RESUMEN
We demonstrate efficient anti reflection coatings based on adiabatic index matching obtained via nano-imprint lithography. They exhibit high total transmission, achromaticity (99.5% < T < 99.8% from 390 to 900 nm and 99% < T < 99.5% from 800 to 1600 nm) and wide angular acceptance (T > 99% up to 50 degrees). Our devices show high laser-induced damage thresholds in the sub-picosecond (>5 J/cm2 at 1030 nm, 500 fs), nanosecond (>150 J/cm2 at 1064 nm, 12 ns and >100 J/cm2 at 532 nm, 12 ns) regimes, and low absorption in the CW regime (<1.3 ppm at 1080 nm), close to those of the fused silica substrate.
RESUMEN
Large scale and low-cost nanopatterning of materials is of tremendous interest for optoelectronic devices. Nanoimprint lithography has emerged in recent years as a nanofabrication strategy that is high-throughput and has a resolution comparable to that of electron-beam lithography (EBL). It is enabled by pattern replication of an EBL master into polydimethylsiloxane (PDMS), that is then used to pattern a resist for further processing, or a sol-gel that could be calcinated into a solid material. Although the sol-gel chemistry offers a wide spectrum of material compositions, metals are still difficult to achieve. This gap could be bridged by using colloidal nanoparticles as resist, but deep understanding of the key parameters is still lacking. Here, we use supported metallic nanocubes as a model resist to gain fundamental insights into nanoparticle imprinting. We uncover the major role played by the surfactant layer trapped between nanocubes and substrate, and measure its thickness with subnanometer resolution by using gap plasmon spectroscopy as a metrology platform. This enables us to quantify the van der Waals (VDW) interactions responsible for the friction opposing the nanocube motion, and we find that these are almost in quantitative agreement with the Stokes drag acting on the nanocubes during nanoimprint, that is estimated with a simplified fluid mechanics model. These results reveal that a minimum thickness of surfactant is required, acting as a spacer layer mitigating van der Waals forces between nanocubes and the substrate. In the light of these findings we propose a general method for resist preparation to achieve optimal nanoparticle mobility and show the assembly of printable Ag and Au nanocube grids, that could enable the fabrication of low-cost transparent electrodes of high material quality upon nanocube epitaxy.
RESUMEN
We combine a gas-adsorbent microporous hybrid silica layer and a dense TiO2 Mie resonator array (metasurface), both obtained by sol-gel deposition and nanoimprint lithography, to form nanocomposite systems with high sensitivity for refractive index (RI) variations induced by gas adsorption. Using optical transduction based on direct specular reflection, we show spectral shifts of 4470 nm/RIU corresponding to 0.2 nm/ppm gas (air concentration) and reflection intensity changes of R* = 17 (R/RIU) and 0.55 × 10-3 R/ppm (air concentration). The metasurface is composed of hexagonally arranged TiO2 nanopillar arrays, whereas the surrounding sensitive material is a class II microporous hybrid silica, containing methyl and phenyl covalently bonded organic functions. This hybrid layer shows efficient adsorption capability of volatile organic molecules such as isopropanol, which is used to induce slight variations of RI around the TiO2 antennas. Specular reflectance variations at 45° incidence and refractive index measurements are performed using a spectroscopic ellipsometer. The presence of the titania metasurface enhances the signal by almost an order of magnitude with respect to the 2D counterpart (simulated as an effective medium approximation) and is attributed to the antenna effect, enhancing the interaction of the confined electromagnetic wave with the sensitive microporous medium. This sol-gel nanocomposite system presents many advantages such as high throughput and low-cost elaboration of elements and a high chemical, mechanical, and thermal resistance, ensuring high stability as a potential gas-sensitive nanocomposite layer for long periods. This work is a case study of improving the sensitivity of sol-gel gas-sensitive materials in optical transduction, which will be exploited in further works to develop artificial noses.
RESUMEN
Fabrication and scaling of disordered hyperuniform materials remain hampered by the difficulties in controlling the spontaneous phenomena leading to this novel kind of exotic arrangement of objects. Here, we demonstrate a hybrid top-down/bottom-up approach based on sol-gel dip-coating and nanoimprint lithography for the faithful reproduction of disordered hyperuniform metasurfaces in metal oxides. Nano- to microstructures made of silica and titania can be directly printed over several cm2 on glass and on silicon substrates. First, we describe the polymer mold fabrication starting from a hard master obtained via spontaneous solid-state dewetting of SiGe and Ge thin layers on SiO2. Then, we assess the effective disordered hyperuniform character of master and replica and the role of the thickness of the sol-gel layer on the metal oxide replicas and on the presence of a residual layer underneath. Finally, as a potential application, we show the antireflective character of titania structures on silicon. Our results are relevant for the realistic implementation over large scales of disordered hyperuniform nano- and microarchitectures made of metal oxides, thus opening their exploitation in the framework of wet chemical assembly.