Driving strontium titanate ferroelectricHidden phases are metastable collective states of matter that are typically not accessible on equilibrium phase diagrams. Nova et al. used infrared pulses to ...excite higher-frequency lattice modes that drive the crystal into a metastable ferroelectric phase, a phase that can persist for many hours. X. Li et al. used terahertz fields to drive the soft mode that moves the ions in the crystal into the positions they occupy in the new phase. The ferroelectric phase in this case was transient, lasting on the order of 10 picoseconds. Because these hidden phases can host exotic properties in otherwise conventional materials, the accessibility to and control of such hidden phases may broaden potential functionality and applications.Science, this issue p. 1075, p. 1079Fluctuating 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. Strontium titanate (SrTiO3) is a quantum paraelectric in which dipolar fluctuations grow upon cooling, although a long-range ferroelectric order never sets in. Here, we show that optical excitation of lattice vibrations can induce polar order. This metastable polar phase, observed up to temperatures exceeding 290 kelvin, persists for hours after the optical pump is interrupted. Furthermore, hardening of a low-frequency vibration points to a photoinduced ferroelectric phase transition, with a spatial domain distribution suggestive of a photoflexoelectric coupling.
Many non-equilibrium phenomena have been discovered or predicted in optically-driven quantum solids
. Examples include light-induced superconductivity
and Floquet-engineered topological phases
. ...These are short lived effects that should lead to measurable changes in electrical transport, which can be characterized using an ultrafast device architecture based on photoconductive switches
. Here, we report the observation of a light-induced anomalous Hall effect in monolayer graphene driven by a femtosecond pulse of circularly polarized light. The dependence of the effect on a gate potential used to tune the Fermi level reveals multiple features that reflect a Floquet-engineered topological band structure
, similar to the band structure originally proposed by Haldane
. This includes an approximately 60 meV wide conductance plateau centered at the Dirac point, where a gap of equal magnitude is predicted to open. We find that when the Fermi level lies within this plateau, the estimated anomalous Hall conductance saturates around 1.8±0.4 e
/h.
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We report on the demonstration of ultrafast optical reversal of the ferroelectric polarization in LiNbO_{3}. Rather than driving the ferroelectric mode directly, we couple to it indirectly by ...resonant excitation of an auxiliary high-frequency phonon mode with femtosecond midinfrared pulses. Because of strong anharmonic coupling between these modes, the atoms are directionally displaced along the ferroelectric mode and the polarization is transiently reversed, as revealed by time-resolved, phase-sensitive, second-harmonic generation. This reversal can be induced in both directions, a key prerequisite for practical applications.
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Light fields at terahertz and mid-infrared frequencies allow for the direct excitation of collective modes in condensed matter, which can be driven to large amplitudes. For example, excitation of the ...crystal lattice has been shown to stimulate insulator-metal transitions, melt magnetic order or enhance superconductivity. Here, we generalize these ideas and explore the simultaneous excitation of more than one lattice mode, which are driven with controlled relative phases.
Abstract
Excitation of high-
T
c
cuprates and certain organic superconductors with intense far-infrared optical pulses has been shown to create non-equilibrium states with optical properties that are ...consistent with transient high-temperature superconductivity. These non-equilibrium phases have been generated using femtosecond drives, and have been observed to disappear immediately after excitation, which is evidence of states that lack intrinsic rigidity. Here we make use of a new optical device to drive metallic K
3
C
60
with mid-infrared pulses of tunable duration, ranging between one picosecond and one nanosecond. The same superconducting-like optical properties observed over short time windows for femtosecond excitation are shown here to become metastable under sustained optical driving, with lifetimes in excess of ten nanoseconds. Direct electrical probing, which becomes possible at these timescales, yields a vanishingly small resistance with the same relaxation time as that estimated by terahertz conductivity. We provide a theoretical description of the dynamics after excitation, and justify the observed slow relaxation by considering randomization of the order-parameter phase as the rate-limiting process that determines the decay of the light-induced superconductor.
Parametric amplification of optical phonons Cartella, A.; Nova, T. F.; Fechner, M. ...
Proceedings of the National Academy of Sciences - PNAS,
11/2018, Volume:
115, Issue:
48
Journal Article
Peer reviewed
Open access
We use coherent midinfrared optical pulses to resonantly excite large-amplitude oscillations of the Si–C stretching mode in silicon carbide. When probing the sample with a second pulse, we observe ...parametric optical gain at all wavelengths throughout the reststrahlen band. This effect reflects the amplification of light by phonon-mediated four-wave mixing and, by extension, of optical-phonon fluctuations. Density functional theory calculations clarify aspects of the microscopic mechanism for this phenomenon. The high-frequency dielectric permittivity and the phonon oscillator strength depend quadratically on the lattice coordinate; they oscillate at twice the frequency of the optical field and provide a parametric drive for the lattice mode. Parametric gain in phononic four-wave mixing is a generic mechanism that can be extended to all polar modes of solids, as a means to control the kinetics of phase transitions, to amplify many-body interactions or to control phonon-polariton waves.
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Nonlinear optical techniques at visible frequencies have long been applied to condensed matter spectroscopy. However, because many important excitations of solids are found at low energies, much can ...be gained from the extension of nonlinear optics to mid-infrared and terahertz frequencies. For example, the nonlinear excitation of lattice vibrations has enabled the dynamic control of material functions. So far it has only been possible to exploit second-order phonon nonlinearities at terahertz field strengths near one million volts per centimetre. Here we achieve an order-of-magnitude increase in field strength and explore higher-order phonon nonlinearities. We excite up to five harmonics of the A
(transverse optical) phonon mode in the ferroelectric material lithium niobate. By using ultrashort mid-infrared laser pulses to drive the atoms far from their equilibrium positions, and measuring the large-amplitude atomic trajectories, we can sample the interatomic potential of lithium niobate, providing a benchmark for ab initio calculations for the material. Tomography of the energy surface by high-order nonlinear phononics could benefit many aspects of materials research, including the study of classical and quantum phase transitions.
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Light can interact with the electrons in a crystalline solid, which in turn generates lattice vibrations or phonons. A related phenomenon was proposed 40 years ago in which it is the ions in the ...crystal rather than the electrons that mediate the interaction. This effect, known as ionic Raman scattering, is now observed experimentally.
Unconventional superconductivity in the cuprates coexists with other types of electronic order. However, some of these orders are invisible to most experimental probes because of their symmetry. For ...example, the possible existence of superfluid stripes is not easily validated with linear optics, because the stripe alignment causes interlayer superconducting tunneling to vanish on average. Here we show that this frustration is removed in the nonlinear optical response. A giant terahertz third harmonic, characteristic of nonlinear Josephson tunneling, is observed in La
Ba
CuO
above the transition temperature
= 13 kelvin and up to the charge-ordering temperature
= 55 kelvin. We model these results by hypothesizing the presence of a pair density wave condensate, in which nonlinear mixing of optically silent tunneling modes drives large dipole-carrying supercurrents.
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