New frontiers in Terahertz microscopy: development of novel near-field imaging methodologies

Time-Domain imaging at Terahertz (THz) frequencies exploits illumination with short broadband electromagnetic pulses and has become a crucial tool for coherent 2D spectral analysis since its inception in 1995. Many materials of interest, bio compounds and even explosives exhibit typical revealing spectral signatures in this band. In addition, the interaction of terahertz waves with free carriers is widely used as diagnostic in several conducting and semiconducting media. A peculiar distinction established in the THz field is the ability to access the evolution of the full electromagnetic field in time (in the picosecond time-scale). Established implementations are generally remarkably slow, especially resolution finer than the wavelength are targeted, making them of limited practical significance in many fields. Approaching the desired resolutions with a real-time video rate is still a challenge. This work concerns the development of two novel methodologies that sharply move above state of the art, pushing forward all the typical qualifying features of THz imaging, enabling the field to further grow and even outpace the scope of established imaging techniques. The first methodology I developed, in terms of physical mechanism and methodological approach, regards a way to map ultrafast dynamics of carriers using terahertz imaging with resolution significantly finer than the wavelength and acquisition rate approaching video-rate, which is unprecedented in the field. I conceived a video-rate large-area THz carrier analysis approach that allowed for phase-sensitive measurement of the conductivity changes that photo-excited carriers introduce in samples, significantly surpassing the performances of established approaches to map carrier relaxation dynamics and relate them to material properties in complex structures. This novel technique, the Optical Pump – Terahertz Near-field Microscopy, made THz carrier analysis possible simultaneously on large surfaces, showing how the full microscopic distribution of THz conductivity is affected by a distribution of hot carriers that lies on the sample plane. The technique achieves a video rate of multiple frames per second and also a subwavelength spatial resolution that exceeds ?/20. The second methodology I contributed to in my thesis work regards the establishment of a single-pixel terahertz microscopy approach, which does not require arrays of sensors. THz raster scan imaging approach has been around for a while. However, the achievable signal-to-noise ratio and the exploitation of mechanics scanning mechanisms fundamentally limit their performances. Within a research team, we then introduced the Nonlinear Ghost Imaging an approach that places a sample in the near-field of a optical-to-terahertz converter. My specific contribution regarded the specific methodological approach of sampling the object with a series of terahertz patterns and reconstructing the image by correlating this information with the information obtained with a single-pixel terahertz detector. Again, this approach significantly outperforms the state-of-the-art in terms of fidelity of the reconstructed spectral information and its publication is having a transformative effect on the field. This thesis work serves as a presentation of methodologies and results and also covers related issues such as limitations and future development, with a comprehensive exploration of the motivations behind the work and the general impact in the domain. Because some results and achievements are certainly the result of a teamwork, I will present them in a comprehensive manner, and then I will highlight my specific contribution.