Generation Of Coherent Phonons Research Articles

Optically Driven Quantum Dots as Source of Coherent Cavity Phonons: A Proposal for a Phonon Laser Scheme

We present a microscopically based scheme for the generation of coherent cavity phonons (phonon laser) by an optically driven semiconductor quantum dot coupled to a THz acoustic nanocavity. External laser pump light on an anti-Stokes resonance creates an effective Lambda system within a two-level dot that leads to coherent phonon statistics. We use an inductive equation of motion method to estimate a realistic parameter range for an experimental realization of such phonon lasers. This scheme for the creation of nonequilibrium phonons is robust with respect to radiative and phononic damping and only requires optical Rabi frequencies of the order of the electron-phonon coupling strength.

Spectral responses of phonon optical generation and detection in superlattices

We present a thorough theoretical and experimental analysis of the generation and detection of coherent phonons in superlattices by optical femtosecond pulses. We focus on the spectral responses of both phenomena in pump-probe experiments. The samples consist of two GaAs/AlAs superlattices separated by an intermediate layer through which phonons can propagate. The spatial decoupling of the generation and detection stages in one and the other supperlattice enables the individualization of each phenomenon. We clearly verify experimentally the selection rules for the generation and detection processes, which translate into well-defined maxima at specific frequencies in the corresponding spectral responses. A thickness gradient in one of the superlattices adds a degree of freedom that permits the variation of the spectral detuning between the maxima in the generation and in the detection spectral responses. The zero-detuning case is investigated. In the analysis of both spectral responses, the full size of the sample is found to play a significant role. Thin ($\ensuremath{\sim}$3 $\ensuremath{\mu}$m) and thick ($\ensuremath{\sim}$350 $\ensuremath{\mu}$m) samples are compared. Backreflection of light at sample outer surfaces and inner interfaces induces important alterations to the spectral responses as well.

Coherent phonon generation and detection in ultrathin SrTiO3 grown directly on silicon

AbstractTime‐resolved two color pump‐probe polarization spectroscopy was performed at room temperature on SrTiO3 films grown directly on Si with film thickness varying from 2 nm to 7.8 nm. An E‐symmetry mode with a characteristic frequency of 0.2 THz is impulsively generated and measured in these coherently strained tetragonal phase SrTiO3 thin films. A superimposed exponentially decaying signal observed indicates the possible relaxational hopping of Ti ion between double potential wells. The dependence of the coherent phonon signal on pump and probe laser polarization helps to identify the symmetry of the phonon modes.

Coherent radial-breathing-like phonons in graphene nanoribbons

We have developed a microscopic theory for the generation and detection of coherent phonons in armchair and zigzag graphene nanoribbons using an extended tight-binding model for the electronic states and a valence force field model for the phonons. The coherent phonon amplitudes satisfy a driven oscillator equation with the driving term depending on photoexcited carrier density. We examine the coherent phonon radial-breathing-like mode amplitudes as a function of excitation energies and nanoribbon types. For photoexcitation near the optical absorption edge the coherent phonon driving term for the radial-breathing-like mode is much larger for zigzag nanoribbons where transitions between localized edge states provide the dominant contribution to the coherent phonon driving term. Using an effective mass theory, we explain how the armchair nanoribbon width changes in response to laser excitation.

Control of coherent acoustic phonon generation with external bias in InGaN/GaN multiple quantum wells

Control of acoustic phonon generation with external bias is demonstrated for an InGaN light-emitting diode by performing femtosecond pump–probe measurements. The evolution of both the carrier dynamics and the acoustic phonon strength with an external bias reveals that the acoustic wave is generated during the carrier sweeping process of the depletion field, where electric field screening accompanies stress of the lattice for a piezoelectric material.

Giant Enhancement of Stimulated Brillouin Scattering in the Subwavelength Limit

Stimulated Brillouin scattering in bulk materials and micron-scaled optical fibers has been exploited to realize coherent phonon generation and slow light as well as new light sources. What happens when the size of a light-interacting system is reduced to nanoscales? A qualitatively new, and several-orders-of-magnitude more powerful form of stimulated Brillouin scattering is shown to emerge.

First-Principles Calculation to Explore Mechanisms of Coherent Phonon Generation(Current Topics)

First-Principles Calculation to Explore Mechanisms of Coherent Phonon Generation(Current Topics)

Coherent phonon generation in semiconducting single-walled carbon nanotubes using a few-cycle pulse laser

Coherent phonon dynamics in single-walled carbon nanotubes with several chiralities is investigated by 7.1-fs pump–probe experiments. Vibrational wave-packets corresponding to the radial breathing mode (RBM) and G mode can be detected for four chiral systems, (6,4), (6,5), (7,5) and (8,3). Coherent phonon generation of RBMs is in-depth studied by analyzing the probe photon energy dependent amplitude profiles, which indicates that the real and imaginary parts of the third-order susceptibility can both contribute to the modulation of the probed difference absorbance.

Resonant generation of coherent phonons in a superconductor by ultrafast optical pump pulses

We study the generation of coherent phonons in a superconductor by ultrafast optical pump pulses. The nonequilibrium dynamics of the coupled Bogoliubov quasiparticle-phonon system after excitation with the pump pulse is analyzed by means of the density-matrix formalism with the phonons treated at a full quantum kinetic level. For ultrashort excitation pulses, the superconductor exhibits a nonadiabatic behavior in which the superconducting order parameter oscillates. We find that in this nonadiabatic regime the generation of coherent phonons is resonantly enhanced when the frequency of the order-parameter oscillation is tuned to the phonon energy, a condition that can be achieved in experiments by varying the integrated pump pulse intensity.

Molecular Dynamics Simulation of Coherent Phonon Generation and Its Spectral Characterization in a Nanoribbon upon Localized Pulse Heating

The generation and thermal transport of coherent phonons during instantaneous pulse heating in the presence of diffusion is studied by a molecular dynamics (MD) method. Coherent phonon formation and propagation characteristics are obtained and compared for different shapes of the heating pulse, such as a half-period square, a Gaussian, and a triangle, using the Lennard-Jones (LJ) nanoribbon model. Heating energy exceeding the equilibrium energy distribution of a heated region relaxes by emitting a train of (3 to 5) coherent phonons. As shown in the MD model, the equations of heat flux can resolve coherent phonon motion with high resolution when flux through the boundaries is evaluated with sampling regions of the same size as a single phonon vibration period in the direction of propagation. In the presence of diffusion, the dependence of the generation and decay of phonons on the energy density of the heating pulse is studied for different heating times of the nanoribbon sample. Heating pulses of different duration with a Gaussian profile lead to a higher percentage of heating energy being converted into coherent phonons relative to other pulse shapes. The number of generated phonons and their amplitudes are shown to vary with the pulse duration and shape owing to differences in the energy density of the heating pulses. In the phonon propagation sampling regions, the density of states (DOS) is used to identify coherent phonon frequencies, which are shown to correspond, in terms of the number of identified phonons, to the shape of the thermal envelope for the different pulse shapes and heating times of the nanoribbon sample.

Molecular Dynamics Simulation of Coherent Phonon Generation and Its Spectral Characterization in a Nanoribbon upon Localized Pulse Heating

Molecular Dynamics Simulation of Coherent Phonon Generation and Its Spectral Characterization in a Nanoribbon upon Localized Pulse Heating

Coherent Phonon in Single-Walled Carbon Nanotubes by Using a few-Cycle Pulse Laser

We studied coherent phonon dynamics in Co-Mo-catalyst grown single-walled carbon nanotubes by using sub-5-fs visible pulses. Vibrational wave-packets corresponding to radial breathing mode for four chiral systems, (6,4), (6,5), (7,5) and (8,3), can be separately observed and achieved exact chirality assignments without ambiguity in coherent phonon spectra. Coherent phonon generation of radial breathing modes is in-depth studied by analyzing the probe energy dependent amplitude profiles, which indicates that the real and imaginary parts of the third-order susceptibility can both contribute to the modulation of the probed difference absorbance.

Polarization dependence of coherent phonon generation and detection in highly-aligned single-walled carbon nanotubes

We have investigated the polarization dependence of the generation and detection of radial breathing mode (RBM) coherent phonons (CPs) in highly-aligned single-walled carbon nanotubes. Using polarization-dependent pump-probe differential-transmission spectroscopy, we measured RBM CPs as a function of angle for two different geometries. In Type I geometry, the pump and probe polarizations were fixed, and the sample orientation was rotated whereas, in Type II geometry, the probe polarization and sample orientation were fixed, and the pump polarization was rotated. In both geometries, we observed an almost complete quenching of the RBM CPs when the pump polarization was perpendicular to the nanotubes. For both Type I and II geometries, we have developed a microscopic theoretical model to simulate CP generation and detection as a function of polarization angle and found that the CP signal decreases as the angle goes from 0${}^{ˆ}$ (parallel to the tube) to 90${}^{ˆ}$ (perpendicular to the tube). We compare theory with experiment in detail for RBM CPs created by pumping at the ${E}_{44}$ optical transition in an ensemble of single-walled carbon nanotubes with a diameter distribution centered around 3 nm, taking into account realistic band structure and imperfect nanotube alignment in the sample. We theoretically determined a $\mathrm{cos}{}^{8}(\ensuremath{\theta})$ dependence for Type I CP spectroscopy experiments and a $\mathrm{cos}{}^{4}(\ensuremath{\phi})$ polarization dependence for Type II CP spectroscopy experiments and, after including misalignment effects to our fitting, we determined the nematic order parameter of our sample to be 0.81.

Generation of coherent phonons in bismuth by ultrashort laser pulses in the visible and NIR: Displacive versus impulsive excitation mechanism

We have applied femtosecond pump-probe technique with variable pump wavelength to study coherent lattice dynamics in Bi single crystal. Comparison of the coherent amplitude as a function of pump photon energy for two different in symmetry E g and A 1 g phonon modes with respective spontaneous resonance Raman profiles reveals that their generation mechanisms are quite distinct. We show that displacive excitation, which is the main mechanism for the generation of coherent A 1 g phonons, cannot be reduced to the Raman scattering responsible for the generation of lower symmetry coherent lattice modes.

Coherent phonons in pyrochlore titanatesA2Ti2O7(A=Dy, Gd, Tb): A phase transition in Dy2Ti2O7at 110 K

We study the generation of coherent optical phonons in spin-frustrated pyrochlore single crystals Dy${}_{2}$Ti${}_{2}$O${}_{7}$, Gd${}_{2}$Ti${}_{2}$O${}_{7}$, and Tb${}_{2}$Ti${}_{2}$O${}_{7}$ using femtosecond laser pulses (65 fs, 1.57 eV) in degenerate time-resolved transmission experiments as a function of temperature from 4 to 296 K. At 4 K, two coherent phonons are observed at $~$5.3 THz (5.0 THz) and $~$9.3 THz (9.4 THz) for Dy${}_{2}$Ti${}_{2}$O${}_{7}$ (Gd${}_{2}$Ti${}_{2}$O${}_{7}$), whereas three coherent phonons are generated at $~$5.0, 8.6, and 9.7 THz for Tb${}_{2}$Ti${}_{2}$O${}_{7}$. In the case of spin-ice Dy${}_{2}$Ti${}_{2}$O${}_{7}$, a clear discontinuity is observed in the linewidths of both the coherent phonons as well as in the phase of lower-energy coherent phonon mode, indicating a subtle structural change at 110 K. Another important observation is a phase difference of $\ensuremath{\pi}$ between the modes in all the samples, thus suggesting that the driving forces behind the generation of these modes could be different in nature, unlike a purely impulsive or displacive mechanism.

Coherent phonons in bismuth film observed by ultrafast electron diffraction

The generation of coherent phonons in polycrystalline bismuth film excited with femtosecond laser pulse is observed by ultrafast time-resolved electron diffraction. The dynamics of the diffracted intensities from the (110), (202), and (024) lattice planes show pronounced oscillations at 130–150 GHz. The origin of these coherent acoustic phonons is discussed in view of optical phonon decay into two acoustic phonons. Different drop times in the intensity of the diffraction orders are observed and interpreted as anisotropy in the energy transfer rate of coherent optical phonons.

Generation of coherent LO phonons in optically driven biased quantum wells

AbstractWe present a theoretical analysis and numerical simulations of the resonant generation of coherent LO phonons in an electrically biased quantum well. The system is driven by a laser pulse short enough to simultaneously excite two transitions, which leads to a quantum beat that can resonantly create coherent phonons. The generation of incoherent phonons is also calculated on a quantum kinetic level and found to be dominant in terms of the energy transferred to the lattice.The simulations successfully reproduce the main features found in recent experiments [Mizoguchi et al., Appl. Phys. Lett. 94, 171105 (2009)]. Depending on the excitation conditions different generation mechanisms can be discerned. We discuss their characteristics and their relative efficiency in the special case of the quantum well model under consideration. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Graphitized layer buried in a diamond: photothermal properties and hypersound generation under picosecond optical excitation

We assume that a coherent phonon generation was found out in structures with a single graphitized layer under picosecond laser excitation by means of the Pump/Probe method. The thermal component of the response was calculated that allowed us to define the photothermal coefficient of the graphitized layer as well as its thermal conductivity from the experimental results. The thermal-conductivity coefficient of the graphitized layer appears to grow with the thickness decrease. This makes it possible to assume that the microstructure of the graphitized layer produced during annealing depends on its thickness.

Generation of coherent thermal phonons in amorphous dielectrics at room temperature induced by four-photon interaction

Resonant responses of Mandelshtam-Brillouin scattering from longitudinal and transverse acoustic waves at frequencies of ν LS = ±1.15 cm−1 and ν TS = ±0.7 cm−1, respectively, and at frequency of ν SS = ν TS \( \sqrt 3 \) = ±0.43 cm−1 were detected in K8 optical glass at room temperature using four-photon spectroscopy. We attribute this effect to the induced generation of a second sound wave (coherent thermal phonons). The mechanisms of generation and the possibility of practical application of the observed effect for express diagnostics of the quality of transparent materials are discussed.

Coherent phonon generation in time-dependent density functional theory

We apply the adiabatic time-dependent density functional theory (TDDFT) to the generation of coherent optical phonons in Si crystals by intense laser pulses. The theory reproduces the main phenomena observed experimentally: dependence on polarization, strong growth at the direct band gap, and the change in phase from below to above the band gap. Both show that two mechanisms invoked in phenomenological theory, namely, impulsively stimulated Raman scattering and displacive excitation, are present in the TDDFT. The calculated phase of the coherent phonon is in qualitative agreement with experiment and with phenomenological modeling in the vicinity of the direct band gap. At higher laser frequencies, the TDDFT predicts additional structure not present in the modeling.