Displacive Excitation Of Coherent Phonons Research Articles

Anisotropic electron and lattice dynamics in excitonic insulator Ta2NiSe5

We employ polarization-resolved femtosecond optical pump–probe spectroscopy to investigate the nonequilibrium photocarrier dynamics in excitonic insulator Ta2NiSe5. The electronic dynamics, including hot carrier cooling, exciton formation, and recombination in the timescale ranging from subpicoseconds to a few tens of picoseconds, have been established from the transient reflectivity spectra, showing strong in-plane anisotropy with respect to the probe polarization. Such anisotropic photocarrier dynamics possibly arise from the crystalline orientation dependence of the excitonic polarizability. Furthermore, we find that the amplitude of coherent phonons with a frequency of 1 THz is subject to the probe polarization, whereas it is not sensitive to the pump polarization. This substantiates that the displacive excitation of coherent phonons plays a decisive role in lattice dynamics. In addition, we find that the photo-induced dielectric screening tends to suppress the amplitude of coherent phonons with increasing pump fluence, manifesting a remarkable polarization dependence. Our work provides valuable insights into the excitonic dynamics and the origin of coherent phonon generation and also may contribute to the development of polarization-sensitive photoelectric devices based on Ta2NiSe5.

Optically and thermally driven huge lattice orbital and spin angular momenta from spinning fullerenes

Lattice vibration in solids may carry angular momentum. But unlike the intrinsic spin of electrons, the lattice vibration is rarely rotational. To induce angular momentum, one needs to find a material that can accommodate a twisted normal mode, two orthogonal modes, or excitation of magnons. If excitation is too strong, one may exceed the Lindemann limit, so the material melts. Therefore these methods are not ideal. Here, we theoretically propose a route to phonon angular momentum in a molecular crystal ${\mathrm{C}}_{60}$. We find that a single laser pulse is able to inject a significant amount of angular momentum to ${\mathrm{C}}_{60}$, and the momentum transfer is helicity dependent. Changing from right-circularly polarized light to left-circularly polarized light switches the direction of phonon angular momentum. On the ultrafast timescale, the orbital angular momentum change closely resembles the displacive excitation of coherent phonons, with a cosine-function dependence on time, different from the spin counterpart. Atomic displacements, even under strong laser excitation, remain far below the Lindemann criterion. Under thermal excitation, spinning ${\mathrm{C}}_{60}$ even at room temperature generates a huge angular momentum close to several hundred $\ensuremath{\hbar}$. Our finding opens the door to a large group of fullerenes, from ${\mathrm{C}}_{60},\phantom{\rule{0.16em}{0ex}}{\mathrm{C}}_{70}$ to their endohedral derivatives, where angular momentum can be generated through light or temperature. This paves the way to the phononic control electronic spin and harvesting thermal energy through phonon angular momentum.

Hot carrier transport limits the displacive excitation of coherent phonons in bismuth

We performed femtosecond transient reflectivity measurements on epitaxially grown bismuth (Bi) films in the weak photoexcitation regime. Single crystalline ultrathin Bi films down to a thickness of 7 nm enabled us to determine a clear correspondence between the amplitude of the coherent A1g phonon and the photoexcitation level. We were able to empirically measure the effective hot carrier penetration length that determines the excited carrier density governing the magnitude of the coherent A1g phonon in Bi. Our findings suggest that the transport behavior of hot carriers is to be taken into consideration in order to provide insights into the mechanism for the displacive excitation of coherent phonons.

Observation of strong and anisotropic nonlinear optical effects through polarization-resolved optical spectroscopy in the type-II Weyl semimetal Td−WTe2

The unique bulk crossing points in the band structure of topological Weyl semimetals have been shown to enhance nonlinear optical and optoelectronic properties, even for light at optical wavelengths. Such pronounced nonlinear optical effects have been studied in type-I Weyl semimetals, yet few studies have quantified these effects in type-II Weyl semimetals. We here present an optical experimental study with polarization resolution of two nonlinear optical effects in the type-II Weyl semimetal ${T}_{\mathrm{d}}\text{\ensuremath{-}}\mathrm{W}{\mathrm{Te}}_{2}$. We begin by investigating the bulk optical second-harmonic response of this material using the rotational anisotropy of the second-harmonic generation. This technique allows us to simultaneously investigate the dependence of the second-harmonic response on the symmetry of the crystal and to quantify the size of that response, which we compare to other nonlinear crystals. We then use polarized time-resolved optical reflectivity spectroscopy to identify the nonlinear optical effect of impulsive stimulated Raman scattering as the origin of the coherent oscillations of the 0.25-THz shear mode. We find that the strength of this response in ${T}_{\mathrm{d}}\text{\ensuremath{-}}\mathrm{W}{\mathrm{Te}}_{2}$ is enhanced compared with the observation of other modes excited through the displacive excitation of coherent phonons. Notably, both the second-harmonic response and the coherent excitations of the phonons demonstrate strong anisotropy, displaying a clear dependence on the polarization of the light used in the experiments. We use point-symmetry analyses of the material to guide our understanding of these observations and perform fluence-dependent measurements to investigate the electron-phonon coupling.

Ultrafast optical absorption and coherent phonon generation in monolayer and bilayer transition metal dichalcogenides

When ultrashort laser pulses are applied to the solid-state material within a time scale less than the period of a particular phonon mode, the energy and momentum of the absorbed light are used by photo-excited carriers to interact collectively with the crystal lattices and generate lattice oscillations coherently, known as the coherent phonons. The Fourier transform of the absorption modulation due to the coherent phonon oscillations gives the coherent phonon intensity. In this work, we theoretically investigate the shape of coherent phonon intensity for transition metal dichalcogenides as a function of the polarization angle of the laser pulses. In particular, we focus on the E2g and A1g phonons in monolayer and bilayer molybdenum disulfides. Within the simplified theory of displacive excitation of coherent phonons, we find that the laser pulses uniquely select the electronic states in monolayer and bilayer molybdenum disulfides, resulting in a rich feature of the polar plots of the energy-dependent coherent phonon intensity of these materials.

Ultrafast dynamics in the high-symmetry and in the charge density wave phase of 2H−NbSe2

We investigate carrier and collective mode dynamics in 2H-NbSe$_2$ using time-resolved optical pump-probe spectroscopy and compare the results with first-principle calculations. Broadband ultrafast reflectivity studies of 2H-NbSe$_2$ in a wide temperature interval covering the normal, charge density wave (CDW) and superconducting phase were performed. Spectral features observed in the transient reflectivity experiment were associated with specific optical transitions obtained from band structure calculations. Displacive excitation of coherent phonons showed CDW-associated coherent oscillations of the soft phonon mode across the whole spectral range. Temperature evolution of this coherent phonon mode in the low-excitation linear regime shows softening of the mode down to the CDW transition temperature T$_{CDW}$ with subsequent hardening below T$_{CDW}$. The global fit of the broadband probe data reveals four different relaxation times associated with characteristic electron-electron, electron-phonon and phonon-phonon relaxation processes. From first principle calculations of electron-phonon coupling we associate the few picosecond electron-phonon relaxation time $\tau_2$ with a specific group of phonons with frequencies around 20 meV. On the other hand, the anomalously long relaxation time of $\tau_3$~100 ps is associated with anharmonicity-driven phonon-phonon scattering. All relaxation processes result from anomalies near the second order CDW phase transition that are reflected in the temperature dependencies of the characteristic relaxation times and amplitudes of optical densities. At highest fluences we observe electronic melting of the CDW and disappearance of the mode hardening below T$_{CDW}$.

Femtosecond reflectivity study of coherent phonons in doubly photoexcited bismuth

Ultrafast optical excitation generates coherent A1g optical phonons in bismuth via the mechanism of displacive excitation of coherent phonons (DECP). Here, femtosecond pump-probe reflectivity spectroscopy examines the dynamics of coherent phonons in doubly photoexcited bismuth. Two successive pump pulses are employed to generate optic phonons and thus to investigate the phonon dynamics including bond softening and dephasing with variable pump fluence and inter-pump separation times. Combined with thermal analysis, it distinguishes thermal and nonthermal effects of photoexcitation on the phonon dynamics. It also reveals that photocarriers relax via electron-hole recombination and diffusion out of the optical penetration depth on ultrafast timescales of 10–20 ps.

Access to phases of coherent phonon excitations by femtosecond ultraviolet photoelectron diffraction

Coherent phonons are an excellent tool to investigate the interplay between electronic and structural dynamics. The displacive excitation of coherent phonons in elemental bismuth is one of the most widely studied processes for this purpose. We employ time-resolved photoelectron diffraction to access the structural dynamics by recording the photoemission intensity from one initial state as a function of emission angle. In comparison with tight-binding and single-scattering cluster calculations, this allows electronic and structural effects to be disentangled. Hence, the full dynamics of the hot electron gas and of coherently excited phonons can be accessed in a single experiment. As a major result the phase lag between the coherent phonons and the modulation of the electronic structure can be determined with high precision. The phonon phase lag with respect to the modulation of the electronic structure is about 2.85±0.21 rad, thus significantly smaller than π. The difference is not due to phonon decay by energy dissipation into low-energy modes, but rather caused by the very early evolution of the highly excited electron distribution.

Excitation of Coherent Phonons in the One-Dimensional Bi(114) Surface

We present time-resolved photoemission experiments from a peculiar bismuth surface, Bi(114). The strong one-dimensional character of this surface is reflected in the Fermi surface, which consists of spin-polarized straight lines. Our results show that the depletion of the surface state and the population of the bulk conduction band after the initial optical excitation persist for very long times. The disequilibrium within the hot electron gas along with strong electron-phonon coupling cause a displacive excitation of coherent phonons, which in turn are reflected in coherent modulations of the electronic states. Beside the well-known A(1g) bulk phonon mode at 2.76 THz, the time-resolved photoelectron spectra reveal a second mode at 0.72 THz which can be attributed to an optical surface phonon mode along the atomic rows of the Bi(114) surface.

Measuring optical phonon dynamics in a bismuth thin film through a surface plasmon resonance

Surface plasmon resonances have become a useful tool for measuring coherent motion in solids, ranging from nanoparticle dynamics to acoustic vibrations in thin films. The non-linear electronic response near the surface plasmon resonance can significantly enhance transient optical measurements, making efficient detection of the coherent motion possible. In this work, we measure coherent optical phonon dynamics in a thin bismuth film through a surface plasmon resonance. We observe distinct changes in the measured amplitude and phase of the fully symmetric A1g optical phonon mode that are not explained through the standard model of displacive excitation of coherent phonons. In particular, near the surface plasmon resonance, we observe a strong polarization dependence on the amplitude and phase of the optical phonon. These results are explained through the rapid change of the optical reflectivity as a function of the complex dielectric constant near the surface plasmon resonance.

Excitation mechanisms of coherent phonons unravelled by femtosecond X‐ray diffraction

AbstractCoherent zone‐folded longitudinal acoustic phonons (ZFLAPs) were measured nondestructively with sub‐picometer spatial and sub‐picosecond time resolution via ultrafast X‐ray diffraction (XRD). The femtosecond excitation of a GaAs/AlGaAs superlattice with 1.5 eV photons results in spatially periodic stress, which induces a coherent ZFLAP with a 3.5 ps period. The motion is directly monitored by femtosecond X‐ray pulses at a kilohertz repetition rate. The intensity change ΔR /R = 0.03 of the −1st order satellite peak of the (002) reflection of bulk AlGaAs is directly proportional to the induced lattice strain of Δa /a 0 = 1.5 × 10−4. The phase and amplitude of the oscillatory XRD signal around a new equilibrium demonstrates the displacive excitation of coherent phonons (DECP) as the dominant mechanism for strong excitation. We discuss different mechanisms which account for impulsive stress, and argue that the electronic stress due to the deformation potential dominates. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cage motions induced by electronic and vibrational excitations: Cl2 in Ar

Femtosecond dynamics of molecular vibrations as well as cage motions in the B<--X transition of Cl2 in solid Ar have been investigated. We observed molecular vibrational wave-packet motion in experimental pump-probe spectra and an additional oscillation with a 500 fs period which is assigned to the zone-boundary phonon of the Ar crystal. The cage motion is impulsively driven by the B<--X transition due to the expansion of the electronic cloud of the chromophore. To clarify the underlying mechanism, we performed simulations based on the diatomics-in-molecules method which takes into account the different shapes of the Cl2 electronic wave function in the B and X states as well as the anisotropic interaction with the matrix. The simulation results show that Ar atom motion in the (100) plane is initiated by the electronic transition and that only those Ar atoms oscillate coherently with an approximately 500 fs period which are essentially decoupled from the molecular vibration. Their phase and time evolution are in good agreement with the experimentally observed oscillation, supporting the assignment as a displacive excitation of coherent phonons.

Coherent phonon dynamics: Br2in solid Ar

A long lasting coherent oscillation with a sharp frequency of f(p) = 2 THz is observed in fs pump probe spectra for B <-- X excitation of Br2 in solid argon. It exactly matches the frequency of a coherent zone boundary phonon (ZBP) of the Ar environment. The ZBPs have a vanishing group velocity v(g), thus they stay in the vicinity of the chromophore. They originate from a displacive excitation of coherent phonons (DECP) initiated in the electronic B <-- X transition, because neither f(p) nor the phase of the oscillation do depend on the B state vibrational dynamics. A model calculation shows that an expansion of the electronic density in going from the electronic ground state X to the B state kicks the Ar atoms in the Br2 vicinity. In addition, a group of Ar atoms in the (100) plane is decoupled from the intramolecular dynamics afterwards. The ZBP modulates the solvation energy of the terminal charge transfer states used in the probe transition from the B state and thus the detection sensitivity. The contrast is enhanced by probing the B state wave packet with the cutting edge of the probe window. This is in full accordance with a study for I2 : Kr.

Phonon relaxation in CdSSe semiconductor quantum dots studied by femtosecond time-resolved coherent anti-Stokes Raman scattering

We have observed the relaxation dynamics of coherent longitudinal optical phonons in CdSSe quantum dots embedded in a glass matrix by femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS). The phase relaxation time is directly deduced from the exponential decay of the signal. Oscillations with frequency of the longitudinal optical (LO) phonon are superimposed on the CARS signal. We consider two possible mechanisms, which result in such oscillations. The excitation of a LO phonon wave packet of the fundamental and the overtone would produce quantum beats. The second possibility is displacive excitation of coherent phonons. This mechanism does not contribute to the CARS signal, but it could attenuate it periodically with the phonon frequency.

Displacive excitation of coherent phonons in YBa2Cu3O7.

We suggest a microscopic model that provides a consistent explanation of the recent femtosecond pump-probe experiments on ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, including the fact that the reflectivity change in the superconducting state is much larger than that in the normal state. In this model, not only the oscillatory part of the reflectivity, but also the total reflectivity change, is due to displacive excitation of coherent phonons. The microscopic reason for this excitation is that superconductivity induces small displacements in the equilibrium positions of the ions, since the pairing energy depends on the density of states at the Fermi level, which changes with the ionic positions. When superconductivity is destroyed by the femtosecond laser pulse, the ions are pulled back to their normal equilibrium positions, thus exciting coherent phonons. The relative size of the oscillatory contribution to the reflectivity depends upon the ratio of the phonon period to the time scale of the pair breaking, and when this ratio is small, the oscillations are suppressed, as observed in the experiment. Ab initio caluclations confirm this model. The model also provides an explanation for why the mangitude of the 150 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ mode below ${\mathit{T}}_{\mathit{c}}$ may be smaller than that of the 120 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ mode.

Theory for displacive excitation of coherent phonons.

We report femtosecond time-resolved pump-probe reflection experiments in semimetals and semiconductors that show large-amplitude oscillations with periods characteristic of lattice vibrations. Only ${\mathit{A}}_{1}$ modes are detected, although modes with other symmetries are observed with comparable intensity in Raman scattering. We present a theory of the excitation process in this class of materials, which we refer to as displacive excitation of coherent phonons (DECP). In DECP, after excitation by a pump pulse, the electronically excited system rapidly comes to quasiequilibrium in a time short compared to nuclear response times. In materials with ${\mathit{A}}_{1}$ vibrational modes, the quasiequilibrium nuclear ${\mathit{A}}_{1}$ coordinates are displaced with no change in lattice symmetry, giving rise to a coherent vibration of ${\mathit{A}}_{1}$ symmetry about the displaced quasiequilibrium coordinates. One important prediction of the DECP mechanism is the excitation of only modes with ${\mathit{A}}_{1}$ symmetry. Furthermore, the oscillations in the reflectivity R are excited with a cos(${\mathrm{\ensuremath{\omega}}}_{0}$t) dependence, where t=0 is the time of arrival of the pump pulse peak, and ${\mathrm{\ensuremath{\omega}}}_{0}$ is the vibrational frequency of the ${\mathit{A}}_{1}$ mode. These predictions agree well with our observations in Bi, Sb, Te, and ${\mathrm{Ti}}_{2}$${\mathrm{O}}_{3}$. The fit of the experimental \ensuremath{\Delta}R(t)/R(0) data to the theory is excellent.

Mechanism for displacive excitation of coherent phonons in Sb, Bi, Te, and Ti2O3

Coherent phonons in Sb, Bi, Te, and Ti2O3 can be generated impulsively, and detected in the time domain through reflectivity modulation using 60 fs pulses of laser light at 2 eV. Experimental data for these opaque solids suggest that a direct Raman excitation mechanism is not responsible for coherent phonon generation. Rather, the excitation is attributed to an electronically induced displacement of the ion equilibrium coordinates.