Transparency, Secrecy, and Profiling Using Mutual Scattering in Complex Media
Alfredo Rates is a PhD student in the department Complex Photonic Systems. Promotors are prof.dr. W.L. Vos and prof.dr. A. Lagendijk from the faculty Science & Technology.
In this thesis, we study various wavefront modulations of light for various scientific and applied purposes and scenarios. In summary, the thesis can be distinguished into three projects, where we apply wavefront modulation to 1) study mutual scattering in depth, 2) test the performance of the wavefront shaping technique (WFS), and 3) study speckle correlation for applications in visible light communication (VLC). Modulating the wavefront of light incoming into a complex media is a powerful tool for understanding and controlling light propagation within the media. This technique applies to highly random materials, such as paper, clouds, or biological tissue. It is also applicable to structured materials, such as photonic crystals or integrated circuits. Several techniques and active devices can be utilized to modulate the wavefront, such as spatial light modulators (SLM), digital micromirror devices (DMD), meta-materials, deformable mirrors, electro-optic modulators, and many more.
The main project of this thesis is dedicated to mutual scattering. This thesis presents the first experimental observation of mutual scattering and its first application in light scattering characterization of complex media. Mutual scattering occurs when two or more incoming beams cross in a finite object, and it is the cross-interference between the coherent incident wave of one beam and the scattered wave generated by the other beam. In our experiments, we modulate the incoming beams to change their relative angle and phase, thereby controlling and characterizing the scattering properties of the object.
Besides measuring mutual scattering, we apply WFS in free-form objects. In this project, we intend to defy the standard approach to light scattering in complex media, where the geometry of the object under study is considered a slab. Surprisingly, although WFS is assumed to apply to any scattering media, other geometries have yet to be explored. In this project, we demonstrate that WFS is equally effective in a free-form sample as in a slab geometry, thus confirming our hypothesis.
Last but not least, we explore wavefront modulation in a VLC system, utilizing the properties of complex scattering media for encryption purposes. To achieve this, we employ a scattering layer as a physical unclonable function (PUF), which serves as an additional security layer in the system. We expand on previously studied situations by incorporating two scattering layers, one at the emitter and the other at the receiver. This approach aims to prevent a person-in-the-middle attack, where an attacker intercepts the communication between the emitter and the receiver. The scattering layer at the emitter stops the attacker from exactly copying the source message, and the scattering layer at the receiver stops the attacker from understanding the message and avoids jamming attacks. Moreover, the complexity of the system allows for thousands of available wavefronts to send the same message. The emitter can alternate between the available wavefronts without any added expense, making it exceedingly difficult for an attacker to decipher the message.