Efficient laser-ultrasound generation by using heavily absorbing films as targets.

An efficient all-fiber optic source is presented; it adopts absorbing films, deposed directly over the fiber tip, as targets. It is demonstrated that the use of absorbing films made of pure graphite, or graphite powder mixed with epoxy resin, has produced a conversion efficiency increase of two orders of magnitude with respect to metallic materials. It is observed that the conversion efficiency increases monotonically as thickness is reduced down to the material optical penetration depth. Moreover, the conversion efficiency rises with the concentration of graphite powder. Principal advantages of this kind of source are the ease of production and miniaturization, the excellent electromagnetic compatibility, wide ultrasonic bandwidth and, consequently, high spatial resolution. The ultrasonic bandwidth can be controlled by varying the laser pulse duration. The possibility of generating ultrasonic signals with high frequency and flat spectral distribution makes the proposed device suitable for biological tissue spectral characterization.