71 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
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
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
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
Tensile strain in Ge membranes induced by SiGe nanostressors
The monolithic integration of photonic functionality into silicon microtechnology is widely advanced. Yet, there is no final solution for the realization of a light source compatible with the prevailing complementary metal-oxide-semiconductor technology. A lot of research effort focuses on germanium (Ge) on silicon (Si) heterostructures and tensile strain application to Ge is accepted as one feasible route to make Ge an efficient light emitter. Prior work has documented the special suitability of Ge membranes to reach the high tensile strain. We present a top-down approach for the creation of SiGe stressors on Ge micro-bridges and compare the obtained strain to the case of an attached bulk-like Ge layer. We could show that the Ge influenced by a SiGe stressor is under tensile strain; absolute strain values are of the order of 0.7% for both micro-bridge and bulk. The relative strain induced by the nanostructures in the micro-bridge is 1.3% due to the high sharing of elastic energy between nanostructures and bridges
Dislocation engineering in SiGe on periodic and aperiodic Si(001) templates studied by fast scanning X-ray nanodiffraction
Fast-scanning X-ray nanodiffraction microscopy is used to directly visualize the misfit dislocation network in a SiGe film deposited on a pit-patterned Si substrate at the beginning of plastic relaxation. X-ray real-space diffracted intensity maps are compared to topographic atomic force microscopy images, in which crosshatch lines can be seen. The change in intensity distribution as a function of the incidence angle shows localized variations in strain within the SiGe film. These variations, which reflect the order imposed by the substrate pattern, are attributed to the presence of both bunches of misfit dislocations and defect-free regions
- …