PhD thesis defense to be held on Jule 16, 2024, at 14:30 (NTUA Central Library)

Picture Credit: Anastasios Georgakopoulos

Thesis title: Photon Propagation inside highly Scattering Media. Dynamic Radiative Transfer System

Abstract: A new dynamic-system approach is presented in this thesis to the problem of radiative transfer within media that exhibit both scattering and absorption properties. This methodology is fundamentally grounded in first hand physical principles.

This method, named Dynamic Radiative Transfer System (DRTS), employs dynamical system formalism to model the radiative transfer. Central to this methodology is the construction and use of a global sparse matrix. This matrix is designed to encapsulate and represent the various physical, optical, and geometrical characteristics that define the specific material-volume being analyzed.

In this system, the creation of a new state is generated by this system time-independent matrix, using simple matrix-vector multiplication for each subsequent time step. One of the notable features of DRTS is its ability to compute with high accuracy the temporal evolution of photon propagation within media that are structurally complex and varied in shape.

The flexibility of the DRTS is further evidenced by its capacity to seamlessly integrate multiple elements. These include time-dependent sources, various boundary conditions, diverse types of media and several optical phenomena like reflection and refraction, which are embedded in a unified and consistent way. Various examples of DRTS simulation results are presented ranging from the continuous down to the ultra-fast light pulse 3-D propagation. The results from these simulations are particularly noteworthy for significantly reduced computational demands and resource requirements, especially when compared with other existing methods in the field.

By further applying simple intensity-normalization processing, clean images of the embedded objects can be extracted. The pulse evolution through the medium can be traced and displayed in time-frame sequence, depicting the gradual image formation and fading.

Also, it is demonstrated that illumination from scattered photons in the medium is advantageous, yielding clean full-frame images, eliminating the need for traditional beam scanning techniques. Additionally, image retrieval is significantly enhanced by capturing photons that are directionally selected, thereby revealing three-dimensional objects that are embedded deeper within the medium.

In contrast with the accepted notion that early photons are the only means for image formation, DRTS proves that there exists a capability for image formation using the scattered photons. The forward-problem simulation results have broad-ranging implications and potential applications across various scientific and technological domains.

Supervisor: Associate Professor Konstantinos Politopoulos

PhD Student: Anastasios Georgakopoulos