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International audience; The polariton emission in a microcavity is usually measured from the surface, and little is known about the actual role of guided modes below the light cone in air, that can couple to the exciton transition, collect part of the emission and contribute to polariton losses. In this work we investigate a ZnO planar microcavity with mesas. The emission of the guided modes is observed at the mesa’s edge, together with the vertical emission. The microcavity operates as a polariton laser under pulsed optical pumping [1].The spectra collected under the excitation spot and at the mesa’s edge are very different, as shown on figure 1a. The vertical emission is dominated by the lower polariton branch (LPB) associated to the cavity mode whereas the edge emission forms a broad peak modulated by fringes which spacing decreases as the spot moves away from the edge. While these fringes could indicate Fabry-Perot oscillations of polaritons propagating in-plane, in analogy with radiative polaritons in GaAs microcavity wires [2], a careful analysis shows that they are the signature of beatings between 5 to 10 guided modes, without being necessary to invoke multiple reflections. These modes have a large attenuation length of 27 µm, intermediate between the propagation length of LPB polaritons (a few µm in a ZnO microcavity [3]) and the one of waveguide polaritons (100µm in a GaN microcavity [4]).Under pulsed optical pumping, the guided modes are strongly excited. They form leaky modes for the excitonic reservoir, as well as for the LPB polaritons themselves. Most interestingly, when the vertical microcavity displays polariton lasing, the polariton laser emission feeds the guided modes, which then cascade in energy (Fig. 1b,c).This work evidences therefore that the threshold of the polariton laser in conventional vertical microcavities is increased due to the existence of usually hidden guided modes, that can deplete the exciton reservoir and collect resonantly scattered light from the lasing LPB mode.

International audience

We present recent progress achieved with III-nitride photonic circuits epitaxially grown on silicon(111). The photonic circuits embed two-dimensional photonic crystals and microdisk resonators coupled to suspended waveguides. We discuss the linear and nonlinear response of the resonators through second and third harmonic generation.

Light-emitting devices based on high-Al content (Al,Ga)N suffer from high densities of dislocations or other point defects, counterbalanced by the carrier localization arising from disorder in the ternary alloy. One way to improve the internal quantum efficiency is the controlled Si-doping of AlxGa1−xN/AlyGa1−yN multi-quantum wells, which was assigned to the reduction of point defects by reduction of internal strains, for some ideal concentration of Si. Time-resolved photoluminescence (TR-PL) can be used [S. F. Chichibu et al., Appl. Phys. Lett. 99, 051902 (2011)] to try and correlate the observed PL time-decays with the density of defects, in relation with the nonradiative recombination probability of photoexcited carriers. We present TR-PL studies of MOVPE-grown Si-doped AlxGa1−xN/AlyGa1−yN multi-quantum wells with typical values of x = 0.6 and y = 0.7 and of AlxGa1−xN epilayers with x up to 0.86. High-Al content MQWs and epilayers exhibit similar bi-exponential PL decay dynamics, with the slower component rapidly quenched when T is increased. The fast decay component remains in the nanosecond range at all temperatures and the PL intensity loss is limited by carrier localization.

[EN] The lack of memory effect of silicon makes it unfeasible to store electronic data in photonics. Here we propose a nonvolatile readout photonic memory, which is electronically written/erased and optically read. The memory utilizes indium tin oxide as a floating gate and exploits its epsilon-near-zero regime and electro-optic activity. Extinction ratios greater than 10 dB in a bandwidth of 100 nm for a 5 mu m long memory are obtained. Furthermore, power consumption in the order of microwatts with retention times of about a decade have been predicted. The proposed structure opens a pathway for developing highly integrated electro-optic devices such as memory banks. (C) 2019 Optical Society of America., The authors thank Alba Vicente for her valuable comments. Ministerio de Economia y Competitividad (MINECO) (TEC2016-76849, TEC2017-92037-EXP); Ministerio de Educacion, Cultura y Deporte (MECD) (FPU17/04224); Universitat Politecnica de Valencia (UPV) (FPI-Irene Olivares).

This paper aims to clarify two distinct but complementary questions on economic and legal additionality in the payments for environmental services (PES) debate based on examples from the literature and direct observations made in Madagascar and Mexico. For the economic dimension of additionality, we explain two 'regimes of justification', efficiency on the one hand and social equity on the other, and discuss how analysts position themselves with regard to both regimes. For the legal dimension, we review and analyse specific cases in which PES are implemented in addition to existing environmental regulations. We propose a renewed framework of analysis to distinguish 'compensation' and 'reward' in PES by crossing the opportunity cost dimension and the legal constraint vis-à-vis the environment. We show how difficult it is to fully maintain the objective of efficiency when PES are implemented simultaneously across different combinations of opportunity costs and regulation constraints. We propose policy options to address the contradiction between incentive and coercive instruments. These options are land sparing, social targeting and chronological combinations. Copyright © 2017 John Wiley & Sons, Ltd and ERP Environment.

Programmable switching nodes supporting Software-Defined Networking (SDN) over optical interconnecting technologies arise as a key enabling technology for future disaggregated Data Center (DC) environments. The SDNenabling roadmap of intra-DC optical solutions is already a reality for rack-to-rack interconnects, with recent research reporting on interesting applications of programmable silicon photonic switching fabrics addressing board-to-board and even on-board applications. In this perspective, simplified information addressing schemes like Bloom filter (BF)-based labels emerge as a highly promising solution for ensuring rapid switch reconfiguration, following quickly the changes enforced in network size, network topology or even in content location. The benefits of BF-based forwarding have been so far successfully demonstrated in the Information-Centric Network (ICN) paradigm, while theoretical studies have also revealed the energy consumption and speed advantages when applied in DCs. In this paper we present for the first time a programmable 4x4 Silicon Photonic switch that supports SDN through the use of BF-labeled router ports. Our scheme significantly simplifies packet forwarding as it negates the need for large forwarding tables, allowing for its remote control through modifications in the assigned BF labels. We demonstrate 1x4 switch operation controlling the Si-Pho switch by a Stratix V FPGA module, which is responsible for processing the packet ID and correlating its destination with the appropriate BF-labeled outgoing port. DAC- and amplifier-less control of the carrier-injection Si-Pho switches is demonstrated, revealing successful switching of 10Gb/s data packets with BF-based forwarding information changes taking place at a time-scale that equals the duration of four consecutive packets.

[EN] Photonic integrated circuits are developing as key enabling components for high-performance computing and advanced network-on-chip, as well as other emerging technologies such as lab-on-chip sensors, with relevant applications in areas from medicine and biotechnology to aerospace. These demanding applications will require novel features, such as dynamically reconfigurable light pathways, obtained by properly harnessing on-chip optical radiation. In this paper, we introduce a broadband, high-directivity (>150), low-loss, and reconfigurable silicon photonics nanoantenna that fully enables on-chip radiation control. We propose the use of these nanoantennas as versatile building blocks to develop wireless (unguided) silicon photonic devices, which considerably enhance the range of achievable integrated photonic functionalities. As examples of applications, we demonstrate 160 Gbit·s-1 data transmission over mm-scale wireless interconnects, a compact low-crosstalk 12-port crossing, and electrically reconfigurable pathways via optical beam steering. Moreover, the realization of a flow micro-cytometer for particle characterization demonstrates the smart system integration potential of our approach as lab-on-chip devices., Funding from grant TEC2015-63838-C3-1-R OPTONANOSENS (MINECO/FEDER, UE) is acknowledged. This work was also supported by project TEC2015-73581-JIN (AEI/FEDER, UE), the EU-funded projects FP7-ICT PHOXTROT (No.318240) and H2020-, the EU-funded H2020-FET-HPC EXANEST (No.671553) and the Generalitat Valenciana's PROMETEO grant NANOMET PLUS (PROMETEO II/2014/34) CG-M acknowledges support from Generalitat Valenciana’s VALi+d postdoctoral program (exp. APOSTD/ 2014/044). We thank David Zurita for his help in the design of the data acquisition code for the sensing application.

We demonstrate phase-matched second harmonic generation in GaN microdisks integrated on a photonic circuit on silicon. Phase matching between TM modes is evidenced by varying the disk diameter by steps of 8 nm, one thousand smaller than their diameter. An enhanced conversion efficiency is obtained under cw near infrared excitation when both pump and harmonic modes are resonant with whispering gallery modes (Q factors around 10000) and orbital momentum is conserved (Δm = 0 where m is the azimuthal number). We will compare the second harmonic conversion efficiency in GaN and AlN microdisks to the one obtained with other III-V materials.

We present a series of microdisk lasers realized within the same GaN-on-Si photonic platform scheme, and operating at room temperature under pulsed optical pumping over a broad spectral range extending over λ = 275 nm–470 nm. The III-nitride microdisks embed either binary GaN/AlN multiple quantum wells (MQWs) for UV operation, or ternary InGaN/GaN MQWs for violet and blue operation. This demonstrates the versatility of this nitride-on-silicon platform, and the realization on this platform of efficient active layers for lasing action over a 200 nm broad UV to visible spectral range. We probe the lasing threshold carrier density over the whole spectral range and found that it is similar whatever the emission wavelength for these Q > 1000 microdisk resonators with a constant material quality until quantum confined Stark effect takes over. The threshold is also found independent of microdisk diameters from 3 to 12 μm, with a β factor intermediate between the one of vertical cavity lasers and the one of small modal volume “thresholdless” lasers.

International audience; Excitons in nitride quantum wells (QWs) are naturally indirect due to the strong internal electric field: electron and hole within such excitons are spatially separated, leading to strong dipole moments and long radiative lifetimes. Extensive studies of indirect excitons (IXs) in GaAs-based heterostructures have shown that a combination of these two features results in many interesting properties of IXs: they can propagate over large distances, can be controlled in-situ by light and external gate voltage, cool down to the lattice temperature before recombination, and form cold and dense gas of interacting bosons. Compared to traditional IXs in arsenide heterostrutures, IXs in GaN QWs have much larger binding energies and smaller Bohr radii. This allows exploring IX propagation up to room temperature, and over a much larger density range. Using spatially- and time-resolved photoluminescence experiments, we have investigated the exciton transport in a 7 nm-wide GaN single QW sandwiched between Al0.19Ga0.81N barriers. The IX emission is imaged, with spectral and temporal resolutions, along the sample plane. Therefore the corresponding spatial density profiles are obtained and monitored as a function of (i) temperature, (ii) the exciton density at the excitation spot, (iii) the substrate material (GaN vs Sapphire) and (iv) the excitation regime (continuous vs pulsed excitation) [F. Fedichkin et al., Phys. Rev. B 91, 205424 (2015)]. We provide a comprehensive analysis of the data combined with numerical modeling, and we show that the efficient propagation of IXs takes place in the high density regime where the in-plane disorder is efficiently screened by the dipole-dipole interaction. Under these latter conditions, exciton mobility is almost temperature-independent from 10K up to room temperature. This suggests that exciton-exciton interaction is by far the dominant scattering mechanism, compared to scattering by the interface disorder. However, the exciton transport is maintained up to 300K only in the QW that was epitaxially grown on a GaN substrate, since nonradiative processes dominate the transport in the case of a sapphire substrate, for which high densities of threading dislocations are present.We provide a detailed understanding of the physical mechanisms of IX transport and its temperature and density dependence, and we show that nitride-hosted IXs constitute a promising system for the formation of collective bosonic states in semiconductors. Financial support: projects INDEX (FP7 PITN-GA-2011-289968) and OBELIX (ANR-15-CE30-0020-02).

International audience; Ultra-violet semiconductor lasers have numerous applications for optical storage, biochemistry, sterilization or optical interconnects. However the usual design of deep UV-emitting ridge lasers relies on complex buffer layers or expensive substrates for the growth of the nitride heterostructures [1]. Moreover microlaser and nanophotonic devices with integrated sources require an alternative approach compatible with thin waveguides.In this work we demonstrate a series of nitride-on-silicon microdisk lasers covering a broad spectral range, from the blue (=470 nm) to the deep ultra-violet (=275nm), and operating at room temperature. They rely on the controlled growth of thin AlN buffer layers on silicon substrates, followed by the growth of GaN/AlN or InGaN/GaN multiple quantum wells (MQWs) with a high radiative efficiency up to T=300K. We exploit the possibility to under-etch selectively the silicon substrate in order to form nitride membranes for photonic resonators; this has allowed many demonstrations of microdisk and photonic crystal cavities with quality factors larger than 1000, ranging from the mid-IR [2] to the near-UV spectral range [3,4], now expanding to the UV-C [5].Following the demonstration of a UV-C microlaser operating atat 275 nm and at room temperature [5], the figure 1 presents the emission characteristics of a series of nitride-on-silicon microdisks with different active layers grown on a thin AlN buffer layer. Their whispering gallery modes are probed under continuous wave excitation at 266 nm with a 15 µm laser spot (Figure 1.b); the photoluminescence is collected from the edge of the microdisks. The chosen microdisk diameters in panel (b) are 5-6 µm in order to properly exhibit the modes, except for the 3 µm microdisk emitting at 275 nm. In all cases, the sharpest peaks are observed in the low energy tail of the PL spectrum where the absorption of the active layer is limited. The measured Q factors exceed 1000, and reach 4000 in the best resonators. The lasing operation is obtained under pulsed quasi-resonant optical pumping (266nm, 400ps pulses) with a threshold excitation density ranging from 2 to 40 nJ/pulse (Figure 1.a). The dependence of the lasing threshold will be discussed.In the UV-B and UV-C spectral ranges, these demonstrations underline the interest of simple binary GaN/AlN MQWs, grown on a thin AlN buffer on a silicon substrate, in order to obtain efficient active layers providing optical gain up to room temperature. The microdisk lasing in the visible spectral range, based on more mature InGaN/GaN MQWs, proves the versatility of the nitride-on-silicon photonic platform, that can embed a broad range of integrated sources.

We demonstrate phase-matched second harmonic generation in gallium nitride on silicon microdisks. The microdisks are integrated with side-coupling bus waveguides in a two-dimensional photonic circuit. The second harmonic generation is excited with a continuous wave laser in the telecom band. By fabricating a series of microdisks with diameters varying by steps of 8 nm, we obtain a tuning of the whispering gallery mode resonances for the fundamental and harmonic waves. Phase matching is obtained when both resonances are matched with modes satisfying the conservation of orbital momentum, which leads to a pronounced enhancement of frequency conversion.

Plenary Lecture; International audience; Excitons in polar group-III nitride quantum wells (QWs) are naturally indirect, because electron and hole within such excitons are spatially separated. As a consequence, they have strong dipole moments and long radiative lifetimes. Extensive studies of indirect excitons (IXs) in GaAs-based heterostructures have shown, that a combination of these two features results in many interesting properties of IXs: they can propagate over large distances, can be controlled in-situ by light and external gate voltage, cool down to the lattice temperature before recombination, and form cold and dense gas of interacting bosons. Compared to traditional IXs in arsenide heterostrutures, IXs in nitride QWs have much larger binding energies and smaller Bohr radii. This allows exploring IX propagation up to room temperature, and over two orders of magnitude higher density range. Using spatially- and time-resolved photoluminescence experiments we examined temperature, density, substrate material (GaN vs Sapphire) and excitation regime (cw vs pulsed excitation) dependence of the exciton transport in 7 nm wide GaN QW sandwiched between Al0.19Ga0.81N barriers, the structure is characterized by internal electric field of order of 1 MV/cm. Comprehensive analysis of the data combined with numerical modeling show that in GaN QWs efficient propagation of IX takes place in the high density regime where the in-plane disorder is efficiently screened by the dipole - dipole interaction. Under these conditions, exciton mobility is almost temperature-independent from 10 K up to room temperature. This suggests that exciton-exciton interaction is by far dominant scattering mechanism, compared to scattering on the interface disorder. In this context, we argue that nitride-hosted IXs constitute a promising system for formation of collective bosonic states in semiconductors.

International audience; The recent developments of nitride nanophotonics, based on photonic crystal membrane nanocavities and microdisk resonators, pave the way to a novel nanophotonic platform. Here we present two recent achievements: first we demonstrate the room-temperature operation of a nitride microlaser emitting in the deep UV spectral range (=275 nm) with GaN/AlN multiple quantum wells. Secondly we tailor the second-harmonic generation of a cw near infra-red radiation coupled to a high quality factor photonic crystal cavity.

International audience; The spatial dynamics of the formation of a polariton condensate under a tightly focused excitation is imaged through 2D near-field and far-field 2D tomography in a ZnO microcavity, up to room temperature. The modelling exhibits the role of the outwards polariton flux caused by the reservoir repulsion, that leads to a 3 to 10 fold increase of the condensation threshold and is imprinted in the shape of the polariton condensate

Towards Single Mode Fiber (SMF) based optical connections in Data Centers (DCs) we are presenting the SM platform of the EU FP7 project PhoxTror. The major building blocks envisioned in this area are a 3D transceiver with a capacity of up to 480 Gb/s and a 3D router with 480 Gb/s throughput based on a Si photonics switching matrix. Towards this goal it is presented the basic 2×2 switching elements, the 4×4 switching matrix of the router and the Optical Through Silicon Vias (OTSVs) that enable the 3D functionality in the Si interposer.