Longitudinal Acoustic Mode Research Articles

Characterization of acousto-optical interaction in planar silica optical waveguide by the finite element method

A full-vectorial finite-element-based approach is used to find accurate modal solutions for the acoustic modes in a Ge-doped planar silica optical waveguide. To enhance accuracy of the solution, the existing structural symmetry is exploited, and rigorous analyses of the interactions between the guided acoustic and optical modes are performed. The stimulated Brillouin scattering frequency and the overlaps between the fundamental and the higher-order transverse and longitudinal acoustic modes and the fundamental quasi-TE optical mode are presented.

Evaluation of the isothermal curing process of UV-cured resin in terms of elasticity studied through micro-Brillouin light scattering

ABSTRACTThe isothermal curing process of two photopolymers were studied using micro-Brillouin spectroscopy, which is a powerful tool for probing the local elasticity of condensed matters. The Brillouin frequency shift and the half width of the longitudinal acoustic mode showed significant changes during curing under ultraviolet (UV) irradiation, and became saturated after complete curing. These were closely correlated with the changes in the optical characteristics of the white light-emitting diodes, where the same photopolymers were used as a matrix for phosphor inclusion. These results indicate that micro-Brillouin spectroscopy may be an effective tool for monitoring the curing process of resins in various applications.

High pressure Raman study of layered Mo0.5W0.5S2 ternary compound

Ternary two-dimensional (2D) transition metal dichalcogenide compounds exhibit a tunable electronic structure allowing for control of the interlayer and the intralayer atomic displacement to efficiently tune their physical and electronic properties. Using a diamond anvil cell, hydrostatic pressure was applied to Mo0.5W0.5S2 up to 40 GPa in order to study the optical phonon vibrational modes. Analysis of the high-pressure Raman spectra shows that the two in-plane E2g modes resembling that of pristine MoS2 and WS2, as well as disorder-activated longitudinal acoustic phonon mode, are hardened and suppressed as pressure increases. The two A1g modes of the ternary compound that resemble the A1g modes of pristine MoS2 and WS2, displayed similar Raman shifts to the pristine compounds as pressure increases. A Raman peak at 470 cm−1 that is close to A1g peaks emerges at ∼8 GPa, which represents a disorder-activated pressure-induced out-of-plane Raman mode observed only in the ternary compound under high pressure. At pressures above ∼30 GPa, a Raman peak at approximately 340 cm−1 is observed, signifying additional disorder-activated vibration mode. Our results reveal the enhanced interactions in the structural and vibrational behavior of the MoS2 and WS2 domains in the Mo0.5W0.5S2 compound under hydrostatic pressure. These results could have implications in understanding the electronic, optical, and structural properties of the new 2D ternary compound materials under extreme mechanical conditions.

Phonon Scattering Dynamics of Thermophoretic Motion in Carbon Nanotube Oscillators.

Using phonon wave packet molecular dynamics simulations, we find that anomalous longitudinal acoustic (LA) mode phonon scattering in low to moderate energy ranges is responsible for initiating thermophoretic motion in carbon nanotube oscillators. The repeated scattering of a single mode LA phonon wave packet near the ends of the inner nanotube provides a net unbalanced force that, if large enough, initiates thermophoresis. By applying a coherent phonon pulse on the outer tube, which generalizes the single mode phonon wave packet, we are able to achieve thermophoresis in a carbon nanotube oscillator. We also find the nature of the unbalanced force on end-atoms to be qualitatively similar to that under an imposed thermal gradient. The thermodiffusion coefficient obtained for a range of thermal gradients and core lengths suggest that LA phonon scattering is the dominant mechanism for thermophoresis in longer cores, whereas for shorter cores, it is the highly diffusive mechanism that provides the effective force.

Resonance Raman spectroscopy as a probe of the crystallite size of MoS2 nanoparticles

Abstract Irregularly shaped and inorganic fullerene-like MoS2 compounds were characterized by Resonance Raman spectroscopy using an exciting line at 633 nm. It was shown that the relative intensity of the longitudinal acoustic mode at 226 cm−1 and its overtone strongly depends on the MoS2 crystallite size but not on the size of the particles made of agglomerated crystallites. This technique appeared as a promising probe to characterize in situ very small crystallites that are not observed by XRD.

Classical ferroelectriclike behavior of highly orderedPb(Sc1/2Nb1/2)O3studied by dielectric and Brillouin scattering spectroscopy

The ferroelectric phase transition behavior in the highly ordered $\mathrm{Pb}(\mathrm{S}{\mathrm{c}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}$ has been investigated by the dielectric and Brillouin spectroscopy. The dielectric permittivity ${\ensuremath{\varepsilon}}_{r}$ exhibits a sharp maximum without any frequency dispersion at its Curie temperature ${T}_{c}$. In the temperature range far above ${T}_{c}$, it was noted that the dielectric permittivity exhibits a noticeable deviation from the Curie-Weiss law below the characteristic intermediate temperature ${T}^{*}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}500\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is common to most of the Pb-based oxide perovskite relaxors. Upon cooling, the frequency of the longitudinal acoustic phonon mode exhibits a remarkable softening towards ${T}_{c}$. The relaxation time of the order parameter calculated using the Landau-Khalatnikov approach was determined to be more than one order of magnitude lower than that of the disordered $\mathrm{Pb}(\mathrm{S}{\mathrm{c}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}$ and is very close to that observed in the paraelectric phase of the classical ferroelectric, $\mathrm{BaTi}{\mathrm{O}}_{3}$. The observed dielectric and relaxation features are qualitatively discussed in terms of the difference in the strength of the random electric fields.

Bulk wave excitation from the surface of a LiNbO3 crystal by a system of planar electrodes

Ultrasound generation using planar electrodes deposited on the surface of a lithium niobate crystal is investigated. The analysis is carried out in an acousto-optic device based on the XY cut of a lithium niobate crystal and in an acousto-optic cell designed as a prototype of a modulator of non-polarized radiation. The experimental investigation is conducted at different frequencies of the driving electrical signal. The generated acoustic field is visualized by the acousto-optic method at different polarizations of the incident optical beam in case of slow shear and longitudinal acoustic modes. A relation between diffraction efficiencies for ordinary and extraordinary polarized optical radiation is analyzed. The obtained experimental results on the diffraction efficiencies are compared to the theoretically predicted values. It is demonstrated that the polarization characteristics of the devices differ from the conventional characteristics by the dependence on the structure of the acoustic beam.

Phase Transition Behaviors of Lead-Free (Na1/2Bi1/2)TiO3-BaTiO3 Single Crystals Studied by Inelastic Light Scattering Spectroscopy

Acoustic anomalies of (1-x)(Na1/2Bi1/2)TiO3-xBaTiO3 (NBT-xBT) with x = 0.05 were investigated over a wide temperature range of from 900°C to −200°C by using Brillouin light scattering. The sound velocity of the longitudinal acoustic mode propagating along the [100] direction showed a substantial softening, while the acoustic attenuation coefficient of this mode exhibited a large increase in the paraelectric phase. These anomalies were attributed to the formation and growth of noncentrosymmetric polar regions in the paraelectric phase, which was supported by the appearance of the strong polarized central peak. The absence of any sharp or discontinuous changes in the acoustic properties seems to suggest that the polar regions remain short-ranged and do not form macroscopic polar domains at low temperatures, which may be attributed to the inherent A-site disorder induced by random substitutions of Bi, Na and Ba cations.

Competing mechanism driving diverse pressure dependence of thermal conductivity ofXTe(X=Hg,Cd, and Zn)

Effectively engineering the lattice thermal conductivity of materials is a key interest of the current thermal science community. Pressure or compressive strain is one of the most worthwhile processes to modify the thermal transport property of materials, due to its robust tunability and flexibility of realization. While it is well documented in the literature that the application of hydrostatic pressure normally increases the thermal conductivity of bulk materials, little work has been performed on abnormal pressure-dependent thermal conductivity and the governing mechanism has not been fully understood yet. In this paper, taking bulk telluride systems $X\text{Te}$ $(X=\mathrm{Hg},\phantom{\rule{0.16em}{0ex}}\mathrm{Cd},\phantom{\rule{0.16em}{0ex}}\mathrm{Zn})$ as examples, we show, by combining first-principle calculation and the phonon Boltzmann transport equation, that the thermal conductivity presents diverse pressure dependence although they belong to the same group. The thermal conductivity of ZnTe is independent of pressure, while abnormal negative pressure dependence of thermal conductivity is observed in HgTe. As for CdTe, the trend falls in between HgTe and ZnTe and relies largely on the temperature. By comparing the key contributors of the lattice thermal conductivity, we find that the diverse pressure dependence of the lattice thermal conductivity is governed by the competition between the enhancement of group velocity of longitudinal acoustic and optic modes and the reduction of phonon relaxation time of transverse acoustic modes, with both effects being fully quantified by our calculation. Comparison with traditional bulk systems such as silicon further underpins the governing mechanism. The correlation between the diverse thermal transport phenomena and the nature of the atomic bonding is also qualitatively established. These findings are expected to deepen our understanding of manipulating phonon transport of bulk materials via simple compressive strain and are also helpful for related applications, such as optimizing thermoelectric performance.

Influence of the lattice mismatch on the lattice vibration modes for InAs/GaSb superlattices

Raman scattering study on a group of InAs/GaSb superlattice (SL) samples where the strain is systematically changed from tensile to compressive regime is presented. The effect of the lattice mismatch between the substrate and the epitaxially grown SL layers on particularly the InSb-like interface phonon frequencies is revealed in the backscattering geometry. The higher order folded longitudinal acoustic (FLA) phonon modes are also observed for samples having different superlattice periodicity. An ideality factor is incorporated into the model used for predicting the FLA phonon frequencies to simply express the deviation in the average acoustic velocity in the SL from the one in the homogeneous medium with abrupt transition in the interfaces.

Strong anharmonicity induces quantum melting of charge density wave in2H−NbSe2under pressure

The interplay between charge density wave (CDW) order and superconductivity has attracted much attention. This is the central issue of along standing debate in simple transition metal dichalcogenides without strong electronic correlations, such as 2H-NbSe$\_2$, in which twosuch phases coexist. The importance of anisotropic electron-phonon interaction has been recently highlighted from both theoretical and experimental point of view, and explains some of the key features of the formation of the CDW in this system. On the other hand, other aspects, such as the effects of anharmonicity, remain poorly understood despite their manifest importance in such soft-phonon driven phase transition. At the theoretical level in particular, their prohibitive computational price usually prevents their investigation within conventional perturbative approaches.Here, we address this issue using a combination of high resolution inelastic X-ray scattering measurements of the phonon dispersion, as afunction of temperature and pressure, with state of the art ab initio calculations. By explicitly taking into account anharmonic effects, we obtain an accurate, quantitative, description of the (P,T) dependence of the phonon spectrum, accounting for the rapid destruction of the CDW under pressure by zero mode vibrations - or quantum fluctuations - of the lattice. The low-energy longitudinal acoustic mode that drives the CDW transition barely contributes to superconductivity, explaining the insensitivity of the superconducting critical temperature to the CDW transition.

Ab-initio study of dynamical properties of two dimensional MoS2 under strain

In this paper, we report the biaxial strain induced modifications in the phonon dispersion curves of monolayer MoS2 using first principles calculations in the framework of density functional perturbation theory. We have used the ultrasoft psedopotentials and the exchange correlation energies have been approximated by the local density approximation. There are significant changes in the phonon dispersion curves under strain. A systematic decrease in the optical phonon modes is observed with strain percentage. The longitudinal and transverse acoustic phonon modes of monolayer MoS2 show linear dependency for both without and with strain cases. However, the out of plane acoustic mode (ZA) which behaves quadratically with wave vector k- turns linear under strain. We have also linked ripple in single layer (SL) MoS2 by ZA mode with strain using shell elasticity theory. The strain induced softening of linear behaviour of ZA mode with strain indicates the absence of rippling under strain. At a very high strain there is a possibility of structural phase transition as the ZA mode at zone centre turns imaginary above 15% strain. Our results indicate a close relationship between the morphology and properties enabling the device tailoring and bandgap engineering in SL-MoS2 by manipulating the strain.

Study of confined coherent acoustic phonon modes in a free-standing cubic GaN membrane by femtosecond spectroscopy

Confined longitudinal coherent acoustic phonon modes are excited and detected in a sub-μm-thick free-standing cubic GaN membrane by femtosecond pump-probe spectroscopy. After fs laser excitation, Brillouin oscillation and thickness oscillation with frequencies up to 100 GHz are observed and studied in the time domain. We found an initial expansion of the membrane upon optical excitation at 400 nm. Our experimental results confirmed earlier existing theoretical predictions and experimental observations that the hydrostatic deformation potential of cubic GaN is positive.

Combustion Instability Mode Transition in a Pilot Bluff-Body Stabilized Combustor

The pilot flame’s effects on dynamic stability have been investigated in a pilot bluff-body stabilized combustor at atmospheric pressure. During the experiment, the pilot fuel proportion increased from 6 to 20% with an interval of 2%, while the total equivalence ratio remained unchanged. Simultaneous dynamic pressure and unsteady heat release rate measurement and analysis were used to identify and characterize combustion instability modes. Combustion dynamics were captured by a high-speed camera. High-speed images were further analyzed by using proper orthogonal decomposition. With the increase of pilot fuel proportion, the transition between combustion instability modes of 33 and 130 Hz was observed. Based on acoustic mode analysis, the 33 and 130 Hz modes corresponded to the first and second longitudinal acoustic mode, respectively. The results show that the pilot fuel proportion affects the local equivalence ratio distribution in the shear layer and the recirculation zone. As a result, it has significant effects on the dynamics of pilot and main flame interactions, which lead to different unsteady heat release characteristics. Finally, the phenomenon of mode transition occurs when different acoustic modes have been excited.

Linear magnetotransport in monolayerMoS2

A momentum balance equation is developed to investigate the magnetotransport properties in monolayer molybdenum disulphide when a strong perpendicular magnetic field and a weak in-plane electric field are applied simultaneously. At low temperature, in the presence of intravalley impurity scattering Shubnikov de Haas oscillation shows up accompanying by a beating pattern arising from large spin splitting and its period may halve due to high-order oscillating term at large magnetic field for samples with ultrahigh mobility. In the case of intervalley disorders, there exists a magnetic-field range where the magnetoresistivity almost vanishes. For low-mobility layer, a phase-inversion of oscillating peaks is acquired in accordance with recent experiment. At high temperature when Shubnikov de Haas oscillation is suppressed, the magnetophonon resonances induced by both optical phonons (mainly due to homopolar and Fr\"ohlich modes) and acoustic phonons (mainly due to intravalley transverse and longitudinal acoustic modes) emerge for suspended system with high mobility. For the single layer on a substrate, another resonance due to surface optical phonons may occur, resulting in a complex behavior of the total magnetoresistance. The beating pattern of magnetophonon resonance due to optical phonons can also be observed. However, for nonsuspended layer with low mobility, the magnetoresistance oscillation almost disappears and the resistivity increases with field monotonously.

Test of a virtual cylindrical acoustic resonator for determining the Boltzmann constant

We report progress toward determining the Boltzmann constant kB using the concept of a virtual acoustic resonator, a hypothetical resonator that is mathematically equivalent to a cylindrical cavity with periodic boundary conditions. We derived the virtual resonator by combining the measured frequencies of the longitudinal acoustic modes of two argon-filled, cylindrical cavity resonators in such a way to minimize the effects of the cavities’ ends, including transducers and ducts attached to the ends. The cavities had lengths of 80 mm and 160 mm and were operated in their longitudinal (ℓ,0,0) modes. We explored virtual resonators that combine modes of the two resonators that have nearly the same frequencies. The virtual resonator formed from the (2,0,0) mode of the 80 mm resonator combined with the (4,0,0) mode of the 160 mm resonator yielded a value for kB that is, fractionally, only (0.2 ± 1.5) × 10−6 larger than the 2010 CODATA-recommended value of kB. (The estimated uncertainty is one standard uncertainty corresponding to a 68% confidence level.) The same virtual resonator yielded values of the pressure derivatives of the speed of sound c in argon, (∂c2/∂p)T and (∂c2/∂p2)T, that differed from literature values by 1% and 2%, respectively. By comparison, when each cavity was considered separately, the values of kB, (∂c2/∂p)T, and (∂c2/∂p2)T differed from literature values by up to 7 ppm, 10%, and 5%, respectively. However, combining the results from the (3,0,0) or (4,0,0) modes of shorter resonator with the results from the (6,0,0) or (8,0,0) modes of the longer resonator yielded incorrect values of kB that varied from run-to-run. We speculate that these puzzling results originated in an unmodeled coupling, either between the two cavities (that resonated at nearly identical resonance frequencies in the same pressure vessel) or between the cavities and modes of the pressure vessel.

Unsteady High-Order Simulation of a Liquid Oxygen/Gaseous Hydrogen Rocket Combustor

Coupled Euler–Lagrange simulations are performed for a subcritically operated model rocket combustor at 10 bar pressure (MASCOTTE A-10 test case). A Lagrangian spray model is used to simulate the liquid phase. This liquid oxygen/gaseous hydrogen single element combustor experiment has been the focus of many numerical studies. In contrast to previous investigations, time-accurate unsteady Reynolds-averaged Navier–Stokes simulations are performed. A good spatial resolution is achieved by using a fifth-order multidimensional limiting process scheme to discretize the inviscid fluxes. Instead of assuming axisymmetry, the three-dimensional geometry of the rectangular combustor is taken into account. However, only a quarter of the combustor is simulated, to limit the computational cost. Presented results indicate that three-dimensional effects are important, questioning the assumption of axisymmetry. A second important finding is that time-averaged unsteady Reynolds-averaged Navier–Stokes results agree much better with experimental data than those of a comparable steady-state simulation. Close to the injector, results show a strong unsteadiness of the turbulent flame, which has been observed in the experiment, too. Finally, pressure fluctuations are monitored at several positions in the combustor and analyzed by fast Fourier transform. The peak amplitudes in the pressure spectra could successfully be assigned to the first longitudinal acoustic mode and its higher harmonics.

Elastic anomaly and aging of new type of incommensurate phase transition in ferroelectric barium sodium niobate

The lattice instability of the incommensurate (IC) phase transition of uniaxial ferroelectric Ba2NaNb5O15 (BNN) was investigated by micro-Brillouin scattering. Spectra of the longitudinal acoustic (LA) mode were observed from room temperature to 750 K. In the vicinity of the IC phase transition temperature TIC = 573 K, elastic anomalies in the form of a sharp peak in the sound velocity and thermal hysteresis during the heating and cooling cycle were observed. During this transition, the crystal point group changed from tetragonal 4mm to orthorhombic 2mm along with the IC modulation. In order to deepen our understanding of the thermal hysteresis, aging experiment in the IC phase was conducted. We can conclude that the appearance of thermal hysteresis related to the relaxation of ferroelastic strain is related to the feature of the new type III IC phase transition mechanism of BNN.

Influence of KF substitution on the ferroelectric phase transition of lead titanate single crystals studied by Brillouin light scattering

The elastic properties of KF-substituted perovskite lead titanate ( PbTiO 3) were investigated by dielectric measurements and Brillouin light scattering. The ferroelectric phase transition occurred at substantially lower temperature due to KF substitution, which was attributed to the modification of the covalency in Pb – O and Ti – O bonds. The longitudinal acoustic (LA) mode of KF-substituted PbTiO 3 showed a frequency softening in the paraelectric phase, which was accompanied by increasing acoustic damping. This indicated that polarization fluctuations responsible for the acoustic anomalies were enhanced by KF substitution.

Infrared versus light scattering techniques to monitor the gel to liquid crystal phase transition in lipid membranes

In this study, Fourier transform infrared, Raman and Brillouin spectroscopy have been used to study lipid phase behavior of hydrated as well as dried multilamellar l‐α‐phosphatidylcholine assemblies, in order to compare limitations and potentials of the different techniques. Dried lipid samples have been studied in the presence and absence of trehalose, which is known to affect the phase behavior of these systems. The methylene C‐H stretching (2800–3000 cm−1) region in infrared (IR) and Raman spectra provided mutually consistent information on the rearrangement of lipid acyl chains occurring at the lipid melting temperature. IR spectra have a higher signal‐to‐noise ratio, thus permitting a more precise evaluation of the melting temperature. In the hydrated lipid samples, the C‐H stretching region in the Raman spectra is less affected by the contribution of water compared with that in the IR spectra. Raman spectra are particularly suitable to simultaneously study both lipid and water contributions allowing to distinguish ice from non‐frozen water below 0 °C. Brillouin light scattering was used to probe the collective dynamics, i.e. the propagation velocity and the attenuation of longitudinal acoustic modes in the lipid samples. Lipid phase transitions are evident from a change in the temperature behavior of the acoustic velocity. Moreover, a strong relaxation process with a characteristic time of 14 ps was observed in the sample dried without trehalose with a maximum in acoustic attenuation at about 45 °C, which likely reflects the rearrangement of acyl chains. Copyright © 2015 John Wiley & Sons, Ltd.