Megahertz ultrasonic source induced by femtosecond laser irradiation of graphene foam.

• A graphene foam-based MHz ultrasonic source is excited by femtosecond laser. • Its bandwidth is 100 times larger than that by the piezoelectric transducer. • The ultrasonic source has a dipole-like pressure distribution. 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 graphene-foam-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. [ABSTRACT FROM AUTHOR]