Kuon Inoue

Ultra-fast photonic crystal/quantum dot all-optical switch for future photonic networks

We demonstrated a novel two-dimensional photonic-crystal based Symmetric Mach Zehnder type all-optical switch (PC-SMZ) with InAs quantum dots acting as a nonlinear phase-shift source. The 600-mum-long PC-SMZ exhibited a 15-ps-wide switching-window at sufficiently low optical-energy of ~100 fJ.

Photonic crystal and quantum dot technologies for all-optical switch and logic device

Nano-photonic technologies of GaAs-based two-dimensional photonic crystal (2DPC) slab waveguides (WGs) and InAs-based quantum dots (QDs) are reviewed for a symmetrical Mach?Zehnder (SMZ) type, ultra-small and ultra-fast all-optical switch (PC-SMZ) and logic device. As the first phase, ultra-fast (~ps) and ultra-low energy (~100?fJ) switching has been demonstrated using a chip 600??m?300??m in size. The second phase is to create a PC-SMZ-based ultra-fast photonic logic switch with a latch function for a future ultra-fast photonic digital processor. One of the priority subjects is to establish a new design method, i.e., topology optimization (TO) method of 2DPC-WGs with wide/flat bandwidth, high transmittance and low reflectivity. Another one is to develop selective-area-grown, high-density and highly uniform InAs QDs with large optical nonlinearity (ONL) by using a metal-mask (MM) molecular beam epitaxy (MBE) growth method. Recent results regarding these two subjects encourage us to reach the final goal.

Low propagation loss of 0.76 dB/mm in GaAs-based single-line-defect two-dimensional photonic crystal slab waveguides up to 1 cm in length

Straight single-line-defect photonic crystal (PC) waveguides on GaAs slabs with lengths of 1, 4, and 10 mm have been fabricated. By controlling the Al content of a sacrificial AlGaAs clad layer and the wet etching duration, a PC core layer with a very smooth surface was obtained. Atomic force microscope images indicate that the roughness on the top surface is less than 1 nm. An extremely low propagation loss of 0.76 dB/mm for the GaAs-based PC waveguide was achieved.

Design, fabrication, and characterization of coupling-strength-controlled directional coupler based on two-dimensional photonic-crystal slab waveguides

A directional coupler (DC) with a coupling-strength control defect (CCD) based on a two-dimensional photonic-crystal slab waveguide is numerically and experimentally studied. The structure consists of two parallel-coupled single-defect waveguides separated by a triple-line air-hole array, with some missing air holes in the middle of the triple line. Using a two-dimensional finite-difference time-domain method, a 50% coupler was designed and almost 50% coupling performance was experimentally confirmed at a wavelength of ∼1.3 μm with a coupling length of about 7 μm, about 1/3 in length as compared to the DC without the CCD.

Light amplification by stimulated Raman scattering in AlGaAs-based photonic-crystal line-defect waveguides

The authors performed an amplification experiment of optical signal around 1550 nm by stimulated Raman scattering in a photonic-crystal slab waveguide (WG) of single-line defect. They adopted an air-bridge type AlGaAs-based WG sample to avoid two-photon absorption. Short light pulses of 5 ps in duration operated at 20 MHz and a cw laser were employed for pump (excitation) and probe (signal) lights, respectively. As a result, amplification of a net gain of ∼3 dB (maximum) is achieved with pulse pump energy of 22 pJ for a sample of 1.0 mm in length.

Self-phase modulation in photonic-crystal-slab line-defect waveguides

The authors observed self-phase modulation in spectral domain at 1.50μm in AlxGa1−xAs-based photonic-crystal-slab line-defect waveguides. Adopting a sample with x=0.18 has enabled the authors to avoid two-photon absorption, causing undesirable free-carrier-induced refractive-index change. Variation of the observed spectrum with input pulse peak power is well reproduced by theoretical simulation. They find that a 1.6ps light pulse of 2.5pJ suffices for causing a phase shift of π in a 1mm long sample owing to tight light confinement and nonlinear refractive index n2 due to instantaneous optical Kerr effect is (3.8±1.2)×10−13cm2∕W. A possibility of developing ultrafast all-optical switch due to n2 is discussed

InAs quantum-dot laser utilizing GaAs photonic-crystal line-defect waveguide

We have observed laser action from optically-pumped InAs-quantum-dots embedded in a line-defect waveguide in an air-bridge type GaAs-photonic-crystal slab (an array of air-holes). The lasing is found to occur without any optical cavity such as a set of Fabry-Perot mirrors. Comparison of the observed transmittance spectrum with the calculated band dispersion of the W3 defect-mode enables us to specify the lasing wavelength as that at the band edge. From this fact it follows that distributed feedback mechanism at the band edge with a vanishingly small group-velocity should be responsible for the present lasing. Usefulness of this kind of compact laser in a future ultrafast planar photonic integrated circuit is discussed.

Observation of Raman scattering in GaAs photonic-crystal slab waveguides

We have observed Raman spectra of optical phonons at 1.34 mum in an air-bridge type of GaAs photonic-crystal (PC) slab waveguides (WGs) of 0.6 mm in length. Unlike the bulk GaAs case, both longitudinal (LO)- and transverse (TO) phonons were observed not only in the forward direction, but also in the backward direction. This anomalous feature can be well interpreted as arising from strong confinement of light in PC WGs. The scattering efficiency of LO phonon at 292 cm(-1) is estimated as 1.9 x 10(-7) cm(-1)-sr(-1) in the forward scattering. Based on the present information, the possibility of developing an optical amplifier with use of stimulated Raman scattering in a future ultrafast and ultrasmall PC WGs-based optical circuit is discussed.

Design, fabrication, and characterization of a two-dimensional photonic-crystal symmetric Mach-Zehnder interferometer for optical integrated circuits

We report on the design, fabrication, and optical characterization of a GaAs two-dimensional photonic-crystal-based symmetric Mach-Zehnder interferometer (PC-SMZ) aimed at all-optical switches in planar photonic integrated systems. The directional couplers that constitute the PC-SMZ interferometer prevent the occurrence of ring resonance which degrades the interference, and allow control pulses for the switching operation to be received. We show 50%- and 100%-power couplings from the transmission characteristics of the directional couplers, thus demonstrating their operation as 3-dB dividers in the PC-SMZ. We also show experimentally wavelength selectivity between signal and control pulses with a separation of ∼7nm. These results indicate that the structure is promising as an all-optical switch when a nonlinear material such as a quantum dot is introduced into both arms of the interferometer.

Studies on key nano-fabrication processes for GaAs-based air-bridge-type two-dimensional photonic-crystal slab waveguides

We report successful studies on improved key nano-fabrication processes for AlGaAs-based air-bridge-type two-dimensional photonic-crystal (2DPC) slab waveguides. The first topic is proximity-effect correction in electron beam lithography for the definition of a uniform and precise periodic/non-periodic 2DPC waveguide pattern. The second topic is optimization of the selective wet-etching condition in an air-bridge fabrication process. The resultant nano-fabrication processes have been proved to be quite effective for the realization of 2DPC slab waveguides such as modified and topology-optimized bend waveguides having broad-band and high-transmittance spectra in excellent agreement with the calculated ones.