108 research outputs found
Laser‐driven strong‐field Terahertz sources
A review on the recent development of intense laser‐driven terahertz (THz) sources is provided here. The technologies discussed include various types of sources based on optical rectification (OR), spintronic emitters, and laser‐filament‐induced plasma. The emphasis is on OR using pump pulses with tilted intensity front. Illustrative examples of newly emerging applications are briefly discussed, in particular strong‐field THz control of materials and acceleration and manipulation of charged particles
Terahertz sum-frequency excitation of a Raman-active phonon
In stimulated Raman scattering, two incident optical waves induce a force
oscillating at the difference of the two light frequencies. This process has
enabled important applications such as the excitation and coherent control of
phonons and magnons by femtosecond laser pulses. Here, we experimentally and
theoretically demonstrate the so far neglected up-conversion counterpart of
this process: THz sum-frequency excitation of a Raman-active phonon mode, which
is tantamount to two-photon absorption by an optical transition between two
adjacent vibrational levels. Coherent control of an optical lattice vibration
of diamond is achieved by an intense terahertz pulse whose spectrum is centered
at half the phonon frequency of 40 THz. Remarkably, the carrier-envelope phase
of the driving pulse is directly imprinted on the lattice vibration. New
prospects in infrared spectroscopy, light storage schemes and lattice
trajectory control in the electronic ground state emerge
Transient birefringence of liquids induced by terahertz electric-field torque on permanent molecular dipoles
Collective low-frequency molecular motions have large impact on chemical
reactions and structural relaxation in liquids. So far, these modes have
mostly been accessed indirectly by off-resonant optical pulses. Here, we
provide evidence that intense terahertz (THz) pulses can resonantly excite
reorientational-librational modes of aprotic and strongly polar liquids
through coupling to the permanent molecular dipole moments. We observe a
significantly enhanced response because the transient optical birefringence is
up to an order of magnitude higher than obtained with optical excitation.
Frequency-dependent measurements and a simple analytical model indicate that
the enhancement arises from resonantly driven librations and their coupling to
reorientational motion, assisted by the pump field and/or a cage translational
mode. Our results open up the path to applications such as efficient molecular
alignment, enhanced transient Kerr signals and systematic resonant nonlinear
THz spectroscopy of the coupling between intermolecular modes in liquids
Laser-Driven Strong-Field Terahertz Sources
A review on the recent development of intense laser-driven terahertz (THz) sources is provided here. The technologies discussed include various types of sources based on optical rectification (OR), spintronic emitters, and laser-filament-induced plasma. The emphasis is on OR using pump pulses with tilted intensity front. Illustrative examples of newly emerging applications are briefly discussed, in particular strong-field THz control of materials and acceleration and manipulation of charged particles
Theory of spin-Hall magnetoresistance in the AC (terahertz) regime
In bilayers consisting of a normal metal (N) with spin-orbit coupling and a
ferromagnet (F), the combination of the spin-Hall effect, the spin-transfer
torque, and the inverse spin-Hall effect gives a small correction to the
in-plane conductivity of N, which is referred to as spin-Hall magnetoresistance
(SMR). We here present a theory of the SMR and the associated off-diagonal
conductivity corrections for frequencies up to the terahertz regime. We show
that the SMR signal has pronounced singularities at the spin-wave frequencies
of F, which identifies it as a potential tool for all-electric spectroscopy of
magnon modes. A systematic change of the magnitude of the SMR at lower
frequencies is associated with the onset of a longitudinal magnonic
contribution to spin transport across the F-N interface.Comment: 21 pages, 9 figure
Maximizing the amplitude of coherent phonons with shaped laser pulses
We perform model calculations of coherent lattice vibrations in solids driven
by ultrashort laser pulses. In order to maximize the amplitude of the coherent
phonon in the time domain, an evolutionary algorithm optimizes the driving
laser field. We find that only a Fourier-limited single pulse yields the
maximum phonon amplitude, irrespective of the actual physical excitation
mechanism (impulsive or displacive). This result is in clear contrast to the
widespread intuition that excitation by a pulse train in phase with the
oscillation leads to the largest amplitude of an oscillator. We rationalize
this result by an intuitive model and discuss implications for other nonlinear
processes such as optical rectification
Complex THz and DC inverse spin Hall effect in YIG/CuIr bilayers across a wide concentration range
We measure the inverse spin Hall effect of CuIr thin films on
yttrium iron garnet over a wide range of Ir concentrations (). Spin currents are triggered through the spin Seebeck effect,
either by a DC temperature gradient or by ultrafast optical heating of the
metal layer. The spin Hall current is detected by, respectively, electrical
contacts or measurement of the emitted THz radiation. With both approaches, we
reveal the same Ir concentration dependence that follows a novel complex,
non-monotonous behavior as compared to previous studies. For small Ir
concentrations a signal minimum is observed, while a pronounced maximum appears
near the equiatomic composition. We identify this behavior as originating from
the interplay of different spin Hall mechanisms as well as a
concentration-dependent variation of the integrated spin current density in
CuIr. The coinciding results obtained for DC and ultrafast
stimuli show that the studied material allows for efficient spin-to-charge
conversion even on ultrafast timescales, thus enabling a transfer of
established spintronic measurement schemes into the terahertz regime.Comment: 12 pages, 4 figure
- …