Femtosecond imaging of nonlinear acoustics in gold

Velocimetry of GHz elastic surface waves in quartz and fused silica based on full-field imaging of pump-probe reflectometry

This study reports an imaging method for gigahertz surface acoustic waves in transparent layers using infrared subpicosecond laser pulses in the ablation regime and an optical pump-probe technique. The reflectivity modulations due to the photoelastic effect of generated multimodal surface acoustic waves were imaged by an sCMOS camera illuminated by the time-delayed, frequency-doubled probe pulses. Moving the delay time between 6 . 0 n s to 11 . 5 n s , image stacks of wave field propagation were created. Two representative samples were investigated: wafers of isotropic fused silica and anisotropic x-cut quartz. Rayleigh (SAW) and longitudinal dominant high-velocity pseudo-surface acoustic wave (HVPSAW) modes could be observed and tracked along a circular grid around the excitation center, allowing the extraction of angular profiles of the propagation velocity. In quartz, the folding of a PSAW was observed. A finite element simulation was developed to predict the measurement results. The simulation and measurement were in good agreement with a relative error of 2 % to 5 %. These results show the potential for fast and full-field imaging of laser-generated ultrasonic surface wave modes, which can be utilized for the characterization of thin transparent samples such as semiconductor wafers or optical crystals in the gigahertz frequency range.

Magneto-optical Kerr effect measurement in ultrafast Sagnac interferometry using the Jones matrix approach

We report the Jones matrix formalism of the magneto-optic Kerr effect (MOKE) for ferromagnets using an ultrafast Sagnac interferometer. Compared to the time-resolved MOKE instrument, the Sagnac interferometer has the advantage of obtaining the real and imaginary parts of the differential MOKE signal as well as the differential reflectivity and the lattice displacement at the same time. In addition, a simple method to obtain the static values of Kerr rotation and ellipticity is presented.

Compressive imaging of transient absorption dynamics on the femtosecond timescale

Femtosecond spectroscopy is an important tool used for tracking rapid photoinduced processes in a variety of materials. To spatially map the processes in a sample would substantially expand the method's capabilities. This is, however, difficult to achieve, due to the necessity of using low-noise detection and maintaining feasible data acquisition time. Here, we demonstrate realization of an imaging pump-probe setup, featuring sub-100 fs temporal resolution, by using a straightforward modification of a standard pump-probe technique, which uses a randomly structured probe beam. The structured beam, made by a diffuser, enabled us to computationally reconstruct the maps of transient absorption dynamics based on the concept of compressed sensing. We demonstrate the setup's functionality in two proof-of-principle experiments, where we achieve spatial resolution of 20 μm. The presented concept provides a feasible route to imaging, by using the pump-probe technique and ultrafast spectroscopy in general.

An Overview of Non-Destructive Testing Methods for Integrated Circuit Packaging Inspection

The article provides a review of the state-of-art non-destructive testing (NDT) methods used for evaluation of integrated circuit (IC) packaging. The review identifies various types of the defects and the capabilities of most common NDT methods employed for defect detection. The main aim of this paper is to provide a detailed review on the common NDT methods for IC packaging addressing their principles of operation, advantages, limitations and suggestions for improvement. The current methods such as, X-ray, scanning acoustic microscopy (SAM), infrared thermography (IRT), magnetic current imaging (MCI) and surface acoustic waves (SAW) are explicitly reviewed. The uniqueness of the paper lies in comprehensive comparison of the current NDT methods, recommendations for the improvements, and introduction of new candidate NDT technologies, which can be adopted for IC packaging.

Interferometric analysis of laser-driven cylindrically focusing shock waves in a thin liquid layer

Shock waves in condensed matter are of great importance for many areas of science and technology ranging from inertially confined fusion to planetary science and medicine. In laboratory studies of shock waves, there is a need in developing diagnostic techniques capable of measuring parameters of materials under shock with high spatial resolution. Here, time-resolved interferometric imaging is used to study laser-driven focusing shock waves in a thin liquid layer in an all-optical experiment. Shock waves are generated in a 10 µm-thick layer of water by focusing intense picosecond laser pulses into a ring of 95 µm radius. Using a Mach-Zehnder interferometer and time-delayed femtosecond laser pulses, we obtain a series of images tracing the shock wave as it converges at the center of the ring before reemerging as a diverging shock, resulting in the formation of a cavitation bubble. Through quantitative analysis of the interferograms, density profiles of shocked samples are extracted. The experimental geometry used in our study opens prospects for spatially resolved spectroscopic studies of materials under shock compression.

Nonlinear acoustics at GHz frequencies in a viscoelastic fragile glass former

Using a picosecond pump-probe ultrasonic technique, we study the propagation of high-amplitude, laser-generated longitudinal coherent acoustic pulses in the viscoelastic fragile glass former DC704. We observe an increase of almost 10% in acoustic pulse propagation speed at the highest optical pump fluence which is a result of the supersonic nature of nonlinear propagation in the viscous medium. From our measurement, we deduce the nonlinear acoustic parameter of the glass former in the gigahertz frequency range across the glass transition temperature.