46 research outputs found
Fabrication and Optical Properties of a Fully Hybrid Epitaxial ZnO-Based Microcavity in the Strong Coupling Regime
In order to achieve polariton lasing at room temperature, a new fabrication
methodology for planar microcavities is proposed: a ZnO-based microcavity in
which the active region is epitaxially grown on an AlGaN/AlN/Si substrate and
in which two dielectric mirrors are used. This approach allows as to
simultaneously obtain a high-quality active layer together with a high photonic
confinement as demonstrated through macro-, and micro-photoluminescence
({\mu}-PL) and reflectivity experiments. A quality factor of 675 and a maximum
PL emission at k=0 are evidenced thanks to {\mu}-PL, revealing an efficient
polaritonic relaxation even at low excitation power.Comment: 12 pages, 3 figure
LO-phonon assisted polariton lasing in a ZnO based microcavity
Polariton relaxation mechanisms are analysed experimentally and theoretically
in a ZnO-based polariton laser. A minimum lasing threshold is obtained when the
energy difference between the exciton reservoir and the bottom of the lower
polariton branch is resonant with the LO phonon energy. Tuning off this
resonance increases the threshold, and exciton-exciton scattering processes
become involved in the polariton relaxation. These observations are
qualitatively reproduced by simulations based on the numerical solution of the
semi-classical Boltzmann equations
Stakeholder perceptions of policy tools in support of sustainable food consumption in Europe: Policy implications
Transitioning agri-food systems towards increased sustainability and resilience requires that attention be paid to sustainable food consumption policies. Policy-making processes often require the engagement and acceptance of key stakeholders. This study analyses stakeholders' solutions for creating sustainable agri-food systems, through interviews with a broad range of stakeholders including food value chain actors, non-governmental organizations, governmental institutions, research institutions and academic experts. The study draws on 38 in-depth, semi-structured interviews conducted in four European countries: France, Iceland, Italy and the UK, as well as three interviews with high-level EU experts. The interviewees' solutions were analysed according to a five-category typology of policy tools, encompassing direct activity regulations, and market-based, knowledge-based, governance and strategic policy tools. Most of the identified solutions were located in the strategic tools category, reflecting shared recognition of the need to integrate food policy to achieve long-term goals. Emerging solutions-those which were most commonly identified among the different national contexts-were then used to derive empirically-grounded and more universally applicable recommendations for the advancement of sustainable food consumption policies
Silicon CMOS photonics platform for enabling high-speed DQPSK transceivers
In this work we review the results obtained under the framework of FP7-HELIOS project for integrated DQPSK transceivers in silicon photonics. A differential DQPSK receiver with balanced zero biased Germanium photodiodes has been demonstrated at 10Gbit/s with an error floor around 10(-15). Furthermore, DPSK modulation up to 10Gbit/s with a bit error rate below 10(-9) is also demonstrated using a silicon push-pull operated dual-drive Mach-Zehnder modulator (MZM) based on carrier depletion. The results indicate the potential of the silicon CMOS photonics platform for boosting next-generation optical networks based on advanced modulation formats
Silicon optical modulators for high data rate applications
Abstract In this work we describe the carrier depletion MZI modulators, slow wave structures for modulation enhancement and the QCSE modulator which are under development in the European HELIOS project and the UK Silicon Photonics project. Introduction High performance silicon optical modulators are key to many silicon based photonic applications. Over the previous decade the development seen in the performance of silicon optical modulators has been vast. Several routes to modulation have been used to overcome the lack of a strong electro optic effect in silicon. These include the plasma dispersion effect, III-V hybrid device fabrication, SiGe devices, Polymer and Strain induced electro-optic effects. Reported performances now regularly range from 10Gbit/s up to 40Gbit/s. HELIOS, which is a European FP7 funded project and the UK silicon photonics project (UKSP), funded by the EPSRC both involve the development of the different photonic components required to form photonic circuits with a range of functionality. Within both projects there is strong modulator activity with carrier depletion based modulation, QCSE modulation and structures to gain enhancement of the modulation effect under development. Carrier depletion modulation Optical modulators based upon free carrier depletion are widely regarded as being the simplest approach to achieve high performance modulation in silicon. They operate by reverse biasing a diode structure which is incorporated in or around an optical waveguide. The depletion of free carriers therefore interacts with the propagating light causing a change in phase through the plasma dispersion effect. Within the HELIOS project two phase modulators based upon this approach are under investigation using both PN and PIPIN diodes. Cross sectional diagrams of these devices are shown in figure 1. The first structure is based in silicon-on-insulator (SOI) of 220nm thickness. The waveguide section and the slab to one side is doped p type. The slab on the other side of the waveguide is then doped n type setting up a pn junction at the edge of the waveguide rib. The concentration of the n type doping is made larger than the p type doping such that the depletion region extends mainly into the waveguide during reverse bias. These lightly doped p and n type regions extend out to meet highly doped regions which in turn provide ohmic contacts to coplanar waveguide electrodes which are used to drive the device. The devic
Silicon optical modulators
Optical technology is poised to revolutionize short-reach interconnects. The leading candidate technology is silicon photonics, and the workhorse of such an interconnect is the optical modulator. Modulators have been improved dramatically in recent years, with a notable increase in bandwidth from the megahertz to the multigigahertz regime in just over half a decade. However, the demands of optical interconnects are significant, and many questions remain unanswered as to whether silicon can meet the required performance metrics. Minimizing metrics such as the device footprint and energy requirement per bit, while also maximizing bandwidth and modulation depth, is non-trivial. All of this must be achieved within an acceptable thermal tolerance and optical spectral width using CMOS-compatible fabrication processes. This Review discusses the techniques that have been (and will continue to be) used to implement silicon optical modulators, as well as providing an outlook for these devices and the candidate solutions of the future
Incentivising Forest Conservation: Payments for Environmental Services and Reducing Carbon Emissions from Deforestation
The Economic and Legal Sides of Additionality in Payments for Environmental Services
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 âregimes of justificationâ, 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 âcompensationâ and ârewardâ in PES by crossing the opportunity cost dimension and the legal constraint vis-Ă -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
Room-Temperature Transport of Indirect Excitons in (Al,Ga)N/GaN Quantum Wells
We report on the exciton propagation in polar Ă°Al; GaĂN=GaN quantum wells over several micrometers and up to room temperature. The key ingredient to achieve this result is the crystalline quality of GaN quantum wells grown on GaN substrate that limits nonradiative recombination. From the comparison of the spatial and temporal dynamics of photoluminescence, we conclude that the propagation of excitons under continuous-wave excitation is assisted by efficient screening of the in-plane disorder. Modeling within drift-diffusion formalism corroborates this conclusion and suggests that exciton propagation is still limited by the exciton scattering on defects rather than by exciton-exciton scattering so that improving interface quality can boost exciton transport further. Our results pave the way towards room-temperature excitonic devices based on gate-controlled exciton transport in wide-band-gap polar heterostructures