Having pursued laser ultrasonics research for the past decade, this success raises questions regarding the direction that current and future laser ultrasonics research efforts should follow in the university environment. Several of the directions in which the current work in laser ultrasonics has progressed are described as follows:

Hybrid Laser Ultrasonics Systems
(Graduate students Johanna Bernstein, Tobias Berndt, Kevin Baldwin): In many applications, the ability to use a laser-based receiver for ultrasound is not accomplished easily owing to the physical condition of the material surface. Additionally, many applications where the use of laser ultrasonics could be beneficial do not warrant the costs associated with laserbased receivers. Consequently, work is being conducted to assess the capabilities of hybrid laser ultrasonics systems. These systems use a laserbased ultrasonic transmitter, but the reception of ultrasound is accomplished using electromagnetic acoustic transducers (EMATs) or air-coupled transducers, both of which are noncontacting transduction methods.

Ultrafast Laser Ultrasonics
(Graduate student Chris Richardson): Typical laser ultrasonics apparatus uses a Q-switched pulsed laser as the ultrasonic transmitter. Such a source produces ultrasound with a broad frequency content extending from 0 Hz to 50 MHz. For many applications, the high frequency content of laser ultrasound allows for sufficiently precise time-of-flight measurements to assess materials properties in a range of structures. Unfortunately, this frequency range is not sufficient to measure time-of-flight in thin structures such as coatings that are less than I gm thick. However, using ultrafast laser ultrasonic techniques, these coating thicknesses may be measured. Ultrafast techniques employ lasers that produce pulses with sub-picosecond durations. These pulses generate ultrasonic waves with frequencies into the 100 GHz range that are capable of interrogating thin structures. For the case of thin coatings, not only can the thickness of the coating be determined, but the interface between the coating and the substrate may be characterized as well.

Transient Laser Stressing Methods
(Graduate student John Champion): For many material systems, ultrasonic attenuation limits the usefulness of using laser ultrasonic methods to characterize the material. Essentially, ultrasound cannot propagate from the transmitter to the receiver. However, the need to interrogate the elastic properties of the material remotely might still exist. For these materials, low frequency deformations of the body produced by laser excitation may be used to characterize the material structure. Laser stressing methods produce whole body deformations of the material by thermal expansion that are measured using shearographic methods. These methods allow for full-field visualization and analysis of the material deformations. By recording and analyzing these deformations as a function of time after the initiation of laser heating, information about the material structure may be gained concerning the depth and location of defects in the structure. Even though these methods are not ultrasonic, they are remote and non-contacting and can yield useful information about the material.

Materials Interactions Studies
(Undergraduate student Suzanne Wallace): For many materials processing applications, the use of laser ultrasonics is desirable not only because of its remote and non-contacting nature, but also because the laser source produces longitudinal and shear waves simultaneously. Consequently, information about the longitudinal and shear moduli of the material may be obtained in a single measurement and may be used to assess the state of the material microstructure. Work is currently under way to characterize the interaction of laser ultrasound with materials microstructure as an aid to real-time, in-process control of materials properties.

Obviously, laser ultrasonics, generalized to laser-induced material deformations, has expanded to include a host of related techniques that provide much useful information about materials in a variety of environments.