Analysis of High-Frequency Vibrational Modes Through Laser Pulses

The authors are currently doing fatigue tests, by using sonotrodes working at 20 kHz, and compression tests on solid samples of rock materials. The specimens are monitored by laser sources focused on spots of the external surface. The main purpose is to identify in the crystal lattice mechanical oscillations in a frequency range higher than the Acoustic Emission (kHz) and comprised between MHz and THz. Such vibrational modes in the THz regime will be the signature of developing anomalies in piezoelectric or piezomagnetic materials, such as neutron and/or alpha particle emissions, and compositional changes. Moreover, it is also known that laser current pulses should be effective in exciting vibrational modes in the high-frequency range in conductive samples. A photodetector connected to an usual oscilloscope, measuring the intensity of reflected light, detects vibrations in the range from MHz up to few GHz. To reduce the limitations in the ability to acquire high frequency signals, additional experiments are planned exploiting the Raman effect. Through the scattered light spectrum analysis, capable of detecting radiation in the visible light field, it should be obtained information on the intermolecular interactions that during solicitations induce resonant vibrations in the field of THz.