172 research outputs found
Probing the Interatomic Potential of Solids by Strong-Field Nonlinear Phononics
Femtosecond optical pulses at mid-infrared frequencies have opened up the
nonlinear control of lattice vibrations in solids. So far, all applications
have relied on second order phonon nonlinearities, which are dominant at field
strengths near 1 MVcm-1. In this regime, nonlinear phononics can transiently
change the average lattice structure, and with it the functionality of a
material. Here, we achieve an order-of-magnitude increase in field strength,
and explore higher-order lattice nonlinearities. We drive up to five phonon
harmonics of the A1 mode in LiNbO3. Phase-sensitive measurements of atomic
trajectories in this regime are used to experimentally reconstruct the
interatomic potential and to benchmark ab-initio calculations for this
material. Tomography of the Free Energy surface by high-order nonlinear
phononics will impact many aspects of materials research, including the study
of classical and quantum phase transitions
Metastable ferroelectricity in optically strained
Fluctuating orders in solids are generally considered high-temperature
precursors of broken symmetry phases. However, in some cases these fluctuations
persist to zero temperature and prevent the emergence of long-range order, as
for example observed in quantum spin and dipolar liquids. is a
quantum paraelectric in which dipolar fluctuations grow when the material is
cooled, although a long-range ferroelectric order never sets in. We show that
the nonlinear excitation of lattice vibrations with mid-infrared optical pulses
can induce polar order in up to temperatures in excess of 290 K. This
metastable phase, which persists for hours after the optical pump is
interrupted, is evidenced by the appearance of a large second-order optical
nonlinearity that is absent in equilibrium. Hardening of a low-frequency mode
indicates that the polar order may be associated with a photo-induced
ferroelectric phase transition. The spatial distribution of the optically
induced polar domains suggests that a new type of photo-flexoelectric coupling
triggers this effect
Dynamical Multiferroicity
An appealing mechanism for inducing multiferroicity in materials is the
generation of electric polarization by a spatially varying magnetization that
is coupled to the lattice through the spin-orbit interaction. Here we describe
the reciprocal effect, in which a time-dependent electric polarization induces
magnetization even in materials with no existing spin structure. We develop a
formalism for this dynamical multiferroic effect in the case for which the
polarization derives from optical phonons, and compute the strength of the
phonon Zeeman effect, which is the solid-state equivalent of the
well-established vibrational Zeeman effect in molecules, using density
functional theory. We further show that a recently observed behavior -- the
resonant excitation of a magnon by optically driven phonons -- is described by
the formalism. Finally, we discuss examples of scenarios that are not driven by
lattice dynamics and interpret the excitation of Dzyaloshinskii-Moriya-type
electromagnons and the inverse Faraday effect from the viewpoint of dynamical
multiferroicity
Parametric amplification of optical phonons
Amplification of light through stimulated emission or nonlinear optical
interactions has had a transformative impact on modern science and technology.
The amplification of other bosonic excitations, like phonons in solids, is
likely to open up new remarkable physical phenomena. Here, we report on an
experimental demonstration of optical phonon amplification. A coherent
mid-infrared optical field is used to drive large amplitude oscillations of the
Si-C stretching mode in silicon carbide. Upon nonlinear phonon excitation, a
second probe pulse experiences parametric optical gain at all wavelengths
throughout the reststrahlen band, which reflects the amplification of
optical-phonon fluctuations. Starting from first principle calculations, we
show that the high-frequency dielectric permittivity and the phonon oscillator
strength depend quadratically on the lattice coordinate. In the experimental
conditions explored here, these oscillate then at twice the frequency of the
optical field and provide a parametric drive for lattice fluctuations.
Parametric gain in phononic four wave mixing is a generic mechanism that can be
extended to all polar modes of solids, as a new means to control the kinetics
of phase transitions, to amplify many body interactions or to control
phonon-polariton waves
Probing optically silent superfluid stripes in cuprates
Unconventional superconductivity in the cuprates emerges from, or coexists
with, other types of electronic order. However, these orders are sometimes
invisible because of their symmetry. For example, the possible existence of
superfluid charge stripes in the normal state of single layer cuprates cannot
be validated with infrared optics, because interlayer tunneling fluctuations
vanish on average. Similarly, it is not easy to establish if charge orders are
responsible for dynamical decoupling of the superconducting layers over broad
ranges of doping and temperatures. Here, we show that TeraHertz pulses can
excite nonlinear tunneling currents between linearly de-coupled charge-ordered
planes. A giant TeraHertz third harmonic signal is observed in
La1.885Ba0.115CuO4 far above Tc=13 K and up to the charge ordering temperature
TCO = 55 K. We model these results by considering large order-parameter-phase
oscillations in a pair density wave condensate, and show how nonlinear mixing
of optically silent tunneling modes can drive large dipole-carrying
super-current oscillations. Our results provide compelling experimental support
for the presence of hidden superfluid order in the normal state of cuprates.
These experiments also underscore the power of nonlinear TeraHertz optics as a
sensitive probe of frustrated excitations in quantum solids.Comment: 9 pages main text, 5 figures, 12 page supplementar
Experimental evidence for Wigner's tunneling time
Tunneling of a particle through a potential barrier remains one of the most
remarkable quantum phenomena. Owing to advances in laser technology, electric
fields comparable to those electrons experience in atoms are readily generated
and open opportunities to dynamically investigate the process of electron
tunneling through the potential barrier formed by the superposition of both
laser and atomic fields. Attosecond-time and angstrom-space resolution of the
strong laser-field technique allow to address fundamental questions related to
tunneling, which are still open and debated: Which time is spent under the
barrier and what momentum is picked up by the particle in the meantime? In this
combined experimental and theoretical study we demonstrate that for
strong-field ionization the leading quantum mechanical Wigner treatment for the
time resolved description of tunneling is valid. We achieve a high sensitivity
on the tunneling barrier and unambiguously isolate its effects by performing a
differential study of two systems with almost identical tunneling geometry.
Moreover, working with a low frequency laser, we essentially limit the
non-adiabaticity of the process as a major source of uncertainty. The agreement
between experiment and theory implies two substantial corrections with respect
to the widely employed quasiclassical treatment: In addition to a non-vanishing
longitudinal momentum along the laser field-direction we provide clear evidence
for a non-zero tunneling time delay. This addresses also the fundamental
question how the transition occurs from the tunnel barrier to free space
classical evolution of the ejected electron.Comment: 31 pages, 15 figures including appendi
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