Enhanced switching law for synchronized switch damping on inductor with bimodal excitation

Piezoelectric shunt damping is an emerging field of research. In recent years, a multitude of different electrical circuits have been developed aiming to increase the damping performance and robustness. Synchronized switch damping on inductor (SSDI) is a semi-active control technique that utilizes a passive inductance to build-up a voltage on the piezoceramics that is synchronized with the mechanical vibration. For a single mode excitation the voltage inversion should occur at the moments of maximum deformation, but for multimodal vibrations such a switching law may not be optimal.In this paper a novel switching law for bimodal vibrations is presented using a modal observer. An enhanced voltage build-up is generated by utilizing the vibration energy of the second mode. The amplification of dissipated energy is calculated in an analytical way using normalized parameters, yielding a general result which includes the influence of the frequency and amplitude ratio of the excitation signal. Measurements on a clamped beam test rig are conducted in order to validate the proposed method. An increase of nearly 350 percent in energy dissipation compared to the classical SSDI has been achieved. Furthermore, the increase in energy dissipation is higher than for a previously suggested, comparable switching law.     

Laser excitation and ionic motion in small clusters

We discuss the properties of small metal and hydrogen clusters under irradiation by intense lasers. We use a fully non adiabatic model of coupled electrons and ions, based on density functional theory. We demonstrate the quality of such an approach in comparison to experimental data on atomic and molecular properties as well as to the optical response of small sodium clusters. We next address the response of small clusters to various lasers allowing to scan a broad span of dynamical regimes from the linear to the non linear domain. We finally discuss the impact of ionic motion in the case of hydrogen clusters irradiated by short laser pulses in the 10 ¹³-10¹⁵ W cm^-2 intensity range.     

Laser induced excitation transfer in helium glow discharge

Some experimental results on the temporal decay of the fluorescence induced by a resonant laser pulse excitation focused onto a helium gas discharge are presented. In particular, excitation transfer between singlet-singlet and singlet- triplet sub-levels has been studied when 2¹ S → 3¹ P and 2¹ P → 4¹ D transitions of He I are optically pumped.     

C4分子在激光场中的密度泛函理论研究

采用含时局域密度近似与分子动力学相结合的方法计算了C4分子的光学吸收谱及它在激光场中的响应.研究表明,在激光场中C4分子的诱导偶极矩与电离之间有着直接的联系,最外层电子对电离的贡献最大.此外,通过对激光的极化方向与C4分子的激发的关系的研究表明,x方向的激光场唯一的激发起C4分子x方向的偶极矩,电离增强而且离子的振动增强.     

C4分子在激光场中的密度泛函理论研究

采用含时局域密度近似与分子动力学相结合的方法计算了C4分子的光学吸收谱及它在激光场中的响应。研究表明,在激光场中C4分子的诱导偶极矩与电离之间有着直接的联系,最外层电子对电离的贡献最大。此外,通过对激光的极化方向与C4分子的激发的关系的研究表明,x方向的激光场唯一的激发起C4分子x方向的偶极矩,电离增强而且离子的振动增强。     

Laser excitation of positronium in the Paschen-Back regime

Zeeman mixing of singlet and triplet 2P states of positronium (Ps) atoms, followed by decay back to the ground state, can effectively turn a long-lived triplet atom into a short-lived singlet state, which would seem to preclude laser cooling of Ps in a magnetic field. Here we report experiments which show that, in fact, because of the large splitting of the n=2 states in a high magnetic field (the Paschen-Back regime), the amount of such mixing diminishes approximately exponentially with an increasing magnetic field >0.01 T and is essentially eliminated above ～2 T. Thus, laser cooling of Ps should be feasible at high fields, which will facilitate the production of a Ps Bose-Einstein condensate.     

A new sample holder for excitation of transverse sound waves

A simple non-resonant sample holder with inclined electrodes is described by means of which transverse sound waves in the GHz range are easily generated by an evaporated piezoelectric layer. Because of its small size it is especially well suited for cryostat applications.     

Laser excitation of a fracture source for elastic waves

We show that elastic waves can be excited at a fracture inside a transparent sample by focusing laser light directly onto this fracture. The associated displacement field, measured by a laser interferometer, has pronounced waves that are diffracted at the fracture tips. We confirm that these are tip diffractions from direct excitation of the fracture by comparing them with tip diffractions from scattered elastic waves excited on the exterior of the sample. Being able to investigate fractures-in this case in an optically transparent material-via direct excitation opens the door to more detailed studies of fracture properties in general.     

Laser excitation spectra of He2

Metastable $\text{a(2sσ)}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ He₂ molecules are produced by a dc discharge in a flowing He stream. Laser excitation downstream of the discharge produces excitation spectra for a number of He₂ states. LIF spectra are observed for the $\text{(npπ)}{}^3\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{+}}$ series for n = 4–9, excepting 5 and the $\text{(npπ)}{}^3\text{Π}{}_{\mathrm{g}}$ series for n = 5–15.     

Laser excitation of long-lived oscillatory states in a dusty-plasma trap

Vertical oscillations of a single particle in a monolayer dusty-plasma sheath have been excited by a laser radiation pulse. The particle oscillates in a self-sustained regime at a characteristic frequency of about ?? = 25 Hz. It is established that phenomena related to the particle recharge and the delay of its charge variation relative to the equilibrium value cannot account for the self-sustained regime of oscillations. It is suggested that the experimentally observed vertical auto-oscillations of a single particle take place due to resonant pumping of the kinetic energy of the chaotic motion of particles in the dusty-plasma sheath to the energy of vertical oscillations.     

Laser excitation of surface electronic states for a one-dimensional semiconductor

The electronic band structure of a finite one-dimensional semiconductor is examined. The surface states are identified and the absorption cross-section for transitions to these states is calculated. The use of a laser to enhance surface charge density is discussed.     

Spatiotemporal communication with synchronized optical chaos

We propose a model system that allows communication of spatiotemporal information using an optical chaotic carrier waveform. The system is based on broad-area nonlinear optical ring cavities, which exhibit spatiotemporal chaos in a wide parameter range. Message recovery is possible through chaotic synchronization between transmitter and receiver. Numerical simulations demonstrate the feasibility of the proposed scheme, and the benefit of the parallelism of information transfer with optical wave fronts.