Fast control of light through acousto-optics

Abstract

The capability to structure the light in space or time is key in many important scientific and industrial fields, such as telecommunications, imaging, and manufacturing. So far, the technological advancements have been pushed by a variety of different beam-shaping tools, which typically provide either a high degree of customization or high-speed, but not both. This trade-off hinders the throughput of many photonic-based technologies. To address this issue, we developed multiple strategies to control the light at high-speed exploiting the acousto-optic effect. This latter is a well-known phenomenon that consists of the diffraction of light by ultrasonic waves. Notably, the spatio-temporal properties of the diffracted photons are directly related to those of the acoustic wave, which can be easily controlled with an electronic driver. Additionally, ultrasonic frequencies range from MHz to GHz, enabling high-speed generation of tailored light. Those exceptional features led us to the design and implementation of a new instrument called the acousto-opto-fluidic device. It is an acoustic resonant cavity immersed in a fluid, capable of generating ultrasonic standing waves that can diffract a laser beam in multiple beamlets. Thus, the device parallelizes the output of a laser source. However, the beamlets can be easily recombined to create interference fringes. In this case, the device works as a structured light generator. Importantly, the structure of the light patterns can be tuned using the driving parameters, and it is possible to switch between different patterns in less than a microsecond. In this thesis, we present a complete theoretical model of the acousto-opto-fluidic device and full experimental characterization of its optical performance. Additionally, we provide proof-of-principle experiments to demonstrate how our novel device can be successfully integrated into a laser-direct-writing station to increase its throughput greatly. The last important contribution of this thesis is the design and development of an all-acousto-optic light-sheet microscope. Indeed, light-sheet fluorescence microscopy enables gentle volumetric imaging of large samples, but it is limited in speed by the movement of bulky components required to produce a z-stack. We used a tunable acoustic gradient lens -- namely a resonant varifocal lens -- to perform a fast axial scan of the sample, which enables the acquisition of images of different samples with no mechanical movements. However, this approach degrades the signal-to-noise ratio of the images. To compensate, we designed a strategy to illuminate simultaneously multiple planes at choice, using a couple of acousto-optic deflectors. The parallelized excitation enables the acquisition of multiple planes in a single frame, thus requiring a decoding process. The retrieval of the individual images is performed via a simple algorithm, which returns a decoded z-stack with an enhanced signal-to-noise ratio. Therefore, our novel imaging technique enables fast volumetric imaging without sacrificing the quality of the images. In conclusion, the work presented in this thesis paves the way for the fast and tailored generation of tailored light, opening new roads for high-throughput material processing and microscopy.