Sum-frequency ionic Raman scattering

Juraschek, Dominik M.; Maehrlein, Sebastian F.

doi:10.1103/physrevb.97.174302

Cited by 78 publications

(71 citation statements)

References 61 publications

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“…This process allows the coherent excitation of both Raman-active and silent phonon modes in contrast to conventional difference and sum-frequency ionic Raman scattering that only allow coupling of Raman-active to IR-active phonons. The energy conversion efficiency of the parametric excitation thereby outperforms ionic Raman scattering, in which only a fraction of the energy is transferred to other modes [41,[66][67][68], and is at par with predictions for the transfer of energy in the resonant phonon upconversion process sum-frequency ionic Raman scattering [41,42,69].…”

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confidence: 60%

“…, the lowest-order symmetry-allowed nonlinear phonon coupling between the Higgs and Goldstone modes is of the form Q H Q 2 G . A phonon coupling squared in the IR-active mode and linear in the other one that is required for ionic Raman scattering [38,41] is forbidden by symmetry for the silent B 1 mode here. The potential energy of the phonons can therefore be written in a minimal model as…”

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confidence: 95%

“…We model the electric field component of the terahertz pulse as E(t) = E 0 exp(−t 2 /(2(τ / √ 8ln2) 2 )) cos(ω 0 t), where E 0 is the peak electric field, ω 0 is the center frequency, and τ is the full width at half maximum duration of the pulse [41]. In order to couple to the dipole moment of the Higgs mode, the electric field component has to be aligned with the hexagonal axis of the crystal.…”

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Parametric Excitation of an Optically Silent Goldstone-Like Phonon Mode

2020

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It has recently been indicated that the hexagonal manganites exhibit Higgs-and Goldstone-like phonon modes that modulate the amplitude and phase of their primary order parameter. Here, we describe a mechanism by which a silent Goldstone-like phonon mode can be coherently excited, which is based on nonlinear coupling to an infrared-active Higgs-like phonon mode. Using a combination of first-principles calculations and phenomenological modeling, we describe the coupled Higgs-Goldstone dynamics in response to the excitation with a terahertz pulse. Besides theoretically demonstrating coherent control of crystallographic Higgs and Goldstone excitations, we show that the previously inaccessible silent phonon modes can be excited coherently with this mechanism.Order parameters are physical observables that are used to quantify the different states of matter. Their amplitudes and phases can be excited by external stimuli, such as a laser pulse, leading to exotic states of matter that cannot be accessed in equilibrium [1]. Two particular excitations are Higgs and Goldstone modes, which correspond to the modulation of the amplitude and phase of an order parameter that breaks a continuous symmetry. Originally discussed in the context of particle physics [2] and superconductivity [3], Higgs and Goldstone excitations were found in cold atom systems, such as superfluids [4][5][6][7][8] or supersolids [9]. Higgs and Goldstone modes are well-studied in superconductors today [10][11][12][13][14][15][16][17][18][19][20][21], and similar manifestations have recently been reported in charge density wave (CDW) systems [22][23][24], antiferromagnets [25][26][27], and excitonic insulators [28]. It has been debated whether the optical and acoustic vibrational modes of solids can be considered Higgs and Goldstone excitations of the crystal lattice [29]. Several complex transition metal oxide compounds have shown signatures of optical Goldstone-like phonon modes that live in their symmetry-broken potential energy landscape [30][31][32][33][34].Coherent control over Raman-active phonons via impulsive stimulated Raman scattering using visible light pulses is well-established [35,36]. Recently, the excitation of infrared (IR)-active phonons with large amplitudes via IR absorption has become feasible through the development of high intensity terahertz and mid-IR sources. This progress has enabled selective control over the dynamics of the crystal lattice through nonlinear phonon interactions using ionic Raman scattering [37-39] and two-particle absorption mechanisms [40][41][42][43][44][45]. In contrast, phonon modes that do not respond to IR absorption or Raman scattering techniques, so called silent modes, can only be detected through hyper-Raman scattering. As hyper-Raman scattering is a third-order interaction of the electric field component of light with a * djuraschek@seas.harvard.edu † prineha@seas.harvard.edu phonon mode, it is inefficient and no coherent excitation has been achieved yet. Higgs and Goldstone modes, similar to silent p...

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Parametric Excitation of an Optically Silent Goldstone-Like Phonon Mode

2020

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“…In this THz pump-THz probe experiment, the relative change δE probe (t pp ) in the transmitted probe field with and without the pump has been explicitly computed and compared to the experiments. This allowed the authors to link the observed oscillations to the Leggett mode, which contributes in MgB 2 to the Raman kernel, and to show that the basic underlying excitation mechanism is a SFP, in full correspondence with the case of phonons 12,17 .…”

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confidence: 92%

Theory of coherent-oscillations generation in terahertz pump-probe spectroscopy: From phonons to electronic collective modes

2019

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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.In the last decade, a significant advance in the investigation of complex systems has been gained thanks to the huge experimental progress in time-resolved spectroscopic techniques 1,2 . On very general grounds, the basic idea behind any pump-probe protocol is to first excite the system with a short and very intense electromagnetic pulse (pump), and then to monitor its relaxation towards equilibrium by using a secondary, weak pulse (probe) applied with a finite time delay with respect to the pump. This general protocol can then be implemented in several different ways, according to the nature of the spectroscopic measurement (angle-resolved photoemission, optical reflection or transmission, etc.) or to the wavelength of the pump/probe fields. However, in all cases one has to face with two phenomena which mark the difference with respect to ordinary equilibrium spectroscopies: (i) the use of an intense pulse triggers in general non-linear optical processes; (ii) the subsequent relaxation encodes by definition non-equilibrium phenomena on time scales which depend on the characteristics of the experiment and of the system under investigation. Due in part to these innovative aspects, many pump-probe protocols still lack a general theoretical framework able to connect the measured quantities to the material properties. More specifically, while Kubo linear-response theory 3 represents nowadays the standard theoretical tool needed to compute the optical response of any system to a weak external perturbation, an analogous protocol for timeresolved spectroscopies has not been established yet.The present work aims at filling in part this knowl- * mat...

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“…3a) reveals two strong IR-active phonons along the a-axis (probe direction) with resonant peaks at ω 1,2 IR ∼1.5 and 2.5THz in the 4.1K trace (red line). These IR phonon modes can contribute to the the Raman A 1g phonon generation via the ionic Raman mechanism that involves the IR modes as mediator and IR-Raman coupling due to anharmonicity [16]. At elevated temperatures, these modes progressively shift to higher frequencies up to 200K (magenta line).…”

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Light-Driven Raman Coherence as a Nonthermal Route to Ultrafast Topology Switching in a Dirac Semimetal

et al. 2020

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A grand challenge underlies the entire field of topology-enabled quantum logic and information science: how to establish topological control principles driven by quantum coherence and understand the time-dependence of such periodic driving? Here we demonstrate a THz pulse-induced phase transition in Dirac materials that is periodically driven by vibrational coherence due to excitation of the lowest Raman-active mode. Above a critical field threshold, there emerges a longlived metastable phase with unique Raman coherent phonon-assisted switching dynamics, absent for optical pumping. The switching also manifest itself by non-thermal spectral shape, relaxation slowing down near the Lifshitz transition where the critical Dirac point (DP) occurs, and diminishing signals at the same temperature that the Berry curvature induced Anomalous Hall Effect varnishes. These results, together with first-principles modeling, identify a mode-selective Raman coupling that drives the system from strong to weak topological insulators, STI to WTI, with a Dirac semimetal phase established at a critical atomic displacement controlled by the phonon pumping. Harnessing of vibrational coherence can be extended to steer symmetry-breaking transitions, i.e., Dirac to Weyl ones, with implications on THz topological quantum gate and error correction applications.

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Sum-frequency ionic Raman scattering

Cited by 78 publications

References 61 publications

Parametric Excitation of an Optically Silent Goldstone-Like Phonon Mode

Parametric Excitation of an Optically Silent Goldstone-Like Phonon Mode

Theory of coherent-oscillations generation in terahertz pump-probe spectroscopy: From phonons to electronic collective modes

Light-Driven Raman Coherence as a Nonthermal Route to Ultrafast Topology Switching in a Dirac Semimetal

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