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A method to integrate nanowire-based quantum dot single photon sources on-chip using evanescent coupling is demonstrated. By deterministically placing an appropriately tapered III-V nanowire, containing a single quantum dot, on top of a silicon-based ridge waveguide, the quantum dot emission directed toward the taper can be transferred to the ridge waveguide with calculated efficiencies close to 100%. As the evanescent coupling is bidirectional, the source can be optically pumped in both free-space and through the ridge waveguide. The latter configuration paves the way toward a self-contained, all-fiber, plug-and-play solution for applications requiring a bright on-demand single photon source. Using InAsP quantum dots embedded in InP nanowire waveguides, coupling efficiencies to a SiN ridge waveguide of 74% with a single photon purity of 97% are demonstrated. The technique to demonstrate deterministic placement of single quantum emitters onto pre-fabricated waveguides is used, an important step toward the fabrication of complex quantum photonic circuits.

A planar waveguide Fourier-transform spectrometer with densely arrayed Mach-Zehnder interferometers is demonstrated. Subwavelength gratings are used to produce an optical path difference without waveguide bends. The fabricated device comprises of an array of 32 Mach-Zehnder interferometers, which produce a spatial interferogram without any moving parts, yielding a spectral resolution of 50 pm and a free-spectral range of 0.78 nm. As a result of similar propagation losses in subwavelength grating waveguides and conventional strip waveguides, loss imbalance is minimized and high interferometic extinction ratio of -25 to -30 dB is obtained. Furthermore, phase and amplitude errors arising from normal fabrication variation are compensated by the spectral retrieval process using calibration measurements.

Multimode interference couplers (MMIs) are fundamental building blocks in photonic integrated circuits. Here it is experimentally demonstrated, for the first time, a two-fold length reduction in an MMI coupler without any penalty on device performance. The design is based on a slotted 2 × 2 MMI fabricated on a commercial silicon-on-insulator (SOI) substrate. The slot is implemented with a subwavelength grating (SWG) comprising holes fully etched down to the oxide cladding, thereby allowing single etch step fabrication. The device has been designed using an in-house tool based on the Fourier Eigenmode Expansion Method. It has a footprint of only 3.5 μm x 23 μm and it exhibits a measured extinction ratio better than 15 dB within the full C-band (1530 nm‒1570 nm). SWG engineered slots thus offer excellent perspectives for the practical realization of MMIs couplers with substantially reduced footprint yet with outstanding performance.

Photoluminescence from individual InAs/InP quantum dots embedded within a planar n–i structure is studied as a function of vertical electric field. We demonstrate control of the electron number and determine the Stark shift and built-in dipole at zero electric field. In contrast to the well studied InAs/GaAs quantum dot material system, we obtain a built-in dipole which indicates that the electron lies above the hole at zero electric field. The magnitude and direction of the measured dipole suggests a uniform quantum dot composition. The gating principles we demonstrate can be applied to pre-positioned InAs/InP quantum dots, such that arrays of initialized single spins can be employed for quantum information applications. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Grating couplers are key elements enabling the coupling of light between planar waveguide circuits and optical fibers. In this work, it is demonstrated using simulations and experiments that a high coupling efficiency can be achieved for an arbitrary buried oxide thickness by judicious adjustment of the grating radiation angle. The coupler strength is engineered by subwavelength structure, allowing straightforward apodization and single etch step fabrication. The design has been implemented using Fourier-eigenmode expansion and finite difference time domain methods. The measured coupling loss of a continuously apodized grating is -2.16 dB with a 3 dB bandwidth of 64 nm, therefore opening promising prospects for low-cost and high-volume fabrication using 193 nm deep-ultraviolet lithography. It is also shown by simulations that a coupling loss as low as -0.42 dB is predicted for a modified coupler structure with bottom mirror.

Periodic structures with a sub-wavelength pitch have been known since Hertz conducted his first experiments on the polarization of electromagnetic waves. While the use of these structures in waveguide optics was proposed in the 1990s, it has been with the more recent developments of silicon photonics and high-precision lithography techniques that sub-wavelength structures have found widespread application in the field of pho- tonics. This review first provides an introduction to the physics of sub-wavelength structures. An overview of the applications of sub-wavelength structures is then given including: anti-reflective coatings, polarization rotators, high-efficiency fiber-chip cou- plers, spectrometers, high-reflectivity mirrors, athermal waveg- uides, multimode interference couplers, and dispersion engi- neered, ultra-broadband waveguide couplers among others. Particular attention is paid to providing insight into the design strategies for these devices. The concluding remarks provide an outlook on the future development of sub-wavelength structures and their impact in photonics.

The growth of GaN/AIGaN high electron mobility transistor structures (HEMT) on 4H-SiC by ammonia molecular beam epitaxy (a-MBE) is reported. Structures were grown using a magnetron sputter epitaxy deposited AlN buffer layer prior to the MBE growth of a carbon-doped insulating GaN isolation layer, undoped GaN channel layer and AIGaN cap layer. No ex-situ or in-situ high temperature pre-treatment of the SiC substrate was used. The electrical characteristics of the layers was excellent with RT mobilities of >1100 cm 2 /Vs for a sheet carrier density of >1 x 10 13 cm -2 . 2 HEMTs fabricated from layers gave an f t and f max of 36 and 80 GHz, respectively with a maximum saturated drain current of 450 mA/mm and transconductance of 160 mS/mm for a gate length of 0.3 μm.

Cette publication montre que les algorithrnes de raisonnement qualitatif manipulent les qualities par une caracterisation des valeurs reelles. Un exemple simple fait ressortir la signification qualitative du raisonnement de QSIM (Qualitative SIMulation), son calcul par intervalles et ses limitations. Par la suite, on propose une extension de QSIM pour integrer differentes connaissances (regles de production, raisonnernent qualitatif, ordre de grandeur, calcul quantitatit) dans un algorithme de simulation qualitative-quantitative (QQSIM) en qualifiant les donnees sous forme d'intervalles. Cette extension permet l'utilisation du raisonnement qualitatif sur un systeme de traitement d'eaux usees. Pour terminer, on montre que cette approche s'applique dans un contexte de systemes-experts. This paper shows that qualitative reasoning algorithms manipulate qualities through characterisation of real values. A simple example outlines the qualitative meaning of QSIM reasoning (Qualitative SIMulation). its interval arithmetic and its limits. An extension of QSIM is proposed to integrate various knowledge (production, rules qualitative reasoning, order of magnitude, quantitative computation) in a qualitative-quantitative simulation algorithm (QQSIM) by qualifying data as intervals. This extension enables qualitative reasoning to be used on a waste-water treatment plant. Finally, this approach is shown to apply to expert systems.

We examine the optical mode structure of two-dimensional, photonic crystal microcavities based on triple missing hole defects in hexagonal symmetry, etched air-hole, suspended InP membranes. Polarisation dependent photoluminescence from InAs/InP quantum dots embedded within the cavities is used to explore mode energies and quality factors, Q, as a function of cavity design parameters. Optimised Q values are in excess of 7,500 for the modified y-dipole mode. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Recent developments in single and double electrostatically confined quantum dots have shown that these devices are promising candidates for the implementation of quantum computation schemes in solid state systems. One of the obstacles for such an accomplishment is scalability. We present here the first realization of a triple lateral quantum dot with a few number of electrons, going down to fundamental configurations such as the vacuum state (no electrons in the system) or one electron in each quantum dot. New phenomena unseen in single or double dots are revealed involving charge and spin reconfigurations within the system, reminiscent of quantum cellular automata (QCA) functionalities. More importantly, it is necessary to understand these phenomena for they will be present in any three or more coupled quantum dots system. An equivalent circuit analysis of the system is used to reproduce these new features and to highlight some differences between experiments and the model, possibly arising from quantum phenomena not included in the circuit analysis. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optimization of the electrical characteristics of GaN/AlGaN HFETs grown on SiC substrates by ammonia-MBE is reported. By optimizing the growth conditions, GaN/AlGaN HFETs with room temperature mobilities of ∼1000 cm 2 /Vs and electron sheet densities of up to 1.9 x 10 13 cm -2 have been grown on SiC substrates with good reproducibility. These characteristics are comparable to the best characteristics of the GaN/AlGaN HFETs grown on sapphire. Devices fabricated from these optimized HFET layers showed excellent characteristics, e.g. maximum drain current density of 1.3 A/mm, f T of 70 GHz, f MAX of 130 GHz were measured for devices with 0.25 μm gate length and 100 μm gate width.