The present work showcases an innovative optimization methodology based on deep learning that combines Multi- Valued Artificial Neural Networks and back-propagation optimization. The methodology addresses the inherent limitations of conventional approaches when employed in isolation. We applied the proposed methodology to design structural color filters that surpasses the sRGB gamut while preserving fabrication constraints.
We present a novel approach to create spinodal-like structures appropriately modulating the instability of the solid state dewetting: many materials, indeed, 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, as light management and topologically protected electronic states. These applications require scalable fabrication, which is hard to achieve with available top-down approaches. These spinodal materials are used by a hybrid top-down/bottom-up approach based on sol-gel dip-coating and nano-imprint lithography for the faithful reproduction of the disordered metasurfaces in metal oxides (SiO2 and TiO2).
Nanoimprint lithography (NIL) is a method to (nano-) structure inorganic materials from sol-gel liquid formulations
and colloidal suspensions onto a surface. This technique, first inspired by embossing techniques, was developed for soft polymer processing, as final or intermediate materials, but is today fully adapted to hard inorganic materials with a high dielectric constant, such as metal oxides, with countless chemical compositions provided by the sol−gel chemistry. Consequently, NIL has become a versatile, high-throughput, and highly precise fabrication method that is mature for lab developments and scaling up.
I will first describe generalities of sol-gel dip coating and NIL method for metal oxides (e.g. SiO2, TiO2) and review some of the recent results we obtained in this field, including fabrication of ordered and disordered optical metasurfaces, structural color, anti-reflection coatings, refractive index sensing and enhanced light extraction.
Fabrication and scaling of disordered hyperuniform (dHU) materials remain hampered by the difficulties in controlling the spontaneous phenomena leading to this novel kind of exotic arrangement of objects. In this work, we demonstrate a hybrid top-down/bottom-up approach based on sol-gel dip-coating and nano-imprint lithography for the faithful reproduction of dHU metasurfaces in metal oxides (MOx). Nano- to micro-structures made of silica and titania can be directly printed over several cm2 on glass and on silicon substrates. Firstly, we describe the polymer mold fabrication starting from a hard master obtained via spontaneous solid-state dewetting. Then we address the effective dHU character of the master and of the replica and the role of the initial thickness of the sol-gel layer on the MOx replicas. Finally, these structures will be optimized towards their exploitation in many potential photonic applications like photonic devices (anti-reflection coatings, quantum emitters).
This article demonstrates that the combination of all-dielectric metal oxides sol-gel sensitive materials and metasurfaces, prepared by simple sol-gel methods (dip-coating and soft-Nano Imprint Lithography), can lead to nanocomposite systems with high sensitivity for RI variation and VOC concentration in air detection in spectral shift mode: 4500 nm / RIU ; 0.2 nm / ppm, and in direct reflectance mode: FOM* = 17 ; 0.55 10-3 R / ppm. The metasurface is composed of TiO2 high aspect ratio nano pillars array, replicated from a commercial anti-reflective polymer surface, while the sensitive materials embedding the latter are class II hybrid silica microporous materials containing various types of covalently bonded organic functions. These hybrid layers showed relative significant differences in chemical affinity with different VOCs, which can be exploited to eliminate interferences with air moisture and for qualitative analysis of gas mixtures. We also demonstrated that the presence of the TiO2 metasurface is responsible for the signal intensity increase by almost an order of magnitude in simple reflection mode. This improvement compared to simple Fabry-Perot bi-layer is due to the antenna effect, enhancing the interaction of the confined electromagnetic wave with the sensitive medium. This sol-gel nanocomposite system presents many advantages such as high throughput and low-cost elaboration of the elements, high chemical mechanical and thermal stability ensuring a high stability for detection for long period of time.
In this work, mechanically, chemically, and thermally resistant broadband and broad-angle antireflection coatings were prepared on 10 cm diameter glass substrates combining sol−gel deposition with nanoimprint lithography. The coatings are composed of water-repellent methylated silica (Si4O7Me2) and exhibit a transverse refractive index gradient created by tapered, nipple-dimple, subwavelength nanostructures, featuring a record vertical aspect ratio of ∼1.7. The structure is composed of hexagonal arrays of nanopillars (∼200 nm height, ∼120 nm width) and holes (∼50 nm depth, ∼100 nm width) with a 270 nm pitch. The corresponding effective refractive index is between 1.2 and 1.26, depending on the fabrication conditions. Total transmission for double-face nanoimprint wafers reaches 96−97% in the visible range; it is limited by specular reflection and mostly by the intrinsic diffusion of the glass substrate. The antireflective effect is effective up to an ∼60° incidence angle. We address the robustness of the inorganic-based coating in various realistic and extreme conditions, comparing them to the organic perfluoropolyether (PFPE) counterpart (master reference). The sol−gel system is extremely stable at high temperature (up to 600 °C, against 200 °C for the polymer reference). Both systems showed excellent chemical stability, except in strong alkaline conditions. The inorganic nanostructure showed an abrasion resistance of more than 2 orders of magnitude superior to the polymer one with less than 20% loss of antireflective performance after 2000 rubbing cycles under an ∼2 N cm−2 pressure. This difference springs from the large elastic modulus of the sol−gel material combined with an excellent adhesion to the substrate and to the specific nipple-dimple conformation. The presence of holes allows maintaining a refractive index gradient profile even after tearing out part of the nanopillar population. Our results are relevant to applications where transparent windows with broadband and broad-angle transmission are needed, such as protective glasses on photovoltaic cells or C-MOS cameras.
Transverse patterns in polariton fluids were recently studied as promising candidates for all-optical low-intensity switching. Here, we demonstrate these patterns in a specifically designed double-cavity system. We theoretically and experimentally analyse their formation and optical control. Our detailed theoretical analysis of the coupled nonlinear dynamics of the optical fields inside the double-cavity and the excitonic excitations inside the embedded semiconductor quantum wells is firmly based on a microscopic many-particle theory. Our calculations in the time domain enable us to study both the ultrafast transient dynamics of the patterns and their steady-state behavior under stationary excitation conditions. The patterns we report and analyze go beyond what can be observed and understood in a simple scalar quantum field. We find that polarization-selective excitation of the polaritons leads to a complex interplay between longitudinal-transverse splitting of the cavity modes and the spin-dependent interactions of the polaritons' excitonic component.
Optical and spin properties of individual GaAs droplet dots in AlGaAs barriers are studied in photoluminescence
experiments at 4K. First we report strong mixing of heavy hole-light hole states. Using the neutral and charged
exciton emission as a monitor we observe the direct consequence of quantum dot symmetry reduction in this strain free system. By fitting the polar diagram of the emission with simple analytical expressions obtained from k•p theory we are able to extract the mixing that arises from the heavy-light hole coupling due to the geometrical asymmetry of the quantum dot. Second we report optical orientation experiments. Circularly polarized optical excitation yields strong circular polarization of the resulting photoluminescence. Optical injection of spin polarized electrons into a GaAs dot gives rise to dynamical nuclear polarization that considerably changes the exciton Zeeman splitting (Overhauser shift). We show that the created nuclear polarization is bistable and present a direct measurement of the build-up time of the nuclear polarization in a single GaAs dot in the order of one second.
We investigated on the growth, by molecular beam epitaxy, of InAs quantum dots on nanoscale areas of the GaAs(001) surface defined by an e-beam lithographed SiO2 mask. The selective self-assembling of the InAs dots inside the holes of the mask was obtained by a suitable choice of the growth parameters and of the pattern size. Photoluminescence from the spatially confined dots showed a blue-shifted emission but a radiative decay comparable to that of dots nucleated on the extended GaAs surface.
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