arXiv subjects

Mattia Udina

Publications and source records attributed to Mattia Udina.

17 recordsLinked to original sources

arXiv2026

Measuring momentum-resolved dissipation of phonon-polaritons in LiNbO$_3$ with terahertz driving

Mapping the dispersion of polaritons, hybrid quasiparticles arising from light-matter coupling, can provide key insights into the material dielectric response, coupling strength, and energy transfer pathways with other excitations. In this work, we present THz pump-Raman probe (TP-RP) as a versatile method for mapping the polariton dispersion in polar non-centrosymmetric materials, demonstrated here for the case of phonon-polaritons in LiNbO$_3$. By resonantly driving polaritonic modes with a broadband THz pump and probing them with a tunable NIR Raman pulse, TP-RP allows for the extraction of the momentum-dependence of both their frequency and damping rate with high accuracy. The spectral features observed in the pump-probe signal, including the polaritonic response as well as pulse artifacts, are reproduced within a many-body theoretical approach. Applying the technique to study the E(TO$_1$) phonon of LiNbO$_3$ enables the combined analysis of theory and experiments to uncover a nontrivial frequency dependence of the phonon intrinsic damping rate, revealing possible anharmonic couplings to other modes.

cond-mat.mtrl-sci↗

arXiv2026

Two-dimensional THz spectroscopy in electronic systems: a many-body diagrammatic approach

The term two-dimensional coherent spectroscopy (2DCS) usually refers to experimental setups where a coherently generated electric field in a sample is recorded over many runs as a function of two time variables: the delay $τ$ between two consequent excitation pulses and the time $t$ over which the signal is emitted. While its implementation in the femtosecond time domain for studying vibrational molecular states has been developed for over two decades, its experimental application in the THz domain to interacting electronic systems remains in its infancy. This work provides a general theoretical framework for describing and interpreting 2DCS using a many-body language based on a perturbative diagrammatic expansion, as widely applied in linear spectroscopy. Focusing on centrosymmetric systems, we show that interpreting the 2D maps can be recast into two complementary problems. The first is the evaluation of a third-order response function to the gauge field. In the velocity gauge, this leads to semi-analytical expressions that both reduce computational complexity and assist in assigning spectral features to microscopic processes, as shown using a toy model of electrons undergoing a charge-density wave transition. The second is a careful treatment of multi-wave propagation effects, which, in bulk systems, can obscure the intrinsic nonlinear response, demonstrated here for soft superconducting Josephson plasmons. Our results provide a solid foundation for extending 2DCS to complex interacting systems and offer a flexible method to realistically model nonlinear responses across arbitrary spectral widths.

cond-mat.supr-con↗

arXiv2026

Anharmonic phonon coupling enabled by local inversion symmetry breaking at domain walls in ferroelastics

In ferroelastic materials, spontaneous symmetry breaking leads to the formation of twin domains. Although the bulk crystal typically remains centrosymmetric, inversion symmetry can be locally broken at the domain walls, potentially changing phonon selection rules and enabling local anharmonic phonon coupling. Here we report direct evidence of such anharmonic coupling in ferroelastic LaAlO$_3$ using two-dimensional Raman-terahertz spectroscopy. We attribute the cross-peaks observed in the two-dimensional spectra to both mechanical and electrical anharmonicity between the $A_{1g}$ Raman-active phonon and the $E_g$ phonon, which acquires finite infrared activity through local inversion symmetry breaking at ferroelastic domain walls. These findings provide new insight into the complex lattice dynamics of ferroelastic materials and highlight the potential of two-dimensional Raman-terahertz spectroscopy to uncover subtle symmetry breaking through the detection of intrinsically weak anharmonic signals.

cond-mat.mtrl-sci↗

arXiv2026

Antisymmetric Raman response

We develop the theory of antisymmetric Raman response, defined as the difference between the Raman signals of two scattering geometries related by an exchange of mutually perpendicular incoming and the outgoing photon polarizations. Such responses, finite in orthorhombic or lower symmetry systems, are related to cross-susceptibilities of two Raman operators and are characterized by the absence of intraband terms. This is in contrast to standard Raman responses which measure auto-susceptibilities where both intra- and interband processes contribute. We illustrate the theory with examples from the charge density wave rare-earth tritellurides and the excitonic insulator Ta$_2$NiSe$_5$. Our theory establishes antisymmetric Raman response as a unique tool to probe microscopic features such as interband energy scales and to detect reflection symmetry breaking.

cond-mat.str-el↗

arXiv2026

Revealing the nature of the charge density wave order of ErTe$_3$ via Raman scattering under anisotropic strain

The nature of the charge density wave (CDW) order of the rare-earth tritelluride ErTe3 is investigated by Raman scattering under anisotropic strain. The CDW state of ErTe3 is unconventional since it is accompanied by an unusual mirror symmetry breaking, whose origin remains to be understood. Studying the polarization-resolved Raman spectrum of the CDW amplitude mode as a function of strain and temperature, we find that the mirror symmetry breakings of the CDW state are not independent, arguing against the recently proposed ferro-axial multi-component CDW order. Instead, we show that a single component CDW order parameter with an ordering wavevector tilted away from the principle crystallographic axis can reproduce the observed mirror symmetry breakings and their manifestation in the Raman spectra.

cond-mat.str-el↗

arXiv2025

Terahertz light driven coherent excitation of a zone-folded Raman-active phonon mode in the Spin-Ladder System $α'$-NaV$_2$O$_5$

We investigate the out-of-equilibrium lattice dynamics in the spin-ladder system $α'$-NaV$_2$O$_5$ using intense terahertz (THz) pump and near-infrared (NIR) probe spectroscopy. When quasi-single-cycle THz pulses interact with $α'$-NaV$_2$O$_5$ in its low-temperature, dimerized charge-ordered phase, they induce coherent oscillations in the time domain at the zone-folded Raman-active phonon frequency of 1.85 THz. By combining pump-probe measurements with lattice dynamics modeling based on equation-of-motion approach, we propose that these oscillations arise from a nonlinear coupling between Raman-active and infrared (IR)-active phonon modes, with the latter being resonantly excited by the THz pulses. In contrast, excitation with NIR femtosecond laser pulses does not produce measurable vibrational dynamics, highlighting the unique potential of THz-driven, nonlinear light-matter interactions for the coherent and selective control of structural dynamics in quantum materials.

cond-mat.mtrl-sci↗

arXiv2025

Investigating Josephson plasmons in layered cuprates via nonlinear terahertz spectroscopy

Josephson plasmons in layered superconductors represent a natural source of optical non-linearity, thanks to their intrinsically anharmonic nature. Here we derive the selection rules behind non-linear plasmonics showing its dependence on plasmonic branches hidden to other spectroscopies, like RIXS. We benchmark our results for the case of layered cuprates, showing how in a layered system the combined effect of plasmon dispersion and light polarization can move the resonance of the bilayer system away from the plasma edge measured in linear spectroscopy. Our results demonstrate the dependence of the non-linear THz response on the convoluted plasmon dispersion in a momentum region complementary to RIXS, and offer a possible perspective for the generation of THz pulses by artificially designed Josephson heterostructures.

cond-mat.supr-con↗

arXiv2024

Phonon-Polaritons in Non-Centrosymmetric Systems: Theory of Terahertz Pump-Optical Probe Spectroscopy

Hybrid lattice-light modes, known as phonon-polaritons, represent the backbone of advanced protocols based on THz pumping of infrared modes. Here we provide a theoretical framework able to capture the different roles played by phonon-polaritons in experimental protocols based either on Raman-like pump and probe schemes, typical of four-wave-mixing processes, or on THz pump-visible probe three-wave mixing protocols. By using a many-body description of the nonlinear optical kernel, along with a perturbative solution of nonlinear Maxwell's equations, we highlight the advantages of exploiting broadband THz pumps to enlarge the phase space of the phonon-polariton dispersion accessible in a single experiment. Besides providing a quantitative description of existing and future experiments, our results offer a general framework for the theoretical modeling of the hybridization between light and lattice degrees of freedom in time-resolved experiments.

cond-mat.mtrl-sci↗

arXiv2024

Terahertz ionic Kerr effect: Two-phonon contribution to the nonlinear optical response in insulators

The THz Kerr effect measures the birefringence induced in an otherwise isotropic material by a strong THz pulse driving the Raman-active excitations of the systems. Here we provide experimental evidence of a sizable Kerr response in insulating SrTiO3 due to infrared-active lattice vibrations. Such a signal, named ionic Kerr effect, is associated with the simultaneous excitation of multiple phonons. Thanks to a theoretical modeling of the time, polarization and temperature dependence of the birefringence we can disentangle the ionic Kerr effect from the off-resonant electronic excitations, providing an alternative tunable mechanism to modulate the refractive index on ultrashort time-scales via infra-red active phonons.

cond-mat.mtrl-sci↗

arXiv2024

Revealing novel aspects of light-matter coupling in terahertz two-dimensional coherent spectroscopy: the case of the amplitude mode in superconductors

Recently developed terahertz (THz) two-dimensional coherent spectroscopy (2DCS) is a powerful technique to obtain materials information in a fashion qualitatively different from other spectroscopies. Here, we utilized THz 2DCS to investigate the THz nonlinear response of conventional superconductor NbN. Using broad-band THz pulses as light sources, we observed a third-order nonlinear signal whose spectral components are peaked at twice the superconducting gap energy $2Δ$. With narrow-band THz pulses, a THz nonlinear signal was identified at the driving frequency $Ω$ and exhibited a resonant enhancement at temperature when $Ω= 2Δ$. General theoretical considerations show that such a resonance can only arise from a disorder-activated paramagnetic coupling between the light and the electronic current. This proves that the nonlinear THz response can access processes distinct from the diamagnetic Raman-like density fluctuations, which are believed to dominate the nonlinear response at optical frequencies in metals. Our numerical simulations reveal that even for a small amount of disorder, the $Ω=2Δ$ resonance is dominated by the superconducting amplitude mode over the entire investigated disorder range. This is in contrast to other resonances, whose amplitude-mode contribution depends on disorder. Our findings demonstrate the unique ability of THz 2DCS to explore collective excitations inaccessible in other spectroscopies.

cond-mat.supr-con↗

arXiv2023

Tracing the dynamics of superconducting order via transient third harmonic generation

Ultrafast optical control of quantum systems is an emerging field of physics. In particular, the possibility of light-driven superconductivity with ultrashort laser pulses has attracted much of attention. To identify non-equilibrium superconductivity, it is necessary to measure fingerprints of superconductivity on ultrafast timescales. Recently non-linear THz third harmonic generation (THG) was shown to directly probe the collective degrees of freedoms of the superconducting condensate including particularly the Higgs mode. Here we extend this idea to light-driven non-equilibrium states in superconducting La2-xSrxCuO4 establishing a protocol to access the transient superconducting (SC) order-parameter fluctuations. We perform an optical pump-THz-THG drive experiment and use a two-dimensional spectroscopy approach to disentangle the driven third-harmonic response of optically excited quasiparticles and the pure condensate response. In this way, 2D spectroscopy separately probes both the ultrafast pair breaking dynamics and transient pairing amplitude of the condensate.

cond-mat.supr-con↗

arXiv2022

Contribution of Collective Excitations to Third Harmonic Generation in Multiband Superconductors: the case of MgB$_2$

Multiband superconductors can host collective excitations with marked differences with respect to their single-band counterpart. We first study the spectrum of collective amplitude fluctuations in a clean two-bands superconductor, showing that the spectral weight of the Higgs mode rapidly deviates from the naive extension of the single band case as the interband coupling is turned on. These results are then used to critically analyze the non-linear optical response in MgB$_2$, providing an explanation for the apparently contradictory results of recent experiments, pointing towards a selective relevance either of the Leggett mode or of the amplitude fluctuations at twice the lower gap. By using exact numerical simulations and realistic estimate of disorder we compute the relative contribution of the quasiparticle, amplitude and phase fluctuations to the non-linear optical response. We show that at low pumping frequency only the resonance at twice the smaller gap emerges, as due to the BCS response, while the Leggett mode dominates only in a narrow range of higher pumping frequencies matching its low-temperature value. Our findings provide a fresh perspective on the potential of non-linear THz spectroscopy to detect collective modes in other multiband systems, as e.g. iron-based superconductors.

cond-mat.supr-con↗

arXiv2022

Terahertz displacive excitation of a coherent Raman-active phonon in V$_2$O$_3$

Nonlinear processes involving frequency-mixing of light fields set the basis for ultrafast coherent spectroscopy of collective modes in solids. In certain semimetals and semiconductors, generation of coherent phonon modes can occur by a displacive force on the lattice at the difference-frequency mixing of a laser pulse excitation on the electronic system. Here, as a low-frequency counterpart of this process, we demonstrate that coherent phonon excitations can be induced by the sum-frequency components of an intense terahertz light field, coupled to intraband electronic transitions. This nonlinear process leads to charge-coupled coherent dynamics of Raman-active phonon modes in the strongly correlated metal V$_2$O$_3$. Our results show a new up-conversion pathway for the optical control of Raman-active modes in solids mediated by terahertz-driven electronic excitation.

cond-mat.str-el↗

arXiv2022

THz non-linear optical response in cuprates: predominance of the BCS response over the Higgs mode

Recent experiments with strong THz fields in unconventional cuprates superconductors have clearly evidenced an increase of the non-linear optical response below the superconducting critical temperature Tc. As in the case of conventional superconductors, a theoretical estimate of the various effects contributing to the non-linear response is needed in order to interpret the experimental findings. Here we report a detailed quantitative analysis of the non-linear THz optical kernel in cuprates within a realistic model, accounting for the band structure and disorder level appropriate for these systems. We show that the BCS quasiparticle response is the dominant contribution for cuprates, and its polarization dependence accounts very well for the third-harmonic generation measurements. On the other hand, the polarization dependence of the THz Kerr effect is only partly captured by our calculations, suggesting the presence of additional effects when the system is probed using light pulses with different central frequencies.

cond-mat.supr-con↗

arXiv2020

Non-linear Terahertz Driving of Plasma Waves in Layered Cuprates

The hallmark of superconductivity is the rigidity of the quantum-mechanical phase of electrons, responsible for superfluid behavior and Meissner effect. The strength of the phase stiffness is set by the Josephson coupling, which is strongly anisotropic in layered superconducting cuprates. So far, THz light pulses have been efficiently used to achieve non-linear control of the out-of-plane Josephson plasma mode, whose frequency scale lies in the THz range. However, the high-energy in-plane plasma mode has been assumed to be insensitive to THz pumping. Here, we show that THz driving of both low-frequency and high-frequency plasma waves is possible via a general two-plasmon excitation mechanism. The anisotropy of the Josephson couplings leads to marked differences in the thermal effects among the out-of-plane and in-plane response, consistently with the experiments. Our results link the observed survival of the in-plane THz non-linear driving above $T_c$ to enhanced fluctuating effects in the phase stiffness in cuprates, paving the way to THz impulsive control of phase rigidity in unconventional superconductors.

cond-mat.supr-con↗

arXiv2019

Theory of coherent oscillations detection in THz pump-probe spectroscopy: from phonons to electronic collective modes

Time-resolved spectroscopies using intense THz pulses appear as a promising tool to address collective electronic excitations in condensed matter. In particular recent experiments showed the possibility to selectively excite collective modes emerging across a phase transition, as it is the case for superconducting and charge-density-wave (CDW) systems. One possible signature of these excitations is the emergence of coherent oscillations of the differential probe field in pump-probe protocols. While the analogy with the case of phonon modes suggests that the basic underlying mechanism should be a sum-frequency stimulated Raman process, a general theoretical scheme able to describe the experiments and to define the relevant optical quantity is still lacking. Here we provide this scheme by showing that coherent oscillations as a function of the pump-probe time delay can be linked to the convolution in the frequency domain between the squared pump field and a Raman-like non-linear optical kernel. This approach is applied and discussed in few paradigmatic examples: ordinary phonons in an insulator, and collective charge and Higgs fluctuations across a superconducting and a CDW transition. Our results not only account very well for the existing experimental data in a wide variety of systems, but they also offer an useful perspective to design future experiments in emerging materials.

cond-mat.str-el↗

arXiv2019

Raman response in the nematic phase of FeSe

Raman experiments on bulk FeSe revealed that the low-frequency part of $B_{1g}$ Raman response $R_{B_{1g}}$, which probes nematic fluctuations, rapidly decreases below the nematic transition at $T_n \sim 85$K. Such behavior is usually associated with the gap opening and at a first glance is inconsistent with the fact that FeSe remains a metal below $T_n$, with sizable hole and electron pockets. We argue that the drop of $R_{B_{1g}}$ in a nematic metal comes about because the nematic order drastically changes the orbital content of the pockets and makes them nearly mono-orbital. In this situation $B_{1g}$ Raman response gets reduced by the same vertex corrections that enforce charge conservation. The reduction holds at low frequencies and gives rise to gap-like behavior of $R_{B_{1g}}$, in full agreement with the experimental data.

cond-mat.str-el↗