Non-equilibrium control of complex solids by nonlinear phononics

Spectroscopy of phononic switching of magnetization in the terahertz gap

All-optical schemes for switching magnetization offer a pathway towards the creation of more advanced data-storage technologies, both in terms of recording speed and energy-efficiency. It has previously been shown that picosecond-long optical pulses with central frequencies ranging between 12 and 30 THz are capable of driving magnetic switching in yttrium-iron-garnet films, provided that the excitation frequency matches the characteristic frequency of longitudinal optical phonons. Here, we explore how the phononic mechanism of magnetic switching in three distinct ferrimagnetic iron-garnet films evolves at optical frequencies below 10 THz, within the so-called terahertz gap. We find that at long wavelengths the magnetic switching rather correlates with phonon modes associated with the substrate. Our results show that the process of phononic switching of magnetization, previously discovered in the mid- to far-infrared spectral range, becomes much more complex at frequencies within the terahertz gap.

Epsilon-near-zero regime for ultrafast opto-spintronics

Over the last two decades, breakthrough works in the field of non-linear phononics have revealed that high-frequency lattice vibrations, when driven to high amplitude by mid- to far-infrared optical pulses, can bolster the light-matter interaction and thereby lend control over a variety of spontaneous orderings. This approach fundamentally relies on the resonant excitation of infrared-active transverse optical phonon modes, which are characterized by a maximum in the imaginary part of the medium's permittivity. Here, in this Perspective article, we discuss an alternative strategy where the light pulses are instead tailored to match the frequency at which the real part of the medium's permittivity goes to zero. This so-called epsilon-near-zero regime, popularly studied in the context of metamaterials, naturally emerges to some extent in all dielectric crystals in the infrared spectral range. We find that the light-matter interaction in the phononic epsilon-near-zero regime becomes strongly enhanced, yielding even the possibility of permanently switching both spin and polarization order parameters. We provide our perspective on how this hitherto-neglected yet fertile research area can be explored in future, with the aim to outline and highlight the exciting challenges and opportunities ahead.

Phononic switching of magnetization by the ultrafast Barnett effect

The historic Barnett effect describes how an inertial body with otherwise zero net magnetic moment acquires spontaneous magnetization when mechanically spinning1,2. Breakthrough experiments have recently shown that an ultrashort laser pulse destroys the magnetization of an ordered ferromagnet within hundreds of femtoseconds3, with the spins losing angular momentum to circularly polarized optical phonons as part of the ultrafast Einstein-de Haas effect4,5. However, the prospect of using such high-frequency vibrations of the lattice to reciprocally switch magnetization in a nearby magnetic medium has not yet been experimentally explored. Here we show that the spontaneous magnetization gained temporarily by means of the ultrafast Barnett effect, through the resonant excitation of circularly polarized optical phonons in a paramagnetic substrate, can be used to permanently reverse the magnetic state of a heterostructure mounted atop the said substrate. With the handedness of the phonons steering the direction of magnetic switching, the ultrafast Barnett effect offers a selective and potentially universal method for exercising ultrafast non-local control over magnetic order.

Phonon Pseudoangular Momentum in α-MoO3

In recent studies, it has been discovered that phonons can carry angular momentum, leading to a series of investigations into systems with three-fold rotation symmetry. However, for systems with two-fold screw rotational symmetry, such as α-MoO3, there has been no relevant discussion. In this paper, we investigated the pseudoangular momentum of phonons in crystals with two-fold screw rotational symmetry. Taking α-MoO3 as an example, we explain the selection rules in circularly polarized Raman experiments resulting from pseudoangular momentum conservation, providing important guidance for experiments. This study of pseudoangular momentum in α-MoO3 opens up a new degree of freedom for its potential applications, expanding into new application domains.

Light-Driven Spontaneous Phonon Chirality and Magnetization in Paramagnets

Spin-phonon coupling enables the mutual manipulation of phonon and spin degrees of freedom in solids. In this study, we reveal the inherent nonlinearity within this coupling. Using a paramagnet as an illustration, we demonstrate the nonlinearity by unveiling spontaneous symmetry breaking under a periodic drive. The drive originates from linearly polarized light, respecting a mirror reflection symmetry of the system. However, this symmetry is spontaneously broken in the steady state, manifested in the emergence of coherent chiral phonons accompanied by a nonzero magnetization. We establish an analytical self-consistency equation to find the parameter regime where spontaneous symmetry breaking occurs. Furthermore, we estimate realistic parameters and discuss potential materials that could exhibit this behavior. Our findings shed light on the exploration of nonlinear phenomena in magnetic materials and present possibilities for on-demand control of magnetization.

Light-Induced Ideal Weyl Semimetal in HgTe via Nonlinear Phononics

Interactions between light and matter allow the realization of out-of-equilibrium states in quantum solids. In particular, nonlinear phononics is one of the most efficient approaches to realizing the stationary electronic state in nonequilibrium. Herein, by an extended ab initio molecular dynamics method, we identify that long-lived light-driven quasistationary geometry could stabilize the topological nature in the material family of HgTe compounds. We show that coherent excitation of the infrared-active phonon mode results in a distortion of the atomic geometry with a lifetime of several picoseconds. We show that four Weyl points are located exactly at the Fermi level in this nonequilibrium geometry, making it an ideal long-lived metastable Weyl semimetal. We propose that such a metastable topological phase can be identified by photoelectron spectroscopy of the Fermi arc surface states or ultrafast pump-probe transport measurements of the nonlinear Hall effect.

The persistence of memory in ionic conduction probed by nonlinear optics

Predicting practical rates of transport in condensed phases enables the rational design of materials, devices and processes. This is especially critical to developing low-carbon energy technologies such as rechargeable batteries1-3. For ionic conduction, the collective mechanisms4,5, variation of conductivity with timescales6-8 and confinement9,10, and ambiguity in the phononic origin of translation11,12, call for a direct probe of the fundamental steps of ionic diffusion: ion hops. However, such hops are rare-event large-amplitude translations, and are challenging to excite and detect. Here we use single-cycle terahertz pumps to impulsively trigger ionic hopping in battery solid electrolytes. This is visualized by an induced transient birefringence, enabling direct probing of anisotropy in ionic hopping on the picosecond timescale. The relaxation of the transient signal measures the decay of orientational memory, and the production of entropy in diffusion. We extend experimental results using in silico transient birefringence to identify vibrational attempt frequencies for ion hopping. Using nonlinear optical methods, we probe ion transport at its fastest limit, distinguish correlated conduction mechanisms from a true random walk at the atomic scale, and demonstrate the connection between activated transport and the thermodynamics of information.

Unusual Low-Energy Collective Charge Excitations in High-Tc Cuprate Superconductors

Despite decades of intensive experimental and theoretical efforts, the physics of cuprate high-temperature superconductors in general, and, in particular, their normal state, is still under debate. Here, we report our investigation of low-energy charge excitations in the normal state. We find that the peculiarities of the electronic band structure at low energies have a profound impact on the nature of the intraband collective modes. It gives rise to a new kind of mode with huge intensity and non-Lorentzian spectral function in addition to well-known collective excitations like conventional plasmons and spin fluctuation. We predict two such modes with maximal spectral weight in the nodal and antinodal directions. Additionally, we found a long-living quasi-one-dimensional plasmon becoming an intense soft mode over an extended momentum range along the antinodal direction. These modes might explain some of the resonant inelastic X-ray scattering spectroscopy data.

Coherent-Phonon-Driven Intervalley Scattering and Rabi Oscillation in Multivalley 2D Materials

Resolving the complete electron scattering dynamics mediated by coherent phonons is crucial for understanding electron-phonon couplings beyond equilibrium. Here we present a time-resolved theoretical investigation on strongly coupled ultrafast electron and phonon dynamics in monolayer WSe_{2}, with a focus on the intervalley scattering from the optically "bright" K state to "dark" Q state. We find that the strong coherent lattice vibration along the longitudinal acoustic phonon mode [LA(M)] can drastically promote K-to-Q transition on a timescale of ∼400  fs, comparable with previous experimental observation on thermal-phonon-mediated electron dynamics. Further, this coherent-phonon-driven intervalley scattering occurs in an unconventional steplike manner and further induces an electronic Rabi oscillation. By constructing a two-level model and quantitatively comparing with ab initio dynamic simulations, we uncover the critical role of nonadiabatic coupling effects. Finally, a new strategy is proposed to effectively tune the intervalley scattering rates by varying the coherent phonon amplitude, which could be realized via light-induced nonlinear phononics that we hope will spark experimental investigation.

In-line synthesis of multi-octave phase-stable infrared light

Parametric downconversion driven by modern, high-power sources of 10-fs-scale near-infrared pulses, in particular intrapulse difference-frequency generation (IPDFG), affords combinations of properties desirable for molecular vibrational spectroscopy in the mid-infrared range: broad spectral coverage, high brilliance, and spatial and temporal coherence. Yet, unifying these in a robust and compact radiation source has remained a key challenge. Here, we address this need by employing IPDFG in a multi-crystal in-line geometry, driven by the 100-W-level, 10.6-fs pulses of a 10.6-MHz-repetition-rate, nonlinearly post-compressed Yb:YAG thin-disk oscillator. Polarization tailoring of the driving pulses using a bichromatic waveplate is followed by a sequence of two crystals, LiIO₃ and LiGaS₂, resulting in the simultaneous coverage of the 800-cm^-1-to-3000-cm^-1 spectral range (at -30-dB intensity) with 130 mW of average power. We demonstrate that optical-phase coherence is maintained in this in-line geometry, in theory and experiment, the latter employing ultra-broadband electro-optic sampling. These results pave the way toward coherent spectroscopy schemes like field-resolved and frequency-comb spectroscopy, as well as nonlinear, ultrafast spectroscopy and optical-waveform synthesis across the entire infrared molecular fingerprint region.

Dynamic self-organisation and pattern formation by magnon-polarons

Magnetic materials play a vital role in energy-efficient data storage technologies, combining very fast switching with long-term retention of information. However, it has been shown that, at very short time scales, magnetisation dynamics become chaotic due to internal instabilities, resulting in incoherent spin-wave excitations that ultimately destroy magnetic ordering. Here, contrary to expectations, we show that such chaos gives rise to a periodic pattern of reversed magnetic domains, with a feature size far smaller than the spatial extent of the excitation. We explain this pattern as a result of phase-synchronisation of magnon-polaron quasiparticles, driven by strong coupling of magnetic and elastic modes. Our results reveal not only the peculiar formation and evolution of magnon-polarons at short time-scales, but also present an alternative mechanism of magnetisation reversal driven by coherent packets of short-wavelength magnetoelastic waves.

Generating Coherent Phonon Waves in Narrow-Band Materials: A Twisted Bilayer Graphene Phaser

Twisted bilayer graphene (TBG) exhibits extremely low Fermi velocities for electrons, with the speed of sound surpassing the Fermi velocity. This regime enables the use of TBG for amplifying vibrational waves of the lattice through stimulated emission, following the same principles of operation of free-electron lasers. Our Letter proposes a lasing mechanism relying on the slow-electron bands to produce a coherent beam of acoustic phonons. We propose a device based on undulated electrons in TBG, which we dub the phaser. The device generates phonon beams in a terahertz (THz) frequency range, which can then be used to produce THz electromagnetic radiation. The ability to generate coherent phonons in solids breaks new ground in controlling quantum memories, probing quantum states, realizing nonequilibrium phases of matter, and designing new types of THz optical devices.

Surface acoustic wave induced phenomena in two-dimensional materials

Surface acoustic wave (SAW)-matter interaction provides a fascinating key for inducing and manipulating novel phenomena and functionalities in two-dimensional (2D) materials. The dynamic strain field and piezo-electric field associated with propagating SAWs determine the coherent manipulation and transduction between 2D excitons and phonons. Over the past decade, many intriguing acoustic-induced effects, including the acousto-electric effect, acousto-galvanic effect, acoustic Stark effect, acoustic Hall effect and acoustic exciton transport, have been reported experimentally. However, many more phenomena, such as the valley acousto-electric effect, valley acousto-electric Hall effect and acoustic spin Hall effect, were only theoretically proposed, the experimental verification of which are yet to be achieved. In this minireview, we attempt to overview the recent breakthrough of SAW-induced phenomena covering acoustic charge transport, acoustic exciton transport and modulation, and coherent acoustic phonons. Perspectives on the opportunities of the proposed SAW-induced phenomena, as well as open experimental challenges, are also discussed, attempting to offer some guidelines for experimentalists and theorists to explore the desired exotic properties and boost practical applications of 2D materials.

Recent Development of Ultrafast Optical Characterizations for Quantum Materials

The advent of intense ultrashort optical pulses spanning a frequency range from terahertz to the visible has opened a new era in the experimental investigation and manipulation of quantum materials. The generation of strong optical field in an ultrashort time scale enables the steering of quantum materials nonadiabatically, inducing novel phenomenon or creating new phases which may not have an equilibrium counterpart. Ultrafast time-resolved optical techniques have provided rich information and played an important role in characterization of the nonequilibrium and nonlinear properties of solid systems. Here, some of the recent progress of ultrafast optical techniques and their applications to the detection and manipulation of physical properties in selected quantum materials are reviewed. Specifically, the new development in the detection of the Higgs mode and photoinduced nonequilibrium response in the study of superconductors by time-resolved terahertz spectroscopy are discussed.

Light-Driven Topological and Magnetic Phase Transitions in Thin Layer Antiferromagnets

We theoretically study the effect of low-frequency light pulses in resonance with phonons in the topological and magnetically ordered two-septuple layer (2-SL) MnBi₂Te₄ (MBT) and MnSb₂Te₄ (MST). These materials share symmetry properties and an antiferromagnetic ground state in pristine form but present different magnetic exchange interactions. In both materials, shear and breathing Raman phonons can be excited via nonlinear interactions with photoexcited infrared phonons using intense laser pulses that can be attained in the current experimental setups. The light-induced transient lattice distortions lead to a change in the sign of the effective interlayer exchange interaction and magnetic order accompanied by a topological band transition. Furthermore, we show that moderate antisite disorder, typically present in MBT and MST samples, can facilitate such an effect. Therefore, our work establishes 2-SL MBT and MST as candidate platforms for achieving non-equilibrium magneto-topological phase transitions.

Cavity-dumping a single infrared pulse from a free-electron laser for two-color pump-probe experiments

Electromagnetic radiation in the mid- to far-infrared spectral range represents an indispensable tool for the study of numerous types of collective excitations in solids and molecules. Short and intense pulses in this terahertz spectral range are, however, difficult to obtain. While wide wavelength-tunability is easily provided by free-electron lasers, the energies of individual pulses are relatively moderate, on the order of microjoules. Here, we demonstrate a setup that uses cavity-dumping of a free-electron laser to provide single, picosecond-long pulses in the mid- to far-infrared frequency range. The duration of the Fourier-limited pulses can be varied by cavity detuning, and their energy was shown to exceed 100 µJ. Using the aforementioned infrared pulse as a pump, we have realized a two-color pump-probe setup facilitating single-shot time-resolved imaging of magnetization dynamics. We demonstrate the capabilities of the setup first on thermally induced demagnetization and magnetic switching of a GdFeCo thin film and second by showing a single-shot time-resolved detection of resonant phononic switching of the magnetization in a magnetic garnet.

Spiers Memorial Lecture: From Optical to THz control of materials

Advances over the past decade have presented new avenues to achieve control over material properties using intense pulses of electromagnetic radiation, with frequencies ranging from optical (approximately 1 PHz, or 10¹⁵ Hz) down to below 1 THz (10¹² Hz). Some of these new developments have arisen from new experimental methods to drive and observe transient material properties, while others have emerged from new computational techniques that have made nonequilibrium dynamics more tractable to our understanding. One common issue with most attempts to realize control using electromagnetic pulses is the dissipation of energy, which in many cases poses a limit due to uncontrolled heating and has led to strong interest in using lower frequency and/or highly specific excitations to minimize this effect. Emergent developments in experimental tools using shaped X-ray pulses may in the future offer new possibilities for material control, provided that the issue of heat dissipation can be resolved for higher frequency light.

The concept of using appropriately shaped pulses of light to control the properties of materials has a range of potential applications, and relies on an understanding of intricate couplings within the material.

Phonon-induced disorder in dynamics of optically pumped metals from nonlinear electron-phonon coupling

The non-equilibrium dynamics of matter excited by light may produce electronic phases, such as laser-induced high-transition-temperature superconductivity, that do not exist in equilibrium. Here we simulate the dynamics of a metal driven at initial time by a spatially uniform pump that excites dipole-active vibrational modes which couple nonlinearly to electrons. We provide evidence for rapid loss of spatial coherence, leading to emergent effective disorder in the dynamics, which arises in a system unitarily evolving under a translation-invariant Hamiltonian, and dominates the electronic behavior as the system evolves towards a correlated electron-phonon long-time state, possibly explaining why transient superconductivity is not observed. Our framework provides a basis within which to understand correlation dynamics in current pump-probe experiments of vibrationally coupled electrons, highlight the importance of the evolution of phase coherence, and demonstrate that pumped electron-phonon systems provide a means of realizing dynamically induced disorder in translation-invariant systems.

Superconductivity reported in metals driven away from equilibrium via optical pumping has been proposed to arise from nonlinear coupling between electrons and optically excited phonons. The authors use an exact approach to show that here, disorder, which disfavors superconductivity, emerges even though the system is translationally invariant.

Visualizing Nonlinear Phononics in Layered ReSe2

Nonlinear phononics has recently been demonstrated as a viable approach for dynamically modifying materials' properties. Conventionally, nonlinearity in the lattice dynamics is introduced via the "ionic" Raman scattering, in which infrared-active phonons (i.e., coherent ionic vibrations) serve as the intermediate state for transferring energy to Raman-active phonons. Here we report that it is also possible to achieve phononic nonlinearity through the "electronic" route, a process that relies on excited electronic states to initiate energy exchange among Raman-active phonons. Taking layered ReSe₂ as a model system, we use coherent phonon spectroscopy with a pump energy larger than the band gap to follow lattice dynamics and observe the nonlinear coupling between both Raman-active intralayer atomic oscillations and interlayer breathing modes. In addition, we show that such nonlinear phononic coupling is highly dependent on the environment temperature. This work, which demonstrates a different and novel mechanism, may enrich the toolkit for controlling material properties by means of nonlinear phononics.

Time-resolved resonant elastic soft x-ray scattering at Pohang Accelerator Laboratory X-ray Free Electron Laser

Resonant elastic x-ray scattering has been widely employed for exploring complex electronic ordering phenomena, such as charge, spin, and orbital order, in particular, in strongly correlated electronic systems. In addition, recent developments in pump-probe x-ray scattering allow us to expand the investigation of the temporal dynamics of such orders. Here, we introduce a new time-resolved Resonant Soft X-ray Scattering (tr-RSXS) endstation developed at the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL). This endstation has an optical laser (wavelength of 800 nm plus harmonics) as the pump source. Based on the commissioning results, the tr-RSXS at PAL-XFEL can deliver a soft x-ray probe (400 eV-1300 eV) with a time resolution of ∼100 fs without jitter correction. As an example, the temporal dynamics of a charge density wave on a high-temperature cuprate superconductor is demonstrated.

Photoinduced Electron Pairing in a Driven Cavity

We demonstrate how virtual scattering of laser photons inside a cavity via two-photon processes can induce controllable long-range electron interactions in two-dimensional materials. We show that laser light that is red (blue) detuned from the cavity yields attractive (repulsive) interactions whose strength is proportional to the laser intensity. Furthermore, we find that the interactions are not screened effectively except at very low frequencies. For realistic cavity parameters, laser-induced heating of the electrons by inelastic photon scattering is suppressed and coherent electron interactions dominate. When the interactions are attractive, they cause an instability in the Cooper channel at a temperature proportional to the square root of the driving intensity. Our results provide a novel route for engineering electron interactions in a wide range of two-dimensional materials including AB-stacked bilayer graphene and the conducting interface between LaAlO_{3} and SrTiO_{3}.

Electron-phonon-driven three-dimensional metallicity in an insulating cuprate

The role of the crystal lattice for the electronic properties of cuprates and other high-temperature superconductors remains controversial despite decades of theoretical and experimental efforts. While the paradigm of strong electronic correlations suggests a purely electronic mechanism behind the insulator-to-metal transition, recently the mutual enhancement of the electron-electron and the electron-phonon interaction and its relevance to the formation of the ordered phases have also been emphasized. Here, we combine polarization-resolved ultrafast optical spectroscopy and state-of-the-art dynamical mean-field theory to show the importance of the crystal lattice in the breakdown of the correlated insulating state in an archetypal undoped cuprate. We identify signatures of electron-phonon coupling to specific fully symmetric optical modes during the buildup of a three-dimensional (3D) metallic state that follows charge photodoping. Calculations for coherently displaced crystal structures along the relevant phonon coordinates indicate that the insulating state is remarkably unstable toward metallization despite the seemingly large charge-transfer energy scale. This hitherto unobserved insulator-to-metal transition mediated by fully symmetric lattice modes can find extensive application in a plethora of correlated solids.

Measuring non-equilibrium dynamics in complex solids with ultrashort X-ray pulses

Strong interactions between electrons give rise to the complexity of quantum materials, which exhibit exotic functional properties and extreme susceptibility to external perturbations. A growing research trend involves the study of these materials away from equilibrium, especially in cases in which the stimulation with optical pulses can coherently enhance cooperative orders. Time-resolved X-ray probes are integral to this type of research, as they can be used to track atomic and electronic structures as they evolve on ultrafast timescales. Here, we review a series of recent experiments where femtosecond X-ray diffraction was used to measure dynamics of complex solids. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

Generation of narrowband, high-intensity, carrier-envelope phase-stable pulses tunable between 4 and 18 THz

We demonstrate the generation of narrowband (<1 THz) high-energy (~2 uJ) carrier-envelope phase-stable pulses, tunable between 4 and 18 THz as achieved by difference-frequency mixing between chirped near-infrared pulses in organic DSTMS.

Floquet Engineering in Quantum Chains

We consider a one-dimensional interacting spinless fermion model, which displays the well-known Luttinger liquid (LL) to charge density wave (CDW) transition as a function of the ratio between the strength of the interaction U and the hopping J. We subject this system to a spatially uniform drive which is ramped up over a finite time interval and becomes time periodic in the long-time limit. We show that by using a density matrix renormalization group approach formulated for infinite system sizes, we can access the large-time limit even when the drive induces finite heating. When both the initial and long-time states are in the gapless (LL) phase, the final state has power-law correlations for all ramp speeds. However, when the initial and final state are gapped (CDW phase), we find a pseudothermal state with an effective temperature that depends on the ramp rate, both for the Magnus regime in which the drive frequency is very large compared to other scales in the system and in the opposite limit where the drive frequency is less than the gap. Remarkably, quantum defects (instantons) appear when the drive tunes the system through the quantum critical point, in a realization of the Kibble-Zurek mechanism.

Ab Initio Investigation of Nonlinear Mode Coupling in C60

Strong light fields may be used to control molecular structure, thus providing a route to new, light-induced phases of matter. In this context, we present an ab initio molecular dynamics investigation of nonlinear mode coupling in C₆₀ and show that, under suitable conditions, resonant infrared excitation induces significant structural changes in the system. Surprisingly, exciting the highest-frequency infrared mode at field strengths employed in a recent experiment [Nature 2016, 530, 461-464], we observe no significant structural change. However, when targeting the first two infrared modes using stronger fields, all Raman modes gain energy through nonlinear coupling to the infrared modes, leading to large-amplitude vibrational excitations. We find a strong response of the H_g(5) mode, which is symmetric with respect to the equilibrium structure. For sufficiently strong field, it is of the same order as the infrared modes' excitation. This encourages further investigations into the light-induced superconducting properties of alkali-doped fullerenes.

Perspective: THz-driven nuclear dynamics from solids to molecules

Recent years have seen dramatic developments in the technology of intense pulsed light sources in the THz frequency range. Since many dipole-active excitations in solids and molecules also lie in this range, there is now a tremendous potential to use these light sources to study linear and nonlinear dynamics in such systems. While several experimental investigations of THz-driven dynamics in solid-state systems have demonstrated a variety of interesting linear and nonlinear phenomena, comparatively few efforts have been made to drive analogous dynamics in molecular systems. In the present Perspective article, we discuss the similarities and differences between THz-driven dynamics in solid-state and molecular systems on both conceptual and practical levels. We also discuss the experimental parameters needed for these types of experiments and thereby provide design criteria for a further development of this new research branch. Finally, we present a few recent examples to illustrate the rich physics that may be learned from nonlinear THz excitations of phonons in solids as well as inter-molecular vibrations in liquid and gas-phase systems.

Real-Time Observation of Phonon-Mediated σ-π Interband Scattering in MgB_{2}

In systems having an anisotropic electronic structure, such as the layered materials graphite, graphene, and cuprates, impulsive light excitation can coherently stimulate specific bosonic modes, with exotic consequences for the emergent electronic properties. Here we show that the population of E_{2g} phonons in the multiband superconductor MgB_{2} can be selectively enhanced by femtosecond laser pulses, leading to a transient control of the number of carriers in the σ-electronic subsystem. The nonequilibrium evolution of the material optical constants is followed in the spectral region sensitive to both the a- and c-axis plasma frequencies and modeled theoretically, revealing the details of the σ-π interband scattering mechanism in MgB_{2}.

Ultrafast Reversal of the Ferroelectric Polarization

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.

Narrowband carrier-envelope phase stable mid-infrared pulses at wavelengths beyond 10 μm by chirped-pulse difference frequency generation

We report on the generation of narrowband carrier-envelope phase stable mid-infrared (MIR) pulses between 10 and 15 μm. High pulse energies and narrow bandwidths are required for the selective nonlinear excitation of collective modes of matter that is not possible with current sources. We demonstrate bandwidths of <2% at 12.5 μm wavelength through difference frequency generation between two near-infrared (NIR) pulses, which are linearly chirped. We obtain a reduction in bandwidth by one order of magnitude, compared to schemes that make use of transform-limited NIR pulses. The wavelength of the narrowband MIR pulse can be tuned by changing the optical delay between the two chirped NIR pulses.