Yusui Nakamura

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.

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.

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.

Nano-dot array formation using nano-jet probe for photonics application

We propose herein a new nano-probe-assisted technique that enables the formation of site-controlled InAs quantum dots. High-density two-dimensional indium (In) nano-dot arrays on a GaAs substrate were fabricated using a specially designed atomic-force-microscope probe, referred to as the Nano-Jet Probe. This probe has a hollow pyramidal tip with a submicron-sized aperture at the apex and an In-reservoir tank within the stylus. A voltage pulse was applied between the pyramidal tip and the sample to extract In clusters from the reservoir tank within the stylus through the aperture, resulting in In nano-dot formation. These In nano-dots were converted directly into InAs arrays by the subsequent annealing with irradiation of arsenic flux. The proposed technique has potential applications in photonics, including regular arrays of quantum bits and single photon emitters for quantum computers and quantum communications.

Control of inas quantum dot emission wavelengths in narrow regions by selective formation of GaInAs covered layers grown with in-situ mask

We have succeeded in controlling the emission wavelength of a self-assembled InAs quantum dot (QD) structure in a narrow region. The emission wavelength of the QDs was varied locally by a covered GaInAs layer grown with an in situ mask, which can be fitted to the sample holder and removed in an ultra-high-vacuum environment. This mask enables the selective growth of high-quality self-assembled QDs with the desired emission wavelengths ranging from 1.23 µm to 1.32 µm. This technique has potential applications in the integration of microstructures with QDs into optoelectronic functional devices.

Large enhancement of optical nonlinearity using quantum dots embedded in a photonic crystal structure for all-optical switch applications

An ultrafast semiconductor all-optical switch using a third-order optical nonlinearity is attractive for a future OTDM system. This type of switch essentially requires both ultrafast and low optical-power operations. However, the tradeoff between them, based on the optical nonlinearity in general, often makes it difficult to achieve such a switch in practical use. This paper proposes a novel enhancement method of the third order nonlinearity by using quantum dots (QDs) embedded in a photonic crystal (PC), thus reducing the optical consumption power.

Uniform ZnO epitaxial films formed at atmospheric pressure by high-speed rotation-type mist chemical vapor deposition

Uniform ZnO epitaxial films were formed on 2-in.-diameter m-plane sapphire substrates by high-speed rotation-type mist chemical vapor deposition at atmospheric pressure, without using any vacuum equipment. The ZnO films were characterized by scanning electron microscopy, X-ray diffraction in θ–2θ and scanning modes, electron backscatter diffraction, and room-temperature photoluminescence measurements. Experimental results show that m-plane ZnO films were epitaxially grown on the m-plane sapphire substrates with high uniformity of not only thickness but also crystallinity and optical properties. These results will promote the progress of ZnO-based devices such as light-emitting diodes.

Site-Control Technology for InAs Quantum Dot Formation by Direct Deposition of Indium Nano-Dots with a Nano-Jet Probe

We propose a new nano-probe-assisted technique which enables the formation of site-controlled InAs quantum dots (QDs). High-density two-dimensional indium (In) nano-dot arrays on a GaAs substrate were fabricated by using a specially designed atomic-force-microscope (AFM) probe (the Nano-Jet Probe). This developed probe has a hollow pyramidal tip with a sub-micron size aperture on the apex and an In-reservoir tank within the stylus. By applying a voltage pulse between the pyramidal tip and the sample, In clusters were extracted from the reservoir tank within the stylus through the aperture, resulting in the In nano-dot formation. These In nano-dots can be directly converted to InAs QD arrays by subsequent irradiation of arsenic flux.

Selective formation of InAs quantum dot arrays by direct deposition of indium nano-dots using a nano-jet probe

We propose a new nano-probe-assisted technique which enables the formation of site-controlled InAs quantum dots (QDs). High-density two-dimensional indium (In) nano-dot arrays on a GaAs substrate were fabricated by using a specially designed atomic-force-microscope (AFM) probe (the nano-jet probe). This developed probe has a hollow pyramidal tip with a sub-micron size aperture on the apex and an In-reservoir tank within the stylus. By applying a voltage pulse between the pyramidal tip and the sample, In clusters were extracted from the reservoir tank within the stylus through the aperture, resulting in the In nano-dot formation. These In nano-dots were directly converted to InAs arrays by subsequent annealing with irradiation of arsenic flux.

Time-resolved optical nonlinearity measurement by a two color pump-probe ellipsometry in InAs quantum dot waveguide

We have developed a two color pump-probe ellipsometry for the measurement of the optical nonlinear phase shift in a waveguide with quantum dot (QD) core layers. The key point of the method is to utilize the large polarization-dependent property in the optical nonlinearity arising from a flat-dome-like shape of the QDs. In this paper we report the time resolved behavior of the nonlinear response in the QD waveguides obtained by this method. Optical phase shift of more than /spl pi//2 rad was obtained by a pumping of 30 pJ//spl mu/m/sup 2/. Delay time dependence of the phase shift is discussed in connection with the carrier dynamics.