A novel laser-based ultrasonic technique for the inspection of thin plates and membranes is presented, in which a modulated continuous-wave laser source is used to excite narrow bandwidth Lamb waves. The dominant feature in the acoustic spectrum is a sharp resonance peak that occurs at the minimum frequency of the first-order symmetric Lamb mode, where the group velocity of the Lamb wave goes to zero while the phase velocity remains finite. Experimental results with the laser source and receiver on epicenter demonstrate that the zero-group velocity resonance generated with a low-power modulated excitation source can be detected using a Michelson interferometer coupled to a lock-in amplifier. This resonance peak is sensitive to the thickness and mechanical properties of plates and may be suitable, for example, for the measurement and mapping of nanoscale thickness variations.

Recent reports on the thermoelastic generation of Lamb waves in isotropic elastic plates show that a laser source efficiently excites a resonance that occurs at the minimum frequency of the first order symmetric (S1) Lamb mode. The group velocity of the Lamb wave goes to zero at this frequency while the phase velocity remains finite, and the resonance is referred to as the S1 zero group velocity (S1 ZGV) resonance. The S1 ZGV resonance can be employed for the nondestructive evaluation of the elastic properties of plates or plate thickness. A model for the generation of elastic waves in plates using an intensity-modulated continuous wave laser source is developed and used to study the behavior of the S1 ZGV resonance. The effects of the laser source parameters on the generation of the S1 ZGV resonance are explored, and the spatial distribution of the displacement produced at the resonance frequency is determined. The predicted displacement spectrum of Lamb waves generated in micron scale plates is found to compare well with experimental measurements. In addition, experimental measurements demonstrate that the S1 ZGV resonance can be used to map subsurface features in thin (4μm) membranes at high ultrasonic frequencies (700MHz).

In this paper, an innovative method to create embedded microchannels is presented. The presented technology is based on a direct-write technique using a scanning laser system to pattern a single layered SU-8. The enormous flexibility of the scanning laser system can be seen in two key features: the laser pulsing can be controlled spot-by-spot with variable exposure doses, and the laser intensity penetrating into samples can be adjusted by varying the laser focus level. The UV laser direct-write method greatly simplifies the fabrication processes. Moreover, it can be set up in a conventional manufacturing environment without the need for clean room facilities. The second part of this paper describes the underlying theory and method to determine Young's modulus of exposed SU-8 by using a laser acoustic microscopy system. The laser-based ultrasonic technique offers a non-contact, non-destructive means of evaluation and material characterization. This paper will determine Young's modulus of UV exposed SU-8 generated with different exposure doses. Measurements show that Young's modulus is highly dependent on exposure dose. Young's modulus ranges from 3.8 to 5.4 GPa when the thickness of a fully cross-linked SU-8 microbeam varies from 100 to 205 µm with a gradually increased UV exposure dose.

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A laser-based acoustic microscopy system has been developed that uses an amplified electroabsorption modulated diode laser for narrow bandwidth acoustic wave generation at frequencies up to 200 MHz. The detection bandwidth reduction afforded by this technique allows for a significant improvement in signal-to-noise ratio over systems using pulsed-laser excitation and broadband detection. Femtometer range displacement sensitivity is demonstrated, allowing for materials characterization with only minimal surface heating. The source modulation frequency is scanned over the bandwidth of interest and the transient response of the specimen is reconstructed from the frequency domain data. This signal processing approach allows for easy identification of individual acoustic arrivals or multiple acoustic modes.