170 research outputs found
Theory of nonlinear phononics for coherent light-control of solids
We present a microscopic theory for ultrafast control of solids with
high-intensity terahertz frequency optical pulses. When resonant with selected
infrared-active vibrations, these pulses transiently modify the crystal
structure and lead to new collective electronic properties. The theory predicts
the dynamical path taken by the crystal lattice using first-principles
calculations of the energy surface and classical equations of motion, as well
as symmetry considerations. Two classes of dynamics are identified. In the
perturbative regime, displacements along the normal mode coordinate of
symmetry-preserving Raman active modes can be achieved by cubic
anharmonicities. This explains the light-induced insulator-to-metal transition
reported experimentally in manganites. We predict a regime in which ultrafast
instabilities that break crystal symmetry can be induced. This nonperturbative
effect involves a quartic anharmonic coupling and occurs above a critical
threshold, below which the nonlinear dynamics of the driven mode displays
softening and dynamical stabilization.Comment: updated to reflect the published versio
Cavity-mediated electron-photon superconductivity
We investigate electron paring in a two-dimensional electron system mediated
by vacuum fluctuations inside a nanoplasmonic terahertz cavity. We show that
the structured cavity vacuum can induce long-range attractive interactions
between current fluctuations which lead to pairing in generic materials with
critical temperatures in the low-Kelvin regime for realistic parameters. The
induced state is a pair density wave superconductor which can show a transition
from a fully gapped to a partially gapped phase - akin to the pseudogap phase
in high- superconductors. Our findings provide a promising tool for
engineering intrinsic electron interactions in two-dimensional materials.Comment: 11 page
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
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
Transiently enhanced interlayer tunneling in optically driven high-Tc superconductors
Recent pump-probe experiments reported an enhancement of superconducting transport along the c axis of underdoped YBa2Cu3O6+δ (YBCO), induced by a midinfrared optical pump pulse tuned to a specific lattice vibration. To understand this transient nonequilibrium state, we develop a pump-probe formalism for a stack of Josephson junctions, and we consider the tunneling strengths in the presence of modulation with an ultrashort optical pulse. We demonstrate that a transient enhancement of the Josephson coupling can be obtained for pulsed excitation and that this can be even larger than in a continuously driven steady state. Especially interesting is the conclusion that the effect is largest when the material is parametrically driven at a frequency immediately above the plasma frequency, in agreement with what is found experimentally. For bilayer Josephson junctions, an enhancement similar to that experimentally is predicted below the critical temperature Tc. This model reproduces the essential features of the enhancement measured below Tc. To reproduce the experimental results above Tc, we will explore extensions of this model, such as in-plane and amplitude fluctuations, elsewhere.Deutsche Forschungsgemeinschaft; SFB 925; EXC 1074; Joachim Herz StiftungFirst author draf
Nonlinear light-matter interaction at terahertz frequencies
Strong optical pulses at mid-infrared and terahertz frequencies have recently
emerged as a powerful tool to manipulate and control the solid state and
especially complex condensed matter systems with strongly correlated electrons.
The recent developments in high-power sources in the 0.1-30 THz frequency
range, both from table-top laser systems and Free-Electron Lasers, has provided
access to excitations of molecules and solids, which can be stimulated at their
resonance frequencies. Amongst these, we discuss free electrons in metals,
superconducting gaps and Josephson plasmons in layered superconductors,
vibrational modes of the crystal lattice (phonons), as well as magnetic
excitations. This Review provides an overview and illustrative examples of how
intense THz transients can be used to resonantly control matter, with
particular focus on strongly correlated electron systems and high-temperature
superconductors.Comment: 55 pages, 34 figure
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
Terahertz field control of interlayer transport modes in cuprate superconductors
We theoretically show that terahertz pulses with controlled amplitude and
frequency can be used to switch between stable transport modes in layered
superconductors, modelled as stacks of Josephson junctions. We find pulse
shapes that deterministically switch the transport mode between
superconducting, resistive and solitonic states. We develop a simple model that
explains the switching mechanism as a destablization of the centre of mass
excitation of the Josephson phase, made possible by the highly non-linear
nature of the light-matter coupling
Proposed cavity Josephson plasmonics with complex-oxide heterostructures
We discuss how complex-oxide heterostructures that include high-Tc
superconducting cuprates can be used to realize an array of sub-millimeter
cavities that support Josephson plasmon polaritons. These cavities have several
attractive features for new types of light matter interaction studies and we
show that they promote "ultrastrong" coupling between THz frequency radiation
and Josephson plasmons. Cavity electrodynamics of Josephson plasmons allows to
manipulate the superconducting order-parameter phase coherence. As an example,
we discuss how it could be used to cool superconducting phase fluctuations with
light
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