Coherent lattice dynamics in opaque crystals: Testing the adequacy of two-tensor model

Misochko, O. V.; Лебедев, М. В.

doi:10.1103/physrevb.94.184307

Cited by 9 publications

(8 citation statements)

References 49 publications

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“…The intensity of the A 2 1g mode is stronger than that of the A 1 1g mode and decays with a time constant of τ A2 ≈1.13 ps (obtained from an exponential fit), whereas the A 1 1g mode decays more slowly with a time constant of τ A1 ≈2.79 ps. The lifetime of the A 2 1g mode is much shorter than the A 1 1g mode, a result also found in previous studies on Bi 2 Te 3 28,35,36 , Sb 2 Te 3 33 , and Bi 2 Se 3 37,38 . In our experiment, however, a new aspect of coherent phonon generation is uncovered; the two modes reach a maximum intensity with different time constants and the time lag (∆τ ) between the two maximums of the A 2 1g and A 1 1g modes was found to be ≈0.5 ps, as indicated in Figs.…”

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confidence: 86%

“…1(a) consists of oscillatory components with a non-oscillatory background. The oscillatory components are attributed to coherent phonon modes while the non-oscillatory component is related to the excitation and relaxation of nonequilibrium electrons and the subsequent lattice heating 28 . To obtain the oscillatory signal, the non-oscillatory components were subtracted from the transient reflectivity signal by fitting the data to a linear combination of exponentially decaying functions 28 , as shown by the dashed line in Fig.…”

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confidence: 99%

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Coherent Dirac plasmons in topological insulators

et al. 2018

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We explore the ultrafast reflectivity response from photo-generated coupled phonon-surface Dirac plasmons in Sb2Te3 topological insulators several quintuple layers thick. The transient coherent phonon spectra obtained at different time frames exhibit a Fano-like asymmetric line shape of the A 2 1g mode, which is attributed to quantum interference between continuum-like coherent Diracplasmons and phonons. By analyzing the time-dependent asymmetric line shape using the twotemperature model (TTM), it was determined that a Fano-like resonance persisted up to ≈1 ps after photo-excitation with a relaxation profile dominated by Gaussian decay at ≤200 fs. The asymmetry parameter could be well described by the TTM for ≥200 fs, therefore suggesting the coherence time of the Dirac plasmon is ≈200 fs. PACS numbers: 78.47.jg, 63.20.kd Coherent states in condensed media are quantum mechanically described in terms of the annihilation operator, as described by Glauber 1,2 . The concept of coherent states has succeeded in producing the laser 3 . In the last two decades, moreover, the importance of coherent states in solid state physics has greatly increased and new physical effects, such as Bose-Einstein condensation have been discovered 4 . In addition, spin relaxation in quantum spin Hall (QSH) systems 5 has been often studied coupled with the recent discovery of topological insulators 5,6 , and the loss of coherence exhibited by its Gaussian relaxation behavior 7,8 , a characteristic significantly different from exponential relaxation. Thus, Gaussian relaxation is often referred to as a signature of a coherent state.A topological insulator (TI) 6 is a quantum electronic material, which is characterized by an insulating gap in the bulk, while gapless surface states (SSs) exist at the interface with the vacuum or other dielectric materials. The metallic surface states are characterized by massless Dirac quasiparticles, whose scattering is prohibited by time reversal symmetry 9 . Exploiting the birth and decay of quasiparticles on the surface of TIs provides a novel paradigm for future application of TIs to quantum computation 10 , spin electronics 11 , and optical devices 12 . The dynamics of the quasiparticles on a TI surface, however, have been exclusively investigated by means of time-and angle-resolved photoemission spectroscopy under vacuum conditions and only limited information has been obtained 13 .Recently Dirac plasmons have been observed on the surface of TIs in the form of a polariton wave using metamaterials (MMs) 14 . Without the help of MMs, in general, optical techniques are unable to characterize the dynamics of Dirac plasmon-polaritons on the surface of TIs, since momentum conservation requires large wavevectors for the plasmon wave to couple to a photon. Instead of using MMs, one can excite Dirac plasmons by direct coupling using a ≈1.5 eV photon to access the TI surface states 15 . In this alternative case, a film thinner than ∼15 nm for a Bi 2 Se 3 TI is required. Under the conditions that both the Dirac ...

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Coherent Dirac plasmons in topological insulators

et al. 2018

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“…In the case of Sb, a good agreement has been reported for the excitation wavelength λ = 815nm [9]. In the case of Bi 2 Te 3 , on the other hand, a large discrepancy between the ratio has been reported [37]. One of the possible origin is the difference in the relaxation processes during and after the irradiation of pump-pulse [38].…”

Section: Resultsmentioning

confidence: 90%

Dynamic Jahn-Teller viewpoint for generation mechanism of asymmetric modes of coherent phonons

Kayanuma

Nakamura

2017

Phys. Rev. B

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We propose a dynamic Jahn-Teller approach to elucidate the generation mechanism of lowsymmetry modes of coherent phonons induced in crystals by irradiation with a short optical pulse in the opaque energy region. This is a natural extension of the impulsive excitation model of symmetric modes to multi dimensions in the configuration coordinate space. We show that the two generation mechanisms of coherent phonons coexist in this case; namely the impulsive absorption (IA) mechanism and impulsive stimulated Raman scattering (ISRS) mechanism. The dependence of the phonon amplitude on the polarization of the pump-pulse is exactly the same in IA and ISRS processes, and is in agreement with the prediction of the argument based on Raman tensors. The dependence of the excitation efficiency of the coherent phonons on the frequency of the pump-pulse is calculated using a simplified model of the optical response function of the crystal. Generally, the IA mechanism predominates in the opaque region, although ISRS makes a comparable contribution to phonon generation in the near edge opaque region. The initial phase of the coherent phonon is always cosine-like in IA, but depends on the excitation frequency in ISRS.

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“…The similarity of the dependence of the coherent phonon amplitude and the absorption coefficient on photon en- ergy implies that the generation mechanism of the coherent phonon is governed by the so-called two tensor model. 7,17,21,22 As shown in Fig. 4, both the FT amplitude and the absorption coefficient are enhanced when the photon energy becomes larger.…”

Section: Enhancement Of Coherent Phonon Amplitude In Phase-change Matmentioning

confidence: 83%

“…11,19 Note that the ini- tial phase of the excited optical phonon is cosine-like, indicating the generation mechanism is either DECP or resonant ISRS. 20,21 The transient transmission ∆T/T observed in crystalline GST225 for various wavelengths is displayed in Fig. 3(a), together with the corresponding FT spectra in Fig.…”

Section: Enhancement Of Coherent Phonon Amplitude In Phase-change Matmentioning

confidence: 99%

Enhancement of coherent phonon amplitude in phase-change materials by near-infrared laser irradiation

Saito

Fons

Kolobov

et al. 2017

Applied Physics Letters

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We have examined the effect of pump-probe photon energy on the amplitude of coherent optical phonons in a prototypical phase change material using a femtosecond time-resolved transmission technique. The photon energy was varied between 0.8 and 1.0 eV (corresponding to the wavelength of 1550 and 1200 nm), a range over which there is significant optical contrast between the crystalline and amorphous phases of Ge 2 Sb 2 Te 5 (GST225). It was found that in crystalline GST225 the coherent phonon amplitude monotonically increases as the photon energy increases, indicating that the phonon amplitude is enhanced by interband optical absorption, which is associated with the imaginary part of the dielectric function. In amorphous GST225, in contrast, the coherent phonon amplitude does not depend on the photon energy, remaining almost constant over the tuning range. A possible contribution from the polarizability associated with resonant bonding nature of GST225 is discussed.Modern optical data storage technology was developed based upon phase change materials (PCMs), which exhibit a large optical contrast between the crystalline and amorphous states.1,2 Ge 2 Sb 2 Te 5 (GST225) is one of the best-performing chalcogenide alloys and has been utilized as the recording material in Digital Versatile Disc Random Access Memory (DVD-RAM) since it exhibits a large optical contrast, typically more than ≈ 20 %.2,3 The optical properties of DVD-RAM have been extensively investigated experimentally and theoretically, where optical absorption occurs from the visible to the nearinfrared wavelength region due to the narrow band-gap energy range of 0.5 -0.75 eV.4 For crystalline GST225 the real part of the dielectric function ε 1 has a maximum at ≈ 0.7 eV, while the imaginary part ε 2 has a maximum at 1.2 eV.4,5 For the case of amorphous GST225, on the other hand, ε 1 has a maximum at ≈ 1.2 eV, while ε 2 has a maximum at 1.9 eV.4,5 For GST225, in the crystalline phase the real part ε 1 is usually associated with resonant-bonding and the imaginary part ε 2 is associated with interband photo-absorption.6 When the laser photon energy is tuned through these critical energies, the resonant excitation of coherent phonons via either resonant impulsive stimulated Raman scattering (ISRS) 7 or displacive excitation of coherent phonon (DECP) 8 is expected to occur. The photon energy dependence of the coherent phonon amplitude excited via either resonant ISRS or DECP can be described by the two-tensor model, which includes ε 1 and ε 2 . 7It is unusual to investigate the dependence of the amplitude of phonon spectra with laser photon energy; typically the frequency shift has been studied as a function of pump fluence. [9][10][11] In this paper, we have investigated the

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Coherent lattice dynamics in opaque crystals: Testing the adequacy of two-tensor model

Cited by 9 publications

References 49 publications

Coherent Dirac plasmons in topological insulators

Coherent Dirac plasmons in topological insulators

Dynamic Jahn-Teller viewpoint for generation mechanism of asymmetric modes of coherent phonons

Enhancement of coherent phonon amplitude in phase-change materials by near-infrared laser irradiation

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