305 research outputs found
Perturbative regime of terahertz high-harmonics generation in topological insulators
In this Letter, terahertz high harmonic generation processes in topological
insulators of the bismuth and antimony chalcogenides family are investigated.
Field conversion efficiencies are determined and clean cubic and quintic
power-law scaling is observed for third and fifth harmonics, up to driving
terahertz fields of 140 kV/cm. This is in contrast to all previous experiments
on terahertz harmonics generation in Dirac materials where a non-perturbative
regime has been observed already at few 10s kV/cm driving fields. Our nonlinear
THz spectroscopy experiments are complemented by THz pump - optical probe
measurements showing distinctly different relaxation dynamics of the carriers
in the topologically-protected Dirac states at the surfaces and the bulk. The
THz-induced dynamics of surface states reveal ultrafast relaxation that
prevents accumulation effects, and results in a clear perturbative regime of
THz harmonics generation that is different to graphene or Dirac semimetals with
their slower relaxation times in the few ps regime
Probing photo-induced melting of antiferromagnetic order in La0.5Sr1.5MnO4 by ultrafast resonant soft X-ray diffraction
Photo-excitation in complex oxides1 transfers charge across semicovalent
bonds, drastically perturbing spin and orbital orders2. Light may then be used
in compounds like magnetoresistive manganites to control magnetism on nanometre
lengthscales and ultrafast timescales. Here, we show how ultrafast resonant
soft x-ray diffraction can separately probe the photo-induced dynamics of spin
and orbital orders in La0.5Sr1.5MnO4. Ultrafast melting of CE antiferromagnetic
spin order is evidenced by the disappearance of a (1/4,1/4,1/2) diffraction
peak. On the other hand the (1/4,1/4,0) peak, reflecting orbital order, is only
partially reduced. Cluster calculations aid our interpretation by considering
different magnetically ordered states accessible after photo-excitation.
Nonthermal coupling between light and magnetism emerges as a primary aspect of
photo-induced phase transitions in manganites.Comment: 7 pages manuscript, 4 figure
Terahertz signatures of ultrafast Dirac fermion relaxation at the surface of topological insulators
Topologically protected surface states present rich physics and promising spintronic, optoelectronic, and photonic applications that require a proper understanding of their ultrafast carrier dynamics. Here, we investigate these dynamics in topological insulators (TIs) of the bismuth and antimony chalcogenide family, where we isolate the response of Dirac fermions at the surface from the response of bulk carriers by combining photoexcitation with below-bandgap terahertz (THz) photons and TI samples with varying Fermi level, including one sample with the Fermi level located within the bandgap. We identify distinctly faster relaxation of charge carriers in the topologically protected Dirac surface states (few hundred femtoseconds), compared to bulk carriers (few picoseconds). In agreement with such fast cooling dynamics, we observe THz harmonic generation without any saturation effects for increasing incident fields, unlike graphene which exhibits strong saturation. This opens up promising avenues for increased THz nonlinear conversion efficiencies, and high-bandwidth optoelectronic and spintronic information and communication applications.Parts of this research were carried out at ELBE at the Helmholtz-Zentrum Dresden-Rossendorf e.V., a member of the Helmholtz Association. The films are grown in IRE RAS within the framework of the state task. This work was supported by the RFBR grants Nos. 18-29-20101, 19-02-00598. N.A., S.K., and I.I. acknowledge support from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 737038 (TRANSPIRE). T.V.A.G.O. and L.M.E. acknowledge the support by the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter (ct.qmat). K.-J.T. acknowledges funding from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 804349 (ERC StG CUHL) and financial support through the MAINZ Visiting Professorship. ICN2 was supported by the Severo Ochoa program from Spanish MINECO Grant No. SEV-2017-0706
Dissecting spin-phonon equilibration in ferrimagnetic insulators by ultrafast lattice excitation
To gain control over magnetic order on ultrafast time scales, a fundamental understanding of the way electron spins interact with the surrounding crystal lattice is required. However, measurement and analysis even of basic collective processes such as spin-phonon equilibration have remained challenging. Here, we directly probe the flow of energy and angular momentum in the model insulating ferrimagnet yttrium iron garnet. After ultrafast resonant lattice excitation, we observe that magnetic order reduces on distinct time scales of 1 ps and 100 ns. Temperature-dependent measurements, a spin-coupling analysis, and simulations show that the two dynamics directly reflect two stages of spin-lattice equilibration. On the 1-ps scale, spins and phonons reach quasi-equilibrium in terms of energy through phonon-induced modulation of the exchange interaction. This mechanism leads to identical demagnetization of the ferrimagnet’s two spin sublattices and to a ferrimagnetic state of increased temperature yet unchanged total magnetization. Finally, on the much slower, 100-ns scale, the excess of spin angular momentum is released to the crystal lattice, resulting in full equilibrium. Our findings are relevant for all insulating ferrimagnets and indicate that spin manipulation by phonons, including the spin Seebeck effect, can be extended to antiferromagnets and into the terahertz frequency range
TESLA Technical Design Report Part III: Physics at an e+e- Linear Collider
The TESLA Technical Design Report Part III: Physics at an e+e- Linear
ColliderComment: 192 pages, 131 figures. Some figures have reduced quality. Full
quality figures can be obtained from http://tesla.desy.de/tdr. Editors -
R.-D. Heuer, D.J. Miller, F. Richard, P.M. Zerwa
High-field high-repetition-rate sources for the coherent THz control of matter
Ultrashort flashes of THz light with low photon energies of a few meV, but strong electric or magnetic field transients have recently been employed to prepare various fascinating nonequilibrium states in matter. Here we present a new class of sources based on superradiant enhancement of radiation from relativistic electron bunches in a compact electron accelerator that we believe will revolutionize experiments in this field. Our prototype source generates high-field THz pulses at unprecedented quasicontinuous-wave repetition rates up to the MHz regime. We demonstrate parameters that exceed state-of-the-art laser-based sources by more than 2 orders of magnitude. The peak fields and the repetition rates are highly scalable and once fully operational this type of sources will routinely provide 1 MV/cm electric fields and 0.3 T magnetic fields at repetition rates of few 100 kHz. We benchmark the unique properties by performing a resonant coherent THz control experiment with few 10 fs resolution
Terahertz-slicing -- an all-optical synchronization for 4th generation light sources
A conceptually new approach to synchronizing accelerator-based light sources and external laser systems is presented. The concept is based on utilizing a sufficiently intense accelerator-based single-cycle terahertz pulse to slice a thereby intrinsically synchronized femtosecond-level part of a longer picosecond laser pulse in an electro-optic crystal. A precise synchronization of the order of 10 fs is demonstrated, allowing for real-time lock-in amplifier signal demodulation. We demonstrate successful operation of the concept with three benchmark experiments using a 4th generation accelerator-based terahertz light source, i.e. (i) far-field terahertz time-domain spectroscopy, (ii) terahertz high harmonic generation spectroscopy, and (iii) terahertz scattering-type scanning near-field optical microscopy
Gas-to-particle partitioning of major biogenic oxidation products: a study on freshly formed and aged biogenic SOA
Secondary organic aerosols (SOAs) play a key role in climate change and air
quality. Determining the fundamental parameters that distribute organic
compounds between the phases is essential, as atmospheric lifetime and
impacts change drastically between the gas and particle phase. In this work,
gas-to-particle partitioning of major biogenic oxidation products was
investigated using three different aerosol chemical characterization
techniques. The aerosol collection module, the collection thermal desorption unit, and the
chemical analysis of aerosols online are different aerosol sampling inlets connected to a proton-transfer reaction time-of-flight
mass spectrometer (ACM-PTR-ToF-MS, TD-PTR-ToF-MS, and CHARON-PTR-ToF-MS, respectively, referred to hereafter as
ACM, TD, and CHARON). These techniques
were deployed at the atmosphere simulation chamber SAPHIR to perform
experiments on the SOA formation and aging from different monoterpenes
(β-pinene, limonene) and real plant emissions (Pinus sylvestris L.). The saturation mass
concentration C* and thus the volatility of the individual ions was
determined based on the simultaneous measurement of their signal in the gas and particle phase.A method to identify and exclude ions affected by thermal dissociation
during desorption and ionic dissociation in the ionization chamber of the
proton-transfer reaction mass spectrometer (PTR-MS) was developed and tested for each technique. Narrow volatility
distributions with organic compounds in the semi-volatile (SVOCs – semi-volatile
organic compounds) to
intermediate-volatility (IVOCs – intermediate-volatility organic compounds) regime were found for all systems studied.
Despite significant differences in the aerosol collection and desorption
methods of the proton-transfer-reaction (PTR)-based techniques, a comparison of the C* values obtained
with different techniques was found to be in good agreement (within 1 order
of magnitude) with deviations explained by the different operating
conditions of the PTR-MS.The C* of the identified organic compounds were mapped onto the
two-dimensional volatility basis set (2D-VBS), and results showed a decrease in C* with increasing oxidation state. For all experiments conducted in
this study, identified partitioning organic compounds accounted for
20–30 % of the total organic mass measured from an aerosol mass spectrometer (AMS). Further
comparison between observations and theoretical calculations was performed
for species found in our experiments that were also identified in previous
publications. Theoretical calculations based on the molecular structure of
the compounds showed, within the uncertainties ranges, good agreement with
the experimental C* for most SVOCs, while IVOCs deviated by up to a factor of
300. These latter differences are discussed in relation to two main
processes affecting these systems: (i) possible interferences by thermal and
ionic fragmentation of higher molecular-weight compounds, produced by
accretion and oligomerization reactions, that fragment in the m∕z range
detected by the PTR-MS and (ii) kinetic influences in the distribution
between the gas and particle phase with gas-phase condensation, diffusion in
the particle phase, and irreversible uptake.</p
Tropopause and hygropause variability over the equatorial Indian Ocean during February and March 1999.
Measurements of temperature, water vapor, total water, ozone, and cloud properties were made above the western equatorial Indian Ocean in February and March 1999. The cold-point tropopause was at a mean pressure-altitude of 17 km, equivalent to a potential temperature of 380 K, and had a mean temperature of 190 K. Total water mixing ratios at the hygropause varied between 1.4 and 4.1 ppmv. The mean saturation water vapor mixing ratio at the cold point was 3.0 ppmv. This does not accurately represent the mean of the measured total water mixing ratios because the air was unsaturated at the cold point for about 40% of the measurements. As well as unsaturation at the cold point, saturation was observed above the cold point on almost 30% of the profiles. In such profiles the air was saturated with respect to water ice but was free of clouds (i.e., backscatter ratio <2) at potential temperatures more than 5 K above the tropopause and hygropause. Individual profiles show a great deal of variability in the potential temperatures of the cold point and hygropause. We attribute this to short timescale and space-scale perturbations superimposed on the seasonal cycle. There is neither a clear and consistent “setting” of the tropopause and hygropause to the same altitude by dehydration processes nor a clear and consistent separation of tropopause and hygropause by the Brewer-Dobson circulation. Similarly, neither the tropopause nor the hygropause provides a location where conditions consistently approach those implied by a simple “tropopause freeze drying” or “stratospheric fountain” hypothesis
The 2017 Terahertz Science and Technology Roadmap
Science and technologies based on terahertz frequency electromagnetic radiation (100GHz-30THz) have developed rapidly over the last 30 years. For most of the 20th century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to “real world” applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2016, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 17 sections that cover most of the key areas of THz Science and Technology. We hope that The 2016 Roadmap on THz Science and Technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies
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