Coherent diffractive imaging beyond the projection approximation: waveguiding at extreme ultraviolet wavelengths

Abstract

We study extreme-ultraviolet wave propagation within optically thick nanostructures by means of high-resolution coherent diffractive imaging using high-harmonic radiation. Exit waves from different objects are reconstructed by phase retrieval algorithms, and are shown to be dominated by waveguiding within the sample. The experiments provide a direct visualization of extreme-ultraviolet guided modes, and demonstrate that multiple scattering is a generic feature in extruded nanoscale geometries. The observations are successfully reproduced in numerical and semi-analytical simulations.

Nanoscale magnetic imaging using circularly polarized high-harmonic radiation

“…In addition, the reconstructed exit wave of the reference holes ( Fig. 2F ) includes multiple guided modes ( 22 ), with modulation at the level of a single pixel (60 nm in this case) corresponding to scattering that reaches the edges of the CCD detector. Thus, the diffracted reference field interferes with the object wave over the entire detector.…”

“…Finally, while FTH resolution suffers for larger reference holes, the CDI algorithm is fully capable of reconstructing their complex exit-wave patterns, leading to a diffraction-limited resolution. Specifically, guided modes with high spatial frequencies suffer less attenuation upon propagation in wider reference holes ( 22 ) and thus can enhance the high-resolution information at large spatial frequencies. In these experiments, the scattering from large reference holes provides substantial diffraction intensity across the detector (see rings in Figs.…”

Attosecond soft X-ray high harmonic generation

“…There would appear to be no barrier to further improvement to repetition rates tens to hundreds of times higher, and pulse energies an order of magnitude higher, suggesting harmonic fluxes of one to three orders of magnitude higher (around 10 9 photons s −1 /1%BW) without any further improvements in harmonic conversion efficiency. Such high fluxes have proved sufficient for diffraction-limited holographic imaging in the XUV [37]. Though the flux required to reach the diffraction limit increases as the wavelength decreases, the elemental, bond and magnetic sensitivity of SXR absorption make even nondiffraction limited SXR holography very valuable, for instance in imaging phase co-existence [38].…”

Multi-color imaging of magnetic Co/Pt heterostructures

“…However, in the XUV energy range, the wavelengths are on the order of 20 nm and start to approach the size of the reference hole diameters such that the wave guiding effects have to be taken into account. These are expected to exhibit a subtle dependence on the exact shape of the reference hole and on the XUV wavelengths and, hence, will result in an additional and a priori unknown reference wave phase shift 30 . Therefore, we define a measure for the total magnetic contrast as the sum of the real and imaginary part of the reconstructions.…”