Excitation Of Coherent Phonons Research Articles

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.

Coherent Acoustic Phonons in Semiconductor Superlattices

We investigate the excitation of coherent acoustic phonons in GaAs/AlAs superlattices with femtosecond laser pulses. Zone-folded acoustic modes of different order are observed in the time-domain. The excitation of the modes is based on an impulsive stimulated Raman process. The coherent phonon amplitudes can be traced over more than 100 ps which allows us to distinguish propagation and decay of the phonons. The corresponding linewidths of the time-domain data are below <0.2 cm—1 in the frequency domain. This is a new approach for high resolution spectroscopy of low energy phonons in semiconductors.

Coherent Acoustic Phonons in Semiconductor Superlattices

We investigate the excitation of coherent acoustic phonons in GaAs/AlAs superlattices with femtosecond laser pulses. Zone-folded acoustic modes of different order are observed in the time-domain. The excitation of the modes is based on an impulsive stimulated Raman process. The coherent phonon amplitudes can be traced over more than 100 ps which allows us to distinguish propagation and decay of the phonons. The corresponding linewidths of the time-domain data are below <0.2 cm—1 in the frequency domain. This is a new approach for high resolution spectroscopy of low energy phonons in semiconductors.

Coherent Phonon Dynamics Studied by Impulsive Stimulated Raman Scattering

In the first part of this work, impulsive stimulated Raman scattering (ISRS) with tunable fs optical pulses is applied to investigate the mechanism of coherent phonon excitation in opaque materials. Comparison of theoretical values for the phonon-induced change in reflectivity as a function of photon energy in Sb with experimental data yields satisfactory agreement without the need to invoke other mechanisms of generation besides stimulated Raman scattering. In the second part, measurements of the phonon/polariton dephasing time in GaP for mode frequencies between 340 and 364 cm—1 are presented. The experimental data are well described by a classical oscillator fit assuming anharmonic interaction with acoustic combination modes.

Coherent Phonons in Semiconductors and Semiconductor Heterostructures

The impulsive excitation and phase-sensitive detection of coherent phonons enable the study of dynamical properties of the lattice. In pump-probe experiments with femtosecond time resolution the amplitude and phase of the coherent lattice motion can be detected with high sensitivity. Especially in semiconductors and semiconductor heterostructures, where a coherent phonon mode and free carriers are exited at the same time, important information about the carrier-phonon system far away from equilibrium is obtained. In bulk GaAs coherent plasmon-phonon modes can be traced via the associated macroscopic field. In GaAs-AlGaAs based heterostructures coupled phonon-intersubband plasmons, coupled Bloch-phonon oscillations, and coherent acoustic phonons are observed.

Coherent phonons in alkali metal-doped C60

We report the femtosecond impulsive excitation of coherent phonons in alkali metal-doped fullerenes. These excitations appear as small oscillations of the reflectivity (ΔR/R∼10−7) produced by the pump in an optical pump-probe experiment. The observed phonons include previously unobserved highly damped modes near 150 cm−1 as well as long-lived Ag(1) modes near 490 cm−1.

Spectral dependence of femtosecond relaxation and coherent phonon excitation inC60films

Spectral dependence of femtosecond relaxation and coherent phonon excitation in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">C</mml:mi></mml:mrow><mml:mrow><mml:mn>60</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>films

Dynamical theories of dark-field imaging using diffusely scattered electrons in STEM and TEM

Dynamical theories of atomic number sensitive image (or Z-contrast image) formed by thermal diffusely scattered (TDS) electrons are proposed based on first-principles considerations. `Exact' theories are derived for simulating images obtained either in scanning transmission electron microscopy (STEM) using an annular dark-field detector or in transmission electron microscopy (TEM) using an on-axis objective aperture under hollow-cone beam illumination. The atom thermal vibrations are described using lattice dynamics with consideration of phase correlations. The effects that are comprehensively covered in the theory include: dynamical diffraction of the beam before and after TDS, thickness-dependent beam broadening or channelling, Huang scattering from defect regions, coherence of the thermal diffusely scattered electrons generated from the atomic layers packed within the coherent length, multiphonon and multiple phonon excitations, and the detector geometry. Simplified theories have been derived from this unified approach under various approximations. It has been shown that the incoherent imaging theory is a much simplified case of the practical imaging condition, and can be applied only for qualitative image interpretation. The coherent length in the z direction varies with the change of atomic mass in the column. It is thus possible that the z coherence may disappear for heavy elements. Finally, the theory of Huang scattering in high-angle dark-field TEM imaging has been illustrated, and the theoretically expected results have been observed experimentally.

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.

Impulsive excitation of coherent phonons observed in reflection in bismuth and antimony

We report time domain observations of coherent lattice vibrations in bismuth and antimony. Phonons are impulsively generated, and detected through reflectivity modulation with 70 fs pulses of laser light at 1.98 eV. With this technique, we demonstrate that coherent lattice oscillations can be studied by reflection in opaque materials, but with selection rules which may differ from conventional impulsive stimulated Raman scattering.

Low-mode submillimeterwave resonators for piezoelectric surface excitation of sound

The efficient excitation of coherent THz phonons requires an electric field as high as possible at the surface of the piezoelectric transducer material. This can be done below 10 11Hz by means of a coaxial resonator or a cavity resonator, respectively. At still higher frequencies new kinds of suitable resonators must be developed. In this paper two types of low-mode submillimeterwave resonators, a dielectric waveguide resonator and a two-layer Fabry-Perot resonator, are investigated with respect to their electromagnetic properties in the THz range.

Impulsive stimulated scattering: General importance in femtosecond laser pulse interactions with matter, and spectroscopic applications

It is shown that ‘‘impulsive’’ stimulated Raman scattering (ISRS) should occur, with no laser intensity threshold, when a sufficiently short laser pulse passes through many types of matter. ISRS excitation of coherent optic phonons, molecular vibrations, and other excitations (including rotational, electronic, and spin) may play important roles in femtosecond pulse interactions with molecules, crystals, glasses (including optical fibers), semiconductors, and metals. Spectroscopic applications of ISRS, including time-resolved spectroscopy of vibrationally distorted molecules and crystals, are discussed.

Femtosecond time-resolved measurements of optic phonon dephasing by impulsive stimulated raman scattering in α-perylene crystal from 20 to 300 K

Excitation and detection of coherent optic phonons by impulsive stimulated Raman scattering (ISRS) is reported. The technique permits femtosecond time-resolved observations of optic phonon oscillations and dephasing. Decay rates of librational and translational optic phonons in α-perylene crystals have been measured in the temperature range 20–30 K.

On residual RF losses and tunnel currents caused by interface states

Superconducting rf cavities show large residual losses R res which limit the application of such cavities Beside the coherent phonon excitation by interface states (IS), the up till now neglected incoherent phonon excitation will be discussed as cause for residual losses R res . But IS also weaken the superconducting interaction and enhance the current through the oxide by resonance tunneling, e.g., as super, - leakage - or assisted current in tunnel junctions. Parameters influencing the IS and the residual losses, the weakening of the superconducting interaction and the enhanced tunnel currents will be worked out for Nb-Nb 2 O 5 -interfaces.