Shunsuke Ohkouchi

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.

Mode identification of high-quality-factor single-defect nanocavities in quantum dot-embedded photonic crystals

We investigate the quality (Q) factor and the mode dispersion of single-defect nanocavities based on a triangular-lattice GaAs photonic-crystal (PC) membrane, which contain InAs quantum dots (QDs) as a broadband emitter. To obtain a high Q factor for the dipole mode, we modulate the radii and positions of the air holes surrounding the nanocavity while keeping sixfold symmetry. A maximum Q of 17 000 is experimentally demonstrated with a mode volume of V=0.39(λ∕n)3. We obtain a Q∕V of 44 000(n∕λ)3, one of the highest values ever reported with QD-embedded PC nanocavities. We also observe ten cavity modes within the first photonic band gap for the modulated structure. Their dispersion and polarization properties agree well with the numerical results.

Nonlinear optical phase shift in InAs quantum dots measured by a unique two-color pump/probe ellipsometric polarization analysis

The nonlinear optical phase shift in self-assembled InAs quantum dots (QDs) under resonant excitation in a ground-state transition was measured by a unique two-color pump∕probe ellipsometric polarization analysis. This ellipsometric analysis makes use of the large optical birefringence of SK-QD [(SK) — Stranski-Krastanov] originating from the asymmetric structure. A phase shift of 0.5π rad was obtained at an input pump pulse energy density of 30pJ∕μm2, a detuning of 11meV, and a time delay of 20ps in a 1mm long waveguide having QDs with a peak wavelength of 1290nm, a volume density of 4×1015cm−3, and inhomogeneous broadening of 35meV. Analysis revealed that the phase shift is mainly attributed to the absorption saturation for TE-polarized light, though other mechanisms also could contribute at higher pumping. The calculation, based on the two-level approximation, revealed that the minimum energy density for π shift is 240fJ∕μm2, calculated under ideal conditions.

InAs Island Formation Aligned along the Steps on a GaAs(001) Vicinal Surface

InAs island formation on a GaAs(001) substrate misoriented by 1° toward the [110] direction was investigated by scanning tunneling microscopy. On a 2.0ML InAs-deposited GaAs surface, three-dimensional islands were observed; some of the islands were aligned along the [10] direction. That is, the islands were selectively formed at steps running relatively straight along the [10] direction on the GaAs surface. These results show the possibility of controlling the arrangement of InAs islands on a surface by controlling the step structure on the surface, which induces selective island formation at the steps.

Atomic resolution imaging of InP(110) surface observed with ultrahigh vacuum atomic force microscope in noncontact mode

True atomic resolution imaging of the cleaved semi‐insulating InP(110) surface was demonstrated using an ultrahigh vacuum atomic force microscope (AFM) in noncontact mode. The force gradient acting on the tip was detected by the frequency modulation method. The rectangular lattice could be clearly observed. The image contrast suddenly changed during the scan, which suggests that the noncontact AFM imaging is performed under the condition of nearly monoatomic tip–sample force interaction. Atomic defects have been clearly and reproducibly observed. These results suggest that noncontact AFM has the potential for true atomic‐scale lateral resolution and is quite effective for atomic surface structure analysis in real space.

Scanning tunneling microscopy of molecular‐beam epitaxially grown GaAs (001) surfaces

A multichamber ultrahigh vacuum (UHV) scanning tunneling microscope (STM) system which includes a molecular‐beam epitaxy (MBE) growth chamber, as well as a STM chamber, has been constructed for the investigation of processed GaAs surfaces. We observed MBE‐grown GaAs surfaces using this system. Samples were grown on GaAs (001) surfaces and cooled to certain temperatures in an As4 flux before being transferred into an UHV. STM images of c(4×4), 2×4, and mixed c(4×2) and 2×2 structures were obtained for the samples cooled to 330, 570, and 470 °C in an As4 flux, respectively. The mixed c(4×2) and 2×2 structure seems to be formed by desorption of As atoms from the c(4×4) surface in an UHV. Also, a new reconstructed structure which has a 2.8‐nm periodicity along both the [110] and [110] directions was observed. It seems to be a local structure which has a larger arsenic dimer density than that of the c(4×4) structure.

Observation of the InP surface thermally cleaned in an arsenic flux using a scanning tunneling microscope

An InP surface thermally cleaned in an arsenic flux was observed using an ultrahigh‐vacuum scanning tunneling microscope (UHV‐STM). In the STM image, about 1.6 nm period lines of 0.8 nm width with two rows were observed along the [110] direction. This result suggests that the surface comprises two In‐In dimers and two missing dimers per (4×2) cells.

Room temperature pulsed oscillation of GaAlAs/GaAs surface emitting junction laser

Theoretical and experimental work has been done to reduce the threshold current density J th of a GaAlAs/GaAs surface emitting junction laser emitting at a wavelength of 0.87 μm. The threshold current density J th has been obtained as a function of active layer thickness d and the reflectivity of the mirrors. The necessary reflectivity for threshold is 95 percent in the case of d = 1-2 \mu m to achieve J_{th} = 25 kA/cm2at room temperature. In order to realize this high reflectivity without deteriorating the p-side ohmic contact, we have proposed a new surface emitting laser structure with a ring electrode for the p-side contact. Room temperature pulsed oscillation has been realized. The minimum threshold current was about 310 mA at room temperature under pulsed condition. The corresponding J th at the active layer is about 25 kA/cm2. This indicates that we have realized the high reflectivity. The maximum light output peak power was more than 5 mW. The beam was as sharp as 10° (FWHM) and linearly polarized. A single longitudinal mode was observed against the temperature variation of 80 K. Fabrication processes and some lasing performances are described.

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.