Ayumi Takeda

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.

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.

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.

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.

Sensitivity improvement of Schottky-type plasmonic detector

A surface plasmon polariton (SPP) is composed of collective electron oscillations that confine optical energies in nanoscale beyond the diffraction limit. This advantage of SPPs has promoted the development of high-density optoelectronic integrated circuits (OEICs) using SPPs. Schottky-type plasmonic detectors have attracted particular attention, because these devices show sensitivity in the telecommunications wavelength range and can be integrated into Si-based electronic circuits with a simple fabrication process. We have developed an Au/Si Schottky-type plasmonic detector with nano slits that excites SPPs at the Au/Si interface. In this report, we demonstrate a novel nano-slit arrangement that provides a sensitivity improvement for the detector. Using the finite-difference time-domain method, we have shown that the highest electric field intensity in the SPP mode on the Au/Si interface is generated by positioning slits with twice the pitch of the SPP wavelength at the Au/Si interface. Using this slit pitch, a weaker SPP mode intensity on the air/Au interface and a stronger SPP mode intensity at the Au/Si interface have also been confirmed. Nano slits with different slit pitches were formed in the Au film of the detector, and the slit pitch dependence of the photocurrent was measured. The experimental results showed similar tendencies to the simulation results. This novel nano-slit arrangement can provide an efficient plasmonic detector for future high-speed data processing applications.

Optimal design of photodetector with multi-slit grating

We propose an optimal structure for an Au/Si Schottky-type photodetector with a multi-slit grating that excites surface plasmon polaritons (SPPs). The intensity of the SPPs excited by the grating is simulated using the finite-difference time-domain method. The calculation results show that the optimum Au film thickness and slit pitch were determined by the resonance effects of SPPs inside the slit and the in-phase interference of the SPPs generated by each slit, respectively. Using these results, we fabricate and evaluate an optimal photodetector with the grating. We also confirm the operation of metal-oxide-semiconductor field-effect transistors by the SPP-enhanced photocurrent.

Polarization-independent photodetector with ring-type grating

We propose a polarization-independent Au/Si Schottky-type photodetector with a ring-type grating that excites surface plasmon polaritons. The electromagnetic fields in ring-type and rectangular-type grating structures are calculated using the finite-difference time-domain method. For samples with the two geometries (slit width of 100 nm, slit pitch of 440 nm, and Au film thickness of 300 nm), the polarization dependence of the photocurrent is measured and compared. We confirm that the ring-type grating photodetector has a polarization-independent photocurrent, whereas the rectangular-type grating photodetector has only higher sensitivity at specific polarization angles.

Metal-oxide-semiconductor field-effect transistors operated by surface plasmon polaritons

The operation of a metal-oxide-semiconductor field-effect transistor (MOSFET) by a surface plasmon (SP) signal was demonstrated. The SP detector, composed of a gold/silicon Schottky diode with a nano-slit grating, was monolithically integrated with the MOSFETs on a silicon substrate. SP generation by the nano-slit gating (slit width of 100 nm, slit pitch of 440 nm, and slit depth of 300 nm) was confirmed by analytical calculations based on the finite-difference timedomain method. The SP detector operated at a photon energy (0.80 eV) that was below the bandgap energy of silicon (1.1 eV), with responsivity of 24 nA/mW and a dark current of 1.7 nA under reverse bias of 5.0 V. The photocurrent generated by the SP detector controlled the drain current of a monolithically integrated MOSFET.

Photoluminescence characteristics of dye-doped polymer nanofibers excited by surface plasmon polaritons

Grating inscription in azo-dye doped polymers is an interesting phenomenon because of its high diffraction performance and applicability to real-time 3D displays. Although some of these materials were investigated under no external electric field with symmetric optical alignments in preceding studies, they often showed a phase shift of periodic modulation of refractive index from the interference fringe formed by irradiation beams, resulting in asymmetric energy exchange between two coupled beams. The mechanism of the behavior has been usually attributed to the molecular motions triggered by trans-cis isomerization, but their details are still unknown. Therefore, studies on temporal evolution of the process and their translation into physical meaning are necessary. In order to investigate the evolution of grating inscription and phase shift, several methods have been developed. In this study, we analyzed the coupled wave equations proposed by Kogelnik, and derived general solution applicable to the system with both phase and amplitude gratings with arbitrary phase relationship. We showed that the analysis based on the equation can give a direct evidence of the phase shift between the phase and amplitude gratings if it exists. This method was applied to the fringe pattern inscribed in thick films of PMMA doped with an azo-carbazole dye, showing that observed signals indicated the phase deviation between two types of gratings.