The advent of high harmonic technology provides a new generation of high-quality light sources for modern high-resolution imaging applications. The high harmonic sources can provide highly coherent extreme ultraviolet/soft X-ray ultrashort pulses, leading to the rapid development of desktop high-resolution imaging technology in the past 20 years. Meanwhile, this technique can achieve nanometer-level spatial resolution and femtosecond-level temporal resolution by a variety of lensless diffraction imaging methods and has many applications in biology and materials. The isolated attosecond pulses, born in the 21st century, provide us with unprecedented temporal resolution, but the chromatic aberration introduced by their broad-spectrum characteristics has become a new problem in diffraction imaging methods.

There have been numerous reports on imaging applications in biological and material sciences by adopting quasi-monochromatic sources via various imaging techniques including traditional optical imaging, coherent diffractive imaging, Fourier transformation holography, and ptychography as shown in Figs. 1~9. Especially in recent years, there have been a lot of studies focusing on ptychography, including reflective modes and transmissive modes. Furthermore, the vortex structure has shown its unique advantages in ptychography and significantly improved the imaging qualities. Although compared with other methods, ptychography has the best imaging quality and algorithm stability, it is relatively difficult to combine with pump detection technology, and the ultrafast process should be highly repeatable. Thus, most of these ptychography experiments are subjected to static samples, which makes it difficult to show the time resolution advantages of high harmonics. Furthermore, long exposure time has always been a major shortcoming of ptychography. Although single-shot ptychography has been reported, it is only applicable to visible/IR regimes and still challenging under high harmonic illuminations. Imaging using attosecond pulse trains or isolated attosecond pulses is more challenging than quasi-monochromatic sources. The main difficulty is their broad spectrum characteristics, resulting in more unknown quantities to be solved from limited equations. Existing research mainly focuses on two solutions. One is to add a priori condition to reduce the number of unknowns, which can only conduct imaging on specific samples that meet the corresponding prior condition, limiting its application scope. The other is to match the number of unknowns by increasing the number of equations, which requires position scans or delayed scans. With the increasing spectral components, the required number of scan points will increase simultaneously to ensure a sufficient number of equations, thus resulting in the generally limited spectral resolution of such methods. Additionally, these methods are not compatible with single-shot imaging, limiting their applications in ultrafast sciences. Therefore, the development of new imaging methods and algorithms is still an important research direction for achieving combinations of high temporal and spatial resolution of attosecond pulses in imaging.

As high harmonic light sources are characterized by small size, selectable wavelength, good spatial coherence, and short pulse width, they become one of the most preferred choices for extreme ultraviolet/soft X-ray nanoimaging applications. As a quasi-monochromatic light source, the imaging algorithm of the filtered high harmonic light source has been relatively mature. The high harmonics generated by high-energy driving lasers can achieve single-pulse imaging, which provides the possibility to study ultrafast dynamics. Meanwhile, the high harmonics generated by the driving laser in the order of mJ can meet the needs of most static imaging applications and can be applied to many research fields such as biology and materials science, thus having the potential to become a new generation of high-end nanoscopy equipment. Nowadays, high harmonics generated from gases are constantly developing toward higher stability, higher energy, and shorter wavelengths to pave the way for ultrafast imaging with higher imaging quality and higher resolution. With the rapid development of high harmonic generation technology in the water window band, desktop water window live-cell imaging will become the next important research direction. Additionally, solid-state high harmonics are also a rising star developing rapidly in recent years. Although compared to gas-based high harmonics, solid-state high harmonics have low photon energy and damage to the sample, the peak intensity requirements of the driving laser are low, with higher stability and no need for vacuum. Therefore, the development of solid-state high harmonic lasers is also expected to further miniaturize the size of high harmonic microscopy equipment and reduce the cost. In addition to the common Gaussian light source, high harmonics of different spatial or polarization structures can also be obtained by light field synthesis or shaping techniques. With further development of high harmonics field control technology, new structures may also promote the further development of high harmonic imaging technology. As for imaging by adopting attosecond pulse trains or isolated attosecond pulses, there isn’t a satisfying solution. However, the rise of new technologies such as compressive sensing has injected new energy into imaging research, with many new methods developed. Compressed ultrafast imaging and compressed spectral imaging can introduce an additional temporal or spectral dimension to 2D imaging to achieve dynamical or hyperspectral imaging, which is similar to what we are eager to achieve in attosecond imaging. Meanwhile, deep learning is an attractive direction to explore. In the future, the combination of compressed sensing and deep learning with wide-spectrum CDI, wide-spectrum FTH, and wide-spectrum ptychography may provide more opportunities for us.