FREQUENCY SPECTRUM OF LASER-GENERATED ULTRASONIC WAVES

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Abstract
The frequency spectrum of ultrasonic waves generated by short laser pulses due to the thermoelastic effect has been measured in polycrystalline aluminum and in the ceramic material silicon nitride. The spectrum exhibits a maximum at 1/2 πτ if the temporal laser profile is gaussian (2τ is the pulse width). For frequencies smaller than 1/2πτ, the ratio of the thermal skin depth to the sound wavelength determines the frequency spectrum, whereas at high frequencies the spectrum is limited by the Fourier components of the laser pulse envelope. The absolute magnitudes of the sound amplitudes are of the order of Å for optical intensities of ≈10 MW/cm2

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