23 research outputs found
Transient reflectance of photoexcited Cd\u3csub\u3e3\u3c/sub\u3eAs\u3csub\u3e2\u3c/sub\u3e
We report ultrafast transient-grating measurements of crystals of the three-dimensional Dirac semimetal cadmium arsenide, Cd3As2, at both room temperature and 80 K. After photoexcitation with 1.5-eV photons, charge-carriers relax by two processes, one of duration 500 fs and the other of duration 3.1 ps. By measuring the complex phase of the change in reflectance, we determine that the faster signal corresponds to a decrease in absorption, and the slower signal to a decrease in the light\u27s phase velocity, at the probe energy. We attribute these signals to electrons\u27 filling of phase space, first near the photon energy and later at lower energy. We attribute their decay to cooling by rapid emission of optical phonons, then slower emission of acoustic phonons. We also present evidence that both the electrons and the lattice are strongly heated
Photon-number-resolution with sub-30-ps timing using multi-element superconducting nanowire single photon detectors
A photon-number-resolving detector based on a four-element superconducting
nanowire single photon detector is demonstrated to have sub-30-ps resolution in
measuring the arrival time of individual photons. This detector can be used to
characterize the photon statistics of non-pulsed light sources and to mitigate
dead-time effects in high-speed photon counting applications. Furthermore, a
25% system detection efficiency at 1550 nm was demonstrated, making the
detector useful for both low-flux source characterization and high-speed
photon-counting and quantum communication applications. The design, fabrication
and testing of this detector are described, and a comparison between the
measured and theoretical performance is presented.Comment: 13 pages, 5 figure
Similar ultrafast dynamics of several dissimilar Dirac and Weyl semimetals
Recent years have seen the rapid discovery of solids whose low-energy
electrons have a massless, linear dispersion, such as Weyl, line-node, and
Dirac semimetals. The remarkable optical properties predicted in these
materials show their versatile potential for optoelectronic uses. However,
little is known of their response in the picoseconds after absorbing a photon.
Here we measure the ultrafast dynamics of four materials that share non-trivial
band structure topology but that differ chemically, structurally, and in their
low-energy band structures: ZrSiS, which hosts a Dirac line node and Dirac
points; TaAs and NbP, which are Weyl semimetals; and
SrMnSb, in which Dirac fermions coexist with broken
time-reversal symmetry. After photoexcitation by a short pulse, all four relax
in two stages, first sub-picosecond, and then few-picosecond. Their rapid
relaxation suggests that these and related materials may be suited for optical
switches and fast infrared detectors. The complex change of refractive index
shows that photoexcited carrier populations persist for a few picoseconds
Using coherent phonons for ultrafast control of the Dirac node of SrMnSb\u3csub\u3e2\u3c/sub\u3e
SrMnSb2 is a candidate Dirac semimetal whose electrons near the Y point have the linear dispersion and low mass of a Dirac cone. Here we demonstrate that ultrafast, 800-nm optical pulses can launch coherent phonon oscillations in Sr0.94Mn0.92Sb2, particularly an Ag mode at 4.4 THz. Through first-principles calculations of the electronic and phononic structure of SrMnSb2, we show that high-amplitude oscillations of this mode would displace the atoms in a way that transiently opens and closes a gap at the node of the Dirac cone. The ability to control the nodal gap on a subpicosecond timescale could create opportunities for the design and manipulation of Dirac fermions
Experimental electronic structure of the electrically switchable antiferromagnet CuMnAs
Tetragonal CuMnAs is a room temperature antiferromagnet with an electrically
reorientable N\'eel vector and a Dirac semimetal candidate. Direct measurements
of the electronic structure of single-crystalline thin films of tetragonal
CuMnAs using angle-resolved photoemission spectroscopy (ARPES) are reported,
including Fermi surfaces (FS) and energy-wavevector dispersions. After
correcting for a chemical potential shift of meV (hole doping),
there is excellent agreement of FS, orbital character of bands, and Fermi
velocities between the experiment and density functional theory calculations.
Additionally, 2x1 surface reconstructions are found in the low energy electron
diffraction (LEED) and ARPES. This work underscores the need to control the
chemical potential in tetragonal CuMnAs to enable the exploration and
exploitation of the Dirac fermions with tunable masses, which are predicted to
be above the chemical potential in the present samples.Comment: Submitted to Physical Review X. 20 pages. 9 figure
Mouse models of neurodegenerative disease: preclinical imaging and neurovascular component.
Neurodegenerative diseases represent great challenges for basic science and clinical medicine because of their prevalence, pathologies, lack of mechanism-based treatments, and impacts on individuals. Translational research might contribute to the study of neurodegenerative diseases. The mouse has become a key model for studying disease mechanisms that might recapitulate in part some aspects of the corresponding human diseases. Neurode- generative disorders are very complicated and multifacto- rial. This has to be taken in account when testing drugs. Most of the drugs screening in mice are very di cult to be interpretated and often useless. Mouse models could be condiderated a ‘pathway models’, rather than as models for the whole complicated construct that makes a human disease. Non-invasive in vivo imaging in mice has gained increasing interest in preclinical research in the last years thanks to the availability of high-resolution single-photon emission computed tomography (SPECT), positron emission tomography (PET), high eld Magnetic resonance, Optical Imaging scanners and of highly speci c contrast agents. Behavioral test are useful tool to characterize di erent ani- mal models of neurodegenerative pathology. Furthermore, many authors have observed vascular pathological features associated to the di erent neurodegenerative disorders. Aim
of this review is to focus on the di erent existing animal models of neurodegenerative disorders, describe behavioral tests and preclinical imaging techniques used for diagnose and describe the vascular pathological features associated to these diseases