Masashi Fukuhara

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.

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.

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.

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.

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.

Plasmonic and electronic device integrated circuits and their characteristics

This paper presents a type of plasmonic circuit that is monolithically integrated with electronic devices on a silicon substrate and discusses the concept behind this circuit. Surface plasmon waveguides and detectors are integrated with metal-oxide-semiconductor field-effect transistors (MOSFETs) on the substrate. In the circuits, surface plasmon signals are generated by light at a wavelength at which silicon is transparent, and propagate along the waveguide before being converted into electrical signals by the detector. These electrical signals drive the MOSFETs during both dc and ac operation. The measured performances of these devices indicate that the surface plasmons propagate on the metal surface at the speed of light and drive the electronic devices without any absorption in silicon.