116 research outputs found
Direct observation of Dirac cones and a flatband in a honeycomb lattice for polaritons
Two-dimensional lattices of coupled micropillars etched in a planar
semiconductor microcavity offer a workbench to engineer the band structure of
polaritons. We report experimental studies of honeycomb lattices where the
polariton low-energy dispersion is analogous to that of electrons in graphene.
Using energy-resolved photoluminescence we directly observe Dirac cones, around
which the dynamics of polaritons is described by the Dirac equation for
massless particles. At higher energies, we observe p orbital bands, one of them
with the nondispersive character of a flatband. The realization of this
structure which holds massless, massive and infinitely massive particles opens
the route towards studies of the interplay of dispersion, interactions, and
frustration in a novel and controlled environment
Nanoporous Ge thin film production combining Ge sputtering and dopant implantation
International audienceIn this work a novel process allowing for the production of nanoporous Ge thin films is presented. This process uses the combination of two techniques: Ge sputtering on SiO 2 and dopant ion implantation. The process entails four successive steps: (i) Ge sputtering on SiO 2 , (ii) implantation preannealing, (iii) high-dose dopant implantation, and (iv) implantation postannealing. Scanning electron microscopy and transmission electron microscopy were used to characterize the morphology of the Ge film at different process steps under different postannealing conditions. For the same postannealing conditions, the Ge film topology was shown to be similar for different implantation doses and different dopants. However, the film topology can be controlled by adjusting the postannealing conditions
Photoneutralization and slow capture of carriers in quantum dots probed by resonant excitation spectroscopy
International audienceWe investigate experimentally and theoretically the resonant emission of single InAs/GaAs quantum dots in a planar microcavity. Due to the presence of at least one residual charge in the quantum dots, the resonant excitation of the neutral exciton is blocked. The influence of the residual doping on the initial quantum dots charge state is analyzed, and the resonant emission quenching is interpreted as a Coulomb blockade effect. The use of an additional non-resonant laser in a specific low power regime leads to the carrier draining in quantum dots and allows an efficient optical gating of the exciton resonant emission. A detailed population evolution model, developed to describe the carrier draining and the optical gate effect, perfectly fits the experimental results in the steady state and dynamical regimes of the optical gate with a single set of parameters. We deduce that ultra-slow Auger- and phonon-assisted capture processes govern the carrier draining in quantum dots with relaxation times in the 1 - 100 microsecond range. We conclude that the optical gate acts as a very sensitive probe of the quantum dots population relaxation in an unprecedented slow-capture regime
Photon correlation in GaAs self-assembled quantum dots
We report on photon coincidence measurement in a single GaAs self-assembled
quantum dot (QD) using a pulsed excitation light source. At low excitation,
when a neutral exciton line was present in the photoluminescence (PL) spectrum,
we observed nearly perfect single photon emission from an isolated QD at 670 nm
wavelength. For higher excitation, multiple PL lines appeared on the spectra,
reflecting the formation of exciton complexes. Cross-correlation functions
between these lines showed either bunching or antibunching behavior, depending
on whether the relevant emission was from a biexciton cascade or a charged
exciton recombination.Comment: 5 pages, 3 figure
One-dimensional microcavity-based optical parametric oscillator: generation of balanced twin beams in strong and weak coupling regime
International audienceWe report on a detailed experimental investigation of interbranch parametric scattering processes in onedimensional semiconductor microcavities. Band dispersion and corresponding far field emission patterns are studied by polarization resolved and power dependence measurements under resonant and non-resonant excitation at normal incidence. We demonstrate the realization of optical parametric oscillation of balanced twin beams which are degenerate in energy and split in momentum space. This achievement is shown for both the strong and the weak coupling regime highlighting the versatility of this peculiar microcavity system
Back-Propagation Optimization and Multi-Valued Artificial Neural Networks for Highly Vivid Structural Color Filter Metasurfaces
We introduce a novel technique for designing color filter metasurfaces using
a data-driven approach based on deep learning. Our innovative approach employs
inverse design principles to identify highly efficient designs that outperform
all the configurations in the dataset, which consists of 585 distinct
geometries solely. By combining Multi-Valued Artificial Neural Networks and
back-propagation optimization, we overcome the limitations of previous
approaches, such as poor performance due to extrapolation and undesired local
minima. Consequently, we successfully create reliable and highly efficient
configurations for metasurface color filters capable of producing exceptionally
vivid colors that go beyond the sRGB gamut. Furthermore, our deep learning
technique can be extended to design various pixellated metasurface
configurations with different functionalities.Comment: To be published. 25 Pages, 17 Figure
Preventing Corrosion of Aluminum Metal with Nanometer-Thick Films of Al2O3 Capped with TiO2 for Ultraviolet Plasmonics
Extending plasmonics into the ultraviolet range imposes the use of aluminum
to achieve the best optical performance. However, water corrosion is a major
limiting issue for UV aluminum plasmonics, as this phenomenon occurs
significantly faster in presence of UV light, even at low laser powers of a few
microwatts. Here we assess the performance of nanometer-thick layers of various
metal oxides deposited by atomic layer deposition (ALD) and plasma-enhanced
chemical vapor deposition (PECVD) on top of aluminum nanoapertures to protect
the metal against UV photocorrosion. The combination of a 5 nm Al2O3 layer
covered by a 5 nm TiO2 capping provides the best resistance performance, while
a single 10 nm layer of SiO2 or HfO2 is a good alternative. We also report the
influence of the laser wavelength, the laser operation mode and the pH of the
solution. Properly choosing these conditions significantly extends the range of
optical powers for which the aluminum nanostructures can be used. As
application, we demonstrate the label-free detection of streptavidin proteins
with improved signal to noise ratio. Our approach is also beneficial to promote
the long-term stability of the aluminum nanostructures. Finding the appropriate
nanoscale protection against aluminum corrosion is the key to enable the
development of UV plasmonic applications in chemistry and biology
Hyperuniform monocrystalline structures by spinodal solid-state dewetting
Materials featuring anomalous suppression of density fluctuations over large
length scales are emerging systems known as disordered hyperuniform. The
underlying hidden order renders them appealing for several applications, such
as light management and topologically protected electronic states. These
applications require scalable fabrication, which is hard to achieve with
available top-down approaches. Theoretically, it is known that spinodal
decomposition can lead to disordered hyperuniform architectures. Spontaneous
formation of stable patterns could thus be a viable path for the bottom-up
fabrication of these materials. Here we show that mono-crystalline
semiconductor-based structures, in particular SiGe layers
deposited on silicon-on-insulator substrates, can undergo spinodal solid-state
dewetting featuring correlated disorder with an effective hyperuniform
character. Nano- to micro-metric sized structures targeting specific
morphologies and hyperuniform character can be obtained, proving the generality
of the approach and paving the way for technological applications of disordered
hyperuniform metamaterials. Phase-field simulations explain the underlying
non-linear dynamics and the physical origin of the emerging patterns.Comment: 6 pages, 3 figures, supplementary information (7 pages) enclose
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