Ptychographic imaging and wavefront sensing at extreme-ultraviolet wavelengths

Microscopy at extreme ultraviolet (EUV) wavelengths has the potential to transform nanotechnology and materials science. The short wavelengths allow for high resolution, while absorption edges in the EUV range enable element-selective imaging. Because of the complexity of imaging optics at EUV wavelengths, lensless coherent diffractive imaging methods are particularly attractive. Compact and fully coherent EUV sources based on high-harmonic generation (HHG), combined with powerful lensless imaging techniques such as ptychography, form the main ingredients for table-top-scale EUV imaging systems. Ptychography is a particularly powerful concept, based on coherent diffractive imaging combined with translational diversity. It has been shown to allow for wavelength-scale-resolution imaging, and has the unique ability to provide images of both the object under study and the incident light field used for imaging. As such, ptychography can be used as an imaging method, but also as a wavefront sensing technique with unparalleled resolution and sensitivity. Importantly, ptychography allows multiplexed imaging, using for instance beams at multiple wavelengths in parallel. We have used both the imaging and wavefront sensing capabilities of ptychography, using multi-wavelength HHG beams as the light source. Through ptychographic wavefront sensing, we have characterized HHG wavefronts in unprecedented detail, allowing measurements of intrinsic chromatic aberration arising from the HHG process itself, and enabling us to identify how wavefront aberrations are transferred from the fundamental beam to the harmonics. Furthermore, I will present our recent results on ptychographic EUV imaging of complex nanostructures, and the future applications enabled by this work.