9 research outputs found
Ultrafast Tunable Terahertz-to-Visible Light Conversion through Thermal Radiation from Graphene Metamaterials
Several technologies, including photodetection, imaging, and data communication, could greatly benefit from the availability of fast and controllable conversion of terahertz (THz) light to visible light. Here, we demonstrate that the exceptional properties and dynamics of electronic heat in graphene allow for a THz-to-visible conversion, which is switchable at a sub-nanosecond time scale. We show a tunable on/off ratio of more than 30 for the emitted visible light, achieved through electrical gating using a gate voltage on the order of 1 V. We also demonstrate that a grating-graphene metamaterial leads to an increase in THz-induced emitted power in the visible range by 2 orders of magnitude. The experimental results are in agreement with a thermodynamic model that describes blackbody radiation from the electron system heated through intraband Drude absorption of THz light. These results provide a promising route toward novel functionalities of optoelectronic technologies in the THz regime
Tunable room temperature nonlinear Hall effect from the surfaces of elementary bismuth thin films
The nonlinear Hall effect (NLHE) with time-reversal symmetry constitutes the
appearance of a transverse voltage quadratic in the applied electric field. It
is a second-order electronic transport phenomenon that induces frequency
doubling and occurs in non-centrosymmetric crystals with large Berry curvature
-- an emergent magnetic field encoding the geometric properties of electronic
wavefunctions. The design of (opto)electronic devices based on the NLHE is
however hindered by the fact that this nonlinear effect typically appears at
low temperatures and in complex compounds characterized by Dirac or Weyl
electrons. Here, we show a strong room temperature NLHE in the centrosymmetric
elemental material bismuth synthesized in the form of technologically relevant
polycrystalline thin films. The () surface electrons of this material
are equipped with a Berry curvature triple that activates side jumps and skew
scatterings generating nonlinear transverse currents. We also report a boost of
the zero field nonlinear transverse voltage in arc-shaped bismuth stripes due
to an extrinsic geometric classical counterpart of the NLHE. This electrical
frequency doubling in curved geometries is then extended to optical second
harmonic generation in the terahertz (THz) spectral range. The strong nonlinear
electrodynamical responses of the surface states are further demonstrated by a
concomitant highly efficient THz third harmonic generation which we achieve in
a broad range of frequencies in Bi and Bi-based heterostructures. Combined with
the possibility of growth on CMOS-compatible and mechanically flexible
substrates, these results highlight the potential of Bi thin films for THz
(opto)electronic applications.Comment: 44 pages, 21 figure
Spin-orbit interaction driven terahertz nonlinear dynamics in transition metals
The interplay of electric charge, spin, and orbital polarizations, coherently
driven by picosecond long oscillations of light fields in spin-orbit coupled
systems, is the foundation of emerging terahertz spintronics and orbitronics.
The essential rules for how terahertz light interacts with these systems in a
nonlinear way are still not understood. In this work, we demonstrate a
universally applicable electronic nonlinearity originating from spin-orbit
interactions in conducting materials, wherein the interplay of light-induced
spin and orbital textures manifests. We utilized terahertz harmonic generation
spectroscopy to investigate the nonlinear dynamics over picosecond timescales
in various transition metal films. We found that the terahertz harmonic
generation efficiency scales with the spin Hall conductivity in the studied
films, while the phase takes two possible values (shifted by {\pi}), depending
on the d-shell filling. These findings elucidate the fundamental mechanisms
governing non-equilibrium spin and orbital polarization dynamics at terahertz
frequencies, which is relevant for potential applications of terahertz spin-
and orbital-based devices.Comment: 11 pages, 4 figure
Impulsive Fermi magnon-phonon resonance in antiferromagnetic
Understanding spin-lattice interactions in antiferromagnets is one of the
most fundamental issues at the core of the recently emerging and booming fields
of antiferromagnetic spintronics and magnonics. Recently, coherent nonlinear
spin-lattice coupling was discovered in an antiferromagnet which opened the
possibility to control the nonlinear coupling strength and thus showing a novel
pathway to coherently control magnon-phonon dynamics. Here, utilizing intense
narrow band terahertz (THz) pulses and tunable magnetic fields up to 7 T, we
experimentally realize the conditions of the Fermi magnon-phonon resonance in
antiferromagnetic . These conditions imply that both the spin and the
lattice anharmonicities harvest energy transfer between the subsystems, if the
magnon eigenfrequency is twice lower than the frequency of the phonon
. Performing THz pump-infrared probe spectroscopy in conjunction
with simulations, we explore the coupled magnon-phonon dynamics in the vicinity
of the Fermi-resonance and reveal the corresponding fingerprints of an
impulsive THz-induced response. This study focuses on the role of nonlinearity
in spin-lattice interactions, providing insights into the control of coherent
magnon-phonon energy exchange
Ultrafast Tunable Terahertz-to-Visible Light Conversion through Thermal Radiation from Graphene Metamaterials [Dataset]
6 pages. -- Supplementary Note 1, Sample Preparation. -- Supplementary Note 2, Experimental. -- Supplementary Note 3, Calculations of electron temperature. -- Supplementary Note 4, THz fluence and intensity. -- Supplementary Figures. -- Supplementary References.Several technologies, including photodetection, imaging, and data communication, could greatly benefit from the availability of fast and controllable conversion of terahertz (THz) light to visible light. Here, we demonstrate that the exceptional properties and dynamics of electronic heat in graphene allow for a THz-to-visible conversion, which is switchable at a sub-nanosecond time scale. We show a tunable on/off ratio of more than 30 for the emitted visible light, achieved through electrical gating using a gate voltage on the order of 1 V. We also demonstrate that a grating-graphene metamaterial leads to an increase in THz-induced emitted power in the visible range by 2 orders of magnitude. The experimental results are in agreement with a thermodynamic model that describes blackbody radiation from the electron system heated through intraband Drude absorption of THz light. These results provide a promising route toward novel functionalities of optoelectronic technologies in the THz regime.Peer reviewe
Terahertz Harmonic Generation from Graphite Pencil Drawings
The third harmonic generation (THG) of graphite layers on paper substrate upon excitation with intense (up to 100 kV cm-1 ) narrowband terahertz (THz) pulses is studied. Highest THG efficiencies are comparable with those of chemical vapor deposition-grown single-layer graphene. Samples are hand drawn, using commercially available pencils. The THG response shows high sensitivity regarding the hatching direction relative to the THz polarization orientation. Using Raman spectroscopy, the occurrence of graphene-like structures in the samples is confirmed. The findings demonstrate the feasibility of virtually no-cost and easy to-fabricate materials for THz nonlinear optics
A tunable room-temperature nonlinear Hall effect in elemental bismuth thin films
The nonlinear Hall effect with time-reversal symmetry is a second-order electronic transport phenomenon-seen as a quadratic voltage transverse to an applied electric field-that induces frequency doubling and occurs in non-centrosymmetric crystals with large Berry curvature. Optoelectronic devices based on this effect are limited because it typically appears at low temperatures and in complex compounds characterized by Dirac or Weyl electrons. Here we report a room-temperature nonlinear Hall effect in polycrystalline thin films of the centrosymmetric elemental material bismuth. The electrons at the (111) surface possess a Berry curvature triple that activates side jumps and skew scatterings, which generate nonlinear transverse currents. We show that the zero-field nonlinear transverse voltage can be boosted in arc-shaped bismuth stripes due to an extrinsic geometric classical counterpart of the nonlinear Hall effect. The electrical frequency doubling in curved geometries can be extended to optical second-harmonic generation in the terahertz spectral range. We also demonstrate efficient third-harmonic generation in polycrystalline bismuth films and bismuth-based heterostructures across a broad range of terahertz frequencies.Polycrystalline thin films of elemental bismuth exhibit a room-temperature nonlinear transverse voltage due to geometric effects of surface electrons that is tunable and can be extended to efficient high-harmonic generation at terahertz frequencies
Impact of the COVID-19 pandemic on patients with paediatric cancer in low-income, middle-income and high-income countries: a multicentre, international, observational cohort study
OBJECTIVES: Paediatric cancer is a leading cause of death for children. Children in low-income and middle-income countries (LMICs) were four times more likely to die than children in high-income countries (HICs). This study aimed to test the hypothesis that the COVID-19 pandemic had affected the delivery of healthcare services worldwide, and exacerbated the disparity in paediatric cancer outcomes between LMICs and HICs. DESIGN: A multicentre, international, collaborative cohort study. SETTING: 91 hospitals and cancer centres in 39 countries providing cancer treatment to paediatric patients between March and December 2020. PARTICIPANTS: Patients were included if they were under the age of 18 years, and newly diagnosed with or undergoing active cancer treatment for Acute lymphoblastic leukaemia, non-Hodgkin's lymphoma, Hodgkin lymphoma, Wilms' tumour, sarcoma, retinoblastoma, gliomas, medulloblastomas or neuroblastomas, in keeping with the WHO Global Initiative for Childhood Cancer. MAIN OUTCOME MEASURE: All-cause mortality at 30 days and 90 days. RESULTS: 1660 patients were recruited. 219 children had changes to their treatment due to the pandemic. Patients in LMICs were primarily affected (n=182/219, 83.1%). Relative to patients with paediatric cancer in HICs, patients with paediatric cancer in LMICs had 12.1 (95% CI 2.93 to 50.3) and 7.9 (95% CI 3.2 to 19.7) times the odds of death at 30 days and 90 days, respectively, after presentation during the COVID-19 pandemic (p<0.001). After adjusting for confounders, patients with paediatric cancer in LMICs had 15.6 (95% CI 3.7 to 65.8) times the odds of death at 30 days (p<0.001). CONCLUSIONS: The COVID-19 pandemic has affected paediatric oncology service provision. It has disproportionately affected patients in LMICs, highlighting and compounding existing disparities in healthcare systems globally that need addressing urgently. However, many patients with paediatric cancer continued to receive their normal standard of care. This speaks to the adaptability and resilience of healthcare systems and healthcare workers globally
Twelve-month observational study of children with cancer in 41 countries during the COVID-19 pandemic
Childhood cancer is a leading cause of death. It is unclear whether the COVID-19 pandemic has impacted childhood cancer mortality. In this study, we aimed to establish all-cause mortality rates for childhood cancers during the COVID-19 pandemic and determine the factors associated with mortality