Coherent Phonon Oscillations Research Articles

Theoretical study on ultrafast dynamics of coherent acoustic phonons in semiconductor nanocrystals

We present a theoretical study on the ultrafast dynamics of coherent acoustic phonons in semiconductor quantum dots using continuum model calculations. The excitonic states and the coherent acoustic vibrational modes of semiconductor quantum dots are calculated using the effective mass approximation and continuum elastic medium model, respectively. By solving the Liouville–von Neumann equation and the equation of motion, we obtain the oscillation of coherent acoustic phonon amplitude excited by a pump pulse laser. Owing to the ultrafast excitation of coherent phonons, both the amplitude and the phase of the coherent phonon oscillation are constant with time. This coherent phonon oscillation results in conservation of the coherence of the exciton state, which cannot exist in a system interacting with incoherent phonons. We further study the amplitude and the period of coherent acoustic phonon oscillation as a function of pump pulse energy detuning, quantum dot size, and material.

Phonon Josephson junction with nanomechanical resonators

We study coherent phonon oscillations and tunneling between two coupled nonlinear nanomechanical resonators. We show that the coupling between two nanomechanical resonators creates an effective phonon Josephson junction which exhibits two different dynamical behaviors: Josephson oscillation (phonon-Rabi oscillation) and macroscopic self-trapping (phonon blockade). Self-trapping originates from mechanical nonlinearities, meaning that when the nonlinearity exceeds its critical value, the energy exchange between the two resonators is suppressed, and phonon-Josephson oscillations between them are completely blocked. An effective classical Hamiltonian for the phonon Josephson junction is derived and its mean-field dynamics is studied in phase space. Finally, we study the phonon-phonon coherence quantified by the mean fringe visibility, and show that the interaction between the two resonators may lead to the loss of coherence in the phononic junction.

Probing superconductivity and pairing symmetry by coherent phonons in multiorbital superconductors

We show that the phase information contained in coherent phonon oscillations generated by a laser pulse in a multi- orbital superconductor can be used as an experimental tool to probe super- conductivity and pairing symmetries. The phase difference between the normal and superconducting states is proportional to the superconducting order parameter just below the superconducting transition temperature, Tc. It also exhibits different behaviors for superconducting states with different pairing symmetries. In particular, if there is an orbital-dependent internal sign change state, the phase difference can have a discontinuous jump below Tc.

Chirality assignment for metallic species via coherent phonon oscillations in arc-discharge single-walled carbon nanotubes

We performed transient absorption measurements with sub-10-fs pulses to observe coherent phonon (CP) oscillations of species in a micelle-suspended arc-discharge single-walled carbon nanotube (SWNT) ensemble. We applied a spectral-resolved measurement scheme to investigate the photon-wavelength dependence of CP oscillations of the radial breathing mode (RBM) specific to each species, covering a broad photon wavelength range from 700 to 1000 nm (1.771 - 1.240 eV). With a linear prediction singular value decomposition (LPSVD) method as a robust alternative for resolving closely-overlapped vibration modes, we divided multiple RBM peaks into two components, the M11-excited metallic species and the S22-excited semiconducting species, respectively. We resolved the RBM peaks into 22 metallic and 29 semiconducting species. Resolved metallic tubes showed a wide distribution in diameters from 1.3 to 2.3 nm, forming a chirality distribution from (2n + m) = 33 family to 51 family.

The influence of charge and magnetic order on polaron and acoustic phonon dynamics in LuFe2O4

Femtosecond optical pump-probe spectroscopy is used to reveal the influence of charge and magnetic order on polaron dynamics and coherent acoustic phonon oscillations in single crystals of charge-ordered, ferrimagnetic LuFe2O4. We experimentally observed the influence of magnetic order on polaron dynamics. We also observed a correlation between charge order and the amplitude of the acoustic phonon oscillations, due to photoinduced changes in the lattice constant that originate from the photoexcited electrons. This provides insight into the general behavior of coherent acoustic phonon oscillations in charge-ordered materials.

Electronic and vibrational dynamics of hollow au nanocages embedded in cu2 o shells.

We have synthesized hollow Au nanocages embedded within thick porous shells of cuprous oxide (Cu2 O). The shell causes a significant redshift of the localized surface plasmon resonance of Au into the near-IR. Electron-phonon coupling in the Au nanocage is 3-6 times faster in the core-shell structure due to the higher thermal conductivity of Cu2 O compared to water. Coherent phonon oscillations within the Au lattice are characterized by a breathing mode of the entire structure for both bare and core-shell nanocages, an assignment made through the use of structural mechanics simulations. The experimental frequencies are obtained through simulations by selectively applying a force to the shell of the core-shell structure. We interpret this as rapid thermal expansion of the gold leading to a mechanical force that acts on the shell.

Coexistence of excited polarons and metastable delocalized states in photoinduced metals

We study how polaronic states form as a function of time due to strong electron-phonon coupling, starting from a hot electron distribution which is representative of a photo-induced metallic state immediately after laser excitation. For this purpose we provide the exact solution of the single-electron Holstein model within nonequilibrium dynamical mean-field theory. In particular, this allows us to reveal key features of the transient metallic state in the numerically most challenging regime, the adiabatic regime, in which phonon frequencies are smaller than the electronic bandwidth: Initial coherent phonon oscillations are strongly damped, leaving the system in a mixture of excited polaron states and metastable delocalized states. We compute the time-resolved photoemission spectrum, which allows to disentangle two contributions. The existence of long-lived delocalized states suggest ways to externally control transient properties of photo-doped metals.

Origin of coherentG-band phonon spectra in single-wall carbon nanotubes

Coherent phonons in single wall carbon nanotubes (SWNTs) are observed as oscillations of the differential absorption coefficient as a function of time by means of pump-probe spectroscopy. For the radial breathing mode (RBM) of a SWNT, the coherent phonon signal is understood to be a result of the modulated diameter-dependent energy gaps due to the coherent RBM phonon oscillations. However, this mechanism might not be the dominant contribution to other phonon modes in the SWNT. In particular, for the G band phonons, which correspond to bond-stretching motions, we find that the modulation of the interatomic dipole matrix element gives rise to a strong coherent G band phonon intensity comparable to the coherent RBM phonon intensity. We also further discuss the dependence of coherent G band and RBM phonon amplitudes on the laser excitation pulse width.

Energy transport and coherence properties of acoustic phonons generated by optical excitation of a quantum dot

The energy transport of acoustic phonons generated by the optical excitation of a quantum dot as well as the coherence properties of these phonons are studied theoretically both for the case of a pulsed excitation and for a continuous wave (CW) excitation switched on instantaneously. For a pulsed excitation, depending on pulse area and pulse duration, a finite number of phonon wave packets is emitted, while for the case of a CW excitation a sequence of wave packets with decreasing amplitude is generated after the excitation has been switched on. We show that the energy flow associated with the generated phonons is partly related to coherent phonon oscillations and partly to incoherent phonon emission. The efficiency of the energy transfer to the phonons and the details of the energy flow depend strongly and in a non-monotonic way on the Rabi frequency exhibiting a resonance behavior. However, in the case of CW excitation it turns out that the total energy transferred to the phonons is directly linked in a monotonic way to the Rabi frequency.

Direct measurement of coherent phonon dynamics in solution-processed stibnite thin films

We report observations of coherent phonon oscillations in solution-processed polycrystalline stibnite $({\mathrm{Sb}}_{2}$${\mathrm{S}}_{3})$ photovoltaic thin films using transient absorption spectroscopy. Detailed optical spectroscopy correlated by extensive first-principles lattice dynamics calculations indicates that the coherent ${B}_{3g}$ longitudinal optical phonon mode with a frequency of 63.74 \ifmmode\pm\else\textpm\fi{} 0.05 ${\mathrm{cm}}^{\ensuremath{-}1}$ (or 1.911 \ifmmode\pm\else\textpm\fi{} 0.002 THz) at room temperature is generated via the impulsive stimulated-Raman-scattering mechanism. These strong electron-phonon interactions indicate a dominant energy-loss channel in these materials that could impose a fundamental limit on their solar energy conversion efficiency.

Coherent transverse-optical phonon generation induced by lattice defects in nitrogen-ion-implanted GaAs

We describe our observation of coherent phonon oscillations of X-point zone-boundary transverse-optical (TO) mode, TO(X), in nitrogen-ion-implanted GaAs that has been annealed at high temperatures. With the TO(X) mode being forbidden from the Raman selection rule in pure zinc-blende GaAs, the lattice defects have provided additional momentum for phonon generation. Annealing-induced structural modifications were demonstrated through X-ray diffraction, transmission electron microscopy, and Raman scattering measurements. The polarization dependence of the TO(X) mode was compared with that of the longitudinal optical mode, and the temperature dependence of the TO(X) phonon dephasing was also investigated.

Coherent acoustic phonon generation and detection by femtosecond laser pulses in ZnTe single crystals

We report experiments of coherent acoustic phonon generations and detections using a two-color femtosecond pump-probe technique in [001] and [110] oriented ZnTe single crystals. The above-bandgap pump excitation initiates strain waves to yield coherent acoustic phonons. The generation and propagation of coherent acoustic waves not only modulate the intensity of the reflected probe light but also change its polarization periodically due to strain-induced birefringence. The nonuniform excitation due to Gaussian spatial distribution of pump pulses takes an important role in the polarimetric detection of coherent acoustic phonon oscillations.

Excitonic effects on coherent phonon dynamics in single-wall carbon nanotubes

We discuss how excitons can affect the generation of coherent radial breathing modes in the ultrafast spectroscopy of single-wall carbon nanotubes. Photoexcited excitons can be localized spatially and give rise to a spatially distributed driving force in real space which involves many phonon wave vectors of the exciton-phonon interaction. The equation of motion for the coherent phonons is modeled phenomenologically by the Klein-Gordon equation, which we solve for the oscillation amplitudes as a function of space and time. By averaging the calculated amplitudes per nanotube length, we obtain time-dependent coherent phonon amplitudes that resemble the homogeneous oscillations that are observed in some pump-probe experiments. We interpret this result to mean that the experiments are only able to see a spatial average of coherent phonon oscillations over the wavelength of light in carbon nanotubes and the microscopic details are averaged out. Our interpretation is justified by calculating the time-dependent absorption spectra resulting from the macroscopic atomic displacements induced by the coherent phonon oscillations. The calculated coherent phonon spectra including excitonic effects show the experimentally observed symmetric peaks at the nanotube transition energies, in contrast to the asymmetric peaks that would be obtained if excitonic effects were not included.

Interband excitation and carrier relaxation as displacive driving force for coherent phonons

We report on displacive excitation of large amplitude coherent lattice motions in TiO2 under resonant conditions using sub-6-femtosecond UV pulses. Calculations of non-equilibrium potential energy surfaces reveal a new displacive mechanism for the coherent phonon generation, where a unidirectional displacement of the phonon potential occurs both instantaneously due to carrier excitation and dynamically due to cooling of the hot photo-excited carriers. The carrier cooling dynamics in TiO2 are faster than the phonon period, resulting in an initially anharmonic lattice motion. This effect manifests itself in an effective phase shift of the coherent phonon oscillation, in good agreement between our experiments and the calculations.

Coherent phonon frequency comb generated by few-cycle femtosecond pulses in Si

We explore the coherent phonon induced refractive index modulation of a Si(001) surface upon the excitation in near-resonance with the direct band gap of Si. Through the anisotropic e-h pair generation and coherent Raman scattering, ~ 10-fs laser pulses exert a sudden electrostrictive force on Si lattice launching coherent LO phonon oscillations at 15.6 THz frequency. The concomitant oscillatory change in the optical constants modulates the reflected probe light at the fundamental LO phonon frequency, generating a broad comb of frequencies at exact integer multiples of the fundamental frequency extending to beyond 100 THz. On the basis of an analytical model, we show that the simultaneous amplitude and phase modulation of the reflected light by the coherent lattice polarization at 15.6 THz generates the frequency comb.

Mapping Carrier Diffusion in Single Silicon Core–Shell Nanowires with Ultrafast Optical Microscopy

Recent success in the fabrication of axial and radial core-shell heterostructures, composed of one or more layers with different properties, on semiconductor nanowires (NWs) has enabled greater control of NW-based device operation for various applications. (1-3) However, further progress toward significant performance enhancements in a given application is hindered by the limited knowledge of carrier dynamics in these structures. In particular, the strong influence of interfaces between different layers in NWs on transport makes it especially important to understand carrier dynamics in these quasi-one-dimensional systems. Here, we use ultrafast optical microscopy (4) to directly examine carrier relaxation and diffusion in single silicon core-only and Si/SiO(2) core-shell NWs with high temporal and spatial resolution in a noncontact manner. This enables us to reveal strong coherent phonon oscillations and experimentally map electron and hole diffusion currents in individual semiconductor NWs for the first time.

Development of a Novel Surface Processing System Using Femtosecond Pulse Train

When a laser pulse interacts with metal or semiconductor target, the coherent phonon, which is the coherent motions of lattice and molecule vibrations in solids, is excited by the interaction of electrons and high latitude electric field. It has unique properties of decaying in approximately several picoseconds and substance specific frequency. Owing to that, femtosecond laser processing is the local processing with little heat diffusion and little thermal damage to the target, due to the ultrafast time scales. We propose a novel femtosecond pulse ablation process with oscillation of the coherent phonon by femtosecond pulse train. The pulse train is shaped using Spatial Light Modulator (SLM), which shift the phase of the passing light. And coherent phonon oscillations are enhanced and decayed due to the controlling the shape of pulse train. It is able to activate the lattice motion for processing efficiently, and hence the target is expected to be ablated with high accuracy and less thermal damage.

Coherent Electronic and Phononic Oscillations in Single-Walled Carbon Nanotubes

Free induction decay of the coherent electronic transition and coherent phonon oscillations of the radial breathing mode in single-walled carbon nanotubes are simultaneously observed via direct resonant excitation of the lowest E(11) optical transition in the near-infrared region from 0.939 to 1.1 eV. We show that coherent electronic oscillations corresponding to the detuning of the probe energy from resonance can be exploited for the chirality assignment of carbon nanotubes, together with the robust assignment of the coherent lattice vibrations resonantly excited by femtosecond pulses. Excitation spectra show a large number of pronounced peaks that map out chirality distributions in great detail.

Classical Fano oscillator

Starting from the quantum-mechanical Fano-Anderson Hamiltonian, we derive classical equations of motion for coordinates associated with the discrete and continuum states. The frequency-dependent absorption spectrum associated with this classical system exhibits the same Fano line shape as the quantum-mechanical system when appropriate correspondences between classical and quantum variables are made. In the time domain, the response of this classical Fano oscillator depends upon the asymmetry parameter $q$ that appears in the expression for the Fano line shape. In particular, under the influence of impulsive driving of the system, the discrete oscillator's phase changes by $\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2$ as $q$ increases from zero (maximum asymmetry in the frequency domain) to $\ensuremath{\mp}\ensuremath{\infty}$ (minimum asymmetry). Previously published ultrafast-laser-pulse-driven coherent-phonon oscillations in degenerate $p$-type Si [K. Kato et al., Jpn. J. Appl. Phys. 48, 100205 (2009)] are discussed in light of these theoretical results.

Application of the Linear Prediction Singular Value Decomposition Method for Studying the Coherent Phonon Oscillations in Single-walled Carbon Nanotubes

Application of the Linear Prediction Singular Value Decomposition Method for Studying the Coherent Phonon Oscillations in Single-walled Carbon Nanotubes