78 research outputs found
Study of Through-Hole Micro-Drilling in Sapphire by Means of Pulsed Bessel Beams
Ultrashort Bessel beams have been used in this work to study the response of a 430-m-thick
monocrystalline sapphire sample to laser–matter interaction when injecting the beam orthogonally
through the whole sample thickness. We show that with a 12 Bessel beam cone angle, we are able to
internally modify the material and generate tailorable elongated microstructures while preventing
the formation of surface cracks, even in the picosecond regime, contrary to what was previously
reported in the literature. On the other hand, by means of Bessel beam machining combined with a
trepanning technique where very high energy pulses are needed, we were able to generate 100 m
diameter through-holes, eventually with negligible cracks and very low taper angles thanks to an
optimization achieved by using a 60-m-thick layer of Kapton Polyimide removable tape
Micro-Hole Generation by High-Energy Pulsed Bessel Beams in Different Transparent Materials
Micro-drilling transparent dielectric materials by using non-diffracting beams impinging orthogonally to the sample can be performed without scanning the beam position along the sample thickness. In this work, the laser micromachining process, based on the combination of picosecond pulsed Bessel beams with the trepanning technique, is applied to different transparent materials. We show the possibility to create through-apertures with diameter on the order of tens of micrometers, on dielectric samples with different thermal and mechanical characteristics as well as different thicknesses ranging from two hundred to five hundred micrometers. Advantages and drawbacks of the application of this technique to different materials such as glass, polymer, or diamond are highlighted by analyzing the features, the morphology, and the aspect-ratio of the through-holes generated. Alternative Bessel beam drilling configurations, and the possibility of optimization of the quality of the aperture at the output sample/air interface is also discussed in the case of glass
Germanium-based nearly hyperuniform nanoarchitectures by ion beam impact
We address the fabrication of nano-architectures by impacting thin layers of amorphous Ge deposited on SiO2 with a Ga+ ion beam and investigate the structural and optical properties of the resulting patterns. By adjusting beam current and scanning parameters, different classes of nano-architectures can be formed, from elongated and periodic structures to disordered ones with a footprint of a few tens of nm. The latter disordered case features a significant suppression of large length scale fluctuations that are conventionally observed in ordered systems and exhibits a nearly hyperuniform character, as shown by the analysis of the spectral density at small wave vectors. It deviates from conventional random fields as accounted for by the analysis of Minkowski functionals. A proof of concept for potential applications is given by showing peculiar reflection properties of the resulting nano-structured films that exhibit colorization and enhanced light absorption with respect to the flat Ge layer counterpart (up to one order of magnitude at some wavelength). This fabrication method for disordered hyperuniform structures does not depend on the beam size. Being ion beam technology widely adopted in semiconductor foundries over 200 mm wafers, our work provides a viable pathway for obtaining disordered, nearly-hyperuniform materials by self-assembly with a footprint of tens of nanometers for electronic and photonic devices, energy storage and sensing
Spectral Imaging of UV-Blocking Carbon Dot-Based Coatings for Food Packaging Applications
Nowadays, there is an increased demand to develop alternative non-plastic packaging to be used in the food industry. The most popular biodegradable films are cellulose and poly(lactic acid) (PLA); however, there is still the need to increase their UV absorption to protect the packaging content. In this work, we have covered those biodegradable films with thin coatings based on carbon dots (CDs) dispersed in polyvinyl alcohol (PVA) deposited by spin- or spray-coating techniques. We report a strong increase in the UV light-absorbing properties, together with a detailed morphological characterization; moreover, we show the results of a new microscopy and spectral imaging technique applied to the coated samples. The scientific and technological novelty of this approach is the possibility of characterizing large areas of the material surface by the simultaneous detection of PL spectra in all the pixels of a highly spatially-resolved two-dimensional (2D) map of the surface. We report UV-excited PL maps whose detailed information allows us to clearly identify regions with different spectral behaviors and to compare their characteristic signals for different CDs:PVA deposition techniques
Light scattering features induced by residual layers in dielectric dewetted nanoparticles
All-dielectric, sub-micrometric particles obtained through solid state dewetting of thin SiGe-films have been shown to support Mie resonances together with a high-quality monocrystalline composition and atomically smooth facets. Recently, a precise study on the impact given by the effective complex morphology of a SiGe dewetted nanoparticle to the Mie scattering properties has been provided and carried on through a novel experimental technique called Dark-field Scanning Optical Microscopy. In this work, by means of the same experimental technique and numerical simulations of light scattering, we show how the presence of a pedestal enriched with silicon placed under the SiGe-nanoparticle results in a sharp peak at high energy in the total scattering cross-section. Exploiting a tilted illumination to redirect scattered light, we are able to discriminate the spatial localization of the pedestal-induced resonance. Our results contribute to extending the practical implementations of dewetted Mie resonators in the field of light scattering directionality, sensing applications and show further engineering options beyond the simple isolated-island case
Large-Area Nanocrystalline Caesium Lead Chloride Thin Films: A Focus on the Exciton Recombination Dynamics
Caesium lead halide perovskites were recently demonstrated to be a relevant class of semiconductors for photonics and optoelectronics. Unlike CsPbBr3 and CsPbI3, the realization of high-quality thin films of CsPbCl3, particularly interesting for highly efficient white LEDs when coupled to converting phosphors, is still a very demanding task. In this work we report the first successful deposition of nanocrystalline CsPbCl3 thin films (70–150 nm) by radio frequency magnetron sputtering on large-area substrates. We present a detailed investigation of the optical properties by high resolution photoluminescence (PL) spectroscopy, resolved in time and space in the range 10–300 K, providing quantitative information concerning carriers and excitons recombination dynamics. The PL is characterized by a limited inhomogeneous broadening (~15 meV at 10 K) and its origin is discussed from detailed analysis with investigations at the micro-scale. The samples, obtained without any post-growth treatment, show a homogeneous PL emission in spectrum and intensity on large sample areas (several cm2). Temperature dependent and time-resolved PL spectra elucidate the role of carrier trapping in determining the PL quenching up to room temperature. Our results open the route for the realization of large-area inorganic halide perovskite films for photonic and optoelectronic devices
Evaluation of microscale crystallinity modification induced by laser writing on Mn3O4 thin films
Defining microstructures and managing local crystallinity allow the
implementation of several functionalities in thin film technology. The use of
ultrashort Bessel beams for bulk crystallinity modification has garnered
considerable attention as a versatile technique for semiconductor materials,
dielectrics, or metal oxide substrates. The aim of this work is the
quantitative evaluation of the crystalline changes induced by ultrafast laser
micromachining on manganese oxide thin films using micro-Raman spectroscopy.
Pulsed Bessel beams featured by a 1 micrometer-sized central core are used to
define structures with high spatial precision. The dispersion relation of Mn3O4
optical phonons is determined by considering the conjunction between X-ray
diffraction characterization and the phonon localization model. The asymmetries
in Raman spectra indicate phonon localization and enable a quantitative tool to
determine the crystallite size at micrometer resolution. The results indicate
that laser-writing is effective in modifying the low-crystallinity films
locally, increasing crystallite sizes from ~8 nm up to 12 nm, and thus
highlighting an interesting approach to evaluate laser-induced structural
modifications on metal oxide thin films.Comment: 27 page
Tunability and Losses of Mid-infrared Plasmonics in Heavily Doped Germanium Thin Films
Heavily-doped semiconductor films are very promising for application in
mid-infrared plasmonic devices because the real part of their dielectric
function is negative and broadly tunable in this wavelength range. In this work
we investigate heavily n-type doped germanium epilayers grown on different
substrates, in-situ doped in the to cm range, by
infrared spectroscopy, first principle calculations, pump-probe spectroscopy
and dc transport measurements to determine the relation between plasma edge and
carrier density and to quantify mid-infrared plasmon losses. We demonstrate
that the unscreened plasma frequency can be tuned in the 400 - 4800 cm
range and that the average electron scattering rate, dominated by scattering
with optical phonons and charged impurities, increases almost linearly with
frequency. We also found weak dependence of losses and tunability on the
crystal defect density, on the inactivated dopant density and on the
temperature down to 10 K. In films where the plasma was optically activated by
pumping in the near-infrared, we found weak but significant dependence of
relaxation times on the static doping level of the film. Our results suggest
that plasmon decay times in the several-picosecond range can be obtained in
n-type germanium thin films grown on silicon substrates hence allowing for
underdamped mid-infrared plasma oscillations at room temperature.Comment: 18 pages, 10 figure
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