PL) from aligned polyethyleneoxide: polyphenylenevinylene derivative composite nanofibers. We demonstrate PL polarization ratios (parallel to perpendicular) greater than 13. This ratio is further increased (up to ∼25) by stretching the nanofibers. Stretching also results in an increase in conjugation length, fiber density, and PL lifetime. We argue that the effect of stretching is equivalent to applying a permanent and strong pressure. Our results open up the possibility for new optoelectronic devices and fundamental science studies based on polymer nanofibers.

Monolithic Integration of Surface Plasmon Detector and Metal–Oxide–Semiconductor Field-Effect Transistors

The monolithic integration of a silicon-based plasmonic detector with metal- oxide-semiconductor field-effect transistors (MOSFETs) was demonstrated. The plasmonic detector consisted of a gold film with a nanoslit grating on a silicon substrate and was operated at a free-space wavelength of 1550 nm. The structure of the nanoslit grating was optimized by using the finite-difference time-domain method. The output current from the plasmonic detector was amplified by ~14 000 times using the monolithically integrated MOSFETs. In addition, dynamic operation of the integrated circuit was demonstrated by modulation of the intensity of a beam that was incident to the plasmonic detector.

Fabrication of a submicron-channel organic field-effect transistor using a controllable electrospun single fibre as a shadow mask

We demonstrate a simple and versatile method for the fabrication of a submicron channel for an organic field-effect transistor (OFET) using a single electrospun fibre as a shadow mask. A single electrospun fibre is produced by an alternative switching electrospinning method and is stretched 2.5-fold. The average diameter of the stretched fibres is 302 nm. The stretched fibre is placed on ultrathin dielectric layers of aluminium oxide and a self-assembled monolayer (SAM). During electrode deposition the fibre acts as a very small shadow mask. After removing the fibre, electrodes with very narrow gaps of around 350 nm and with high uniformity are easily obtained. We fabricate an OFET by depositing pentacene as an active layer onto the electrodes. The OFET is operable at low voltages, with a threshold voltage of - 1.1 V and a subthreshold swing of 0.27 V decade(-1), values which are one order of magnitude lower than those obtained with a channel length of 75 µm.

Coherent Plasmonic Interconnection in Silicon-Based Electrical Circuit

This paper presents a feasibility study of optical interconnections using surface plasmon polaritons (SPPs) as coherent carrier waves in a silicon-based electrical circuit. A gold film plasmonic waveguide and a gold/silicon Schottky-type plasmonic detector were monolithically integrated with an electrical circuit based on metal-oxide-semiconductor field-effect transistors on a silicon substrate. A 1550-nm-band laser source was used for SPP excitation, and the photocurrent generated by the plasmonic detector was amplified 16 000 times by the monolithically integrated electrical circuit after SPPs carrying the optical intensity signal propagated over the gold film surface for a distance of 100 μm. The integrated circuit detected an optical beat signal by using a delayed self-homodyne technique, thus demonstrating that SPPs can be used as coherent carrier waves in the circuit. Additionally, optical amplitude- and frequency-modulated signal transmission in a gold film plasmonic waveguide and optical heterodyne detection by amplification of the signal intensity in a gold/silicon Schottky-type plasmonic detector were also demonstrated.

Low-loss waveguiding and detecting structure for surface plasmon polaritons

A simple and low-loss metal/semiconductor surface plasmon polariton (SPP) device consisting of a SPP waveguide and a detector is studied theoretically and experimentally. We demonstrate a simple diffraction structure (a metal grating) where the SPP couples from the waveguide to the detector. The SPP can propagate without large losses at the air/Au interface, and this interface was used for SPP waveguiding. To convert the SPP into an electric signal using internal photoemission, the propagating SPP is coupled into the Au/Si interface by the diffraction structure. The propagation direction of the coupled SPP at the Au/Si interface depends on the slit pitch of the diffraction structure, and the direction can be controlled by adjusting the pitch. The slit pitch is also modeled using a diffraction grating equation, and the results show good agreement with those of simulations using the finite-difference time-domain method. When diffraction structures consisting of a multi-slit structure and a disk array are placed at the end of the waveguide, SPP coupling into the Au/Si interface is also observed. The photocurrents detected at the Au/Si interface are much larger when compared with that detected for the device without the diffraction structure (26 times for the multi-slit structure and 10 times for the disk array). From the polarization angle dependence of the detected photocurrent, we also confirmed that the photocurrent was caused by the SPP propagating at the air/Au interface.

Schottky-type surface plasmon detector with nano-slit grating using enhanced resonant optical transmission

We propose a metal nano-slit structure to enhance the surface plasmon (SP) intensity at the Au/Si interface between a gold film and a silicon substrate. By tuning the phase conditions to be in anti-phase interference at the air/Au interface and in in-phase interference at the Au/Si interface, the SP intensity at the Au/Si interface was enhanced. This structure was numerically designed using the finite-difference time-domain method and was experimentally confirmed by monitoring of the photocurrent of an Au/Si Schottky-type SP detector. This design, with its two phase matching conditions that enhance the SP intensity at the Au/Si interface, was applied to a ring-type metal grating on a silicon substrate, and demonstrated the photocurrent enhancement.

Plasmonic-multimode-interference-based logic circuit with simple phase adjustment

All-optical logic circuits using surface plasmon polaritons have a potential for high-speed information processing with high-density integration beyond the diffraction limit of propagating light. However, a number of logic gates that can be cascaded is limited by complicated signal phase adjustment. In this study, we demonstrate a half-adder operation with simple phase adjustment using plasmonic multimode interference (MMI) devices, composed of dielectric stripes on a metal film, which can be fabricated by a complementary metal-oxide semiconductor (MOS)-compatible process. Also, simultaneous operations of XOR and AND gates are substantiated experimentally by combining 1 × 1 MMI based phase adjusters and 2 × 2 MMI based intensity modulators. An experimental on-off ratio of at least 4.3 dB is confirmed using scanning near-field optical microscopy. The proposed structure will contribute to high-density plasmonic circuits, fabricated by complementary MOS-compatible process or printing techniques.

True photoluminescence spectra revealed in electrospun light-emitting single nanofibers

We demonstrate for the first time that re-absorption and scatterings of photoluminescence (PL) significantly occur among electrospun light-emitting nanofibers. Electrospun nanofibers composed of a blend of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] and poly(ethylene oxide) with and without LiCF3SO3 were prepared in controlled numbers. Different numbers of fibers showed different PL spectra even though the PL spectra from individual fibers were almost the same. Results of simulated PL spectra using UV-vis absorption spectra measured with and without an integrating sphere revealed that the change of PL spectra is due to re-absorption and scatterings of PL among the fibers. Although most PL spectra of electrospun nanofibers have been performed with a sheet or mat of many electrospun fibers, our study clearly shows that measuring the PL spectrum of a single fiber is indispensable for precisely evaluating the aggregation states and the electronic states of conjugated polymers inside the fibers.

Dielectric-loaded surface plasmon polariton crossing waveguides using multimode interference.

A low-loss low-crosstalk multimode interference (MMI) crossing design for dielectric-loaded surface plasmon polariton waveguides (DLSPPWs), which are SiO2 stripes on Au films, is demonstrated numerically and experimentally. DLSPPWs are compatible with strong surface plasmon polariton (SPP) field confinement and maintain relatively low propagation losses. Unlike simpler crossings without MMI structures, low insertion loss of 0.65 dB and low crosstalk of -20.27  dB is confirmed numerically at a crossing angle of 10° when using tilted mirror-imaged MMI crossings. Similar insertion losses were also confirmed experimentally. The proposed structure will be beneficial for plasmonic device miniaturization and flexible patterning of optical interconnections.

Surface-Plasmon Waveguides as Transmission Lines for Optical Signal and Electrical Bias

Using metal plasmonic waveguides as transmission lines for optical signals and an electrical bias is shown to be feasible in Si-based devices with a separation gap formed between the waveguide and Au/Si Schottky-barrier diode (SBD). Optical signal transmission is confirmed by calculating the radiation pattern from the waveguide edge and measuring the photocurrent detected at the SBD. From a finite-difference time-domain simulation, the radiation pattern from the waveguide edge is represented as an interference fringe. The simulation result for the separation-length dependence of the detected photocurrent at the SBD corresponds well with experiment. Moreover, the intensity-modulated optical signal at 10 MHz is also observed across the 3-μm-length separation gap. The electrical bias separation is confirmed by applying a bias voltage between the waveguide and the Si substrate and generating a bias current through the waveguide. The detected photocurrent at the SBD barely increased with changing bias voltage and was clearly smaller than that under changes in optical intensity. In addition, electrical current produced no influence on the surface-plasmon signal on the waveguide.