50 research outputs found
Ultrafast imaging of photoelectron packets generated from graphite surface
We present an electron projection imaging method to study the ultrafast
evolution of photoelectron density distribution and transient fields near the
surface. The dynamical profile of the photoelectrons from graphite reveals an
origin of a thermionic emission, followed by an adiabatic process leading to
electron acceleration and cooling before a freely expanding cloud is
established. The hot electron emission is found to couple with a surface charge
dipole layer formation, with a sheet density several orders of magnitude higher
than that of the vacuum emitted cloud.Comment: 9 pages, 4 figures. Applied Physics Letter, in pres
Coherent modulation of the electron temperature and electron-phonon couplings in a 2D material
Ultrashort light pulses can selectively excite charges, spins and phonons in
materials, providing a powerful approach for manipulating their properties.
Here we use femtosecond laser pulses to coherently manipulate the electron and
phonon distributions, and their couplings, in the charge density wave (CDW)
material 1T-TaSe. After exciting the material with a short light pulse,
spatial smearing of the electrons launches a coherent lattice breathing mode,
which in turn modulates the electron temperature. This indicates a
bi-directional energy exchange between the electrons and the strongly-coupled
phonons. By tuning the laser excitation fluence, we can control the magnitude
of the electron temperature modulation, from ~ 200 K in the case of weak
excitation, to ~ 1000 K for strong laser excitation. This is accompanied by a
switching of the dominant mechanism from anharmonic phonon-phonon coupling to
coherent electron-phonon coupling, as manifested by a phase change of in
the electron temperature modulation. Our approach thus opens up possibilities
for coherently manipulating the interactions and properties of quasi-2D and
other quantum materials using light.Comment: 15 pages, 4 figure
Active spintronic-metasurface terahertz emitters with tunable chirality
The ability to manipulate the electric-field vector of broadband terahertz
waves is essential for applications of terahertz technologies in many areas,
and can open up new possibilities for nonlinear terahertz spectroscopy and
coherent control. Here, we propose a novel laser-driven terahertz emitter,
consisting of metasurface-patterned magnetic multilayer heterostructures. Such
hybrid terahertz emitters can combine the advantages of spintronic emitters for
being ultrabroadband, efficient and flexible, as well as those of metasurfaces
for the unique capability to manipulate terahertz waves with high precision and
degree of freedom. Taking a stripe-patterned metasurface as an example, we
demonstrate the generation of broadband terahertz waves with tunable chirality.
Based on experimental and theoretical studies, the interplay between the
laser-induced spintronic-origin currents and the metasurface-induced transient
charges/currents are investigated, revealing the strong influence on the device
functionality originated from both the light-matter interactions in individual
metasurface units and the dynamic coupling between them. Our work not only
offers a flexible, reliable and cost-effective solution for chiral terahertz
wave generation and manipulation, but also opens a new pathway to
metasurface-tailored spintronic devices for efficient vector-control of
electromagnetic waves in the terahertz regime
Flexible generation of structured terahertz fields via programmable exchange-biased spintronic emitters
Structured light, particularly in the terahertz frequency range, holds
considerable potential for a diverse range of applications. However, the
generation and control of structured terahertz radiation pose major challenges.
In this work, we demonstrate a novel programmable spintronic emitter that can
flexibly generate a variety of structured terahertz waves. This is achieved
through the precise and high-resolution programming of the magnetization
pattern on the emitter surface, utilizing laser-assisted local field cooling of
an exchange-biased ferromagnetic heterostructure. Moreover, we outline a
generic design strategy for realizing specific complex structured terahertz
fields in the far field. Our device successfully demonstrates the generation of
terahertz waves with diverse structured polarization states, including
spatially separated circular polarizations, azimuthal or radial polarization
states, and a full Poincare beam. This innovation opens a new avenue for
designing and generating structured terahertz radiations, with potential
applications in terahertz microscopy, communication, quantum information, and
light-matter interactions
Solitary beam propagation in a nonlinear optical resonator enables high-efficiency pulse compression and mode self-cleaning
Generating intense ultrashort pulses with high-quality spatial modes is
crucial for ultrafast and strong-field science. This can be accomplished by
controlling propagation of femtosecond pulses under the influence of Kerr
nonlinearity and achieving stable propagation with high intensity. In this
work, we propose that the generation of spatial solitons in periodic layered
Kerr media can provide an optimum condition for supercontinuum generation and
pulse compression using multiple thin plates. With both the experimental and
theoretical investigations, we successfully identify these solitary modes and
reveal a universal relationship between the beam size and the critical
nonlinear phase. Space-time coupling is shown to strongly influence the
spectral, spatial and temporal profiles of femtosecond pulses. Taking advantage
of the unique characters of these solitary modes, we demonstrate single-stage
supercontinuum generation and compression of femtosecond pulses from initially
170 fs down to 22 fs with an efficiency ~90%. We also provide evidence of
efficient mode self-cleaning which suggests rich spatial-temporal
self-organization processes of laser beams in a nonlinear resonator
Nonrelativistic and nonmagnetic control of terahertz charge currents via electrical anisotropy in RuO2 and IrO2
Precise and ultrafast control over photo-induced charge currents across
nanoscale interfaces could lead to important applications in energy harvesting,
ultrafast electronics, and coherent terahertz sources. Recent studies have
shown that several relativistic mechanisms, including inverse spin-Hall effect,
inverse Rashba-Edelstein effect and inverse spin-orbit-torque effect, can
convert longitudinally injected spin-polarized currents from magnetic materials
to transverse charge currents, thereby harnessing these currents for terahertz
generation. However, these mechanisms typically require external magnetic
fields and suffer from low spin-polarization rates and low efficiencies of
relativistic spin-to-charge conversion. In this work, we present a novel
nonrelativistic and nonmagnetic mechanism that directly utilizes the
photo-excited high-density charge currents across the interface. We demonstrate
that the electrical anisotropy of conductive oxides RuO2 and IrO2 can
effectively deflect injected charge currents to the transverse direction,
resulting in efficient and broadband terahertz radiation. Importantly, this new
mechanism has the potential to offer much higher conversion efficiency compared
to previous methods, as conductive materials with large electrical anisotropy
are readily available, whereas further increasing the spin-Hall angle of
heavy-metal materials would be challenging. Our new findings offer exciting
possibilities for directly utilizing these photo-excited high-density currents
across metallic interfaces for ultrafast electronics and terahertz
spectroscopy
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Influence of microscopic and macroscopic effects on attosecond pulse generation using two-color laser fields
Attosecond pulses and pulse trains generated by high-order harmonic generation are finding broad applications in advanced spectroscopies and imaging, enabling sub-femtosecond electron dynamics to be probed in atomic, molecular and material systems. To date, isolated attosecond pulses have been generated either by using very short few-cycle driving pulses, or by using temporal and polarization gating, or by taking advantage of phase-matching gating. Here we show that by driving high harmonics with a two-color linearly polarized laser field, the temporal window for time-gated phase matching is shorter than for the equivalent singe-color driving laser. As a result, we can generate quasi-isolated attosecond pulses with a peak width of ∼ 450 as using relatively long 26 femtosecond laser pulses. Our experimental data are in good agreement with theoretical simulations, and show that the phase matching window decreases by a factor of 4 - from four optical cycles in the case of a single-color fundamental driving laser, to one optical cycle in the case of two-color (ω-2ω) laser drivers. Finally, we also demonstrate that by changing the relative delay between the two-color laser fields, we can control the duration of the attosecond bursts from 450 as to 1.2 fs.National Science Foundation (NSF) (1125844); Air Force Office of Scientific Research (FA9550-16-1-0121); REA (328334); Junta de Castilla y León (SA046U16); MINECO (FIS2013-44174-P, FIS2016-75652-P)
Tomographic reconstruction of circularly polarized high-harmonic fields: 3D attosecond metrology
Bright, circularly polarized, extreme ultraviolet (EUV) and soft x-ray high-harmonic beams can now be produced using counter-rotating circularly polarized driving laser fields. Although the resulting circularly polarized harmonics consist of relatively simple pairs of peaks in the spectral domain, in the time domain, the field is predicted to emerge as a complex series of rotating linearly polarized bursts, varying rapidly in amplitude, frequency, and polarization. We extend attosecond metrology techniques to circularly polarized light by simultaneously irradiating a copper surface with circularly polarized high-harmonic and linearly polarized infrared laser fields. The resulting temporal modulation of the photoelectron spectra carries essential phase information about the EUV field. Utilizing the polarization selectivity of the solid surface and by rotating the circularly polarized EUV field in space, we fully retrieve the amplitude and phase of the circularly polarized harmonics, allowing us to reconstruct one of the most complex coherent light fields produced to date.This work was done at JILA. We gratefully acknowledge support from the NSF through the Physics Frontiers Centers Program with grant no. PHY1125844 and the Gordon and Betty Moore Foundation EPiQS (Emergent Phenomena in Quantum Systems) Initiative through Grant GBMF4538 to M.M. C.H.-G. acknowledges support from the Marie Curie International Outgoing Fellowship within the European Union Seventh Framework Programme for Research and Technological Development (2007–2013), under Research Executive Agency grant agreement no. 328334. R.K. acknowledges the Swedish Research Council (VR) for financial support. A.J.-B. was supported by grants from the U.S. NSF (grant nos. PHY-1125844 and PHY-1068706). C.H.-G. and L.P. acknowledge support from Junta de Castilla y León (project SA116U13) and MINECO (Ministerio de Econom a y Competitividad) (FIS2013-44174-P and FIS2015-71933-REDT). This work used the Janus supercomputer, which is supported by the U.S. NSF (grant no. CNS-0821794) and the University of Colorado, Boulder. P.G. acknowledges support from the Deutsche Forschungsgemeinschaft (no. GR 4234/1-1)