Phononic structures and acoustic band gaps based on bulk materials have been researched in length in the past decades. However, few investigations have been performed on phononic structures in thin plates to form surface acoustic wave (SAW) band gaps. In this letter, we report a new type of phononic crystals manufactured by patterning periodical air-filled holes in thin plates. We confirmed the existence of SAW band gaps in the created phononic crystals through laser ultrasonics measurements. Wide multiple SAW band gaps and special structures, such as narrow pass bands within a band gap were observed experimentally.

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ABSTRACT. An automated, non-contact, non-destructive sensor has been developed for measurement of sheet flexural and shear rigidity in paper manufacturing. It was tested on a pilot web handler at web speeds up to 25.4 m/s. A model equation was fitted to the frequency dependence of the phase velocity of A o mode Lamb waves. Ultrasound was generated in paper with a pulsed Nd:YAG laser and detected with a Mach-Zehnder interferometer coupled with a scanning mirror/timing system to compensate for paper motion.

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Broadband laser ultrasonics and two dimensional Fourier transformation are used to characterize the properties of varieties of foils and plates. Laser ultrasonics generation is achieved by use of a pulsed laser which deposits pulsed laser energy on the surface of the specimen. The displacement amplitude of the resulting broadband ultrasonic modes are monitored using a two wave mixing photo-refractive interferometer. By applying a two dimensional Fourier transformation to the detected spatial and temporal displacement waveforms, the images of density of state (DOS) for the excited ultrasounds are obtained. Results are presented for a 150 µm thick paper sample, a 52.8 µm stainless steel foil and a 1.27 mm thick aluminum plate. The DOS image demonstrates the ability to measure the properties of each generated ultrasonic modes and provides a direct, non destructive, measure of elastic moduli of the tested specimens.Techniques that have been used successfully for non-destructive elastic properties evaluation in films fall into two basic categories [1]: laser based ultrasonic methods [2-4] and techniques utilizing Brillouin scattering [5][6][7]. In laser ultrasonics (LU) experiments, ultrasonic waves are externally generated, and either the dispersion of surface propagation modes is measured or the acoustic pulses propagating out of the plane of the specimens are monitored. Brillouin scattering probes the thermally induced vibration modes and measures their dispersions.The LU metrology uses pulsed laser irradiation to induce ultrasounds in a test object via three mechanisms: thermal-elastic, ablation and plasma. The resulting ultrasonic wave packets are often measured in a non-contact, nondestructive manner using an optical interferometer. LU techniques can provide information about the mechanical, thermal, and electronic properties of materials, and can also be used to detect surface and subsurface flaws in structures [2].LU often monitors an ultrasonic waveform in the time domain. From the obtained time-domain waveform, a technique, like phase velocity calculation, can be used to obtain indirectly the phase velocity dispersion curve as a function of frequency [8][9][10]. Using a fit to this curve one can extract the physical properties of the sample. The conventional technique to obtain phase velocity dispersion requires skill, knowledge and knowhow in data processing. In addition, previous knowledge of material properties is also necessary.In this letter, we describe a two dimensional fast Fourier transformation (2D FFT) technique to avoid most problems confronted by the conventional method. It is implemented by taking incremental measurements in separation distance between ultrasound emitter and receiver to obtain a second spatial dimension, correspond to wavevector ( k ) domain in Fourier transformation. The method automatically produces density of state ( DOS ) image as function of frequency ( f ) and wavevector for the generated ultrasounds with minimum human involvement in the process. The DOS image not only b...