Optoacoustic binary holography is applied to gain complete spatio-temporal control over ultrasonic beams. For flexible temporal intensity modulation of the sound-inducing laser light, an electro-optic modulator is used. Furthermore, in order to generate the desired spatial intensity distributions of the ultrasound, a spatial light modulator impresses synthetic binary holograms on the modulated light beam. At a light absorbing surface the optoacoustic effect converts the modulated light wave into an ultrasonic beam that propagates into water in the holographically predetermined way. With this approach we have successfully generated amplitude distributions that are difficult to realize with traditional piezo-electric techniques.
A combination of laser-induced ultrasound generation and ultrasonic holography for spatial control of the generated ultrasonic pulse is presented. Ultrasound is produced by absorption of laser pulses at an absorbing layer in a water tank via the optoacoustic effect. In order to produce a defined ultrasonic frequency in the MHz range, the laser pulses are harmonically time-modulated using an acousto-optic modulator (AOM). Additionally, the laser intensity is spatially controlled. This is realized with a high resolution liquid crystal spatial light modulator (LCD). A computer generated pattern is displayed at the LCD and projected by the expanded laser beam to an absorptive layer in the water tank. As a result, the emitted ultrasonic wave emerges in a predetermined way, which is an acoustical analogue to the effect of a "diffractive optical element" in laser optics. The flexible method of optical ultrasound generation and diffractive steering promises new applications in medical and technical ultrasound diagnostics.