721 research outputs found
Terahertz field enhancement via coherent superposition of the pulse sequences after a single optical-rectification crystal
Terahertz electromagnetic pulses are frequently generated by optical rectification of femtosecond laser pulses. In many cases, the efficiency of this process is known to saturate with increasing intensity of the generation beam because of two-photon absorption. Here, we demonstrate two routes to reduce this effect in ZnTe(110) crystals and enhance efficiency, namely, by (i) recycling the generation pulses and by (ii) splitting each generation pulse into two pulses before pumping the crystal. In both methods, the second pulse arrives ∼1 ns after the first one, sufficiently long for optically generated carriers to relax. Enhancement is achieved by coherently superimposing the two resulting terahertz fields
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
Magneto-electric point scattering theory for metamaterial scatterers
We present a new, fully analytical point scattering model which can be
applied to arbitrary anisotropic magneto-electric dipole scatterers, including
split ring resonators (SRRs), chiral and anisotropic plasmonic scatterers. We
have taken proper account of reciprocity and radiation damping for electric and
magnetic scatterers with any general polarizability tensor. Specifically, we
show how reciprocity and energy balance puts constraints on the electrodynamic
responses arbitrary scatterers can have to light. Our theory sheds new light on
the magnitude of cross sections for scattering and extinction, and for instance
on the emergence of structural chirality in the optical response of
geometrically non-chiral scatterers like SRRs. We apply the model to SRRs and
discuss how to extract individual components of the polarizability matrix and
extinction cross sections. Finally, we show that our model describes well the
extinction of stereo-dimers of split rings, while providing new insights in the
underlying coupling mechanisms.Comment: 12 pages, 3 figure
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
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
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