Spatiotemporal control of terahertz waves in random media via Nonlinear Ghost Imaging

Harnessing the spatiotemporal control of complex fields is a critical challenge in a plethora of scientific domains, from photonics to ultrasound imaging. The demand for engineering field manipulation techniques is crucial in many disciplines, for instance, the investigation of imaging methodologies in disordered environments. The control of fields through complex media is an established application domain in microwave and ultrasound imaging. In the last two decades, scientists have developed similar approaches for optical waves. There is a great interest in extending this study to the state-of-the-art at Terahertz (THz) frequencies, particularly in the spatiotemporal field manipulation of ultrafast THz pulses, given the significant difference in methodologies and technologies compared to optical frequencies, for instance. Also, this study would be of great interest in telecom applications at THz frequencies, as communications in this band are expected to be more susceptible to scattering when compared to microwaves. This thesis contains the results obtained in the Emergent Photonics Laboratory, where I have been developing novel field manipulation techniques in random systems. I will illustrate a new route for harnessing the spatiotemporal properties of THz waves by exploiting scattering media as space-time combinatory elements. The state-of-the-art of THz TDS allows approaching wave scattering as a deterministic spatiotemporal event to be used as complex and inexpensive pulse shapers. As a specific case study, I will show the possibility of spatiotemporal superfocusing of ultrafast THz pulse propagating in complex media, corresponding to a simultaneous focusing in space and pulse re-compression in time. It is worth mentioning that the methodology behind the study of field manipulation in complex media is based on the Time-Resolved Nonlinear Ghost Imaging, a novel correlation-based near-field THz imaging that I have contributed to developing throughout my PhD. In this methodology, an electromagnetic image is reconstructed by correlating the known spatial THz patterns projected on the target object with the scattered field measured by a standard time-domain spectroscopy (TDS) detection, a mature approach in the field. The work is mainly presented in the form of a collection of publications (paper-style) but also includes very recent developments that are still unpublished. In its deployment, this thesis offers a general overview of the topic to introduce the subject field to a general Photonics physicist, presents published materials aggregated per topic, and discusses novel material and results in the final chapter. All the material presented has been generated by myself individually or within teamwork unless otherwise specified. Teamwork outputs are presented in full, with my specific contribution clearly highlighted at the beginning of each chapter. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme Grant agreement n° 725046.