Hideo Kosaka

Self-collimating phenomena in photonic crystals

We found that self-determining collimated light is generated in a photonic crystal fabricated on silicon. The divergence of the collimated beam is insensitive to that of the incident beam and much smaller than the divergence that would be generated in conventional Gaussian optics. The incident-angle dependence of the self-collimated light propagation including lens-like divergent propagation was interpreted in terms of the highly modulated dispersion surfaces with inflection points, where the curvature changes from downward to upward corresponding to respectively a concave/convex-lens case. This demonstration is an important step towards controlling beam profile in photonic crystal integrated light circuits and towards developing “photonic crystalline optics.”

Photonic crystals for micro lightwave circuits using wavelength-dependent angular beam steering

Light-beam steering that is extremely wavelength dependent has been demonstrated by using photonic crystals fabricated on Si. The scanning span reached 50° with only a 1% shift of incident wavelength at around 1 μm. The resulting angular dispersion is two orders of magnitude larger than that achieved with conventional prisms or gratings. The application of such superprism phenomena promises to enable the fabrication of integrated micro lightwave circuits that will allow more efficient use of wavelength resources when used in wavelength multiplexers/demultiplexers or dispersion compensators by enabling lower loss and broader bandwidth.

Superprism phenomena in photonic crystals: toward microscale lightwave circuits

The superprism phenomenon, the dispersion of light 500 times stronger than the dispersion in conventional prisms, was demonstrated at optical wavelengths in photonic crystals (PCs) fabricated on Si. Drastic light-beam steering in the PCs was achieved by slightly changing the incident wavelength or angle. The scanning span reached 50/spl deg/ with only a 1% shift of incident wavelength, and reached 140/spl deg/ with only a 14/spl deg/ shift of the incident angle at wavelengths around 1 /spl mu/m. The propagation direction was quantitatively interpreted in terms of highly anisotropic dispersion surfaces derived by photonic band calculation. The physics behind this demonstration will open a novel field called photonic crystalline optics. The application of these phenomena promises to enable the fabrication of integrated microscale lightwave circuits (/spl mu/LCs) on Si with large scale integrated (LSI)-compatible lithography techniques. Such /spl mu/LCs will allow more efficient use of wavelength resources when used in wavelength multiplexers/demultiplexers or dispersion compensators by enabling lower loss and broader bandwidth.

Lightwave propagation through a 120° sharply bent single-line-defect photonic crystal waveguide

We have demonstrated 1.55 μm wavelength lightwave propagation through a 120° sharply bent waveguide formed in a triangular-lattice two-dimensional photonic crystal (2D PC). Such propagation has not previously been experimentally confirmed. The photonic crystal was fabricated in a silicon-on-insulator (SOI) wafer with the top silicon layer of the wafer used as a core layer. A 877-μm-long single-line-defect waveguide was formed in the PC with a sharp 120° bend near the middle of the waveguide. A tapered-hemispherical-end fiber was coupled to the input end of the waveguide for the light input, and the output from the other end of the waveguide was directly observed by scanning its near-field profile with another tapered-hemispherical-end fiber.

SURFACE-EMITTING LASER OPERATION IN VERTICAL-TO-SURFACE TRANSMISSION ELECTROPHOTONIC DEVICES WITH A VERTICAL CAVITY

We demonstrate the first surface‐emitting laser operation in a vertical‐to‐surface transmission electrophotonic device with a vertical cavity. The thyristor‐like current‐voltage characteristics, which are required for optical and electrical switching, are achieved. Threshold current during the on state is as low as 1.2 mA. All 100 devices, which were randomly extracted from a grown wafer, emit laser light.

Complete polarization control of 8×8 vertical‐cavity surface‐emitting laser matrix arrays

Polarization of 8×8 vertical cavity surface emitting laser (VCSEL) arrays is completely controlled. These index‐guided VCSELs have a 6×5 μm rectangular poststructure consisting of DBR mirrors. All 64 VCSELs emit fundamental single‐mode and linearly polarized light with a polarization angle deviation of only 2.9°. Their light output characteristics are almost the same as those of conventional 6×6 μm polarization‐uncontrolled VCSELs.

Single-photon Interference over 150 km Transmission Using Silica-based Integrated-optic Interferometers for Quantum Cryptography

We have demonstrated single-photon interference over 150 km using time-division interferometers for quantum cryptography, which were composed of two integrated-optic asymmetric Mach-Zehnder interferometers, and balanced gated-mode photon detectors. The observed fringe visibility was more than 80% after 150 km transmission.

Photonic-crystal spot-size converter

We have demonstrated a spot-size converter (SSC) that is made of photonic crystals (PCs) and has a conversion ratio of 10:1 for a 1-μm-wavelength light beam. Its real-spatial distribution was narrowed by intentionally broadening its wave vector distribution and increasing effective refractive index. The advantage of this PC-based SSC over conventional bulk-based SSC are compactness (monolithic integration), positional independence, and extremely deep depth of focus. This PC-SSC is a candidate for an interface between photonic-crystal waveguides and conventional optical waveguides.

Coherent transfer of light polarization to electron spins in a semiconductor.

We demonstrate that the superposition of light polarization states is coherently transferred to electron spins in a semiconductor quantum well. By using time-resolved Kerr rotation, we observe the initial phase of Larmor precession of electron spins whose coherence is transferred from light. To break the electron-hole spin entanglement, we utilized the big discrepancy between the transverse g factors of electrons and light-holes. The result encourages us to make a quantum media converter between flying photon qubits and stationary electron-spin qubits in semiconductors.

A neuron-MOS neural network using self-learning-compatible synapse circuits

A circuit technology for self-learning neural network hardware has been developed using a high-functionality device called Neuron MOS Transistor (/spl upsi/MOS) as a key circuit element. A /spl upsi/MOS can perform weighted summation of multiple input signals and thresholding all at a single transistor level based on the charge sharing among multiple capacitors. An electronic synapse cell has been constructed with six transistors by merging a floating-gate EEPROM memory cell into a new-concept /spl upsi/MOS differential-source-follower circuitry. The synapse can represent both positive (excitatory) and negative (inhibitory) weights under single V/sub DD/ power supply and is free from standby power dissipation. An excellent linearity in the weight updating characteristics of the synapse memory has been also established by employing a simple self-feedback regime in each cell circuitry, thus making it fully compatible to the on-chip self-learning architecture of /spl upsi/MOS neural networks. The basic operation of the synapse cell and a /spl upsi/MOS neural network using the synapse has been experimentally verified using test circuits fabricated by a double-polysilicon CMOS process. >