Tailoring ultrasonic waves with optoacoustic holography

We propose to experimentally demonstrate a novel method of laser optical generation and holographic manipulation of ultrasound, which might have interesting applications in medical diagnostics and therapy, and in technical applications. The sound waves are generated directly at the surface (or even within) of an object which will be investigated ultrasonically. This is done via the optoacoustic effect, by illuminating an absorbing layer on (or buried under) the surface of the object with modulated or pulsed laser light. The absorption of the modulated or pulsed light produces a sound wave with the frequency of the light intensity modulation, or a shock wave with the duration of the light pulse, respectively, which propagates into the volume of the medium. A special feature of our approach is to illuminate the absorbing surface not just with a plane light wave, but to project a computer designed image from a liquid crystal display (LCD) projection system at the surface. This image is calculated using the principles of computer designed holographical diffractive elements in such a way that the produced sound wave reconstructs an acoustic hologram at a specified position within the medium. For example, projecting a system of concentric light circles at the absorbing surface would generate a sound wave which focuses at a certain position within the medium, according to the principles of an optical Fresnel lens adapted to the case of acoustic waves. Using more sophisticated computer designed patterns, arbitrary sound fields (like lines, surfaces, or arrays of concomitant focal points) can be generated at specified positions within the volume. Three-dimensional spatial scanning of a focused ultrasonic beam, or a continuous change of the shape of the generated sound field, can be achieved at video-frame-rate by continuously changing the LCD image. Scanning or chirping of the ultrasonic frequency, or switching from a continuous to a pulsed mode, can be done by modulating the laser beam (before it enters the projection system) using straightforward opto-electronic methods, e.g. widely used electro-optic and acousto-optic light modulators. In the case of pulsed sound wave generation, the design principles of so-called phased arrays can be adapted straightforwardly and combined with the holographic method, to steer the pulsed acoustic waves. Thus, complete control over any aspect of the ultrasonic field is obtained optically, without any mechanical parts, and even without mechanical contact to the investigated object. This allows examinations from a distance or even through an optically transparent physical separation, which might be preferable for investigations of hazardous or infectious objects. Such a system could be a first step to an all-optical ultrasonic medical examination, where diagnostic ultrasound might be generated and navigated by projecting computer controlled light patterns at an absorbing layer of gelatine on the skin of a person. In a future step the detection of the backscattered ultrasonic waves might also be performed optically (interferometrially), such that a complete ultrasonic investigation is carried out optically.