44 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
Complexity of dipolar exciton Mott transition in GaN/(AlGa)N nanostructures
The Mott transition from a dipolar excitonic liquid to an electron-hole
plasma is demonstrated in a wide GaN/(Al,Ga)N quantum well at K by means
of spatially-resolved magneto-photoluminescence spectroscopy. Increasing
optical excitation density we drive the system from the excitonic state,
characterized by a diamagnetic behavior and thus a quadratic energy dependence
on the magnetic field, to the unbound electron-hole state, characterized by a
linear shift of the emission energy with the magnetic field. The complexity of
the system requires to take into account both the density-dependence of the
exciton binding energy and the exciton-exciton interaction and correlation
energy that are of the same order of magnitude. We estimate the carrier density
at Mott transition as cm and
address the role played by excitonic correlations in this process. Our results
strongly rely on the spatial resolution of the photoluminescence and the
assessment of the carrier transport. We show, that in contrast to
GaAs/(Al,Ga)As systems, where transport of dipolar magnetoexcitons is strongly
quenched by the magnetic field due to exciton mass enhancement, in GaN/(Al,Ga)N
the band parameters are such that the transport is preserved up to T.Comment: 15 pages 13 figure
Patterned silicon substrates: a common platform for room temperature GaN and ZnO polariton lasers
A new platform for fabricating polariton lasers operating at room temperature
is introduced: nitride-based distributed Bragg reflectors epitaxially grown on
patterned silicon substrates. The patterning allows for an enhanced strain
relaxation thereby enabling to stack a large number of crack-free AlN/AlGaN
pairs and achieve cavity quality factors of several thousands with a large
spatial homogeneity. GaN and ZnO active regions are epitaxially grown thereon
and the cavities are completed with top dielectric Bragg reflectors. The two
structures display strong-coupling and polariton lasing at room temperature and
constitute an intermediate step in the way towards integrated polariton
devices
Optical fiber-to-chip assembly process for ultra-low loss photonic devices based on silicon nitride for space applications
[EN] In this work, we demonstrate an efficient fiber array-to-chip assembly process with a high number of
input/output ports. The proposed approach is based on using a pre-alignment coupling structure to separately align
the input and output ports. The assembling process has been experimentally validated in photonic integrated
circuits fabricated with an ultra-low-loss waveguide technology based on silicon nitride, which features
propagation losses as low as 9.5 dB/m. The developed technology is expected to extend the use of integrated
photonics for space applicationsThis work was supported by EU-funded H2020 project RETINA under grant agreement n° 821943Brimont, ACJ.; Zurita Herranz, D.; Duarte, VC.; Mengual, T.; Chmielak, B.; Suckow, S.; Giesecke, A.... (2020). Optical fiber-to-chip assembly process for ultra-low loss photonic devices based on silicon nitride for space applications. 1-3. http://hdl.handle.net/10251/1786581
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 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
Excitonic spin relaxation in GaN
By performing non-degenerate pump-probe experiments, we
study the relaxation dynamics of spin-polarized A and B excitons in wurtzite
epitaxial GaN. We show that the spin relaxation of the exciton as a whole is
negligible with regard to the spin relaxation of the individual carriers. We
determined  ps for the electron in the conduction band,  ps, and  ps for the heavy hole (HH) and for the light hole
(LH), respectively. The quite long HH relaxation time can be related to the
band structure in which the degeneracy between different spin-valence bands
is lifted