Coherent Optical Phonons Research Articles

Generation of a coherent longitudinal optical phonon plasmon coupled mode in an n-type InSb single crystal in the absence and/or strong suppression of the photo-Dember effect confirmed with the use of terahertz time-domain spectroscopy

We explored the feasibility of generating a coherent longitudinal optical (LO) phonon–plasmon coupled mode in an n-type InSb single crystal with the condition that the photo-Dember effect is strongly suppressed and/or eliminated. We systematically performed terahertz time-domain spectroscopic measurements of the n-type InSb single crystal and undoped InSb crystal. The terahertz electromagnetic wave originating from the photo-Dember effect was extinguished in the n-type InSb single crystal, while the clear signal from the photo-Dember effect was detected in the undoped InSb single crystal. Meanwhile we detected the clear signal of the terahertz wave from the coherent LO-phonon–plasmon coupled mode in the n-type InSb single crystal, indicating that the photo-Dember effect is not dominant in the generation of the coherent LO-phonon–plasmon coupled mode although the photo-Dember effect has been attributed to the dominant generation mechanism of the coherent LO-phonon–plasmon coupled mode so far. We explain the present phenomenon, taking account of the fact that the impulsive stimulated Raman scattering process has a possibility of generating coherent LO-phonon–plasmon coupled modes in light-absorbing solids [Nakamura et al., Phys. Rev. B 99, 180301(R) (2019)].

Intercalation-enhanced topological material Bi2Se3: Tuning charge carrier and phonon dynamics for advanced optoelectronic and terahertz applications

The field of topological materials (TMs) is experiencing significant advancements due to their unique surface states, which offer considerable potential for optoelectronics and terahertz (THz) applications. Intercalation in TMs can effectively modulate these surface states, thereby altering electronic and optical properties and enhancing functionalities. This approach enables the tailoring of material properties for specific applications, broadening the utility of TMs across various technological domains. This study focuses on understanding the charge carrier and phonon dynamics in bismuth selenide (Bi2Se3) intercalated with various metals (AxBi2Se3, where A = Ag, Cr, Ni, Mn, Sm, Zn). We examine carrier trapping, carrier relaxation through optical and acoustic phonons, charge recombination processes, and variations in carrier temperature with probe delay lifetimes. Additionally, we explore coherent optical phonons (COPs) in MIBS, which oscillate at THz frequencies and have potential applications in THz generation and detection. Achieving tunability in the THz frequency of COP modes is a significant challenge; however, our research establishes a correlation between optical and structural properties and the dependence of COP frequencies on effective metal intercalation. This comprehensive investigation elucidates the intricate interplay between surface carriers and phonon dynamics in intercalated TMs, highlighting promising advancements for diverse technological applications, including spintronics, optoelectronics, and THz technology.

Anisotropic phonon dynamics in Dirac semimetal PtTe2 thin films enabled by helicity-dependent ultrafast light excitation

Coherent phonons have aroused considerable attention in condensed matter physics owing to their extraordinary capacity of reflecting and controlling the physical properties of matter. However, the investigation on the interaction between coherent phonons and other microscopic particles on the ultrafast timescale within topological systems continues to be an active and unresolved area. Here, we show the energy transfer of coherent optical phonons (COP) in Dirac semimetal PtTe2 thin films using ultrafast optical pump-probe spectroscopy. Specifically, the helicity-dependent light-driven anisotropic COP signals disclose their direct connection with the light-excited anisotropic spin-polarized electrons via an angular momentum transfer. Furthermore, we observe the notable decreases in the COP oscillation frequency and the decay rate with increasing temperatures due to the anharmonic phonon-phonon scattering and electron-phonon scattering in the COP dissipation process, respectively. Our work paves the way for uncovering the coherent phonons in Dirac semimetals for the potential applications in optoelectronics and opto-spintronics.

Coherent phonon signal amplification technique based on a numerical differentiation method

Coherent optical phonon (COP) and coherent acoustic phonon (CAP) play vital roles in probing ultrafast dynamics across various materials and systems. Their applications range from metamaterial diagnosis to manipulating ferroelectric polarization and self-trapping of carriers in perovskites. However, the inherently small signals associated with COP and CAP pose challenges in extracting valuable information. In this paper, we address this challenge by presenting a numerical differential method for signal amplification and deduction of phonon dephasing time. In addition, application of this method has been demonstrated in a superlattice system, establishing the validity and effectiveness of this technique.

Coherent phonon and unconventional carriers in the magnetic kagome metal Fe3Sn2

Temperature- and fluence-dependent carrier dynamics of the magnetic kagome metal Fe3Sn2 were studied using the ultrafast optical pump-probe technique. Two carrier relaxation processes and a laser-induced coherent optical phonon were observed. We ascribe the shorter relaxation (~1 ps) to hot electrons transferring their energy to the crystal lattice via electron–phonon scattering. The second relaxation (~30 ps), on the other hand, cannot be explained as a conventional process, and we attributed it to the unconventional (localized) carriers in the material. The observed coherent oscillation is assigned to be a totally symmetric A1g optical phonon dominated by Sn displacements out of the kagome planes and possesses a prominently large amplitude, on the order of 10−3, comparable to the maximum of the reflectivity change (ΔR/R). This amplitude is similar to what has been observed for coherent phonons in charge-density-wave (CDW) systems, although no signs of such instability were hitherto reported in Fe3Sn2. Our results suggest an unexpected connection between Fe3Sn2 and kagome metals with CDW instabilities and a strong interplay between phonon and electron dynamics in this compound.

Terahertz sum-frequency excitation of coherent optical phonons in the two-dimensional semiconductor WSe2

Driving fundamental excitations via strong light fields is one of the most important issues in solid state physics, which opens up new avenues to control material properties. Two-dimensional materials are fruitful platforms for future semiconductor applications, including opto-electronic and phononic devices, yet the phonon dynamics and nonlinear phonon–phonon coupling remain under-explored. Here, we demonstrate coherent phonon excitation in thin films of the layered two-dimensional semiconductor WSe2 induced by intense and broadband ultrafast terahertz (THz) pulses. We performed THz Kerr effect spectroscopy and observed coherent phonon oscillations assigned to the E2g optical phonon mode. The phonon amplitude displays a quadratic THz field strength dependence, indicating a sum-frequency THz excitation process. Furthermore, pump–probe polarization and crystal orientation relationships, supported by symmetry analysis of the nonlinear susceptibility and Raman tensors, provide helpful insight into nonlinear phonon–phonon interactions and potential coherent control schemes for the manipulation of phonon polarization and material properties in WSe2.

Uncovering White Light Emission in Pressure‐Treated Sb‐Doped Double Perovskite by Coherent Optical Phonon

AbstractSingle‐component white‐light emitters are ideal for solid‐state lighting applications. Here, the dual‐emission white light in the 3D Cs2NaInCl6:0.1 Sb3+ crystal is successfully achieved by the high‐pressure treatment strategy. Upon compression, the remarkable self‐trapped exciton (STE) emission enhancement is observed at 20.6 GPa due to the tilting and twisting of inorganic octahedron caused by the phase transition of Cs2NaInCl6:0.1 Sb3+ crystal. Intriguingly, after undergoing the high‐pressure treatment, in addition to the blue high‐energy emission from the STEs, [SbCl6]3− exhibits the orange low‐energy emission from STEs, which is ascribed to the remaining local distortion of inorganic octahedron resulting from the presence of residual amorphous phase in the local region upon decompression. Furthermore, coherent phonon generation and density functional theory calculations indicate that the electrons exhibit a strong coupling to A1g stretch mode and T2gIn bending mode in Cs2NaInCl6:0.1 Sb3+. After high‐pressure treatment, the coupling between the electrons and the T2gIn bending mode disappears, suggesting local octahedral symmetry disruption due to residual amorphous structures induced by high‐pressure treatment. This work unveils the relationship between the electron‐phonon coupling and the STE emission and offers a new strategy to design and synthesize the new single‐component white‐light metal halide perovskites.

Enhanced optical conductivity and many-body effects in strongly-driven photo-excited semi-metallic graphite

The excitation of quasi-particles near the extrema of the electronic band structure is a gateway to electronic phase transitions in condensed matter. In a many-body system, quasi-particle dynamics are strongly influenced by the electronic single-particle structure and have been extensively studied in the weak optical excitation regime. Yet, under strong optical excitation, where light fields coherently drive carriers, the dynamics of many-body interactions that can lead to new quantum phases remain largely unresolved. Here, we induce such a highly non-equilibrium many-body state through strong optical excitation of charge carriers near the van Hove singularity in graphite. We investigate the system’s evolution into a strongly-driven photo-excited state with attosecond soft X-ray core-level spectroscopy. We find an enhancement of the optical conductivity of nearly ten times the quantum conductivity and pinpoint it to carrier excitations in flat bands. This interaction regime is robust against carrier-carrier interaction with coherent optical phonons acting as an attractive force reminiscent of superconductivity. The strongly-driven non-equilibrium state is markedly different from the single-particle structure and macroscopic conductivity and is a consequence of the non-adiabatic many-body state.

Intrapulse in situ Raman probing of electron, phonon and structural dynamics in synthetic diamond excited by ultrashort laser pulses: Insights into atomistic structural damage

Synthetic Ib-diamond plate with a minor concentration of substitutional atomic nitrogen C-centers was exposed in its bulk by tightly focused 0.2-ps, 525-nm laser pulses, coming at 80-MHz repetition rate and nJ-level variable pulse energies. Stokes first-order Raman scattering line related to 1332-cm−1 optical phonon was observed in the transmitted laser spectra and its pulse energy-dependent intensity, position wavenumber and half-width were analyzed. The Raman intensity, along with the second-order Raman band (2200–2700 cm−1), demonstrated quadratic dependence on pulse energy due to three-photon inter-band absorption, transforming at higher energies into a linear dependence for saturated absorption of opaque dense electron-hole plasma. Under these conditions, first-order Raman scattering was related to a non-resonant spontaneous process, enhanced by plasma emission of seeding coherent optical phonons. The Raman wavenumber decreased versus pulse energy via photo-injection of plasma and then slowly increased due to the “electronic” stress, until its final drop upon the onset of self-accelerated decay of optical phonons into acoustic ones (quasi-heating). The Raman half-width increased versus pulse energy due to the optical-phonon decay during the 0.2-ps laser pulse. An additional Stokes Raman band of radiation-induced defects was observed in the range of 900–1200 cm−1, exhibiting its yield scaling with generation of energetic Auger electrons. At the maximal pulse energies (stronger intra-pulse quasi-heating) all Raman lines disappeared, implying ultrafast structural disordering in the diamond without graphitization.

Stimulated Raman blockade in coherent phonon generation

We theoretically investigate the behavior of coherent optical phonons in an electron–phonon coupled system at moderate laser intensity: below GW/cm2. We calculate the coherent phonons from impulsive stimulated Raman scattering (ISRS) and impulsive absorption (IA) mechanisms with a nonperturbative model. The generation efficiency of coherent phonons in single-pulse excitation exhibits gradual nonlinear saturation with increasing optical intensity in a nonperturbative regime. The ISRS mechanism exhibits substantial saturation at relatively low pulse intensity, whereas the IA mechanism remains roughly linear. We further examine the influence of double-pulse excitation on quantum path interference. The ISRS mechanism exhibits a pronounced saturation effect near 0 fs of pump–pump delay, whereas the IA process exhibits negligible saturation behavior. These results reproduce well experimental data obtained by double-pulse excitation and support our proposal that ISRS is the primary quantum path for the observed coherent phonons in gallium arsenide.

Ultrafast response of Ge2Sb2Te5 nanoparticles: The benefits of low energy amorphization switching with the same read/write speed of bulk memories

We investigate the ultrafast response of crystalline Ge2Sb2Te5 nanoparticles (NPs) below the phase transformation threshold fluence. The observed rapid change of the optical response and the presence of coherent optical phonons are consistent with the relaxation dynamics in bulk Ge2Sb2Te5 films and, more importantly, occur within the same ultrafast timescales. We conclude that the benefit of the lower energy consumption of memories based on Ge-Sb-Te (GST) NPs aggregates, demonstrated by Casarin et al. (2018), occurs with no disadvantage, as the read/write speed can be as fast as in bulk GSTs.

Coherent Phonon-Mediated Many-Body Interaction in Monolayer WSe2

Due to the strong Coulomb interaction, the optical and electrical properties of two-dimensional transition metal dichalcogenides (TMDCs) are greatly determined by the emergence of many-body complexes such as excitons or trions. To fully realize the potential functionalities of these atomically thin materials, a comprehensive understanding of their many-body interaction mechanism is essential. Here, using the advanced femtosecond two-dimensional electronic spectroscopy technique combined with broadband transient absorption spectroscopy, a strong electron-exciton coupling effect in monolayer WSe2 following the ultrafast photoexcitation is revealed. We demonstrate that such many-body complexes can be generated effectively through the band-edge optical excitation, with a ∼1.5 ps stabilization process. The coherent optical phonon plays a dominant role in this electron-exciton interaction, and the coherence of the electron (exciton)-phonon coupling can last for ∼4.5 ps. This finding offers new insight into the formation mechanism of photoinduced many-body complexes in TMDCs.

Coherent-Phonon-Driven Hot-Carrier Effect in a Superlattice Solar Cell

Carrier thermalization in a superlattice solar cell made of polar semiconductors is studied theoretically by considering a minimal model where electron-phonon scattering is the principal channel of carrier energy loss. Importantly, the effect of an intrinsic quantum mechanical property; the phonon coherence, on carrier thermalization is investigated, within semiclassical picture in terms of phonon wave packet. It turns out that coherent longitudinal optical (LO) phonons weaken the effective electron-phonon coupling, thus supposedly lowering the carrier energy loss rate in solar cell. The resulting thermalization power is indeed significantly reduced by the coherent phonons, resulting in enhanced hot carrier effect, particularly for thin enough well layer where carrier confinement is also strong. A recent experiment on superlattice solar cell prototype is shown to manifest the coherent phonons-driven phenomenon. Our results demonstrate the practical implications of the fundamental quantum coherence property of phonons in semiconductors for improving superlattice solar cell performance, via hot carrier effect.

Observation of photoinduced polarons in semimetal 1T-TiSe2

In this work, ultrafast carrier dynamics of mechanically exfoliated 1T-TiSe2 flakes from the high-quality single crystals with self-intercalated Ti atoms are investigated by femtosecond transient absorption spectroscopy. The observed coherent acoustic and optical phonon oscillations after ultrafast photoexcitation reveal the strong electron–phonon coupling in 1T-TiSe2. The ultrafast carrier dynamics probed in both visible and mid-infrared regions indicate that some photogenerated carriers localize near the intercalated Ti atoms and form small polarons rapidly within several picoseconds after photoexcitation due to the strong and short-range electron–phonon coupling. The formation of polarons leads to a reduction of carrier mobility and a long-time relaxation process of photoexcited carriers for several nanoseconds. The formation and dissociation rates of the photoinduced polarons are dependent on both the pump fluence and the thickness of TiSe2 sample. This work offers new insights into the photogenerated carrier dynamics of 1T-TiSe2, and emphasizes the effects of intercalated atoms on the electron and lattice dynamics after photoexcitation.

Laser-induced excitation and decay of coherent optical phonon modes in an iron garnet

The excitation of optical phonon modes at resonance has been recently identified as offering a promising energy-efficient pathway for the ultrafast switching of magnetization. Here, using time-resolved pump–probe measurements, we explore the characteristics of different optical phonon modes in ferrimagnetic cobalt-doped yttrium iron garnet. By rotating the linear polarization of both the pump and probe, we identify a large spectrum of optical phonons within the frequency band of 3–11 THz. We determine the lifetimes of the different phonons using band filtration, revealing that the phonon mode with the eigenfrequency 3.4 THz survives for up to 13 ps. Its long lifetime, comparable with the relaxation time of photo-magnetic anisotropy in garnets, renders this phonon mode the most promising candidate for ultrafast phononic switching of magnetization.

The influence of initial phonon states on the generation of coherent optical phonons

The generation of coherent phonons is theoretically studied using a thermal equilibrium phonon distribution as the initial condition. The time evolution of the coherent phonon amplitude and phonon population is calculated for 4.5-THz phonons with initial phonon distributions at 10 and 300 K. It is determined that the coherent-phonon amplitudes of state-selected phonons are dependent on the initial phonon distribution. The coherent-phonon amplitudes, when considering all phonon states instead of selecting certain phonon levels, are almost the same for both initial distributions. The variance of the coherent-phonon amplitudes is found to oscillate with twice the phonon frequency, and its oscillation amplitude depends on the initial phonon distribution.

Decay time extension of terahertz electromagnetic waves emitted from coherent longitudinal optical phonons in GaAs epitaxial layers with the use of fast atom bombardment

We succeeded in extending the decay time of terahertz electromagnetic waves from coherent longitudinal optical (LO) phonons in GaAs epitaxial layers with the use of fast atom bombardment, a treatment method for introducing defects and/or disorders at the surface. The decay time becomes long, up to 4.81 ± 0.15 ps, with the bombardment time of 4.0 min. This value is 2.4-times larger than the decay time of 2.04 ± 0.04 ps of the reference sample (untreated sample). We attribute the origin of the present phenomenon to the fact that the reduction of the photogenerated carrier scattering effect on the coherent LO phonon, which is caused by the presence of the surface defects and disorders formed by the fast atom bombardment, results in the extending of the decay time of the terahertz wave from the coherent LO phonon.

Ultrafast dynamics of coherent phonons and phonon-polaritons in lithium niobate crystals

This study investigates the ultrafast dynamics of coherent phonons and phonon-polaritons in lithium niobate (LiNbO3) crystals using reflective pump–probe spectroscopy with 25 fs time resolution. In addition to several coherent optical phonon modes, the electro-optic (EO) sampling measurements explored the coexistence of phonon-polariton E-modes near 3.8 and 14.9 THz, which agrees considerably with the theoretical phonon-polariton dispersion curves. We also discovered that a time lag ( 0.2 ps) between the coherent optical phonon and phonon-polariton originates from the coupling time for propagating phonon-polariton. Our findings provide a possible application of LiNbO3 for ultrafast EO phonon modulators with bandwidths greater than 10 THz.

Pulse-width-dependent critical power for self-focusing of ultrashort laser pulses in bulk dielectrics.

Microscale filamentation of 0.25 NA-focused, linearly and circularly polarized 1030 nm and 515 nm ultrashort laser pulses of variable pulse widths in fused silica, fluorite, and natural and synthetic diamonds demonstrates the Raman-Kerr effect in the form of critical pulse power magnitudes, proportional to squared wavelength and inversely proportional to laser pulse width of 0.3-10 ps. The first trend represents the common spectral relationship between the quantities, while the second indicates its time-integrated inertial contribution of Raman-active lattice polarization, appearing in transmission spectra via ultrafast optical-phonon Raman scattering. The optical-phonon contribution to the nonlinear polarization could come from laser field-induced spontaneous/stimulated Raman scattering and coherent optical phonons generated by electron-hole plasma with its clamped density in the nonlinear focus. Almost constant product value of the (sub)picosecond laser pulse widths and corresponding critical pulse powers for self-focusing and filamentation in the dielectrics ("critical pulse energy") apparently implies constant magnitude of the nonlinear polarization and other "clamped" filamentation parameters at the given wavelength.

Probing phonon dynamics with multidimensional high harmonic carrier-envelope-phase spectroscopy

We explore pump-probe high harmonic generation (HHG) from monolayer hexagonal-boron-nitride, where a terahertz pump excites coherent optical phonons that are subsequently probed by an intense infrared pulse that drives HHG. We find, through state-of-the-art ab initio calculations, that the structure of the emission spectrum is attenuated by the presence of coherent phonons and no longer comprises discrete harmonic orders, but rather a continuous emission in the plateau region. The HHG yield strongly oscillates as a function of the pump-probe delay, corresponding to ultrafast changes in the lattice such as specific bond compression or stretching dynamics. We further show that in the regime where the excited phonon period and the pulse duration are of the same order of magnitude, the HHG process becomes sensitive to the carrier-envelope phase (CEP) of the driving field, even though the pulse duration is so long that no such sensitivity is observed in the absence of coherent phonons. The degree of CEP sensitivity versus pump-probe delay is shown to be a highly selective measure for instantaneous structural changes in the lattice, providing an approach for ultrafast multidimensional HHG spectroscopy. Remarkably, the obtained temporal resolution for phonon dynamics is ∼1 femtosecond, which is much shorter than the probe pulse duration because of the inherent subcycle contrast mechanism. Our work paves the way toward routes of probing phonons and ultrafast material structural changes with subcycle temporal resolution and provides a mechanism for controlling the HHG spectrum.