Stabilization mechanisms of longitudinal pulsations in rotating detonation combustors

This work investigates the stabilization mechanisms of two types of longitudinal pulsations in rotating detonation combustors. The first type is linked to operating modes with two counter-rotating waves in combustors with open outlets and appears as a minor peak in the pressure spectrum. The second type is observed as pulsed operation of the combustor when the outlet is restricted. Different combustor lengths are studied and the susceptibility to these longitudinal pulsations is investigated. Pressure measurements along the length of the combustor and around the perimeter are used to identify the operating mode and to describe the propagation and stabilization mechanisms of the two longitudinal modes. The results show that both modes are linked to the longitudinal acoustic resonance of the combustor. The length-to-perimeter ratio and the mass flux are identified as the driving parameters for the existence of these longitudinal modes. The first mode is shown to be an acoustic resonance supported by the intersections of counter-rotating waves. The second mode is controlled by the reflection of an explosion induced shock wave propagating through a high velocity bulk flow.     

Influence of the slow wave on the relation between the angular resonances and the leaky guided modes properties for a poroelastic plate embedded in water

A numerical study of the guided modes in a water-saturated poroelastic plate that obeys the Biot theory is presented. In the first part, we study the leaky guided modes and the angular resonances when the slow wave does not propagate. Two types of guided modes exist. The first ones occur from coupling of the fast longitudinal wave with the shear wave; most of them propagate whatever the frequency is, provided that it is not close to their cut-off frequencies. The leaky guided modes of the second type occur from coupling of the two longitudinal waves and the shear wave. These modes do not propagate (they are highly damped) as long as the slow wave remains diffusive. We show that the characteristics of the angular resonances can be linked to the leaky guided waves of the first type in the same way as for an elastic plate. The guided modes of the second type may not be associated to angular resonances. In the second part, we consider a thinner plate in a higher frequency range so that the slow wave can propagate. Once again its influence is studied both on the leaky guided modes and on the angular resonances.     

Raman scattering from GaAs-Al0.5Ga0.5As-AlAs polytype superlattices

We report the first Raman study of GaAs-Al_0.5Ga_0.5As-AlAs polytype superlattices grown by molecular-beam epitaxy. Folded longitudinal acoustic phonon modes were observed for all the samples. The frequencies agree well with those calculated using an elastic continuum model. The intensities of the folded modes have novel properties, compared with double-constituent superlattices like GaAs-(AlGa)As.     

Internal second harmonic generation in InGaAsP DH lasers

Self induced second harmonic generation (SHG) accompanying 1.3 μm lasing radiation was first observed in fundamental transverse mode InGaAsP DH lasers. Sum frequency generation by two lasing longitudinal modes was observed, which has a conversion factor of four times greater than that for SHG by a single lasing longitudinal mode and predicts the simultaneous laser oscillation of the several longitudinal modes. The estimated second order nonlinear susceptibility of In_0.76Ga_0.24As_0.55 P_0.45 was about 5 × 10^-20 in rotationalized MKS unit, which is at least an order of magnitude greater than that of GaAs.     

Longitudinal-transverse splitting of dipole modes in hyper-Raman scattering spectra of liquid CCl4

The first observation of the longitudinal-transverse splitting of dipole vibrations in liquid CCl₄ is reported. Using the hyper-Raman scattering technique, we have obtained the spectra of transverse (313, 760 and 782 cm^-1) and longitudinal (?770 and 800 cm^-1) modes in CCl₄. Polarization analysis is shown to be used to distinguish longitudinal and transverse modes of dipole vibrations in hyper-Raman spectra.     

Relative anisotropies of plane waves and guided modes in thin orthorhombic plates: Implication for texture characterization

The angular dependences of the long wavelength velocities of S₀ and SH₀ modes of orthorhombic (orthotropic) plates are compared to those of the velocities of corresponding plane waves. To first order in the anisotropy, many of the phenomena are as expected. The absolute velocities and anisotropy of the SH₀ plate modes are identical to those for plane SH waves and the absolute velocities of the S₀ modes propagating along symmetry axes are reduced from the longitudinal plane wave velocities by an amount explained by the change from the plane strain to the plane stress condition. However, for certain classes of materials such as metal polycrystals, the anisotropy of the S₀ mode can be substantially different from that of the longitudinal plane waves. This effect is explained through an expansion of the crystallite orientation distribution function in terms of generalized spherical harmonics. Implications of the results for the ultrasonic measurement of preferred grain orientation (texture) in polycrystals is indicated.     

Microscopic theory of the long-wavelength modes of two-component plasmas and ionic liquids: II. The longitudinal modes

The modification of the damping rate of the sound modes by Coulomb phenomena is demonstrated from first principles. The heat modes of one- and two-component systems of charged particles are shown to differ by a factor c_p/c_v. Microscopic expressions for the interspecies energy and longitudinal momentum relaxation frequencies are provided. The charge relaxation modes are shown to reduce in the limit of weak-coupling to a pair of plasma oscillations occurring slightly below the plasma frequency while being slightly damped even at infinite wavelength. In the opposite limit of strong-coupling the same pair of charge relaxation modes is shown to split into an interspecies momentum relaxation mode and an approximate hydrodynamic diffusion mode. An Einstein relation between the diffusion constant and the electric conductivity is also demonstrated. All expressions are obtained for arbitrary density and coupling.     

Bi-dipole excited plasmonic modes of an elongated gold nanorod

The first and higher-order longitudinal surface plasmon resonance (SPR) modes of an elongated gold nanorod (GNR) induced by the photoluminescence of two quantum dots (QDs) respectively located at the two ends were studied theoretically. Two configurations of a GNR combined with a symmetric or anti-symmetric bi-dipole were simulated and analyzed using the multiple multipole method. The results show that the local maxima of the radiative and nonradiative powers of the bi-dipole are at these modes. When the aspect ratio (AR) of GNR exceeds a specific value, not only the first mode but also the second, third and even fourth modes are generated. For example, for an elongated GNR (radius: 30 nm, AR=7) in water, the first, second, third and fourth modes are at 1800 nm, 930 nm, 680 nm and 600 nm, respectively. These SPR modes depend on the AR as well as the radius of GNR. The larger the AR is, the more the red-shift of these modes will be. In addition, the red-shift increases as the radius increases. Moreover, the odd modes are induced by the anti-symmetric bi-dipole, but suppressed by the symmetric one. On the contrary, the even modes are induced by the symmetric bi-dipole, but suppressed by the anti-symmetric one. In comparison with the scattering and absorption cross sections of GNR irradiated by a plane wave, the high-order modes, particularly the even modes, can be easily induced by the bi-dipole. Moreover, the mutual excitation rate of the two QDs is also enhanced through these modes of GNR.     

Surface defect linear modes in one-dimensional photonic lattices

We report on the existence of surface defect linear modes at an interface between the defect of one-dimensional photonic lattices and the uniform media. The interface defect can significantly affect the properties of linear modes. Such new type of modes exists in the first bandgap for positive defects; while they exist in the second bandgap for negative defects. Particularly, when a Gaussian beam, which is similar to the linear mode, is launched at the defect site, we find that the Gaussian beam can be strongly confined at defect site and robustness along longitudinal direction for a long distance. When launched at a small angle into the defect site, the Gaussian beam exhibits stable snake propagation.     

A new type of acoustic modes of vibration of thin piezoelectric plates: Quasi-longitudinal normal modes

Using ST-cut quartz crystal plates as an example, a new type of normal modes of acoustic vibrations is described. The modes propagate along the x axis with a velocity close or equal to that of longitudinal bulk waves propagating in the same direction and have a longitudinal component of elastic displacement no less than two orders of magnitude greater than the two other components (the shear-horizontal and shear-vertical ones) throughout the whole plate thickness. The domain of existence of the quasi-longitudinal modes consists of a set of limited zones that contain the “allowed” values of the plate thickness H/λ (H is the plate thickness and λ is the wavelength) and are separated by “forbidden” zones corresponding to common Lamb modes. The closeness (or coincidence) of the velocities of a quasi-longitudinal mode in the plate and a longitudinal bulk wave in an unbounded crystal is a necessary but not sufficient condition for the existence of the aforementioned type of modes in ST,x quartz.     

Linewidth analysis of 40-GHz passively mode-locked multi-mode semiconductor lasers

An experimental analysis on the quality of 40-GHz radio-frequency signals generated by various passively mode-locked semiconductor lasers is addressed. The analysis is performed considering the frequency linewidth of 40-GHz optically generated signals and the number of longitudinal modes selected by the cavity of each laser under study. Four of these devices are multi-quantum wells InAlGaAs Fabry-Pérot lasers. They have been engineered to exhibit a specific number of longitudinal modes: 4, 5, 10, and 22 modes, respectively. Another device under test is a bulk distributed Bragg reflector laser exhibiting 3 lasing modes. The last device under study is a quantum-dash Fabry-Pérot laser characterized by 40 lasing modes. From our experimental results it appears that, regardless the nature of the device cavity and active media, the strength of the passively mode-locked mechanism might be enhanced with the number of longitudinal modes oscillating into the laser cavity, allowing a reduction in the frequency linewidth.     

Modified rate equation model including the photon�Cphoton resonance

We show that, when the longitudinal confinement factor in an edge-emitting laser is treated as a dynamic variable, the modulation transfer function has an extra term. This term produces a supplementary photon?Cphoton resonance peak in the modulation response at a frequency corresponding to the frequency separation between longitudinal modes, when these modes are phase-locked long enough (quasi-phase-locked). The photon?Cphoton resonance peak is strongest when two consecutive quasi-phase-locked dominant longitudinal modes have similar longitudinal envelopes and share equally the photon population.     

Electrostatic and magnetostatic modes in semiconductor superlattices

We present a study of interface optical phonons and magnetostatic modes in semiconductor superlattices. We first review the propagation of longitudinal waves propagating parallel to the superlattice axis in order to show the similarities and differences between these and the interface modes just mentioned. The interface modes are discussed with the aim to interpret experiments carried out with superlattices of diluted magnetic semiconductors.     

Effect of the mode composition of laser radiation on the intracavity formation of periodic surface structures

We investigated the effect of the mode composition (longitudinal and transverse modes) of laser radiation on the basic parameters of the stroke pattern formed on a treated surface placed inside a laser cavity. We found that the contrast of the stroke pattern formed by a laser beam with a complex set of longitudinal modes depends, in the first place, on the width of the laser radiation spectrum and the laser cavity length and is virtually independent of the angle of rotation of the beam. In the region of small rotation angles the broadening of strokes caused by the effect of transverse modes becomes predominant. B. I. Stepanov Institute of Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikaladnoi Spektroskopii, Vol. 64, No. 1, pp. 50–54, January–February, 1997.     

Elastic Waves in Generalized Thermo-Piezoelectric Transversely Isotropic Circular Bar Immersed in Fluid

In this paper, a mathematical model is developed to study the wave propagation in an infinite, homogeneous, transversely isotropic thermo-piezoelectric solid bar of circular cross-sections immersed in inviscid fluid. The present study is based on the use of the three-dimensional theory of elasticity. Three displacement potential functions are introduced to uncouple the equations of motion and the heat and electric conductions. The frequency equations are obtained for longitudinal and flexural modes of vibration and are studied based on Lord-Shulman, Green-Lindsay and Classical theory theories of thermo elasticity. The frequency equations of the coupled system consisting of cylinder and fluid are developed under the assumption of perfect-slip boundary conditions at the fluid-solid interfaces, which are obtained for longitudinal and flexural modes of vibration and are studied numerically for PZT-4 material bar immersed in fluid. The computed non-dimensional frequencies are compared with Lord-Shulman, Green-Lindsay and Classical theory theories of thermo elasticity for longitudinal and flexural modes of vibrations. The dispersion curves are drawn for longitudinal and flexural modes of vibrations. Moreover, the dispersion of specific loss and damping factors are also analyzed for longitudinal and flexural modes of vibrations.     

Microscopic theory of the long-wavelength modes of two component plasmas and ionic liquids: I. The transverse modes

The long wavelength modes of a system of two kinds of particles are studied starting from first principles while accounting for the presence of Coulomb interactions. It is shown that in order to cope with the Coulomb singularity the use of two-fluid hydrodynamical variables is indicated. Microscopic expressions for the various hydrodynamical modes as well as a number of interspecies relaxation modes are obtained for arbitrary density and coupling. In this paper only the transverse modes are considered. The results for the longitudinal modes as well as their contribution to a number of correlation functions will be considered in two compagnon papers.     

Controlling number of lasing modes for designing short-cavity self-mode-locked Nd-doped vanadate lasers

A short-cavity Nd:YVO₄ laser is employed to confirm that a spontaneous mode locking (SML) typically occurs without employing an extra nonlinearity. We further experimentally demonstrate that reducing the number of longitudinal lasing modes can diminish the phase fluctuation and effectively improve the SML pulse stability. Considering the spatial hole-burning (SHB) effect, an analytical expression is derived to accurately estimate the number of longitudinal lasing modes for a practical design guideline.     

磁致伸缩-压电混合激励Janus换能器结构特征参量与纵振频率之间的关系

为了拓宽Janus换能器的工作频带,使用超磁致伸缩材料Terfenol-D与PZT压电陶瓷作为混合激励元件驱动Janus换能器,研究了其结构特征参量与纵振频率之间的关系.首先归纳了磁致伸缩-压电混合激励Janus换能器的12个结构特征参量.随后通过模态分析,证明换能器具有两种纵振动模态.最终经过有限元仿真计算,总结分析...     

Linewidth and relative intensity noise measurements of longitudinal modes in ultrahigh-speed mode-locked laser diodes

The longitudinal linewidth and corresponding relative intensity noise (RIN) of 10- and 40-GHz mode-locked laser diodes are measured for the first time to our knowledge. It is shown that the cavity Q value is a dominant parameter of the linewidth. It is also shown that the linewidth of the individual modes is almost constant. This means that the phase noise of each mode is almost the same in the mode-locked condition. The RIN value is larger for modes that are distant from the center longitudinal mode. This mode dependence is a consequence of the mode partition noise.