Nanoscale Waveguiding Studied by Lensless Coherent Diffractive Imaging using EUV High-Harmonic Generation Source

Abstract

The challenge of nanometric imaging drives intense efforts in applied sciences and fundamental research. Following the physical law of diffraction, the utilization of extreme ultraviolet (EUV) or X-ray radiation for imaging extends the ultimate resolution limit given by the illumination wavelength down to nanoscale dimensions. However, despite the great potential, the resolution is governed by imperfections inherent to optical elements employed. In this cumulative thesis, a table-top source of EUV radiation based on high harmonic generation (HHG), is developed and applied for lensless coherent diffractive imaging (CDI). Real-space images of microscopic objects are obtained solely on the basis of their far-field diffraction patterns, eliminating the need of any optical element and therefore any aberrations they may have. Benefiting from the exceptional coherence of the source developed, a diffraction-limited imaging is demonstrated with a spatial resolution of illuminating wavelength down to 30 nm. The microscopic images obtained via CDI allow for a detailed study of electric field distributions on the exit surface of the investigated structures. The experiments reveal that electric fields emerging from metallic structures are strongly affected by the polarization of the illuminating probe as well as propagation effects within the sample, indicating the limitations of the projection approximation. The above observations are attributed to waveguiding phenomena, thus, demonstrating extreme ultraviolet waveguiding in nanoscale slab structures. By direct imaging of the spatial distribution of the underlying electromagnetic modes exiting the sample, a strong polarization contrast of metallic waveguides with high aspect-ratio is experimentally identified. The guiding and polarization-sensitiveness are explored through an analytical approach based on the expansion in eigenmodes as well as numerical simulations to corroborate the experimental findings. Additionally, a novel approach to analyze and control the polarization state of EUV radiation by means of single image acquisition is developed. Such an analyzer, employing a set of nanoscale slab waveguides provides for an advanced characterization scheme for the polarization state of high harmonics. These results on CDI beyond the projection approximation using high harmonics demonstrate the potential of such a source for imaging, and pave the way towards applications in material science, for example, the spatial investigation of chiral phenomena, such as magnetism, enabling nanoscale magnetic imaging with a compact tabletop setup. Furthermore, HHG enables to date shortest possible pulse durations offering the opportunity to study ultrafast phenomena with unprecedented temporal resolution.