arXiv subjects

A. Cavalleri

Publications and source records attributed to A. Cavalleri.

At least 19 recordsLinked to original sources

arXiv2026

Three-phonon mixing as a source of light-induced chirality

Mid- and far-infrared optical pulses have found wide applicability in experiments in which collective modes of solids are driven nonlinearly, as a means to manipulate functional properties of materials on ultrafast time scales. In the illustrative case of boron phosphate (BPO$_4$), mid-infrared excitation has been used to induce chirality in an otherwise achiral crystal. The pump frequency dependence of photo-induced chirality reveals that, while on-resonance excitation is understood in terms of the well-documented phonon rectification from two-phonon mixing, additional pathways become relevant when the pump pulse is detuned from resonance. We find that a three-mode mixing mechanism becomes important in this case, whereas the contributions from other impulsive Raman terms are negligible. The results reported here provide a quantitative foundation for nonlinear phononic lineshapes in transparent materials, and open up new opportunities for lattice control.

cond-mat.mtrl-sci↗

arXiv2026

The Ultrafast Superconducting Diode Effect

Nonreciprocal transport is generally observed in superconductors in which time reversal and inversion symmetries are simultaneously broken. This effect, which may become one of the backbones for future superconducting electronics, arises because of asymmetric vortex transport in a magnetic field. However, vortex transport is also intrinsically dissipative and limited in speed. Here, we report on the discovery of ultrafast non-reciprocal transport in centrosymmetric superconductors. For NbN films biased with a quasi-DC supercurrent, picosecond current pulses with the same sign as the bias experience resistive impedance, whereas pulses of opposite polarity encounter an inductive response. Strikingly, the effect is at least three orders of magnitude faster than in conventional superconducting diodes, limited only by ultrafast current-induced depairing. We demonstrate rectification of a 100 GHz signal, with dissipation levels of a few fJ per cycle. We foresee potential for superconducting logic elements, operating at THz bit rates with aJ energy dissipation per operation.

cond-mat.supr-con↗

arXiv2026

The optical phonoelectric effect

Piezoelectricity is a technologically important property of certain insulators in which mechanical strain induces an electrical polarization. However, the rate at which a piezoelectric response can be established over a macroscopic volume is limited by the sound velocity, constraining applications in high-bit-rate transduction and sensing. Furthermore, the strength of the piezoelectric effect is not readily tunable, as it depends on intrinsic anharmonic coupling between strain and intra-unit-cell distortions in a given material. Lastly, the maximum amplitude of the effect is bounded by material fracture, which sets in already at percent level strain values. Here we overcome these limitations by realizing a strain-free, piezoelectric-like response driven solely by photo-excited optical phonon distortions. We demonstrate such optical phonoelectricity in the weak piezoelectric BPO$_4$, in which we induce electrical polarization through phonon rectification. This effect is established over macroscopic volumes with four orders of magnitude higher speed than piezoelectric responses, ultimately limited by the speed of light. The maximum induced polarization is estimated to be far in excess of that attainable through strain at the fracture limit. Ultrafast phonoelectricity opens up new opportunities for optical control in quantum materials, but also for device applications.

cond-mat.mtrl-sci↗

arXiv2026

Probing picosecond depairing currents in type-II superconductors

Accessing the intrinsic critical current density (Jc*) in type II superconductors has significant fundamental and technological potential, both as a probe of the microscopic superconducting properties and as a means to increase current limits in high magnetic field devices and in electrical power systems. Yet, the experimental critical current density in type II superconductors (Jc), when measured with DC currents, is generally lower than the intrinsic limit, mostly due to vortex motion and self-heating. Here, we show that ultrafast picosecond electrical pulses, which interact with the material on timescales over which vortices are inertially immobile, carry supercurrents up to the intrinsic depairing limit Jc* >> Jc. We probe picosecond critical currents in NbN and YBa2Cu3O7 (YBCO), as representative s-wave and d-wave superconductors, respectively. In NbN, we find a sharp onset of the picosecond depairing at a current density as large as Jc*=2.2 Jc, a limit that is well described by microscopic dynamics based on BCS theory. In contrast, YBCO exhibits a gradual suppression of superconductivity as a function of the picosecond current, reflecting its d-wave symmetry. These results offer a powerful new probe of superconductors beyond the reach of conventional transport measurements. The ability to reach the depairing current may also lead to robust new platforms for superconducting electronics.

cond-mat.supr-con↗

arXiv2025

Tunable narrowband THz generation in the organic crystal BNA

The generation of tunable narrowband pulses is increasingly being pursued in terahertz science, for example to study the nonlinear response of individual modes of solids and molecules. Here, we extend the chirp-and-delay method to achieve collinear phase-matched difference-frequency generation in the organic crystal N-benzyl-2-methyl-4-nitroaniline (BNA-S), which results in tunable narrowband terahertz pulses. In this configuration, the fundamental frequency of a Ti:sapphire amplifier is used - eliminating the need for optical parametric amplifiers typically required for THz generation in other organic crystals. Chirped-pulse excitation suppresses multiphoton absorption in BNA, improving stability and extending crystal lifetime. The source delivers THz transients tunable from 0.25 THz to 2.1 THz with adjustable spectral width.

physics.optics↗

arXiv2025

The Piezochiral Effect

Chirality is a pervasive property of matter that underpins many important phenomena across physics, chemistry and biology. Given its broad significance, the development of protocols for rational control of chirality in solid state systems is highly desirable, especially if this effect can be tuned continuously and in two directions. Yet, this goal has remained elusive due to the absence of a universal conjugate field that couples linearly to this structural order. Here, we introduce the piezochiral effect, which enables control of chirality through mechanical strain. We first show by symmetry analysis that uniaxial strain induces chirality in a broad class of achiral crystals that host fragments of opposite chirality within each unit cell, an effect that has so far remained unrecognized. The strain-induced handedness can be tuned either by changing the strain direction or by switching between compressive and tensile strain. We experimentally verify this effect in AgGaS2, using measurements of the optical activity under strain. Our discovery establishes a new scheme for chirality control, with potential applications that range from spintronics to asymmetric catalysis, and enantioselective interactions in biosystems.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast non-volatile rewritable ferroaxial switching

Ultrafast switching of ferroic phases is an important research frontier, with significant technological potential. Yet, current efforts are meeting some key challenges, ranging from limited speeds in ferromagnets to intrinsic volatility of switched domains due to uncompensated depolarizing fields in ferroelectrics. Unlike these ferroic systems, ferroaxial materials host bistable states that do not break spatial-inversion or time-reversal symmetry, and are therefore immune to depolarizing fields. Yet, they are difficult to manipulate because external axial fields are not easily constructed with conventional methods. Here, we demonstrate ultrafast switching of ferroaxial order by engineering an effective axial field made up of circularly driven terahertz phonon modes. A switched ferroaxial domain remains stable for many hours and can be reversed back with a second terahertz pulse of opposite helicity. The effects demonstrated here may lead to a new platform for ultrafast information storage.

cond-mat.mtrl-sci↗

arXiv2025

Polaritonic Quantum Matter

Polaritons are quantum mechanical superpositions of photon states with elementary excitations in molecules and solids. The light-matter admixture causes a characteristic frequency-momentum dispersion shared by all polaritons irrespective of the microscopic nature of material excitations that could entail charge, spin, lattice or orbital effects. Polaritons retain the strong nonlinearities of their matter component and simultaneously inherit ray-like propagation of light. Polaritons prompt new properties, enable new opportunities for spectroscopy/imaging, empower quantum simulations and give rise to new forms of synthetic quantum matter. Here, we review the emergent effects rooted in polaritonic quasiparticles in a wide variety of their physical implementations. We present a broad portfolio of the physical platforms and phenomena of what we term polaritonic quantum matter. We discuss the unifying aspects of polaritons across different platforms and physical implementations and focus on recent developments in: polaritonic imaging, cavity electrodynamics and cavity materials engineering, topology and nonlinearities, as well as quantum polaritonics.

cond-mat.mtrl-sci↗

arXiv2025

Signatures of three-dimensional photo-induced superconductivity in YBa$_2$Cu$_3$O$_{6.48}$

Optical excitation of large-amplitude apical oxygen phonon oscillations has been shown to renormalize the electronic properties of YBa$_2$Cu$_3$O$_{6+x}$, inducing a superconducting-like optical response above equilibrium $T_C$. All of the evidence collected so far has been based on the changes of the THz frequency $c$-axis response. In these measurements, the capacitive interlayer coupling was seen to transform into a superconducting-like inductive response. This assignment was strengthened by recent measurements of ultrafast magnetic field expulsion. Here, we report an experimental determination of the transient in-plane optical properties, which has so far been elusive due to the high equilibrium reflectivity and the need to evaluate minute changes in the optical response. We report the appearance of a photo-induced in-plane optical gap $2\Delta\simeq30$ cm$^{-1}$ and a divergent imaginary conductivity, both consistent with photo-induced superconductivity. A global fit to these data suggests that in- and out-of-plane electronic properties never completely equilibrate during the dynamics.

cond-mat.supr-con↗

arXiv2025

Enhanced coherent terahertz emission from critical superconducting fluctuations in YBa$_2$Cu$_3$O$_{6.6}$

Coherent terahertz (THz) emission is emerging as a powerful new tool to probe symmetry breakings in quantum materials. This method relies on second order optical nonlinearities and is complementary to second harmonic generation spectroscopy. Here, we report coherent THz emission from Josephson plasmons in underdoped YBa$_2$Cu$_3$O$_{6+x}$, and find that the amplitude of the emitted field increases dramatically close to the superconducting transition temperature, $T_C$. We show theoretically how emission is enhanced by critical superconducting fluctuations, a nonlinear analogue of critical opalescence. This observation is expected to be of general importance for the study of many thermal and quantum phase transitions.

cond-mat.supr-con↗

arXiv2025

Search for magnetic field expulsion in optically driven K$_3$C$_{60}$

Photoexcited K$_3$C$_{60}$ displays several properties reminiscent of equilibrium superconductivity, including transient optical spectra, pressure dependence, and I-V characteristics. However, these observations do not decisively establish non-equilibrium superconductivity, which would be conclusively evidenced by transient Meissner diamagnetism, as shown recently in driven YBa$_2$Cu$_3$O$_{6.48}$. Here, we search for transient magnetic field expulsion in K$_3$C$_{60}$ by measuring Faraday rotation in a magneto-optic material placed in its vicinity. Unlike in the case of homogeneous, insulating YBa$_2$Cu$_3$O$_{6.48}$, inhomogeneous, metallic K$_3$C$_{60}$ powders reduce the size of the effect. With the $\sim50$ nT magnetic field resolution achieved in our experiments, we provide an upper limit for the photo-induced diamagnetic volume susceptibility ($\chi_v>-0.1$). On this basis, we conclude that the photo-induced phase has weaker diamagnetism than superconducting K$_3$C$_{60}$ at zero temperature. Yet, from recent nonlinear transport measurements in this granular material, we expect a light-induced state similar to the equilibrium superconductor near 0.8 T$_c$, for which $\chi_v>-0.1$. A definitive conclusion on the presence or absence of Meissner diamagnetism cannot be made for K$_3$C$_{60}$ with the current resolution.

cond-mat.supr-con↗

arXiv2024

In-depth analysis of LISA Pathfinder performance results: Time evolution, noise projection, physical models, and implications for LISA

We present an in-depth analysis of the LISA Pathfinder differential acceleration performance over the entire course of its science operations, spanning approximately 500 days. We find that: 1) the evolution of the Brownian noise that dominates the acceleration amplitude spectral density (ASD), for frequencies $f\gtrsim 1\,\text{mHz}$, is consistent with the decaying pressure due to the outgassing of a single gaseous species. 2) between $f=36\,μ\text{Hz}$ and $1\,\text{mHz}$, the acceleration ASD shows a $1/f$ tail in excess of the Brownian noise of almost constant amplitude, with $\simeq 20\%$ fluctuations over a period of a few days, with no particular time pattern over the course of the mission; 3) at the lowest considered frequency of $f=18\,μ\text{Hz}$, the ASD significantly deviates from the $1/f$ behavior, because of temperature fluctuations that appear to modulate a quasi-static pressure gradient, sustained by the asymmetries of the outgassing pattern. We also present the results of a projection of the observed acceleration noise on the potential sources for which we had either a direct correlation measurement, or a quantitative estimate from dedicated experiments. These sources account for approximately $40\%$ of the noise power in the $1/f$ tail. Finally, we analyze the possible sources of the remaining unexplained fraction, and identify the possible measures that may be taken to keep those under control in LISA.

astro-ph.IM↗

arXiv2024

Generation of Ultrafast Magnetic Steps for Coherent Control

A long-standing challenge in ultrafast magnetism and in functional materials research in general, has been the generation of a universal, ultrafast stimulus able to switch between stable magnetic states. Solving it would open up many new opportunities for fundamental studies, with potential impact on future data storage technologies. Ideally, step-like magnetic field transients with infinitely fast rise time would serve this purpose. Here, we develop a new approach to generate ultrafast magnetic field steps, based on an ultrafast quench of supercurrents in a superconductor. Magnetic field steps with millitesla amplitude, picosecond risetimes and slew rates approaching 1 GT/s are achieved. We test the potential of this technique by coherently rotating the magnetization in a ferrimagnet. With suitable improvements in the geometry of the device, these magnetic steps can be made both larger and faster, leading to new applications that range from quenches across phase transitions to complete switching of magnetic order parameters.

cond-mat.supr-con↗

arXiv2024

Breaking symmetry with light: photo-induced chirality in a non-chiral crystal

Chirality is a pervasive form of symmetry that is intimately connected to the physical properties of solids, as well as the chemical and biological activity of molecular systems. However, its control with light is challenging, because inducing chirality in a non-chiral material requires that all mirrors and all roto-inversions be simultaneously broken. Electromagnetic fields exert only oscillatory forces that vanish on average, mostly leading to entropy increase that does not break symmetries, per se. Here, we show that chirality of either handedness can be generated in the non-chiral piezoelectric material BPO$_4$, in which two compensated sub-structures of opposite handedness coexist within the same unit cell. By resonantly driving either one of two orthogonal, doubly degenerate vibrational modes at Terahertz frequency, we rectify the lattice distortion and exert a displacive force onto the crystal. The staggered chirality is in this way uncompensated in either direction, inducing chiral structure with either handedness. The rotary power of the photo-induced phases is comparable to the static value of prototypical chiral alpha-quartz, limited by the strength of the pump laser pulse.

cond-mat.mtrl-sci↗

arXiv2024

Squeezed Josephson plasmons in driven YBa$_2$Cu$_3$O$_{6+x}$

The physics of driven collective modes in quantum materials underpin a number of striking non-equilibrium functional responses, which include enhanced magnetism, ferroelectricity and superconductivity. However, the coherent coupling between multiple modes at once are difficult to capture by single-pump probe (one-dimensional) spectroscopy, and often remain poorly understood. One example is phonon-mediated amplification of Josephson plasmons in YBa$_2$Cu$_3$O$_{6+x}$, in which at least three normal modes of the solid are coherently mixed as a source of enhanced superconductivity. Here, we go beyond previous pump-probe experiments in this system and acquire two-dimensional frequency maps using pairs of mutually delayed, carrier envelope phase stable mid-infrared pump pulses, combined with measurements of the time-modulated second-order nonlinear optical susceptibility. We find that the driven zone-center phonons amplify coherent pairs of opposite-momentum Josephson plasma polaritons, generating a squeezed state of interlayer phase fluctuations. The squeezed state is a potentially important ingredient in the microscopic physics of photo-induced superconductivity in this and other materials.

cond-mat.supr-con↗

arXiv2023

Ultrafast Raman thermometry in driven YBa$_2$Cu$_3$O$_{6.48}$

Signatures of photo-induced superconductivity have been reported in cuprate materials subjected to a coherent phonon drive. A 'cold' superfluid was extracted from the transient Terahertz conductivity and was seen to coexist with 'hot' uncondensed quasi-particles, a hallmark of a driven-dissipative system of which the interplay between coherent and incoherent responses are not well understood. Here, time resolved spontaneous Raman scattering was used to probe the lattice temperature in the photo-induced superconducting state of YBa2Cu3O6.48. An increase in lattice temperature of approximately 80 K was observed by measuring the time dependent Raman scattering intensity of an undriven 'spectator' phonon mode. This is to be compared with an estimated increase in quasi-particle temperatures of nearly 200 K. These temperature changes provide quantitative information on the nature of the driven state and its decay, and may provide a strategy to optimize this effect.

cond-mat.supr-con↗

arXiv2023

Giant resonant enhancement for photo-induced superconductivity in K$_3$C$_{60}$

Photo-excitation at terahertz and mid-infrared frequencies has emerged as a new way to manipulate functionalities in quantum materials, in some cases creating non-equilibrium phases that have no equilibrium analogue. In K$_3$C$_{60}$, a metastable zero-resistance phase was documented with optical properties and pressure dependences compatible with non-equilibrium high temperature superconductivity. Here, we report the discovery of a dominant energy scale for this phenomenon, along with the demonstration of a giant increase in photo-susceptibility near 10 THz excitation frequency. At these drive frequencies a metastable superconducting-like phase is observed up to room temperature for fluences as low as ~400 $μJ/cm^2$. These findings shed light on the microscopic mechanism underlying photo-induced superconductivity. They also trace a path towards steady state operation, currently limited by the availability of a suitable high-repetition rate optical source at these frequencies.

cond-mat.supr-con↗

arXiv2023

Comment on "Light-induced melting of competing stripe orders without introducing superconductivity in La$_{1.875}$Ba$_{0.125}$CuO$_4$" (arXiv:2306.07869v1)

In the manuscript arXiv:2306.07869v1, N. L. Wang and co-authors report the results of a near-infrared pump / terahertz probe study in the stripe-ordered cuprate La$_{1.875}$Ba$_{0.125}$CuO$_4$. They measured a change in optical conductivity, but did not find signatures of transient superconductivity. From this observation they extrapolate that in all cuprates in which striped states have been excited with light, there must be no light-induced superconductivity. They conclude that "transient superconductivity cannot be induced by melting of the competing stripe orders with pump pulses whose photon energy is much higher than the superconducting gap of cuprates." Here we show that this extrapolation is unwarranted. First, the absence of light-induced superconductivity in this particular compound was already reported in a previous paper, which instead showed positive evidence for La$_{1.885}$Ba$_{0.115}$CuO$_4$. In addition, the experiment discussed here used photo-excitation with too low fluence and at a suboptimal wavelength. More broadly, a negative result in one compound is rarely compelling indication of the absence of an effect in an entire class of materials.

cond-mat.supr-con↗