Megahertz ultrasonic source induced by femtosecond laser irradiation of graphene foam

The performance of a photo-thermo-acoustic (PTA) ultrasonic source can be significantly improved by utilizing graphene-based materials. Graphene is an excellent PTA material owing to its wide electromagnetic absorption spectrum, low heat capacity per unit area, and high thermal conductivity. In this study, a broadband graphenefoam-based PTA ultrasonic source covering the frequency range of 50 kHz to 1.8 MHz was excited by a near infrared femtosecond laser beam. The lower and upper frequency limits of the acoustic wave that can be detected in the experiments were determined by the responsivity of the microphone and the attenuation of the acoustic wave in air, respectively. The experimental results show that the sound pressure of this ultrasonic source was independent of the laser polarization and incident angle. The peak-to-peak magnitude of the ultrasonic wave was proportional to laser energy when the single-pulse energy of the femtosecond laser varied from 0.4 to 1.0 mJ. Therefore, the sound intensity of the ultrasonic source could be easily controlled by modulating the laser energy. The experimental results also show that the ultrasonic wave emitted from the graphene foam had a dipole-like acoustic pressure distribution, and its principal emission direction was normal to the sample surface, regardless of the laser incidence angle. This characteristic may benefit future applications in directed message transfer/ acquisition and nondestructive testing/imaging.

Automated summary

The performance of a photo-thermo-acoustic ultrasonic source can be significantly enhanced by using graphene-based materials. This study demonstrates that the sound pressure of the ultrasonic source is independent of laser polarization and incident angle, and the ultrasonic wave emitted from the graphene foam has a dipole-like acoustic pressure distribution with a principal emission direction normal to the sample surface.