Ultrafast Nanoscale Imaging with High Harmonic Sources

The principle of high harmonic generation (HHG) is the basis of a new era of compact, high-flux radiation sources, which deliver short wavelengths at ultrafast timescales. Various metrology techniques reported so far, such as pump–probe spectroscopy and microscopy, are either time-, frequency-, or space-resolved, but relatively few combined approaches exist. Recent advances in both source and algorithm development have enabled multimodal acquisition and data analysis schemes that bridge the gap between these separate domains. Here, we describe emerging techniques in ultrafast lensless imaging, which have gained traction in the HHG community. In particular, this chapter includes a discussion on coherent diffraction imaging (CDI), Fourier transform holography (FTH), and ptychography. Emphasis is given to extending the abovementioned diffractive imaging techniques to broadband experimental conditions – a necessary requirement for imaging at attosecond timescales.