Solitary surface acoustic waves and bulk solitons in nanosecond and picosecond laser ultrasonics

Recent achievements of nonlinear acoustics concerning the realization of solitons and solitary waves in crystals and their surfaces attained by nanosecond and picosecond laser ultrasonics are discussed and compared. The corresponding pump-probe setups are described, which allow an all-optical contact-free excitation and detection of short strain pulses in the broad frequency range between 10 MHz and about 300 GHz. The formation of solitons in the propagating longitudinal strain pulses is investigated for nonlinear media with intrinsic lattice-based dispersion. The excitation of solitary surface acoustic waves is realized by a geometric film-based dispersion effect. Future developments and potential applications of nonlinear nanosecond and picosecond ultrasonics are discussed.     

Diffractive acoustic elements for laser ultrasonics

Laser ultrasonics is an effective means of generating surface acoustic waves (SAWs). We have shown in previous publications how computer-generated holograms (CGHs) can be used to project optical distributions onto the sample surface. These can be used to control both the frequency content and the spatial distribution of the resulting ultrasound field. In this paper the concept is extended further to produce distributions which themselves act as diffractive acoustic elements (DAEs) for SAWs. It is demonstrated how frequency suppression, multiple foci, and frequency selective focusing of Rayleigh waves may be achieved with these elements. Agreement between the distributions predicted from the designs and those actually measured is excellent.     

Generation of acoustic solitons by a single-mode electromagnetic field

Mechanisms of acoustic pulse generation by a single-mode electromagnetic field propagating in a photoelastic material are analyzed. The anisotropy induced by acoustic excitations in an isotropic medium leads to nonlinear coupling between the polarization components of a single-mode electromagnetic field. For different conditions, it is shown that the acoustic-electromagnetic wave interaction due to mixing of the polarization components of light and acoustic waves can give rise to soliton-like coherent acoustic excitations in a thin crystal plate. When spatial dispersion is ignored, the governing system of equations for unidirectional acoustic solitons can be reduced to an integrable model. It is shown that qualitatively different scenarios of formation of acoustic solitons are possible, depending on the directions of deformation and field polarization.     

Generation and detection of incoherent phonons in picosecond ultrasonics

In picosecond ultrasonics experiments the absorption of a femtosecond laser pulse in a thin metallic transducer is used to generate very short acoustic pulses. These pulses are made of coherent longitudinal waves with a frequency spectrum that can reach 100-200 GHz. The laser pulse absorption gives rise to a heating of the film of a few Kelvin within a typical time of 1 ps. Later on, the heat goes in the substrate through an interface thermal resistance and is diffused by thermal conduction. At very low temperature and in pure crystals the thermal phonons emitted by the heated metallic film can propagate ballistically over large distances and produce a so-called heat pulse. We report on the experimental evidence of the coexistence of the coherent acoustic pulse and the incoherent heat pulse generated and detected by laser ultrasonics.     

Generation of multiple gain-guided solitons in a fiber laser

We report the experimental observation of multiple gain-guided solitons in an erbium-doped fiber laser made of all normal-dispersion fibers. Numerical simulations show that, in the case of a narrow gain bandwidth, under the action of the cavity pulse peak clamping effect multiple gain-guided solitons can indeed be formed in a laser.     

Generation and detection of propagating solitons in shearing liquid crystals

Different methods of generating and detecting propagating solitons in uniform and nonuniform, steady and unsteady shearing nematic liquid crystals are proposed, reviewed, and discussed in detail. These include the use of (1) an external plate moving uniformly in the middle and at one end of the liquid crystal (LC) cell, (2) a LC cell with one glass plate moving uniformly and reversing in direction, (3) an external plate as in (1) but moving periodically, (4) pressure gradients along the long axis of the LC cell, (5) circular LC cell with one glass plate rotating, and (6) circular LC cell with radial pressure gradients. In each of these cases, the relevant equations of motion of the LC molecules are derived and analyzed. In the essentially one-deminsional cases of (1) to (3), the governing equation is the damped, driven since-Gorden equation. Analytic and numerical results of single and multisolitons are presented. A multiscale perturbation method is used in the unsteady case of (4). The related case of soliton switch in ferroelectric smecticC ^* is discussed.Invited paper presented at Conference on Transport and Propagation in Nonlinear Systems, Los Alamos, May 21–25, 1984.     

Generation and detection of shear acoustic waves in metal submicrometric films with ultrashort laser pulses

We present experimental and calculational results demonstrating the thermoelastic generation of shear acoustic waves using femtosecond laser pulses in submicrometric isotropic aluminum films. We show that the generation of the shear waves is correlated to the reduction of the width of the optoacoustic source on the surface. The presence of shear waves is related to acoustic diffraction and acoustic mode conversion at the thin film interfaces.     

Measurement of material nonlinearity using surface acoustic wave parametric interaction and laser ultrasonics

A dual frequency mixing technique has been developed for measuring velocity changes caused by material nonlinearity. The technique is based on the parametric interaction between two surface acoustic waves (SAWs): The low frequency pump SAW generated by a transducer and the high frequency probe SAW generated and detected using laser ultrasonics. The pump SAW stresses the material under the probe SAW. The stress (typically <5 MPa) is controlled by varying the timing between the pump and probe waves. The nonlinear interaction is measured as a phase modulation of the probe SAW and equated to a velocity change. The velocity-stress relationship is used as a measure of material nonlinearity. Experiments were conducted to observe the pump-probe interaction by changing the pump frequency and compare the nonlinear response of aluminum and fused silica. Experiments showed these two materials had opposite nonlinear responses, consistent with previously published data. The technique could be applied to life-time predictions of engineered components by measuring changes in nonlinear response caused by fatigue.     

Sensitive detection of laser damage to Mo/Si multilayers by picosecond ultrasonics

Picosecond ultrasonics is used to study the damage or degradation of Mo/Si multilayers caused by laser irradiation. Changes of surface phonon spectra in multilayers due to femtosecond laser damaging are observed in a regime of extremely low fluence level, well before the onset of melting, delamination, distortion, or material interdiffusion. It is found that the damage is shallow in depth (top few layers), and its mechanism is laser-induced changes in acoustic impedances, most likely due to stress relaxation. Its effects on extreme ultraviolet (EUV) reflectance and implications for projection EUV lithography are also discussed. We believe this technique has a potential application as a highly sensitive tool capable of detecting low-degree or early-stage damage of multilayers. PACS 68.65.Cd; 61.80.Ba; 78.47.+p     

Generation of domain-wall solitons in an anomalous dispersion fiber ring laser

We report experimental observations performed using a net anomalous dispersion Er-doped fiber ring laser without polarization-selective elements, highlighting the domain-wall solitary pulses generated under the incoherent polarization coupling. By adjusting the pump power and the polarization state appropriately, bright and dark solitons can stably co-exist in the cavity, both centered at 1562.16 nm with a 3-dB spectral width of ～ 0.15 nm and a repetition rate of 3.83 MHz. Moreover, the 0.8 mm long thulium-doped fiber (TDF) facilitated the mode-locking and self-starting of the laser. This is the first demonstration of a laser being used to generate bright and dark solitons synchronously while using TDF as the saturable absorber (SA). Except possessing the all-fiber structure, the laser exhibits good stability, which may have a significant influence on improvement of the pulse-laser design, and may broaden practical applications in optical sensing, optical communication, and soliton multiplexed systems.     

Non-linear effects of high-power ultrasonics in crystalline solids

The aspects of non-linear elastic wave propagation in real crystalline solids pertinent to the field of high-power ultrasonics are presented. Particular emphasis is placed on the role non-linear elastic wave propagation may play in the ultrasonic softening process called the ‘Blaha effect’.     

Generation and detection of very high frequency acoustic phonons in diamond

Generation and detection of very high frequency acoustic phonons in diamond is reported. We generate phonons at a frequency of 28 THz by defect-induced one-phonon absorption of CO₂ laser radiation and observe, after pulsed excitation, phonon decay products in the frequency range from 1 THz to 7 THz. For detection vibronic sideband spectroscopy is used. We find strongly frequency dependent lifetimes for frequencies above 4 THz which we attribute to spontaneous phonon decay.Dedicated to K. Dransfeld on occasion of his 60th birthday     

Generation and interaction of large-amplitude solitons

The results of a numerical simulation of the interaction and generation of solitons in nonlinear integrable systems which admit the existence of very-large-amplitude solitons are reported. The nonlinear integrable system chosen for study is the Gardner equation, particular examples of which are the Korteweg-de Vries equation (for quadratic nonlinearity) and a modified Korteweg-de Vries equation (for cubic nonlinearity).It is shown that during the evolution process solitons of opposite polarity appear on the crest of the maximum soliton. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 628–633 (10 May 1998)     

Hidden corrosion detection in aircraft aluminum structures using laser ultrasonics and wavelet transform signal analysis

Preliminary results of hidden corrosion detection in aircraft aluminum structures using a noncontact laser based ultrasonic technique are presented. A short laser pulse focused to a line spot is used as a broadband source of ultrasonic guided waves in an aluminum 2024 sample cut from an aircraft structure and prepared with artificially corroded circular areas on its back surface. The out of plane surface displacements produced by the propagating ultrasonic waves were detected with a heterodyne Mach-Zehnder interferometer. Time-frequency analysis of the signals using a continuous wavelet transform allowed the identification of the generated Lamb modes by comparison with the calculated dispersion curves. The presence of back surface corrosion was detected by noting the loss of the S(1) mode near its cutoff frequency. This method is applicable to fast scanning inspection techniques and it is particularly suited for early corrosion detection.     

Trapped electron in ion acoustic waves and solitons

Experimental evidence of trapped electrons in ions acoustic nonlinear waves and solitons propagating in a stable plasma is reported. Agreement between theory and experiment was found for the dependency of the energy of the trapped electrons versus wave amplitude.     

Laser induced decohesion of coatings: probing by laser ultrasonics

The aim of the present study is to investigate a conventional laser-ultrasonics technique for the determination of intrinsic properties of oxide coatings and their adhesion strength on a metallic substrate. The good agreement between experiments and computations in an epicenter configuration allows determining the longitudinal wave velocity as well as the Young's modulus of the oxide coatings versus the porosity. For a critical value of the laser energy, a breakdown at the coating-substrate interface is generated by the laser irradiation. The critical tensile stress field developed at the coating/substrate interface, which leads to the interfacial fracture, can be easily calculated. The value of the practical adhesion which is defined is found to be in accordance with those obtained by classic contact techniques (tensile adhesion test, indentation, bending test). Finally, this work demonstrates that this quantitative, contactless test fits well to simultaneously characterise the oxide coatings and evaluate the coating-substrate adhesion.     

Measurement of elastic nonlinearity using remote laser ultrasonics and CHeap Optical Transducers and dual frequency surface acoustic waves

A nonlinear ultrasonic technique for evaluating material elastic nonlinearity has been developed. It measures the phase modulation of a high frequency (82MHz) surface acoustic wave interacting with a low frequency (1MHz) high amplitude stress inducing surface acoustic wave. A new breed of optical transducers has been developed and used for the generation and detection of the high frequency wave. The CHeap Optical Transducer (CHOT) is an ultrasonic transducer system, optically activated and read by a laser. We show that CHOTs offer advantages over alternative transducers. CHOTs and nonlinear ultrasonics have great potential for aerospace applications. Results measuring changes in ultrasonic velocity corresponding to different stress states of the sample are presented on fused silica and aluminium.     

Excitation of converging ion acoustic solitons

Spherically converging ion acoustic solitons have been observes in a double plasma type device, showing good agreement with the theoretical predictions in their characteristics.