Jean-Marie Moison

Atomic‐scale roughness of GaAs/AlAs interfaces: A Raman scattering study of asymmetrical short‐period superlattices

We present Raman spectra obtained from very short period (a few atomic layers) GaAs/AlAs superlattices with asymmetrical unit cells containing two different GaAs wells. This allows us to analyze quantitatively for the first time the atomic‐scale component of the interface roughness. We demonstrate that it mainly originates at the GaAs on AlAs interface and strongly decreases with the growth temperature and the underlying AlAs layer thickness.

Bimodal distribution of Indium composition in arrays of low-pressure metalorganic-vapor-phase-epitaxy grown InGaAs/GaAs quantum dots

Low-pressure metalorganic-vapor-phase-epitaxy (LP-MOVPE) grown InGaAs/GaAs quantum dots (QDs) emitting around 1.3 μm have been studied by photoluminescence and transmission electron microscopy (TEM). We demonstrate the presence of a bimodal inhomogeneous broadening of the photoluminescence, correlated with a bimodal QDs contrast distribution in the TEM micrographs. Increasing the growth temperature of the dots induces a decrease of the ratio between the number of In-poor and In-rich QDs, illustrating the crucial influence of indium desorption on the LP-MOVPE growth of InGaAs QDs.

Mode structure and ray dynamics of a parabolic dome microcavity.

We consider the wave and ray dynamics of an electromagnetic field in a parabolic dome microcavity. The structure of the fundamental s wave involves a main lobe in which the electromagnetic field is confined around the focal point in an effective volume of the order of a cubic wavelength, while modes with finite angular momentum have a structure that avoids the focal area and have correspondingly larger effective volumes. The ray dynamics indicate that the fundamental s wave is robust with respect to small geometrical deformations of the cavity, while the higher order modes are unstable, giving rise to optical chaos. We discuss the incidence of these results on the modification of the spontaneous emission dynamics of an emitter placed in such a parabolic dome microcavity.

Discrete photonics in waveguide arrays

In homogeneous arrays of coupled waveguides, Floquet-Bloch waves are known to travel freely across the waveguides. We introduce a systematic discussion of the built-in patterning of the coupling constant between neighboring waveguides. Key patterns provide functions such as redirecting, guiding, and focusing these waves, up to nonlinear all-optical routing. This opens the way to light control in a functionalized discrete space, i.e., discrete photonics.

Confining light flow in weakly coupled waveguide arrays by structuring the coupling constant: towards discrete diffractive optics

Structuring the coupling constant in coupled waveguide arrays opens up a new route towards molding and controlling the flow of light in discrete structures. We show coupled mode theory is a reliable yet very simple and practical tool to design and explore new structures of patterned coupling constant. We validate our simulation and technological choices by successful fabrication of appropriate III-V semiconductor patterned waveguide arrays. We demonstrate confinement of light in designated areas of one-dimensional semi-conductor waveguide arrays.

Surface Roughness and Light Scattering in a Small Effective Area Microstructured Fiber

We report here the combined study of air/silica surface roughness and light scattering in a microstructured optical fiber designed for non-linear operation. Side polishing of the fiber gave access to the surface of the holes, and allowed measurements of their roughness by atomic force microscopy. The observed roughness topography, not reported in such fibers until now, consists of a rather regular arrangement of shallow patterns with lateral size in the micron range and amplitude in the 10 nm range. By comparing measured angle-resolved scattering patterns to coupled-mode calculations, we show that roughness-induced scattering loss can be linked to both the roughness and the overlap of fundamental with radiative modes at the air/silica interfaces. The reduction of surface roughness amplitude down to the thermodynamic limit could permit to strongly decrease the threshold of Raman fiber lasers.

Room temperature enhancement and inhibition of spontaneous emission in semiconductor microcavities

We have fabricated planar semiconductor microcavities with metallic mirrors in which we have observed both enhancement and inhibition of spontaneous emission, at room temperature. Our results are quantitatively accounted for by modeling the band-to-band emission as arising from point dipoles. Observation of these cavity quantum electrodynamics effects in room temperature semiconductor structures opens the way to optoelectronic devices with controlled spontaneous emission.

Analytical first-order extension of coupled-mode theory for waveguide arrays.

Coupled mode theory for waveguide arrays is extended to next-nearest neighbor interactions using propagation equations. Both lateral diffraction and propagation of Floquet-Bloch waves are altered respectively by extra coupling and non-orthogonality between isolated waveguide modes. The analytical formula describing the distortions of the diffraction relation is validated by direct numerical simulation for weakly coupled InP and GaAs shallow ridge waveguides and for strongly coupled Si-SiO(2) buried strip waveguides. The impact of extended coupled mode theory on propagation and diffraction design in waveguide arrays is discussed with reference to available experimental work.

Light transmission in multiple or single subwavelength trefoil channels of microstructured fibers

We evaluate the trefoil channels present between the holes of microstructured fibers as a potential dense array of small waveguides. In channels with an inner radius of 330nm, calculations indicate possible propagation with a mode waist of ~350nm at lambda=670nm, near to the diffraction limit. Actual measurements have been performed on a 1-meter fiber section, with injection by a microlensed fiber and mapping of output by near-field scanning optical microscopy. They show that light can be output in individual channels or in several of them, depending on the injection. The observed waist is ~500nm, possibly due to experimental widening. Estimated propagation losses are <20dB/m. Since each channel occupies only 2microm2, this structure opens a way to dense parallel optical processing.

A dense array of small coupled waveguides in fiber technology: trefoil channels of microstructured optical fibers

We calculate the limit to which the density of two-dimensional arrays of diffraction limited fiber waveguides can be reduced while maintaining weakly-coupled characteristics. We demonstrate that this density can be experimentally reached in an array of trefoil channels formed by the air holes of a microstructured optical fiber specially designed to meet limiting size and density specifications at lambda=1.55 microm.