Parametric excitation of acoustohelicon waves in a piezoelectric semiconductor

The parametric excitation of acoustohelicon waves has been studied in a piezoelectric semiconductor in the presence of a strong high frequency oscillatory electric field. The threshold electric field amplitude and the growth rate of the unstable mode have been obtained analytically and for n-InSb at 77 K the unstable mode is found to be propagating with a growth rate ～10³ s^?1 when the crystal is irradiated with a 10.6 μm CO₂ laser.     

Lateral-field-excitation properties of LiNbO3 single crystal

LiNbO 3 has been found attractive for lateral field excitation (LFE) applications due to its high piezoelectric coupling. In this paper, bulk acoustic wave propagation properties for LiNbO 3 single crystal excited by a lateral electric field have been investigated using the extended Christoffel-Bechmann method. It is found that the LFE piezoelectric coupling factor for c mode reaches its maximum value of 95.46% when ψ = 0 for both (yxl)-58 and (yxwl)±60 /58 LiNbO 3 . The acoustic wave phase velocity of c mode TSM (thickness shear mode) changes from 3456 m/s to 3983 m/s as a function of ψ. Here ψ represents the angle between the lateral electric field and the crystallographic X-axis in the substrate major surface. A 5 MHz LFE device of (yxl)-58 LiNbO 3 with ψ = 0 was designed and tested in air. A major resonance peak was observed with the motional resistance as low as 17 and the Q-factor value up to 10353. The test result is well in agreement with the theoretical analysis, and suggests that the LFE LiNbO 3 device can be a good platform for high performance resonator or sensor applications.     

Stimulated raman scattering from plasma modes in magnetoactive semiconductors

Stimulated scattering off electron plasma mode is investigated analytically for the case when the pump wave is an intense circularly polarised electromagnetic wave propagating parallel to a homogeneous dc magnetic field in an isotropic semiconductor-plasma. The threshold electric field of the pump necessary for the stimulated Raman scattering and the growth rate of the parametrically unstable mode have been obtained for two cases (i)B ₀=0 and (ii) B₀ ≠ 0. It is seen that the magnetic field does not significantly affect the threshold electric field as well as the growth rate provided the cyclotron frequency is small compared to the frequency of the pump wave. The threshold conditions are also found to be insensitive to the electron thermal velocity.     

Instability of electromagnetic waves in transversely magnetised semiconductor-plasmas

Using the hydrodynamic model of plasmas the general dispersion relation is derived in the collisiondominated regime when a d.c. magnetic field is applied (Y-axis) transversly to the propagation vector k (Z-axis), and the d.c. electric field is inclined to the Z-axis in the X-Z plane. The dispersion relation is solved for intrinsic and extrinsic semiconductors to explore the possibility of wave instability. The threshold conditions of wave oscillations are obtained. In n-InSb the frequency of the oscillation attains a maximum value when the electron cyclotron frequency is equal to the electron collision frequency. In intrinsic InSb instability is possible only in the long wavelength region for E₀ ? 10 kVm^?1 when B₀> 0.2 T, while for lower values of B₀, E₀ should be greater 20kVm^?1. The energy dependent collision frequency has a significant effect on the threshold frequency of oscillation.     

Morphic effects — III. Effects of an external magnetic field on the long wavelength optical phonons

The changes in the frequencies of the k ≈ 0 optical vibration modes on the application of a static, external magnetic field to a non-magnetic crystal are determined to first order in the field strength. Second order effects are equivalent to the effects of an electric field in second order and they are not considered here. It is shown that the frequency of a nondegenerate mode is not altered to first order in the magnetic field. In the case of the noncubic crystal structures it is found that the magnetic field must have a component along the axis of highest symmetry in order that the doubly degenerate modes at k ≈ 0 have their degeneracy lifted. In the case of the cubic structures a magnetic field applied in any direction can completely split the degeneracy of modes which are triply degenerate at k ≈ 0. Expressions are given for the field induced changes in the normal mode frequencies. The modes whose frequencies are shifted are found to be right or left circularly polarized. A brief discussion is given of spatial dispersion effects, that is, splitting of the mode degeneracy linear in the phonon wave-vector. Finally, a review of the symmetry aspects of acoustical activity and Faraday effects of acoustical phonons is presented.     

Piezoelectric measurements on BiFeO3 single crystal by Raman scattering

The piezoelectric response of BiFeO₃ at low temperature has been investigated by Raman scattering measurements. The application of an external electric field at T=10 K induces frequency shifts of the lowest frequency mode related to the Bi-O bonds and corresponding to the soft mode of the ferroelectric transition. The piezoelectric effect is responsible for the softening of this mode via the tensile stress leading to the expansion of the crystal. The phonon deformation potential associated with the soft mode has been estimated around −200 cm⁻¹/strain units using the linear piezoelectric coefficient d₃₃=16 pm/V. It found in the range of the ones obtained for typical piezoelectrics.     

Bulk magnetization study of a DyNi5 single crystal

Magnetization and susceptibility measurements were performed on a single crystal of DyNi₅ along the three main symmetry axes of the ortho-hexagonal cell. Below its ordering temperature (T_c = 11.6 K), b and c are respectively the easy and hard magnetization axes. The strong anisotropy originates from the crystalline electric field acting on the 4f electrons of the Dy³⁺ ions. A small magnetization is induced on nickel atoms by the applied field and the exchange interactions with the dysprosium atoms. The crystal field parameters, the molecular field coefficients and the susceptibility of nickel atoms are determined from the experimental data.     

Modulated magnetic structure of a nonuniformly stressed FeBO3 single crystal

The magnetization of a nonuniformly stressed FeBO₃ crystal along any of the two specific directions in the basal plane (the easy plane) at a temperature of T < 140 K in a magnetic field exceeding the threshold value H ₀ is found to lead to a transition of the crystal from the uniform magnetic state to the spatially modulated one. The modulated magnetic phase arising under these conditions exists in a certain temperature-dependent field range H ₀ ≤ H ≤ H _c and is representable in the form of a static spin wave that is linearly polarized in the easy plane of the crystal and has a wave vector k oriented at an angle of ～30° to the magnetization axis. The field, temperature, and orientation dependences of k are investigated. A physical mechanism is proposed to explain the modulation of the magnetic order parameter of the crystal under study. The results obtained are discussed in terms of the magnetic ripple theory.     

Special features of the electric components of acoustic waves in the vicinity of nonpiezoactive directions in crystals

The conditions of existence of the zero components of electric field E and electric induction D accompanying a volume acoustic wave propagating in a piezoelectric medium have been studied. General equations describing the positions of the zero-field lines E(m) = 0 and the zero-induction points m₀, such that D(m₀) = 0 on the unit sphere (m² = 1) of the wave propagation directions, are obtained. General theorems determining the conditions ensuring the existence of such lines and points, even in triclinic crystals, are formulated. The relationship between such directions and various elements of the crystal symmetry is analyzed. The vector fields D(m), which are always orthogonal to the wave normals m, in the vicinity of the zero-induction points m₀ exhibit certain orientational singularities characterized by the Poincaré indices n = 0, ±1, ±2. The general analytical expressions are obtained for the n values in crystals with arbitrary anisotropy and specified for a number of crystals belonging to various symmetry classes. The conditions of stability of the orientational singularities with respect to small perturbations of the material moduli and a change in the crystal symmetry are considered.     

The pure-shear mode solidly mounted resonator based on c-axis oriented ZnO film

In this paper, we fabricate a pure-shear mode film bulk acoustic resonator based on c-axis oriented ZnO film. The resonator is consisted of an in-plane electrode, a highly c-axis oriented ZnO film and a SiO₂/W Bragg reflector. The shear mode wave is excited by the lateral electric field. The resonator works in a pure-shear mode with the resonance frequency near 1.5 GHz and the Q-factor of 479 in air. There is no obvious longitudinal mode resonance in the frequency response, which can be explained that the electric field component normal to the surface is very weak and the Bragg reflector has the effective frequency selectivity for the shear mode. Importantly for sensors, the immersion into de-ionized water and glycerol liquid still allows for a Q-factor up to 335 and 220, respectively. This resonator shows the potential as mass loading sensors for biochemical application.     

Propagation regimes of intense circularly polarized laser beam in magnetoactive plasma

This paper presents an analytical and numerical investigation of an intense circularly polarized wave propagating along the static magnetic field parallel to oscillating magnetic field in magnetoactive plasma. In the relativistic regime such a magnetic field is created by pulse itself. The authors have studied different regimes of propagation with relativistic electron mass effect for magnetized plasma. An appropriate expression for dielectric tensor in relativistic magnetoactive plasma has been evaluated under paraxial theory. Two modes of propagation as extraordinary and ordinary exist; because of the relativistic effect, ultra-strong magnetic fields are generated which significantly influence the propagation of laser beam in plasma. The nature of propagation is characterized through the critical-divider curves in the normalized beam width with power plane For given values of normalized density (ω_p/ω) and magnetic field (ω_c/ω) the regions are namely steady divergence (SD), oscillatory divergence (OD) and self-focusing (SF). Numerical computations are performed for typical parameters of relativistic laser-plasma interaction: magnetic field B = 10-100 MG; intensity I = 10¹⁶ to 10²⁰ W/cm²; laser frequency ω = 1.1 × 10¹⁵ s⁻¹; cyclotron frequency ω_c = 1.7 × 10¹³ s⁻¹; electron density n_e = 2.18 × 10²⁰ cm⁻³. From the calculations, we confirm that a circularly polarized wave can propagate in different regimes for both the modes, and explicitly indicating enhancement in wave propagation, beam focusing/self-guiding and penetration of E-mode in presence of magnetic field.     

Beat excitation of terahertz radiation in a semiconductor slab in a magnetic field

Two infrared lasers of frequencies ω₁ and ω₂ propagating in the TM/TE mode along $\stackrel{?}{z}$ direction in a rippled density semiconductor waveguide are shown to resonantly excite terahertz radiation at the beat frequency when ripple wave number is suitably chosen to satisfy the phase matching. The wave vector of the density ripple is along the direction of laser propagation while a static magnetic field is applied transverse to it. The lasers exert a ponderomotive force on the electrons at the beat frequency. This force, in the presence of density ripple and transverse magnetic field, produces a nonlinear current at the terahertz frequency. The magnetic field enhances the amplitude of the terahertz wave. However terahertz yield is significantly higher in the TM mode laser beating than in the TE mode laser beating.     

Low-frequency degenerate surface modes of an electron-hole plasma

The computations of Flahive and Quinn¹ of the dispersion curves of low frequency degenerate surface (DS) modes propagating along the magnetic field in an electron-hole plasma are extended to higher values of the wavenumber. We find that beyond a certain value of the wavenumber the DS mode re-enters the allowed region of surface wave propagation and tends to an asymptotic frequency ω_R (<ω_LH). These low frequency resonances of an electron-hole plasma are discussed with reference to the experimental observations.     

Self-modulation of infrared waves in rutile

Electromagnetic waves carry angular and linear momentums and exert torques on anisotropic dielectrics, arising from the fact of the tensor property of the dielectric constant, that is, the direction of electric displacement is not parallel to the electric field vector of the incident light. The torque per unit volume exerted on a wave plate is given by P×E$\mathit{P}\times \mathit{E}$, where P is the polarization and E is the electric field, which induces the rotations of eigenvibration direction in the crystals. The rotation angles increase with the intensity of the incident light and the dielectric constant of the crystals. Because of the large dielectric constants, self-modulation of the incident light in the infrared frequency region was clearly demonstrated in the infrared transmission spectra of ferroelectric and piezoelectric crystals. Rutile (TiO₂) is a non-ferroelectric and non-piezoelectric crystal, but it also has the large dielectric constants. Rotations of the vibration direction of the ordinary (o-ray) and the extraordinary (e-ray) waves were shown in the infrared transmission spectra recorded by incidence of the plane-polarized light and transmission through a rutile plate. Interference of the o-ray and the e-ray waves transmitted through the crystals confirms the rotations of eigenvibration direction, a self-modulation effect of light in the crystal of large dielectric constants and large birefringence in the infrared range.     

Numerical study of the nonlocal effects in a charge density wave system

When a segment of charge density wave (CDW) conductor is submitted to an external electric field, a voltage arises in the neighboring segment where no external electric field is applied. Despite its long-term correlation, the collective mode of a CDW should not extend beyond current injection electrodes; however, the imposed boundary conditions to the sample at the injection current contacts affect the CDW state's far away the region between these latter. A phase gradient ∇φ is established between the region where the CDW slide and the region where it is still pinned (zero electric field); this can destroy the CDW state, hence a mechanism “phase slip” to remove the phase gradient established in the system.In this letter, we present a numerical study based on the Fukuyama-Lee-Rice (FLR) model of the nonlocal phenomenon in a one-dimensional CDW system in the weak pinning limit case.     

Pulsed NMR frequency shifts in superfluid 3He

We report the first measurements of the NMR frequency in ³He A and B under conditions where the net magnetization, M, is tipped far from its equilibrium direction along H_O. In ³He A the frequency shift ω - γH_O varies from the continuous wave value at tipping angle Φ = 0 to a negative shift at Φ = 180°. In ³He B no frequency shift is observed, however for Φ ? 100° a beat pattern is seen to develop in the free induction decay envelope.     

Investigation of quasi lateral-field-excitation on (yxl)-17° LiNbO3 single crystal

Quasi lateral-field-excitation (LFE) on LiNbO₃ crystal is investigated both theoretically and experimentally. It is found that when the driving electric field direction is parallel to the crystallographic X-axis of the piezoelectric substrate, (yxl)-17° LiNbO₃ LFE bulk acoustic wave devices work on quasi-LFE mode. The experimental results agreed with the theoretical prediction well. The results provide the cut of LiNbO₃ crystal for quasi-LFE bulk acoustic wave devices, which is important for designing high performance LFE sensors on LiNbO₃ substrates.     

Frequency splitting phenomenon of dual transverse modes in a Nd:YAG laser

We observed frequency splitting phenomenon of dual transverse modes (TEM_00q and TEM_01q) in a Nd:YAG microchip standing wave laser utilizing intracavity stress birefringence effects. Four resonance frequencies (ν_00qe, ν_00qo, ν_01qe, and ν_01qo, respectively) were produced and tuned by changing the diametral compression force applied on the laser crystal. The transverse mode frequency spacing for the same longitudinal mode number (Δν_trans) was 1.16 GHz, and the magnitude of frequency splitting (Δν) ranged from 0 MHz to 1.16 GHz. Based on this phenomenon, a four-mode differential standing wave laser, whose signal characteristics were a little like those of a four-mode differential travelling wave laser gyro however with a much larger frequency splitting range, was produced. The theoretical analysis is in good agreement with the experimental results. This phenomenon not only can be used to make lasers with two or more frequency differences, but also can be used to make high-resolution self-sensing laser sensors (e.g. laser force sensors and laser accelerometers).