9 research outputs found
Ultrafast optical properties of stoichiometric and non-stoichiometric refractory metal nitrides TiNx, ZrNx, and HfNx
Refractory metal nitrides have recently gained attention in various fields of
modern photonics due to their cheap and robust production technology,
silicon-technology compatibility, high thermal and mechanical resistance, and
competitive optical characteristics in comparison to typical plasmonic
materials like gold and silver. In this work, we demonstrate that by varying
the stoichiometry of sputtered nitride films, both static and ultrafast optical
responses of refractory metal nitrides can efficiently be controlled. We
further prove that the spectral changes in ultrafast transient response are
directly related to the position of the epsilon-near-zero region. At the same
time, the analysis of the temporal dynamics allows us to identify three time
components - the "fast" femtosecond one, the "moderate" picosecond one, and the
"slow" at the nanosecond time scale. We also find out that the
non-stoichiometry does not significantly decrease the recovery time of the
reflectance value. Our results show the strong electron-phonon coupling and
reveal the importance of both the electron and lattice temperature-induced
changes in the permittivity near the ENZ region and the thermal origin of the
long tail in the transient optical response of refractory nitrides
Impact of surface reflectivity on the ultra-fast laser melting of silicon-germanium alloys
Ultraviolet nanosecond laser annealing (LA) is a powerful tool where strongly
confined heating and melting are desirable. In semiconductor technologies the
importance of LA increases with the increasing complexity of the proposed
integration schemes. Optimizing the LA process along with the experimental
design is challenging, especially when complex 3D nanostructured systems with
various shapes and phases are involved. Within this context, reliable
simulations of laser melting are required for optimizing the process parameters
while reducing the number of experimental tests. This gives rise to a virtual
Design of Experiments (DoE). SiGe alloys are nowadays used for their
compatibility with silicon devices enabling to engineer properties such as
strain, carrier mobilities and bandgap. In this work, the laser melting process
of relaxed and strained SiGe is simulated with a finite element method / phase
field approach. Particularly, we calibrated the dielectric functions of the
alloy for its crystal and liquid phase using experimental data. We highlighted
the importance of reproducing the exact reflectivity of the material in its
different aggregation states, to correctly mimic the process
Calorimetric measurements on Li4C60 and Na4C60
We show specific heat data for Na4C60 and Li4C60 in the range 0.4-350 K for samples characterized by Raman spectroscopy and X-ray diffraction. At high temperatures, the two different polymer structures have very similar specific heats both in absolute values and in general trend. The specific heat data are compared with data for undoped polymeric and pristine C60. At high temperatures, a difference in specific heat between the intercalated and undoped C60 polymers of 100 J K−1 mol−1 is observed, in agreement with the Dulong-Petit law. At low temperatures, the specific heat data for Li4C60 and Na4C60 are modified by the stiffening of vibrational and librational molecular motion induced by the polymer bonds. The covalent twin bonds in Li4C60 affect these motions to a somewhat higher degree than the single intermolecular bonds in Na4C60. Below 1 K, the specific heats ofboth materials become linear in temperature, as expected from the effective dimensionality of the structure. The contribution to the total specific heat from the inserted metal ions can be well described by Einstein functions with TE = 386 K for Li4C60 and TE = 120 K for Na4C60, but for both materials we also observe a Schottky-type contribution corresponding to a first approximation to a two-level system with ΔE = 9.3 meV for Li4C60 and 3.1 meV for Na4C60, probably associated with jumps between closely spaced energy levels inside “octahedral-type” ionic sites. Static magnetic fields up to 9 T had very small effects on the specific heat below 10 K
An innovative silicon photomultiplier digitizing camera for gamma-ray astronomy
The single-mirror small-size telescope (SST-1M) is one of the three proposed
designs for the small-size telescopes (SSTs) of the Cherenkov Telescope Array
(CTA) project. The SST-1M will be equipped with a 4 m-diameter segmented mirror
dish and an innovative fully digital camera based on silicon photo-multipliers
(SiPMs). Since the SST sub-array will consist of up to 70 telescopes, the
challenge is not only to build a telescope with excellent performance, but also
to design it so that its components can be commissioned, assembled and tested
by industry. In this paper we review the basic steps that led to the design
concepts for the SST-1M camera and the ongoing realization of the first
prototype, with focus on the innovative solutions adopted for the photodetector
plane and the readout and trigger parts of the camera. In addition, we report
on results of laboratory measurements on real scale elements that validate the
camera design and show that it is capable of matching the CTA requirements of
operating up to high-moon-light background conditions.Comment: 30 pages, 61 figure