M. Hugues

GaN microwires as optical microcavities: whispering gallery modes Vs Fabry-Perot modes

GaN microwires grown by metalorganic vapour phase epitaxy and with radii typically on the order of 1-5 micrometers exhibit a number of resonances in their photoluminescence spectra. These resonances include whispering gallery modes and transverse Fabry-Perot modes. A detailed spectroscopic study by polarization-resolved microphotoluminescence, in combination with electron microscopy images, has enabled to differentiate both kinds of modes and determined their main spectral properties. Finally, the dispersion of the ordinary and extraordinary refractive indices of strain-free GaN in the visible-UV range has been obtained thanks to the numerical simulation of the observed modes.

Phase-Tuned Entangled State Generation between Distant Spin Qubits

Entanglement between distant nodes is essential for the successful realisation of a distributed quantum network. Photon-mediated entanglement has recently been demonstrated in atomic [1] and diamond defect systems [2] and, very recently, using heavy hole spins in quantum dots [3]. However, until now controlled creation of entangled states with arbitrary phase had yet to be demonstrated in these systems. Here we demonstrate the high frequency creation of Bell states with arbitrary phase using electron-spin qubits confined to distant self-assembled InGaAs quantum dots (QD).

Impact of N on the lasing characteristics of GaInNAs∕GaAs quantum well lasers emitting from 1.29 to 1.52μm

The origin of the degradation with N of the threshold current density (Jth) and external differential quantum efficiency (ηd) of 1.29 to 1.52μmGaInNAs∕GaAs laser diodes is analyzed. Adding N to InGaAs leads to a ∼25% reduction of the carrier injection efficiency and thus to an increase of Jth and a decrease of ηd. This effect is likely related to carrier recombination losses in the barriers and is independent of the N content. The optical absorption losses and the internal transparency current density are found to increase with N content, accounting for the rest of the degradation in Jth. Modeling of the transparency carrier and radiative current densities identifies the increase of the defect-related recombination coefficient in GaInNAs as the dominant effect leading to the N dependence of Jtr.

Optimum indium composition for (Ga,In)(N,As)∕GaAs quantum wells emitting beyond 1.5μm

The influence of indium composition and quantum well (QW) thickness on the photoluminescence (PL) properties of high nitrogen content (Ga,In)(N,As)∕GaAs QWs grown by molecular beam epitaxy has been investigated in order to get an efficient emission in the 1.5–1.7μm range. Strong enhancement of room-temperature PL has been observed for postgrowth annealed QWs. However, the optimum annealing temperature depends on the In composition. Taking into account the effects of thermal annealing, a high In content and a very low growth temperature appear to be the best way to obtain an efficient emission beyond 1.5μm with (Ga,In)(N,As)∕GaAs QW.

Three-dimensional atomic-scale investigation of ZnO-MgxZn1−xO m-plane heterostructures

The structural, compositional, and optical properties of ZnO/MgxZn1−xO m-plane heterostructures are investigated using scanning transmission electron microscopy, laser-assisted atom probe tomography, and micro-photoluminescence. Coupled with electron tomography, atom probe tomography is currently the only technique providing a 3D reconstruction of the position of the atoms of a nanoscale specimen with their chemical nature. The multi-quantum well system investigated exhibits a V-groove grating profile along the a-axis accompanied by the formation of Zn- and Mg-enriched regions corresponding to the edges pointing towards the substrate and towards the upper surface, respectively. The optical signature of these heterostructures has been investigated by performing micro-photoluminescence on atom probe tip specimens. Effective mass calculations based on the 3D microscopy data indicate that the quantum well geometry and barrier composition yield a localization of hole states at the bottom of the V-groove.

Optical analogue of the spin Hall effect in a photonic cavity

We observe anisotropy in the polarization flux generated in a GaAs/AlAs photonic cavity by optical illumination, equivalent to spin currents in strongly coupled microcavities. Polarization rotation of the scattered photons around the Rayleigh ring is due to the TE-TM splitting of the cavity mode. Resolving the circular polarization components of the transmission reveals a separation of the polarization flux in momentum space. These observations constitute the optical analogue of the spin Hall effect.

Homoepitaxy of non-polar ZnO/(Zn,Mg)O multi-quantum wells: From a precise growth control to the observation of intersubband transitions

We have developed a method to grow and characterize the state of the art non-polar ZnO/(Zn,Mg)O multi-quantum wells on m-plane ZnO substrates as a prerequisite for applications based on intersubband transitions. The epilayer interfaces exhibit a low roughness, and the layer thickness remains constant within one monolayer in these heterostructures. The optical properties have been studied in the UV and IR domains by means of photoluminescence and absorption experiments, respectively. In the UV, the photoluminescence is very well described by an excitonic transition, with the clear effect of quantum confinement as a function of the well thickness in the absence of the internal field. In the IR, the intersubband transitions can be precisely modeled if a large depolarization shift is taken into account. Overall, we demonstrate a very good control in the design and fabrication of ZnO quantum wells (QWs) for intersubband transitions. Our result gives a clear understanding of the ISBTs in ZnO QWs.

Successive selective growth of semipolar (11-22) GaN on patterned sapphire substrate

Thanks to the use of two successive selective growths by metal organic chemical vapor deposition reactor, high quality semipolar (11-22) GaN with a homogenous defect repartition over the surface was achieved. The procedure starts with a first selective growth on a patterned sapphire substrate, leading to continuous stripes of three dimensional (3D) GaN crystals of low defect density. Then, a second selective growth step is achieved by depositing a SiNx nano-mask and a low temperature GaN nano-layer on the top of the GaN stripes. Hereby, we demonstrate an original way to obtain a homoepitaxial selective growth on 3D GaN crystals by taking advantage of the different crystallographic planes available. Basal stacking faults (BSFs) are generated during this second selective growth but could be eliminated by using a three-step growth method in which elongated voids are created above the defective area. For a fully coalesced sample grown using the 2 step method, dislocation density of 1.2 × 108 cm−2 and BSFs density of 154 cm−1 with a homogenous distribution have been measured by cathodoluminescence at 80 K. Consequently the material quality of this coalesced semipolar layer is comparable to the one of polar GaN on c-plane sapphire.

Infrared detectors based on InGaAsN∕GaAs intersubband transitions

InGaAsN∕GaAs multiquantum well structures have been grown by molecular beam epitaxy with 1% nitrogen in the well. Intersubband transitions have been observed in the infrared region by transmission spectroscopy. Infrared detectors have been processed and an intersubband transition has been observed in the photocurrent spectrum. All the observations are consistent with each other and in very good agreement with a theoretical calculation. Band to band transitions observed by photoluminescence also confirm the position of the levels in the well.

Analysis of the characteristic temperatures of (Ga,In)(N,As)/GaAs laser diodes

The characteristic temperatures of the threshold current density, T0, and external differential quantum efficiency, T1, of a series of (Ga,In)(N,As)/GaAs quantum well (QW) laser diodes are measured in the wavelength range from 1 to 1.5??m. It is found that both T0 and T1 strongly decrease with increasing lasing wavelength. The origin of this degradation is shown to be, in the case of T0, mostly dominated by a decrease in the transparency current density characteristic temperature, an increase in the optical losses and a decrease in the modal gain. The degradation of T1 is mainly due to the increase in the optical losses. The effective carrier recombination lifetime in the QW is shown to decrease from 1.2 to 0.2?ns with N content up to 2%, in good agreement with previous reports that link this low lifetime to non-radiative monomolecular recombination through defects in the QW. Carrier leakage is ruled out as the dominant process degrading T0 and T1 on the basis of the temperature dependence of the effective carrier recombination lifetime.