Distinguishing Nonlinear Terahertz Excitation Pathways with Two-Dimensional Spectroscopy

Johnson, Courtney E.; Knighton, Brittany E.; Johnson, Jeremy A.

doi:10.1103/physrevlett.122.073901

Cited by 88 publications

(50 citation statements)

References 46 publications

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“…The first one is the case of Raman-active phonons. We show the analogies and differences between our scheme and the approach based on the equation of motions for the displacement phononic field, and we recast in our language some interesting recent results obtained for THz-stimulated phononic oscillations in insulators 12,13,17 .…”

Section: Discussionmentioning

confidence: 92%

“…As a consequence a pump in the THz, with a typical duration of fractions of picoseconds, is about one or two orders of magnitude more efficient than a visible pulse, with a typical duration of tens of femtoseconds. It is also worth mentioning that a possible alternative mechanism to the one discussed here to induce coherent phonon oscillations is via the so-called ionic Raman scattering 13,[47][48][49][50] . In this case the pump electric field excites two infra-red active optical phonons, that can in turn couple and transfer energy to a Raman-active one, thanks to the presence of anharmonic (trilinear) phononic interactions.…”

Section: Pump-probe Spectroscopy Of Phononsmentioning

confidence: 94%

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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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Section: Discussionmentioning

confidence: 92%

Section: Pump-probe Spectroscopy Of Phononsmentioning

confidence: 94%

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

2019

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“…urrently, there is much interest in exploring nonperturbative nonlinear optical phenomena in various types of matter driven by intense terahertz (THz) electric and magnetic fields [1][2][3][4][5][6][7][8][9][10] . This is fueled by the impressive advancements made during the last two decades in the development of femtosecond laser-based table-top sources for strong THz radiation [11][12][13] .…”

mentioning

confidence: 99%

Terahertz strong-field physics in light-emitting diodes for terahertz detection and imaging

Ouyang

et al. 2021

Commun Phys

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Intense terahertz (THz) electromagnetic fields have been utilized to reveal a variety of extremely nonlinear optical effects in many materials through nonperturbative driving of elementary and collective excitations. However, such nonlinear photoresponses have not yet been obeserved in light-emitting diodes (LEDs), let alone employing them as fast, cost-effective, compact, and room-temperature-operating THz detectors and cameras. Here, we report ubiquitously available LEDs exhibiting photovoltaic signals of ~0.8 V and ~2 ns response time with signal-to-noise ratios of ~1300 when being illuminated by THz field strengths ~240 kV/cm. We also demonstrated THz-LED detectors and camera prototypes. These unorthodox THz detectors exhibited high responsivities (>1 kV/W) with response time four orders of magnitude shorter than those of pyroelectric detectors. The mechanism was attributed to THz-field-induced impact ionization and Schottky contact. These findings not only help deepen our understanding of strong THz field-matter interactions but also contribute to the applications of strong-field THz diagnosis.

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“…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][38][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.…”

mentioning

confidence: 99%

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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Distinguishing Nonlinear Terahertz Excitation Pathways with Two-Dimensional Spectroscopy

Cited by 88 publications

References 46 publications

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

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

Terahertz strong-field physics in light-emitting diodes for terahertz detection and imaging

Parametric Excitation of an Optically Silent Goldstone-Like Phonon Mode

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