Yoshimasa Sugimoto

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.

Polarization independent visible color filter comprising an aluminum film with surface-plasmon enhanced transmission through a subwavelength array of holes

The enhanced transmission through periodic arrays of sub-wavelength holes in optically-thick metallic films has many potential applications, such as in wavelength filters, light extraction from light emission diodes, and subwavelength photolithography. A color filter comprising arrays of subwavelength holes in an aluminum film has been fabricated. In addition to the simplicity of the process, the aluminum film enables the excitation of visible-range surface plasmons due to its high plasma frequency. Periodic nanostructures in the aluminum film open the way for new visible color filters.

The effect of higher-order dispersion on slow light propagation in photonic crystal waveguides

We have studied the dispersion of ultrafast pulses in a photonic crystal waveguide as a function of optical frequency, in both experiment and theory. With phase-sensitive and time-resolved near-field microscopy, the light was probed inside the waveguide in a non-invasive manner. The effect of dispersion on the shape of the pulses was determined. As the optical frequency decreased, the group velocity decreased. Simultaneously, the measured pulses were broadened during propagation, due to an increase in group velocity dispersion. On top of that, the pulses exhibited a strong asymmetric distortion as the propagation distance increased. The asymmetry increased as the group velocity decreased. The asymmetry of the pulses is caused by a strong increase of higher-order dispersion. As the group velocity was reduced to 0.116(9)·c, we found group velocity dispersion of -1.1(3)·106 ps2/km and third order dispersion of up to 1.1(4)·105 ps3/km. We have modelled our interferometric measurements and included the full dispersion of the photonic crystal waveguide. Our mathematical model and the experimental findings showed a good correspondence. Our findings show that if the most commonly used slow light regime in photonic crystals is to be exploited, great care has to be taken about higher-order dispersion.

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.

Fabrication and characterization of different types of two-dimensional AlGaAs photonic crystal slabs

Photonic crystals having two-dimensional periodicity on a length scale of 320–450 nm are fabricated by electron beam lithography in combination with high-aspect-ratio dry etching. To achieve three-dimensional control of the optical properties, three kinds of dielectric waveguide structures based on AlGaAs heterostructures, that is, semiconductor-clad, air-bridge, and oxide-clad structures, are investigated. The ability to guide light through such photonic crystal devices is demonstrated by optical transmission measurements. Clear photonic band gap effects resulting in 30 dB attenuation of the transmitted light can be observed in the stop-band regions. The measured results are in good agreement with theoretical band-structure calculations and with numerically computed transmission spectra obtained by the finite-difference time-domain method.

Coupling properties in a 2-D photonic crystal slab directional coupler with a triangular lattice of air holes

Two-dimensional (2-D) photonic crystal (PC) directional couplers (DCs) that have a triangular lattice pattern of air holes in a planar dielectric slab are theoretically and experimentally analyzed. Unlike the 2-D PC DC structure with a dielectric rod in air, which is frequently used in theoretical studies, more practical PC DCs tend to be multimode in nature and exhibit a large group velocity dispersion, thus creating decoupling points in the dispersion relation without any additional modifications to the structure. The multimode nature and large dispersion lead to interference which degrades the coupling properties. By inserting three rows of air holes between neighboring line-defect waveguides in order to separate them, we have successfully reduced the multimode region and obtained a single-mode region. In this case, the large dispersion allows the creation of a PC DC with wavelength selectivity and a coupling length as short as 30 a,/spl sim/10 /spl mu/m for a=345 nm, where a is the lattice constant. The transmission spectra obtained experimentally showed good agreement with the theory whereas their transmission ranges were restricted to those of bent waveguides. These results are encouraging for practical application to optical communications.

Cavity formation on an optical nanofiber using focused ion beam milling technique.

We present the experimental realization of nanofiber Bragg grating (NFBG) by drilling periodic nano-grooves on a subwavelength-diameter silica fiber using focused ion beam milling technique. Using such NFBG structures we have realized nanofiber cavity systems. The typical finesse of such nanofiber cavity is F ∼ 20 - 120 and the on-resonance transmission is ∼ 30 - 80%. Moreover the structural symmetry of such NFBGs results in polarization-selective modes in the nanofiber cavity. Due to the strong confinement of the field in the guided mode, such a nanofiber cavity can become a promising workbench for cavity QED.

Novel electron‐beam lithography for in situ patterning of GaAs using an oxidized surface thin layer as a resist

The first demonstration of in situ electron‐beam (EB) lithography is reported, where a photo‐oxidized surface thin layer of GaAs is used for a resist. The in situ EB lithography sequence consists of five processes, i.e., preparation of a clean GaAs surface, photo‐oxidation for a resist film formation, direct patterning of the oxide resist by EB‐induced Cl2 etching, Cl2 gas etching of GaAs surface for pattern transfer, and thermal treatment in an arsenic ambient for resist removal and surface cleaning. The GaAs wafer is never exposed to air throughout all of the above processes to avoid an unintentional surface contamination. The minimum electron dose required for patterning of the GaAs oxide resist is about 5×1016 cm−2. An overgrown layer on the patterned GaAs surface shows a good surface morphology, which strongly indicates that this technology makes it possible to repeat crystal growth and surface patterning.

Light-propagation characteristics of Y-branch defect waveguides in AlGaAs-based air-bridge-type two-dimensional photonic crystal slabs.

Y-branch defect waveguides (DWGs) in an AlGaAs-based air-bridge-type two-dimensional (2D) photonic crystal (PC) slab were fabricated, and the resulting optical transmission spectra were measured. Optical beam propagation along the waveguide was successfully observed in vidicon IR images. The measured transmission spectra at the two output ports in the Y-branch DWG were remarkably similar in shape and magnitude. From comparison between the measured and calculated transmission spectra, the observed optical beam was identified as the guided mode based on the Y-branch DWG in the 2D PC slab. The results suggest that the Y-branch DWG used in the experiment has the potential to function as an ultrasmall 3-dB beam splitter with good performance.