Adaptive laser-induced ultrasound generation using a micro-mirror array spatial light modulator.

Laser ultrasonics is a powerful technique for contactless investigation of important material parameters such as Young's modulus or thin layer thickness. However, the often employed Gaussian beams result in diverging sound fields of quickly decreasing intensity. Conventionally, changing the laser beam profile requires the slow movement or exchange of optical elements. We present a laser ultrasonics setup for the creation of arbitrary intensity distributions by holographic projection using a MEMS spatial light modulator. High-intensity ultrasound foci with a focus width of 1.6 mm are scanned axially in a sample into depths of up to 7.4 mm by projecting ring-shaped intensity distributions of varying diameter without any mechanical movements. This technique is promising for highly spatially resolved flaw detection or a fast scanning investigation of biological tissue.