Longitudinal Phonon Frequencies Research Articles

Spectroscopy of phononic switching of magnetization in the terahertz gap

All-optical schemes for switching magnetization offer a pathway towards the creation of more advanced data-storage technologies, both in terms of recording speed and energy-efficiency. It has previously been shown that picosecond-long optical pulses with central frequencies ranging between 12 and 30 THz are capable of driving magnetic switching in yttrium-iron-garnet films, provided that the excitation frequency matches the characteristic frequency of longitudinal optical phonons. Here, we explore how the phononic mechanism of magnetic switching in three distinct ferrimagnetic iron-garnet films evolves at optical frequencies below 10 THz, within the so-called terahertz gap. We find that at long wavelengths the magnetic switching rather correlates with phonon modes associated with the substrate. Our results show that the process of phononic switching of magnetization, previously discovered in the mid- to far-infrared spectral range, becomes much more complex at frequencies within the terahertz gap.

Simultaneous optical resonances at visible and mid-infrared frequencies with epitaxial TiN/Al0.72Sc0.28N/TiN metal/polar-dielectric/metal multilayers

Traditionally, light-matter interactions in the visible spectral range utilize tailored plasmonic nanostructure in metals that often require advanced nanofabrication techniques. Concomitantly, polar lattice vibrations in dielectrics are used to achieve mid-to-long wavelength infrared (IR) light-matter coupling. Achieving optical resonances simultaneously in the visible and mid-to-long wavelength IR spectral range in one host medium has been challenging due to mutually conflicting material and optical property requirements. In this article, we show simultaneous light-matter coupling and development of super absorbers in the visible and mid-IR spectral ranges with a lithography-free planar wide-angle metal-(polar)-dielectric-metal (MDM) Fabry-Pérot cavity composed of ultrathin refractory metallic TiN as the top layer and polar dielectric Al0.72Sc0.28N as the spacer layer. Lattice-matched TiN/Al0.72Sc0.28N/TiN MDM cavities exhibit super absorption resonance with maximum absorptivity of ∼99%. The absorption maxima spectral position is tuned with changes in spacer layer thickness and by utilizing multiple cavity interactions. Al0.72Sc0.28N inside the MDM is also used for light coupling to the transverse optical phonon mode and to the Berremann mode near the longitudinal phonon frequency with strong selective absorption. Demonstration of refractory epitaxial TiN/Al0.72Sc0.28N/TiN MDM metamaterials mark significant progress towards developing compact optical meta-structures with on-demand optical resonances at different spectral ranges.

Brillouin Light Scattering Characterisation of Gray Tone 3D Printed Isotropic Materials.

Three-dimensional direct laser writing technology enables one to print polymer microstructures whose size varies from a few hundred nanometers to a few millimeters. It has been shown that, by tuning the laser power during writing, one can adjust continuously the optical and elastic properties with the same base material. This process is referred to as gray-tone lithography. In this paper, we characterize by Brillouin light scattering the complex elastic constant of different reticulated isotropic polymers, at longitudinal phonon frequencies of the order of 16 GHz. We estimate the real part of the constant to vary from 7 to 11 GPa as a function of laser power, whereas its imaginary part varies between 0.25 and 0.6 GPa. The linear elastic properties are further measured at a fixed laser power as a function of temperature, from C to C. Overall, we show that our 3D printed samples have a good elastic quality with high Q factors only ten times smaller than fused silica at hypersonic frequencies.

The Response of Phonon Frequencies, Sound Velocity, Electronic, Optical, and Mechanical Properties of Indium (Phosphide, Arsenide, and Antimonide) to Hydrostatic Pressure

The pressure dependence of phonon vibrations, the velocity of sound, and some other related physical quantities of and semiconductors have been determined. This investigation is done using the empirical pseudopotential method. We proposed a pressure dependence of the pseudopotential form factors; hence we were able to calculate the pressure dependence of the fundamental energy gaps. Using the values of the fundamental band gaps, we were able to calculate the refractive index, static dielectric constant, high-frequency dielectric constant, longitudinal and transverse phonon frequencies, elastic constants, mechanical moduli, and velocity of sound. Our findings may serve as a reference for experimental work at high pressures.

Phononic manipulation of antiferromagnetic domains in NiO

The interaction between the crystal lattice and spins appears to be a very efficient mechanism for non-thermal manipulation of magnetic order. A particularly intriguing question is whether lattice vibrations can also be used to control or even switch domains in collinear antiferromagnets, where all other mechanisms fail. To answer this question, here we study the magnetic effects induced by tunable and narrow-band infrared optical pulses in single crystals of antiferromagnetic nickel oxide. Our measurements reveal that a resonant excitation in the vicinity of the frequency of longitudinal optical phonons leads to the displacement of antiferromagnetic domains in NiO, potentially revealing a promising method to manipulate antiferromagnetic order.

Polaritonic quantization in nonlocal polar materials.

In the Reststrahlen region, between the transverse and longitudinal phonon frequencies, polar dielectric materials respond metallically to light, and the resulting strong light-matter interactions can lead to the formation of hybrid quasiparticles termed surface phonon polaritons. Recent works have demonstrated that when an optical system contains nanoscale polar elements, these excitations can acquire a longitudinal field component as a result of the material dispersion of the lattice, leading to the formation of secondary quasiparticles termed longitudinal-transverse polaritons. In this work, we build on previous macroscopic electromagnetic theories, developing a full second-quantized theory of longitudinal-transverse polaritons. Beginning from the Hamiltonian of the light-matter system, we treat distortion to the lattice, introducing an elastic free energy. We then diagonalize the Hamiltonian, demonstrating that the equations of motion for the polariton are equivalent to those of macroscopic electromagnetism and quantize the nonlocal operators. Finally, we demonstrate how to reconstruct the electromagnetic fields in terms of the polariton states and explore polariton induced enhancements of the Purcell factor. These results demonstrate how nonlocality can narrow, enhance, and spectrally tune near-field emission with applications in mid-infrared sensing.

Elastic, lattice dynamical, thermal, electronic, and optical properties of nano-semiconductor CdTe under the effect of temperature

The energy band gaps of CdTe semiconductor material have been determined using the empirical pseudo-potential method (EPM). The refractive index (n), high frequency (ε∞) and static dielectric constants (εs) of the considered material have been studied. The elastic constants, as well as some associated factors like bulk, (Bu), shear (Cs), Young (Y0) moduli, isotropy factor (A), and Poisson ratio (σ), have been reported for CdTe in zinc blende structure. The longitudinal and transverse phonon frequencies (ωLO,ωTO), and Debye temperature (θD) have also been calculated. All of the variables studied have been investigated in terms of their temperature sensitivity. Generally, Our findings are in good agreement with those published in the literature. The studied material could be useful in the fabrication of optoelectronic devices under high temperatures.

Peculiarities of specular infrared reflection spectra of ZnO-based ceramics

Undoped and Mn-doped ZnO ceramics were theoretically and experimentally investigated using specular infrared reflection method. It was shown that infrared reflection spectra can be modeled using the parameters explored for ZnO single crystals. For ceramic samples, it was shown that ZnO grains with orientation of the C-axis along the normal to the electric field ( ) give the main contribution to IR reflection spectra. It has been ascertained that the surface roughness is manifested in these spectra mainly within the range 450…550 cm–1 giving negligible effect for the frequencies above longitudinal phonon frequency. This allowed the electrophysical parameters of ZnO crystallites to be evaluated. In the case of undoped ceramics, the obtained results were found to be consistent with the values of direct current measurements. This finding supports the utility of infrared spectroscopy for determination of the electrophysical parameters of polycrystalline ceramic materials. For Mn-doped ceramic samples, the conductivity value measured using the direct current method was found to be essentially lower than those determined from simulation of infrared reflection spectra. This phenomenon was explained by barrier formation at the grain boundaries in Mn-doped ZnO ceramics.

Absorption of Powerful Light by Free Electrons in Crystals: Intraband Electron–Phonon Rabi Oscillations

The absorption of high-power visible or near-IR laser radiation by free electrons is calculated using the modified resonance approximation. The probabilities Wexc of intraband transitions at which the electron energy changes by ℏ(ω ± ωl), where ω is the frequency of light and ωl is the frequency of longitudinal optical phonons involved in the process, are determined. It is shown that specific electron–phonon Rabi oscillations at a frequency of ΩR may take place, and that the light is absorbed only up to the moment of time at which the first maximum τ1 on the dependence Wexc(t) is reached, where t is the time elapsed from the beginning of the laser pulse. It is shown that, in the case of prebreakdown light intensities, processes of high orders in the field of the light wave affect substantially Wexc, τ1, and ΩR.

Optical phonons with a negative oscillator strength

We discuss the nature of additional (redundant from the point of view of selection rules) optical phonons observed in most crystals with ion–covalent bonds between atoms, including in their solid solutions. These ‘redundant’ phonons are located in the frequency range of the longitudinal–transverse splitting of fundamental phonons, where the real part of the crystal permittivity is negative. They are also split by the crystal field into transverse and longitudinal phonons such that the frequencies of redundant longitudinal phonons are lower than those of redundant transverse phonons and the oscillator strength of these phonons is negative.

Multipolar scattering of subwavelength interacting particles: Extraction of effective properties between transverse and longitudinal optical modes

The general concern of this investigation is the extraction of the effective electromagnetic properties of agglomerates of randomly located particles, which are small compared to the wavelength. The focus is on the spectral window ranging from the transverse phonon to the longitudinal phonon frequencies, in which resonances may be excited. In this frequency domain, it is shown that limiting the problem to bare dipole-dipole interactions leads to inaccurate calculations of the electromagnetic fields, resulting in a doubtful extraction of the effective properties. We perform the complete electromagnetic calculation by taking into account the higher multipole orders that are activated when the agglomerates are illuminated. Several results, which are not usually observed outside the frequency range highlighted here, are revealed by means of extensive numerical simulations. In particular, we evidence large deviations compared to the predictions provided by the effective medium theories, while we find that the volume of the microstructures (each one with a different internal geometry) that are used to average the fields must be unusually large to avoid a bias in the determination of the effective properties. Furthermore, the evolution of the incoherent component of the fields between the two optical phonon modes is investigated.

Evaluation of structural, optical and dielectric properties of MWCNT-BaTiO3/silica ceramic nanocomposites

In this research, MWCNT-BaTiO3/silica nano-composites were synthesized and analyzed at different MWCNTs loadings (2, 4, 6, and 8% wt). Utilizing the different concentrations of MWCNTs, the optical responses of MWCNT-BaTiO3/silica nanocomposite were investigated. For this purpose, X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) spectroscopy were used. Using the Kramers-Kronig method, the refractive index and dielectric coefficient of MWCNT-BaTiO3/silica nanocomposites were analyzed. Results clearly revealed that the higher incorporation of MWCNTs in nanocomposites led to more strong responses in the real parts of both refractive index and dielectric coefficient. Finally, the transversal (TO)/longitudinal (LO) phonon frequencies shifted to the blue wavenumbers by decreasing the amount of MWCNTs in MWCNT-BaTiO3/silica nanocomposite.

Поглощение мощного света свободными электронами в кристаллах: внутризонные электрон-фононные осцилляции Раби

Absorption of high-intensity visible or near-IR pulsed laser radiation at free electrons in crystals is calculated in the modified resonance approximation. The probabilities $W_{exc}$ of intraband transitions with a change in the electron energy by $\hbar(\omega\pm\omega_l)$, where $\omega$ is the radiation frequency and $/omega_l$ the frequency of longitudinal optical phonons involved in the process, are derived. It is shown that specific electron–phonon Rabi oscillations with the frequency $\Omega_R$ can occur and radiation is absorbed only until the point in time $\tau_1$, at which the first maximum in the dependence $W_{exc} (t)$, where t is the time elapsed from the beginning of the laser pulse, is reached. It is shown that, in the case of prebreakdown radiation intensities, the processes of high orders in field of the radiation wave substantially influence $W_{exc}$, $\tau_1$, and $\Omega_R$.

Influence of Dielectric Screenings on Phonon Frequencies and Acoustic Properties of Pd-Based Bulk Metallic Glasses

The phonon dispersion curves for bulk metallic glasses (BMGs) Pd40Ni10Cu30P20 and Pd64Ni16P20 are computed for the longitudinal and transverse phonon frequencies using the simple model given by Bhatia and Singh. Different dielectric screening functions are employed for the longitudinal mode. We obtain the values of the force constants β and δ calculated from the elastic constants of the material of the respective BMGs for computing the dispersion curves. The computed phonon dispersion curves show appropriate behaviour for both the longitudinal and transverse modes. The transverse sound velocity and the longitudinal sound velocities with various dielectric screenings are calculated in the long wavelength region from the computed dispersion curves for both the BMGs. The first peak position of the static structure factor is predicted from the dispersion curves. The values of sound velocities and the first peak of the static structure factor estimated from the computed dispersion curves show excellent agreement with the experimental values reported in literature for the BMGs under consideration and the results may be used for correlating other properties of the BMGs.

Assessment of the optical and dielectric properties of f-MWCNTs/BaTiO3 nanocomposite ceramics

This research systematically evaluated the fabrication of the functionalized multi-walled carbon nanotubes (f-MWCNTs)/barium titanate (BaTiO3) nanocomposite through the use of a simple two-step mixing route. Different concentrations of f-MWCNTs (0.25%, 0.5%, 1%, 2%, 4%, and 8% wt) were applied to the BaTiO3 matrix. The effects of different concentrations of f-MWCNTs on the structural and optical properties of BaTiO3 were investigated. Scanning Electron Microscopy (SEM) and Fourier Transform Infrared (FTIR) spectra were used for sample analysis. Using the FTIR spectra, the optical parameters of the refractive index and permittivity of the synthesized samples were calculated by Kramers-Kronig relations. The optical constants of f-MWCNTs/ BaTiO3 nanocomposites were strongly related to the concentrations of f-MWCNTs. This is because by increasing the concentrations of f-MWCNTs, both the real/imaginary parts of the refractive index and dielectric function were increased. Consequently, the transversal (TO) and longitudinal (LO) phonon frequencies were detected. The TO and LO frequencies shifted to the red frequencies by increasing the amount of f-MWCNTs in the BaTiO3 matrix.

Phonon dispersion evolution in uniaxially strained aluminum crystal

The influence of loading upon the phonon dispersion of crystalline materials could be highly nonlinear with certain particular trends that depend upon the loading path. In this paper, we have calculated the influence of [100] uniaxial strain on the phonon dispersion and group velocities in fcc aluminum using second moments of position obtained from molecular dynamics (MD) simulation at 300 K. In contrast to nonlinear monotonic variation of both longitudinal and transverse phonon frequencies along the $$\Delta $$ , $$\Lambda $$ and $$\Sigma $$ lines of the first Brillouin zone under tension, transverse phonon branches along the $$\Lambda $$ line show inflection at specific wavevectors when the compressive strain exceeds 5%. Further, the longitudinal group velocities along the high-symmetry $$\Delta $$ line vary non-monotonically with strain, reaching a minimum at 5% compressive strain. Throughout the strain range studied, the equilibrium positions of atoms displace in an affine manner preserving certain static structural symmetry. We attribute the anomalies in the phonon dispersion to the non-affine evolution of second moments of atomic position, and the associated plateauing of force constants under the applied strain path.

Optical and dielectric properties of NiFe2O4 nanoparticles under different synthesized temperature

Abstract In this research, NiFe 2 O 4 nanoparticles was prepared via the simple sol-gel route, using different sintering temperature. This nanoparticle was characterized via X-ray diffraction (XRD) pattern, scanning electron microscopy (SEM), and FTIR spectra. The XRD patterns show by increasing the synthesized temperature, the intensity, and broadening of peaks are decreased so the results are more crystallization and raising the size of nanoparticles. The size distribution in the histogram of the NiFe 2 O 4 nanoparticles is 42, 96, and 315 nm at 750 °C, 850 °C, and 950 °C, respectively. The FTIR spectra were evaluated using Kramers-Kronig method. Results approved the existing of certain relations between sintering temperatures and grain size of nanoparticles. By raising the temperature from 750 °C to 950 °C, the grain size was increased from 70 nm to 300 nm and the optical constants of nanoparticles were strongly related to synthesizing temperature as well. Since by increasing temperature, both real/imaginary parts of the refractive index and dielectric function were decreased. Consequently, the transversal (TO) and longitudinal (LO) phonon frequencies are detected. The TO and LO frequencies have shifted to red frequencies by increasing reaction temperature.

Theoretical Studies on Optical Phonon and Surface Phonon Polariton of Wurtzite AlInN Alloys

The lattice vibrational properties of wurtzite ternary mixed crystal aluminium indium nitride (AlxIn1-xN) are investigated thoroughly using modified random element iso-displacement (MREI) model and Born-Huang procedure. MREI model, which considers the nearest neighbour interactions, is used to predict the composition dependence of longitudinal and transverse optical phonon frequencies. For AlxIn1-xN alloy, oscillator strength of its weak mode is sufficiently significant for composition range of 0 < x < 0.4. As a result, AlxIn1-xN alloy is deduced to exhibit mixed-mode behaviour. Finally, the calculated dielectric functions for the entire composition range (0 < x < 1) are used to simulate the surface phonon polariton characteristics of the AlxIn1-xN alloy.

First-principles study of the mobility ofSrTiO3

We investigate the electronic and vibrational spectra of ${\mathrm{SrTiO}}_{3}$, as well as the coupling between them, using first-principles calculations. We compute electron-phonon scattering rates for the three lowest-energy conduction bands and use Boltzmann transport theory to calculate the room-temperature mobility of ${\mathrm{SrTiO}}_{3}$. The results agree with experiment and highlight the strong impact of longitudinal optical phonon scattering. Our analysis provides important insights into the key factors that determine room-temperature mobility, such as the number of conduction bands and the nature and frequencies of longitudinal phonons. Such insights provide routes to engineering materials with enhanced mobilities.

Quantum hexatic order in two-dimensional dipolar and charged fluids

Recent advances in cold atom experimentation suggest that studies of quantum two-dimensional melting of dipolar molecules, with dipoles aligned perpendicular to ordering plane, may be on the horizon. An intriguing aspect of this problem is that two-dimensional classical aligned dipoles (already studied in great detail in soft matter experiments on magnetic colloids) are known to melt via a two-stage process, with an intermediate hexatic phase separating the usual crystal and isotropic fluid phases. We estimate here the effect of quantum fluctuations on this hexatic phase, for both dipolar systems and charged Wigner crystals. Our approximate phase diagrams rely on a pair of Lindemann criteria, suitably adapted to deal with the effects of thermal fluctuations in two dimensions. As part of our analysis, we determine the phonon spectra of quantum particles on a triangular lattice interacting with repulsive $1/{r}^{3}$ and $1/r$ potentials. A large softening of the transverse and longitudinal phonon frequencies, due to both lattice effects and quantum fluctuations, plays a significant role in our analysis. The hexatic phase is predicted to survive down to very low temperatures.