Coaxial- and counter-optical setups for laser ultrasonics using a photorefractive liquid crystal were fabricated. The laser ultrasonics involves irradiating an object with a laser pulse to produce an ultrasonic vibration, and then using another laser beam to detect the vibration. The phase of the laser beam reflected from the object is shifted by the ultrasonic vibration. By using liquid crystals with photorefractive properties, the resulting phase shift of the laser beam reflected from the material can be detected. Compared to traditional laser ultrasonic methods, this system offers a simpler optical setup and allows for more precise measurements that are unaffected by environmental vibrations.
The photorefractive effect of flexoelectric smectic liquid crystal mixtures was investigated and applied to a laser ultrasonic measurement. Smectic liquid crystal mixtures, composed of smectic-C liquid crystals, photoconductive chiral compounds, and a sensitizer, are known to exhibit a fast photorefractive effect. The principle of the ultrasonic measurement is that a nanosecond laser pulse is shot on an object to cause an ultrasonic vibration, a continuous laser beam is irradiated on the object, and the ultrasonic variation is detected using photorefractive two-beam coupling. This method can be used to investigate the thickness and the internal structure of an object without contact.
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