Excitation Of Coherent Phonons Research Articles

Laser-excited acoustic oscillations in silver and bismuth nanowires

Coherent acoustic oscillations in Bi and Ag nanowire samples were studied with a femtosecond pump-probe technique and detection of the scattered light. The observed optical and acoustic properties reflect the nanostructure of these materials. The electronic and lattice contributions to the excitation of coherent acoustic phonons are described using a two-temperature model. The excitation is performed at different laser fluences, and the high density of optically induced excitations modifies the state of the lattice. A transient state with softening of the lattice was observed in Ag nanowire samples.

Ultrafast soft x-ray absorption spectroscopy with sub-20-fs resolution

The most challenging application of time-resolved spectroscopy is to watch the motion of atoms directly. In the current experiment, we were able to follow the displacement of atoms in silicon after excitation with an intense near infrared femtosecond laser pulse with time-resolved x-ray absorption spectroscopy. The sub-20-fs x-ray probe pulses in an energy range up to several 100eV were generated by high harmonic generation of intense femtosecond laser pulses at a repetition rate of 1kHz. In a proof of principle experiment, we found an oscillatory motion of the atoms which can be explained by the excitation of coherent phonons. Our setup can be easily extended for studying structural dynamics in many important systems not only in solids but also in gases and liquids.

Excitation of Coherent Phonons in Crystalline Bi: Theory for Driving Atomic Vibrations by Femtosecond Pulses

AbstractIn this paper we show time-resolved experiment with 35 fs resolution on bismuth single crystal, for which very high sensitivity detection system has been used. Coherent and incoherent lattice dynamics as well as electrons dynamics can be clearly seen into the reflectivity changes. The complex behaviour of the reflectivity could not be explained in the light of the existing theories. Therefore, we developed a new theory, starting from the very basic principle of laser-matter interaction, which is in good agreement with experimental results. Two main results will be shown: the coherent phonon is excited by the temperature gradient; the changes in reflectivity are related directly to the changes in electron-phonon collision frequency.

Carrier‐induced coherent acoustic phonon excitation in strain engineered InAs/GaAs quantum dot clusters

AbstractStrain engineered InAs/GaAs quantum dot (QD) clusters grown on (311)B GaAs are studied by means of time‐resolved differential reflection spectroscopy revealing coherent excitation of the quasilongitudinal and quasitransverse acoustic phonon modes. Photogenerated carriers which are captured within the ordered lateral QD clusters initiate coherent acoustic phonon excitation, which induces a transient modulation of the local strain‐induced piezoelectric field within the QD clusters. The excited acoustic phonons modulate the optical properties of the QDs through the quantum‐confined Stark effect, causing oscillations of the reflection signal. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Publisher's Note: Control of coherent optical phonon excitations in GaN using femtosecond pulse shaping [Phys. Rev. B74, 113201 (2006)

Publisher's Note: Control of coherent optical phonon excitations in GaN using femtosecond pulse shaping [Phys. Rev. B<b>74</b>, 113201 (2006)

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 &amp; Co. KGaA, Weinheim)

Fundamental aspects of electrospray droplet impact/SIMS

A new ionization method, electrospray droplet impact ionization (EDI), has been developed for matrix-free secondary-ion mass spectrometry (SIMS). The charged droplets formed by electrospraying 1 M acetic acid aqueous solution are sampled through an orifice with a diameter of 400 microm into the first vacuum chamber, transported into a quadrupole ion guide, and accelerated by 10 kV after exiting the ion guide. The droplets impact on a dry solid sample (no matrix used) deposited on a stainless steel substrate. The secondary ions formed by the impact are transported to a second quadrupole ion guide and mass-analyzed by an orthogonal time-of-flight mass spectrometer (TOF-MS). Ten pmol of gramicidin S could be detected with the presence of as much as 10 nmol of NaCl. The ion signal for arginine disappeared with decrease in the substrate temperature below 150 K owing to the formation of ice film over the sample surface. While 10 fmol of gramicidin S could be detected for 30 min, the ionization/desorption efficiency for EDI becomes smaller with an increase in the molecular weight (MW) of a biological sample. The largest protein samples detected to date are cytochrome c and lysozyme. The high sensitivity for EDI is due to the fact that samples only a few monolayers thick are subject to desorption/ionization by EDI, with little fragmentation. A coherent phonon excitation may be the main mechanism for the desorption/ionization of the solid sample.

Coherent acoustic phonons in strain engineered InAs∕GaAs quantum dot clusters

Coherent excitation of the quasilongitudinal and quasitransverse acoustic phonon mode in strain engineered InAs∕GaAs quantum dot (QD) clusters grown on (311)B GaAs is monitored by means of time-resolved differential reflection spectroscopy. Carrier capture within the ordered QD clusters initiate coherent acoustic phonon excitation, which induces a transient modulation of the local strain-induced piezoelectric field within the QD clusters. The excited acoustic phonons then modulate the optical properties of the QDs through the quantum-confined Stark effect, causing distinct oscillations of the differential reflection signal.

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.

Mode-selective excitation of coherent surface phonons on alkali-covered metal surfaces

We demonstrate the mode-selective excitation of coherent phonons at Pt(111) surfaces covered with submonolayer caesium atoms. A burst of 150 fs laser pulses with the repetition rate of 2.0-2.9 THz was synthesized by using a spatial-light modulator, and used for the coherent surface phonon excitation. The coherent nuclear motion was monitored by time-resolved second harmonic generation. By tuning the repetition rate, we succeeded in controlling the relative amplitude of the vibrational coherence of the Cs-Pt stretching mode (2.3-2.4 THz) to that of the Pt surface Rayleigh phonon mode (2.6 or 2.9 THz, depending on the Cs coverage).

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.

Time-resolved X-ray diffraction: a wonderful tool for probing structural photo-induced phase transitions

Laser-driven solid–solid phase transition is a new kind of manipulation of matter by light which offers fascinating possibilities for controlling macroscopic switching of materials at different time scales from hours down to femtosecond. Slow dynamics phenomena are illustrated by a photo-induced spin transition. A sub-nanosecond laser-induced ferroelectric ordering and the opposite transformation have been observed in a molecular charge-transfer material with a 100 ps time resolution. New, exciting scientific challenges at 100 fs such as the excitation of coherent phonons and non-thermal melting are also presented.

Laser-induced coherent phonons in graphite and carbon nanotubes: model and simulations

We present a microscopic description of the excitation of coherent phonons as a response to femtosecond laser excitation. Using molecular dynamics simulations based on a tight-binding electronic Hamiltonian we discuss two examples of laser-induced coherent phonons: (1) excitation of the E2g1 phonon mode in graphite under high external pressure and (2) the displacive excitation of two perpendicular phonon modes in carbon nanotubes. We discuss the influence of these coherent phonons on the ablation mechanisms.

The birth of a quasiparticle in silicon observed in time-frequency space.

The concept of quasiparticles in solid-state physics is an extremely powerful tool for describing complex many-body phenomena in terms of single-particle excitations. Introducing a simple particle, such as an electron, hole or phonon, deforms a many-body system through its interactions with other particles. In this way, the added particle is 'dressed' or 'renormalized' by a self-energy cloud that describes the response of the many-body system, so forming a new entity--the quasiparticle. Using ultrafast laser techniques, it is possible to impulsively generate bare particles and observe their subsequent dressing by the many-body interactions (that is, quasiparticle formation) on the time and energy scales governed by the Heisenberg uncertainty principle. Here we describe the coherent response of silicon to excitation with a 10-femtosecond (10(-14) s) laser pulse. The optical pulse interacts with the sample by way of the complex second-order nonlinear susceptibility to generate a force on the lattice driving coherent phonon excitation. Transforming the transient reflectivity signal into frequency-time space reveals interference effects leading to the coherent phonon generation and subsequent dressing of the phonon by electron-hole pair excitations.

Excitation of coherent acoustic phonons by a femtosecond pulse

Excitation of coherent acoustic phonons in superlattices or layered materials by a femtosecond pulse is considered. In superlattices, coherent acoustic phonons with a wave vector Q=2π/a can be excited, where a is the superlattice period. The number and statistical properties of coherent phonons are estimated.

Femtosecond X-ray measurement of coherent lattice vibrations near the Lindemann stability limit

The study of phase-transition dynamics in solids beyond a time-averaged kinetic description requires direct measurement of the changes in the atomic configuration along the physical pathways leading to the new phase. The timescale of interest is in the range 10(-14) to 10(-12) s. Until recently, only optical techniques were capable of providing adequate time resolution, albeit with indirect sensitivity to structural arrangement. Ultrafast laser-induced changes of long-range order have recently been directly established for some materials using time-resolved X-ray diffraction. However, the measurement of the atomic displacements within the unit cell, as well as their relationship with the stability limit of a structural phase, has to date remained obscure. Here we report time-resolved X-ray diffraction measurements of the coherent atomic displacement of the lattice atoms in photoexcited bismuth close to a phase transition. Excitation of large-amplitude coherent optical phonons gives rise to a periodic modulation of the X-ray diffraction efficiency. Stronger excitation corresponding to atomic displacements exceeding 10 per cent of the nearest-neighbour distance-near the Lindemann limit-leads to a subsequent loss of long-range order, which is most probably due to melting of the material.

Bloch-phonon coupling and tunneling-induced coherent phonon excitation in semiconductor superlattices

The coupling of Bloch oscillations to longitudinal optical phonons is investigated in a narrow-well ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}{\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}}_{0.52}{\mathrm{Al}}_{0.48}\mathrm{As}$ superlattice. A strong increase of coherent phonon amplitudes is observed when the Bloch oscillations are subsequently tuned into resonance with different optical phonon modes. The rapid dephasing of the Bloch oscillations due to field induced tunneling from the weakly bound miniband into above-barrier continuum states leads to an additional and new excitation process of coherent phonons in superlattices at high electric fields. Polarization dependent measurements confirm their generation via Coulomb interaction excluding any contribution through impulsively stimulated Raman scattering.

Phase Transition Dynamics Studied by Coherent Phonon Excitation with Ultrashort Laser Pulses

The low-frequency light scattering spectroscopy always encounters experimentally a difficulty near the phase transition point due to the overlapping between the soft mode spectra and the strong elastic-scattering component originated from the surface and/or imperfections irradiated by the laser light. A time-resolved phonon spectroscopy, the coherent phonon excitation method, has been introduced as a new method to compliment the light scattering method. It observe directly the time dependence of the soft phonons excited artificially in terms of pico- or femtosecond laser pulses. Several cases of the studies on TGS, TGSe, KDP, DKDP and LiNbO 3 are briefly mentioned.

Techniques for coherent phonon excitation and stimulated Brillouin scattering.

The basic principle of a rotating chisel-shaped phonon coupling and detection probe, believed to be novel, is presented for the measurement of coherent phonon propagation and interaction characteristics.

Coherent phonons in InSb and their properties from femtosecond pump–probe experiments

We report the ultrafast impulsive excitation of coherent LO phonons with the frequency of 5.57 THz in InSb. These phonons, when generated by femtosecond laser pulses, exhibit phase-dependent fluctuation properties, that is, the variance of coherent amplitude is a function of time delay. The variance oscillations occur, for short time delays, at twice the frequency of the coherent amplitude.