Phonon Wave Packets Research Articles

Kapitza resistance of Si/SiO2 interface

A phonon wave packet dynamics method is used to characterize the Kapitza resistance of a Si/SiO2 interface in a Si/SiO2/Si heterostructure. By varying the thickness of SiO2 layer sandwiched between two Si layers, we determine the Kapitza resistance for the Si/SiO2 interface from both wave packet dynamics and a direct, non-equilibrium molecular dynamics approach. The good agreement between the two methods indicates that they have each captured the anharmonic phonon scatterings at the interface. Moreover, detailed analysis provides insights as to how individual phonon mode scatters at the interface and their contribution to the Kapitza resistance.

Sub-THz Phonon drag on dislocations by coarse-grained atomistic simulations

In this paper, dislocation–phonon interactions in fcc crystals are simulated using the concurrent atomistic-continuum (CAC) method. With significantly less computational cost than that by full molecular dynamics (MD) simulations, dislocation migration, phonon transport, and their interactions are explicitly modeled. Sub-THz Phonon drag coefficients on moving dislocations are predicted using CAC and compared with MD and experimental results. During the interactions between phonons and dislocations, the local temperature is found to rise due to energy dissipation, and dislocations are observed to exhibit “breathing modes” in which the separation between leading and trailing partials varies in time in a quasi-periodic fashion. Simulation results show that the phonon drag coefficient on dislocation migration increases with phonon wave packet magnitudes or sizes but is insensitive to the incident angles.

Brillouin scattering of visible and hard X-ray photons from optically synthesized phonon wavepackets

We monitor how destructive interference of undesired phonon frequency components shapes a quasi-monochromatic hypersound wavepacket spectrum during its local real-time preparation by a nanometric transducer and follow the subsequent decay by nonlinear coupling. We prove each frequency component of an optical supercontinuum probe to be sensitive to one particular phonon wavevector in bulk material and cross-check this by ultrafast x-ray diffraction experiments with direct access to the lattice dynamics. Establishing reliable experimental techniques with direct access to the transient spectrum of the excitation is crucial for the interpretation in strongly nonlinear regimes, such as soliton formation.

Comparison of Atomistic and Continuum Methods for Calculating Ballistic Phonon Transmission in Nanoscale Waveguides

We compare two methods for the calculation of mode dependent ballistic phonon transmission in nanoscale waveguides. The first method is based on continuum acoustic waveguide theory and uses an eigenmode expansion to solve for phonon transmission coefficients. The second method uses lattice dynamics (LD)-computed mode shapes to excite guided phonon wavepackets in a nonequilibrium molecular dynamics (MD) simulation and calculates phonon transmission from the final distribution of system energy. The two methods are compared for the case of shear-horizontal (SH) phonons propagating in a planar waveguide with a T-stub irregularity, a geometry which has been proposed for the tuning of phonon transmission and nanostructure thermal conductance. Our comparison highlights advantages and disadvantages of the two methods and illustrates regimes when atomistic effects are prominent and continuum approaches are not appropriate.

Investigation on interfacial thermal resistance and phonon scattering at twist boundary of silicon

Grain interfaces in nanocrystalline materials play a critical role in thermal transport. A series of twist boundary thermal resistances in silicon is investigated by the nonequilibrium molecular dynamics simulation so as to find the relationship between the boundary resistance, the twist angle, the boundary energy and temperature. The results indicate that the magnitude of the twist grain boundary (GB) thermal resistance is on the order of 10−9 m2 KW−1, and the GB thermal resistance becomes larger with increasing GB energy at most twist angles, and it drops obviously with increasing temperature. The phonon wave packet dynamic simulation shows that the transmission coefficient of the low frequency phonons with long wavelength is close to 100% at the boundary with different twist angles. The transmission coefficient of the longitudinal phonon wave packet decreases with increasing frequency and transverse phonons are produced due to the scattering. In most cases, higher grain boundary energy corresponds to lower transmission coefficient, leading to larger GB thermal resistance.

Fluctuation properties of acoustic phonons generated by ultrafast optical excitation of a quantum dot

We study theoretically the fluctuation properties of acoustic phonons created in a semiconductor quantum dot after ultrafast optical excitation. An excitation with a single ultrafast pulse creates an exciton confined to the quantum dot, which is coupled to longitudinal acoustic phonons. This leads to the formation of a polaron in the quantum dot accompanied by the emission of a phonon wave packet. We show that the fluctuations of the lattice displacement associated with the wave packet after a single laser pulse excitation in resonance with the exciton transition are always larger than their respective vacuum values. Manipulating the exciton with a second pulse can result in a reduction of the fluctuations below their vacuum limit, which means that the phonons are squeezed. We show that the squeezing properties of the wave packet strongly depend on the relative phase and the time delay between the two laser pulses.

Thermal rectification and phonon scattering in silicon nanofilm with triangle hole

Thermal rectification has potential applications in the thermal management of electronics and energy saving. Discovering thermal rectification phenomena and understanding the mechanism are very essential. Reported in this paper is the thermal rectification in silicon nanofilm with triangle holes by the non-equilibrium molecular dynamics simulation. The results show that in the silicon nanofilm with triangle holes, the difference in thermal rectification coefficient is around 28% with the variation of heat flow direction in a temperature range from 300 K to 1100 K. The phonon wave packet dynamic simulations indicate that transverse phonons are generated during the scattering of longitudinal phonons in the nanofilms. When the phonon transport direction is reversed, the average phonon energy transmission coefficient is changed by about 22% in all the frequency range. The difference in phonon transmissity, which is caused by asymmetric structure, is regarded as being attributed mainly to the thermal rectification in silicon film with triangle holes.

Electrostriction and guidance of acoustic phonons in optical fibers

We investigate the generation of phonon wavepackets in optical fibers via electrostriction from coherent optical waves. Solving the elastodynamic equation subject to the electrostrictive force, we are able to reproduce experimental spectra found in standard and photonic crystal fibers. We discuss the two important practical cases of forward interaction, dominated by elastic resonances of the fiber, and backward interaction, for which an efficient mechanism of phonon guidance by light is found. The latter result describes the formation of the coherent phonon wavepacket involved in stimulated Brillouin scattering.

Point source in a phononic grating: stop bands give rise to phonon-focusing caustics

We use locally-excited gigahertz surface phonon wavepackets in microscopic line structures of different pitches to reveal profound anisotropy in the radiation pattern of a point source in a grating. Time-domain data obtained by an ultrafast optical imaging technique and by numerical simulations are Fourier transformed to obtain frequency-filtered real-space acoustic field patterns and k-space phononic band structure. The numerically-obtained k-space images are processed to reveal an intriguing double-horn structure in the lowest-order group-velocity surface, which explains the observed non-propagation sectors bounded by caustics, noted at frequencies above the bottom of the first stop band. We account for these phonon-focusing effects, analogous to collimation effects previously observed in two- and three-dimensional lattices, with a simple analytical model of the band structure based on a plane wave expansion. As the frequency is increased, a transition to dominant waveguiding effects along the lines is also documented.

Analog Hawking radiation from an acoustic black hole in a flowing polariton superfluid

We theoretically study Hawking radiation processes from an analog acoustic black hole in a flowing superfluid of exciton-polaritons in a one-dimensional semiconductor microcavity. Polaritons are coherently injected into the microcavity by a laser pump with a suitably tailored spot profile. An event horizon with a large analog surface gravity is created by inserting a defect in the polariton flow along the cavity plane. Experimentally observable signatures of the analog Hawking radiation are identified in the scattering of phonon wave packets off the horizon, as well as in the spatial correlation pattern of quantum fluctuations of the polariton density. The potential of these tabletop optical systems as analog models of gravitational physics is quantitatively confirmed by numerical calculations using realistic parameters for state-of-the-art devices.

Thermal rectification and phonon scattering in silicon nanofilm with cone cavity

The thermal rectification has potential application in the thermal management of electronics and energy saving. This paper investigates the thermal rectification in silicon nanofilm with cone cavity by the non-equilibrium molecular dynamics simulation. The results show that the thermal resistance caused by the cone cavity is on the order of 10−9 m2 KW−1. The thermal resistance of cone cavity differs averagely by about 11% from 300 K to 1100 K when the heat flow direction is turned over. The phonon wave packet dynamic simulations indicate that transverse phonons are generated by the cavity scattering of longitudinal phonon. When the phonon transport direction is reversed, the average transmission coefficient is changed by about 14% in all the frequency range and is regarded as the main reason for the thermal rectification in silicon film with cone cavity. The thermal rectification depends on the aspect ratio of the cone cavity.

Coherent phonon dynamics in short-period InAs/GaSb superlattices

We have performed ultrafast pump-probe spectroscopy studies on a series of InAs/GaSb-based short-period superlattice (SL) samples with periods ranging from 46Å to 71Å. We observe two types of oscillations in the differential reflectivity with fast (∼1–2ps) and slow (∼24ps) periods. The period of the fast oscillations changes with the SL period and can be explained as coherent acoustic phonons generated from carriers photoexcited within the SL. This mode provides an alternative method for determining the SL period. The period of the slow mode depends on the wavelength of the probe pulse and can be understood as a propagating coherent phonon wavepacket modulating the reflectivity of the probe pulse as it travels from the surface into the sample.

Thermal rectification and phonon scattering in asymmetric silicon nanoribbons

Thermal rectification is an interesting phenomenon and has important potential applications in improving the thermal management of electronics and saving energy. Exploring thermal rectification phenomena and understanding the mechanism are very necessary and important. This paper reports the investigation of the thermal conductivity and thermal rectification of asymmetric silicon nanoribbons by the non-equilibrium molecular dynamics simulation. The results indicate that the thermal conductivity of the nanoribbon is only on the order of 100 Wm−1K−1. Thermal rectification is observed in silicon nanoribbons at different temperatures, geometry aspects, and ribbon length. The thermal conductivity is apparently larger when heat flows from the thin end to the thick end. The larger the aspect ratio of the thick end to thin end is, the larger the thermal rectification. The rectification coefficient does not change much in the ribbon length ranges from 8.1 nm to 21.7 nm. The longitudinal phonon scattering in the silicon nanoribbon at different frequency is investigated by the phonon wave packet dynamic simulation. The results show that the phonon transmission coefficient is sensitive to frequency and transverse phonons are generated during the scattering. Decreasing the thin end width will reduce the transmission coefficient due to the scattering at the slope boundary.

Single mode phonon scattering at carbon nanotube-graphene junction in pillared graphene structure

Phonon scattering at the carbon nanotube-graphene interface is studied in the pillared graphene structure, using the phonon wave packet method. Qualitatively different scattering characteristics, compared to previous studies of carbon nanotube systems, are observed. The phonon group velocity, dictating the energy transmission in simple carbon nanotube systems, is found to play an insignificant role. Distributing the incoming phonon energy to both sides of the interface more or less equally, the graphene interface provides a strong diffusive scattering site, which features a microscopic understanding of the decisive role of junction on the thermal transport in the pillared graphene hierarchical structure.

Detecting optically synthesized quasi-monochromatic sub-terahertz phonon wavepackets by ultrafast x-ray diffraction

We excite an epitaxial SrRuO3 thin film transducer by a pulse train of ultrashort laser pulses, launching coherent sound waves into the underlying SrTiO3 substrate. Synchrotron-based x-ray diffraction (XRD) data exhibiting separated sidebands to the substrate peak evidence the excitation of a quasi-monochromatic phonon wavepacket with sub-THz central frequency. The frequency and bandwidth of this sound pulse can be controlled by the optical pulse train. We compare the experimental data to combined lattice dynamics and dynamical XRD simulations to verify the coherent phonon dynamics. In addition, we observe a lifetime of 130 ps of such sub-THz phonons in accordance with the theory.

Quantum kinetics of squeezed lattice displacement generated by phonon down conversion

We study the fluctuation properties of longitudinal acoustic (LA) phonons generated by the anharmonic decay of coherent longitudinal optical phonons in a bulk semiconductor and a quantum dot. This process is comparable to the down conversion of photons, which is well known to generate squeezed photons. We use a quantum kinetic model to calculate the fluctuations of lattice displacement and momentum. This allows us to analyze the strength and the spatial distribution of the fluctuations. It is shown that the fluctuations may fall below their vacuum value, i.e., squeezing occurs. The squeezing only persists for short times due to fast dephasing of the different LA phonon modes and the increasing LA phonon population. In the quantum dot case, it is found that although LA phonon wave packets travel out of the quantum dot, the squeezing does not leave the vicinity of the dot but remains confined.

Single mode phonon energy transmission in functionalized carbon nanotubes

Although the carbon nanotube (CNT) features superior thermal properties in its pristine form, the chemical functionalization often required for many applications of CNT inevitably degrades the structural integrity and affects the transport of energy carriers. In this article, the effect of the side wall functionalization on the phonon energy transmission along the symmetry axis of CNT is studied using the phonon wave packet method. Three different functional groups are studied: methyl (-CH(3)), vinyl (-C(2)H(3)), and carboxyl (-COOH). We find that, near Γ point of the Brillouin zone, acoustic phonons show ideal transmission, while the transmission of the optical phonons is strongly suppressed. A positive correlation between the energy transmission coefficient and the phonon group velocity is observed for both acoustic and optical phonon modes. On comparing the transmission due to functional groups with equivalent point mass defects on CNT, we find that the chemistry of the functional group, rather than its molecular mass, has a dominant role in determining phonon scattering, hence the transmission, at the defect sites.

Efficient linear scaling method for computing the thermal conductivity of disordered materials

An efficient order-$N$ real-space Kubo approach is developed for the calculation of the thermal conductivity of complex disordered materials. The method, which is based on the Chebyshev polynomial expansion of the time evolution operator and the Lanczos tridiagonalization scheme, efficiently treats the propagation of phonon wave packets in real space and the phonon diffusion coefficients. The mean free paths and the thermal conductance can be determined from the diffusion coefficients. These quantities can be extracted simultaneously for all frequencies, which is another advantage in comparison with approaches based on the Green's function. Additionally, multiple scattering phenomena can be followed through the time dependence of the diffusion coefficient deep into the diffusive regime, and the onset of weak or strong phonon localization could possibly be revealed at low temperatures for thermal insulators. The accuracy of our computational scheme is demonstrated by comparing the calculated phonon mean free paths in isotope-disordered carbon nanotubes with Landauer simulations and analytical results. Then the upscalability of the method is illustrated by exploring the phonon mean free paths and the thermal conductance features of edge-disordered graphene nanoribbons having widths of $~$20 nm and lengths as long as a micrometer, which are beyond the reach of other numerical techniques. It is shown that the phonon mean free paths of armchair nanoribbons are smaller than those of zigzag nanoribbons for the frequency range which dominates the thermal conductance at low temperatures. This computational strategy is applicable to higher-dimensional systems as well as to a wide range of materials.

Acoustic white holes in flowing atomic Bose–Einstein condensates

We study acoustic white holes in a steadily flowing atomic Bose–Einstein condensate. A white hole configuration is obtained when the flow velocity goes from a super-sonic value in the upstream region to a sub-sonic one in the downstream region. The scattering of phonon wavepackets on a white hole horizon is numerically studied in terms of the Gross–Pitaevskii equation of mean-field theory: dynamical stability of the acoustic white hole is found, as well as a signature of a nonlinear back-action of the incident phonon wavepacket onto the horizon. The correlation pattern of density fluctuations is numerically studied by means of the truncated-Wigner method, which includes quantum fluctuations. Signatures of the white hole radiation of correlated phonon pairs by the horizon are characterized; analogies and differences with Hawking radiation from acoustic black holes are discussed. In particular, a short wavelength feature is identified in the density correlation function, whose amplitude steadily grows in time since the formation of the horizon. The numerical observations are quantitatively interpreted by means of an analytical Bogoliubov theory of quantum fluctuations for a white hole configuration within the step-like horizon approximation.

Dynamics of a one-dimensional Holstein polaron with the Davydov ansätze

Following the Dirac-Frenkel time-dependent variational principle, dynamics of a one-dimensional Holstein polaron is probed by employing the Davydov D2 Ansatz with two sets of variational parameters, one for each constituting particle in the exciton-phonon system, and a simplified variant of the Davydov D1 Ansatz, also known as the $\tilde{D}$ Ansatz, with an additional set of phonon displacement parameters. A close examination of variational outputs from the two trial states reveals fine details of the polaron structure and intricacies of dynamic exciton-phonon interactions. Superradiance coherence sizes, speeds of exciton-induced phonon wave packets, linear optical absorption, and polaron energy compositions are also included in the study.