Laser-induced focused ultrasound for nondestructive testing and evaluation

Abstract

Focused ultrasound pulses generated by photoacoustic transformation at a metal surface immersed in water possess a pronounced compression phase on the nanosecond time scale. For 8 ns laser pump pulses, the spectrum of the initially generated ultrasonic pulse covered a frequency range between 0.1 and 150 MHz. A concave spherical geometry of the light-absorbing metal surface can be used to achieve focusing. In the present experiments a conical ultrasound beam was directed at a solid glass plate or silicon wafer, where the tilt of the normal of the metal mirror defined the efficiency of mode conversion at the water-solid interface. Depending on the configuration, focused bulk waves as well as Rayleigh and Lamb waves could be launched in the sample with this setup. The laser probe-beam-deflection method was employed for local detection of elastic disturbances at the sample surface. Due to the nonlinear elastic response of water and harmonics generation, frequencies >100 MHz were realized, despite a strong attenuation in this frequency range. Gradual increase of the laser power density from 5 to 14 MW/cm2 led to shock formation in the compressive pressure pulse in water and shortening of the Rayleigh pulse induced at the surface of the glass plate. The observed transient surface profiles were highly sensitive to nearby mechanical discontinuities such as a microcrack in glass or an edge discontinuity in silicon. Therefore, laser-induced focused ultrasound seems to be a very promising method of accomplishing diverse tasks of nondestructive evaluation.

A Basic Complete Numerical Toolbox for Picosecond Ultrasonics

“…High frequency acoustic waves into the GHz or into the sub-THz frequency domain can be optically excited and detected by femtoseconde ( f s) laser pulses [4]. Conventional (piezo-electric) transducers approach or photo-acoustic transduction approach involving a metal layer absorbing a nanoseconde (ns) laser pump pulse [5] are unable to reach such ultra-high frequency content [6]. The ultrasonic approach becomes, therefore, the only relevant way to generate and to detect such high frequency ranges.…”

Wave field characterization for non-destructive assessment of elastic properties using laser-acoustic sources in fluids and eye related tissues

“…Starting at the determination of quite simple geometrics [1] they were developed to a versatile tool in several advanced applications like materials characterization, acoustic microscopy [2] or focused nondestructive testing and flaw detection [3]. During the last ten years the influence of laser methods within medical diagnostics and therapy increased considerably.…”

Review of Laser-Generated Ultrasound Transmitters and Their Applications to All-Optical Ultrasound Transducers and Imaging

“…The received acoustic pulse is used to acquire the resonant frequencies of the PDMS thin film. (ii) A focused ultrasonic pressure pulse is used for nondestructive testing (NDT) of the surface of a sample placed in the focal zone [79,80]. Clear changes in the transient acoustic waveform are observed for the sample with cracks at the surface compared with a control sample.…”