Thermoelastic damping effect on in-extensional vibration of rotating thin ring

Sensitive devices such as resonant sensors and radio frequency micro-electro-mechanical system (RF-MEMS) filters etc., require high Quality factors (Q-factors) defined as the ratio of total system energy to dissipation that occurs due to various damping mechanisms. Also, thermoelastic damping is considered to be one of the most important factors to elicit energy dissipation due to the irreversible heat flow of oscillating structures in the micro scales. In this study, the Q-factor for thermoelastic damping is investigated in rotating thin rings with in-plane vibration. First, in order to obtain the temperature profile of the model, a heat conduction equation for the thermal flow across the radial direction is solved based on the bending approximation so-called in-extensional approximation of the ring. Using the temperature distribution coupled with a displacement, a governing equation of the ring model can then be derived. Eventually, an eigen-value analysis is performed to obtain the natural frequency of rotating thin rings, and the analytical and numerical values of Q-factors can then be determined by the definition. Furthermore, the effects of rotating speed, dimensions of the ring, mode numbers and ambient temperatures on the Q-factor are discussed in detail.     

Thermoelastic damping in micro-scale circular plate resonators

The governing equations of coupled thermoelastic problems are established for out-of-plane vibration of a circular plate. The analytical expression for thermoelastic damping is obtained. Then the thermoelastic damping is studied under different environmental temperature, plate dimensions and boundary conditions.     

短脉冲激光加热模型的遴选原则

 利用推导出的1维半空间光致热弹性应力响应的精确解，对激光与材料相互作用的加热模型的选取进行了研究。理论分析和计算结果表明：常用的表面加热模型只有在热穿透深度远大于光吸收长度时有效，若将其应用于超短脉冲激光（ps以下量级）加热的情况，可能会导致较大的误差；皮秒和飞秒激光辐照的热效应须用体加热模型计算。     

Surface cleaning of metals by pulsed-laser irradiation in air

The surface-cleaning effect of metals was investigated using KrF-excimer-laser irradiation of metal surfaces in air. The laser-induced cleaning of copper, stainless steel and aluminum surfaces was studied. It is found that laser cleaning is an effective cleaning process for metals even if the metal surfaces are heavily contaminated. It is also found that short wavelength and pulse duration are necessary for laser surface-cleaning. The energy density of the laser pulse is an important parameter in the cleaning process. Low energy density results in a cleaner surface but a larger pulse number is required, whereas high energy density can achieve higher cleaning efficiency but the temperature rise can cause surface oxidation and secondary contamination. In contrast to the KrF-excimer-laser, the pulsed CO₂ laser is not effective in surface-cleaning. The mechanisms of laser cleaning may include laser photodecomposition, laser ablation and surface vibration due to the impact of the laser pulse. Laser cleaning provides a new dry process to clean different substrate surfaces and can replace the conventional wet cleaning processes such as ultrasonic cleaning with CFC and other organic solvents.     

Thermoelastic damping of the axisymmetric vibration of circular plate resonators

Thermoelastic damping is recognized as a significant loss mechanism at room temperature in micro-scale circular plate resonators. In this paper, the governing equations of coupled thermoelastic problems are established for axisymmetric out-of-plane vibration of circular plate. Then the analytical expression for thermoelastic damping is obtained. The effects of environmental temperature, plate dimensions and boundary conditions on the thermoelastic damping are studied.     

Quality factors for the nano-mechanical tubes with thermoelastic damping and initial stress

The Quality factors (Q-factor) are defined as the ratio of the kinetic and potential energy to dissipation for various damping mechanisms of structures. Therefore, improvement in the Q-factors is an important issue in micro- and nano-resonator applications for the high performance. Also, it is well known that the thermoelastic damping is more crucial than the other damping factors in a device. Thus, the vibration of nano-mechanical circular tube is investigated with thermoelastic damping and initial stress effects in this work. To simplify the shell equations for the transverse displacement-dominated problems, the Donnell-Mushtari-Vlasov (DMV) approach is adopted. Applying the stress function, the equations of motion for deflection, compatibility equation and heat conduction equation are derived. Using an iterative scheme, the natural frequencies and the Q-factors under the initial stress are obtained, and the influences of the dimensions of the shell, the mode numbers and initial stress are discussed in detail.     

Analysis of thermoelastic dissipation in circular micro-plate resonators using the generalized thermoelasticity theory of dual-phase-lagging model

This study investigates the thermoelastic dissipation of micro-plate resonators by using the generalized thermoelasticity theory of dual-phase-lagging model. Explicit formulae of thermoelastic damping and frequency shift are derived. Influences of the plate thickness and vibration frequency on the thermoelastic damping are examined. Phenomena distinct from those of classical theory are observed in the numerical results of thermoelastic damping in micro-plate resonators. These results may bring new insights into the study of thermoelastic damping at submicrometer or nanometer scale.     

Laser hyperacoustic spectroscopy of single crystal germanium

Using a photothermal laser deflection technique the profiles of laser-induced hyperacoustic pulses in single crystal germanium were studied at a subnanosecond time resolution. It is shown that the hyperacoustic pulses are excited due to an electron-deformation interaction of photogenerated carriers with the crystal lattice, which is much more effective than the thermoelastic mechanism of the acoustic wave generation. Evolution of the hyperacoustic pulse profiles related to the diffraction and acoustic absorption effects was studied. An analysis of the hyperacoustic signal profiles allowed us to estimate the coefficient of ambipolar diffusion of the photogenerated charge carriers and the coefficient of hyperacoustic wave damping. It is established that the front of the electron-hole plasma laser-excited in germanium at room temperature propagates at a supersonic velocity.     

Transient analysis of nonlinear Euler–Bernoulli micro-beam with thermoelastic damping,via nonlinear normal modes

In this paper an Euler–Bernoulli model has been used for vibration analysis of micro-beams with large transverse deflection. Thermoelastic damping is considered to be the dominant damping mechanism and introduced as imaginary stiffness into the equation of motion by evaluating temperature profile as a function of lateral displacement. The obtained equation of motion is analyzed in the case of pure single mode motion by two methods; nonlinear normal mode theory and the Galerkin procedure. In contrast with the Galerkin procedure, nonlinear normal mode analysis introduces a nonconventional nonlinear damping term in modal oscillator which results in strong damping in case of large amplitude vibrations. Evaluated modal oscillators are solved using harmonic balance method and tackling damping terms introduced as an imaginary stiffness is discussed. It has been shown also that nonlinear modal analysis of micro-beam with thermoelastic damping predicts parameters such as inverse quality factor, and frequency shift, to have an extrema point at certain amplitude during transient response due to the mentioned nonlinear damping term; and the effect of system?s characteristics on this critical amplitude has also been discussed.     

Numerical modeling of short pulse laser interaction with Au nanoparticle surrounded by water

Short pulse laser interaction with a metal nanoparticle surrounded by water is investigated with a hydrodynamic computational model that includes a realistic equation of state for water and accounts for thermoelastic behavior and the kinetics of electron-phonon equilibration in the nanoparticle. Computational results suggest that, at laser fluences close to the threshold for vapor bubble formation, the region of biological damage due to the laser-induced thermal spike and the interaction of the pressure wave with internal cell structures can be localized within short distances from the absorbing particle comparable to the particle diameter. This irradiation regime is suitable for targeted generation of thermal and mechanical damage at the sub-cellular level.     

Thermoelastic damping in torsion microresonators with coupling effect between torsion and bending

Predicting thermoelastic damping (TED) is crucial in the design of high Q MEMS resonators. In the past, there have been few works on analytical modeling of thermoelastic damping in torsion microresonators. This could be related to the assumption of pure torsional mode for the supporting beams in the torsion devices. The pure torsional modes of rectangular supporting beams involve no local volume change, and therefore, they do not suffer any thermoelastic loss. However, the coupled motion of torsion and bending usually exists in the torsion microresonator when it is not excited by pure torque. The bending component of the coupled motion causes flexural vibrations of supporting beams which may result in significant thermoelastic damping for the microresonator. This paper presents an analytical model for thermoelastic damping in torsion microresonators with the coupling effect between torsion and bending. The theory derives a dynamic model for torsion microresonators considering the coupling effect, and approximates the thermoelastic damping by assuming the energy loss to occur only in supporting beams of flexural vibrations. The thermoelastic damping obtained by the present model is compared to the measured internal friction of single paddle oscillators. It is found that thermoelastic damping contributes significantly to internal friction for the case of the higher modes at room temperature. The present model is validated by comparing its results with the finite-element method (FEM) solutions. The effects of structural dimensions and other parameters on thermoelastic damping are investigated for the representative case of torsion microresonators.     

Radiation damping effects on the interaction of ultraintense laser pulses with an overdense plasma

A strong effect of radiation damping on the interaction of an ultraintense laser pulse with an overdense plasma slab is found and studied via a relativistic particle-in-cell simulation including ionization. Hot electrons generated by the irradiation of a laser pulse with a radiance of I lambda(2)>10(22) W microm(2)/cm(2) and duration of 20 fs can convert more than 35% of the laser energy to radiation. This incoherent x-ray emission lasts for only the pulse duration and can be intense. The radiation efficiency is shown to increase nonlinearly with laser intensity. Similar to cyclotron radiation, the radiation damping may restrain the maximal energy of relativistic electrons in ultraintense-laser-produced plasmas.     

Characterization of laser-induced ultrasound signal by reduced graphene oxide thickness and laser intensity

A nanosecond pulse laser generates acoustic waves on a water-material interface. The absorbed beam energy heats and thermoelastically expands the material. The thermoelastic stress of a material is dependent on its absorbance and expansion coefficient. In this work, we used a composite of reduced graphene oxide (RGO) and aluminum thin film to increase the efficiency of conversion from beamed energy to thermoelastic stress. A laser shadowgraph showed enhanced acoustic waves propagating at ~1,500 m/s under water. The effect of RGO on ultrasound generation is examined for different thicknesses of RGO at several laser fluences. The pressure of laser-induced ultrasound on the RGO–aluminum composite was measured to be up to 59 times greater than that produced with an aluminum film alone, and the frequency of laser-induced ultrasound was determined by the thermoelastic response. The strong intensity and broad bandwidth of the laser-induced acoustic wave suggested enhanced repetition time and resolution required for biomedical imaging.     

Photoacoustic cavitation in spherical and cylindrical absorbers

Photomechanical damage in absorbing regions or particles surrounded by a non-absorbing medium is investigated experimentally and theoretically. The damage mechanism is based on the generation of thermoelastic pressure by absorption of pulsed laser radiation under conditions of stress confinement. Principles of photoacoustic sound generation predict that the acoustic wave generated in a finite-size absorbing region must contain both compressive and tensile stresses. Time-resolved imaging experiments were performed to examine whether the tensile stress causes cavitation in absorbers of spherical or cylindrical shape. The samples were absorbing water droplets and gelatin cylinders suspended in oil. They were irradiated with 6-ns-long pulses from an optical parametric oscillator. Photoacoustic cavitation was observed near the center of the absorbers, even if the estimated temperature caused by absorption of the laser pulse did not exceed the boiling point. The experimental findings are supported by theoretical simulations that reveal strong tensile stress in the interior of the absorbers, near the center of symmetry. Tensile stress amplitudes depend on the shape of the absorber, the laser pulse duration, and the ratio of absorber size to optical absorption length. The photoacoustic damage mechanism has implications for the interaction of ns and sub-nslaser pulses with pigmented structures in biological tissue. Received: 9 October 1998 / Accepted: 5 January 1999 / Published online: 31 March 1999     

Femtosecond laser-induced dynamic alignment in Coulomb explosion of CO: Roles of laser wavelength, intensity and pulse duration

Dependences of dynamic alignment of CO molecules induced by intense femtosecond laser fields on laser wavelength, intensity and pulse duration are investigated by numerical simulations. A counting approach and a fourth-order Runge-Kutta algorithm are used to calculate the angular distribution and the time evolution of molecules. A two-step Coulomb explosion model of diatomic molecules in intense laser fields is used to determine the instant that CO molecular dynamic alignment is over. Our calculating results show that the linear polarizability and the damping force play an important role in the angular rotation of CO molecule in conditions of 800 nm laser wavelength and 10¹⁵ W/cm² laser intensity. The contributions of the second-order field-induced dipole moment and the higher-order correction term to molecular rotation acceleration comparing to the linear polarizability and damping force are negligible. The extent of dynamic alignment of CO molecules reduces with the increasing of laser intensity. The dynamic alignment time of CO molecules is tightly connected to the laser pulse duration. The angular distributions of CO molecules as the laser pulse length varied from 50 to 250 fs at laser intensity of 3×10¹⁴ W/cm² are shown and discussed.     

Damping in micro-scale generalized thermoelastic circular plate resonators

The out-of-plane vibrations of a generalized thermoelastic circular plate are studied under different environmental temperature, plate dimensions and boundary conditions. The analytical expressions for thermoelastic damping of vibration and phase velocity of circumferential surface wave modes are obtained. It is noticed that the damping of vibrations and phase velocities of circumferential surface wave modes significantly depend on thermal relaxation time in addition to thermoelastic coupling in circular plates under resonance conditions. The surface conditions also impose significant effects on the vibrations of such resonators. The expressions for displacement and temperature fields in the plate resonator are also derived and obtained. Some numerical results have also been presented for illustration purpose in case of silicon material plate.     

Nonlinear longitudinal waves in the presence of the interaction between the strain and temperature fields and the field of nonequilibrium atomic defects

The propagation of nonlinear longitudinal waves in a plate is studied by taking into account the interaction of the longitudinal displacement component with the temperature field and the field of concentration of nonequilibrium atomic point defects. A nonlinear evolution equation is derived for describing the self-consistent thermoelastic longitudinal strain fields. It is shown that the thermoelastic effect on the strain waves manifests itself in the appearance of dissipative terms, which describe the heat transfer and the thermoelastic interaction caused by the strain-induced heat release due to the recombination of nonequilibrium atomic defects. The soliton solutions to the evolution equation are investigated, and the characteristic features of their damping are considered with allowance for the low-frequency and high-frequency losses.     

Analytical solution of thermoelastic interaction in a half-space by pulsed laser heating

In this article, we consider the problem of a two-dimensional thermoelastic half-space in the context of generalized thermoelastic theory with one relaxation time. The surface of the half-space is taken to be traction free and thermally insulated. The solution of the considered physical quantity can be broken down in terms of normal modes. The nonhomogeneous basic equations have been written in the form of a vector-matrix differential equation, which is then solved by an eigenvalue approach. The exact analytical solution is adopted for the temperature, the components of displacement and stresses. The results obtained are presented graphically for the effect of laser pulse to display the phenomena physical meaning. The graphical results indicate that the thermal relaxation time has a great effect on the temperature, the components of displacement and the components of stress.     

The effect of errors and detector noise on sensitive detection of chirp and pulse asymmetry of ultrashort laser pulses from interferometric autocorrelation signals

We have studied the effect of various types of systematic and non-systematic errors on unbalanced spectrally modified interferometric autocorrelation signals for detection of pulse chirp and asymmetry of ultrashort laser pulses. The effect of systematic errors arising due to limited number of data points per fringe, scan rate etc, and non-systematic errors due to phase noise, additive noise, multiplicative noise and quantization of interferometric autocorrelation (IAC) signals is illustrated using a linearly chirped asymmetric laser pulse. It is seen that the spectrally modified IAC signals based on difference of normalized envelope functions corresponding to different frequencies are not much sensitive to various noises, permitting their use for sensitive detection of pulse chirp and asymmetry. The analysis is supported by experimentally recorded IAC signals under different conditions.     

Thermoelastic wave induced by pulsed laser heating

In this work, a generalized solution for the thermoelastic plane wave in a semi-infinite solid induced by pulsed laser heating is developed. The solution takes into account the non-Fourier effect in heat conduction and the coupling effect between temperature and strain rate, which play significant roles in ultrashort pulsed laser heating. Based on this solution, calculations are conducted to study stress waves induced by nano-, pico-, and femtosecond laser pulses. It is found that with the same maximum surface temperature increase, a shorter pulsed laser induces a much stronger stress wave. The non-Fourier effect causes a higher surface temperature increase, but a weaker stress wave. Also, for the first time, it is found that a second stress wave is formed and propagates with the same speed as the thermal wave. The surface displacement accompanying thermal expansion shows a substantial time delay to the femtosecond laser pulse. On the contrary, surface displacement and heating occur simultaneously in nano- and picosecond laser heating. In femtosecond laser heating, results show that the coupling effect strongly attenuates the stress wave and extends the duration of the stress wave. This may explain the minimal damage in ultrashort laser materials processing. Received: 23 May 2000 / Accepted: 26 May 2000 / Published online: 20 September 2000