Ultra-broadband diffractive imaging with unknown probe spectrum

BCDI uses wideband illumination to improve photon efficiency across the full spectrum, eliminating the need for coherent light. However, this extension introduces diffraction aliasing and significant decoherence, hindering CDI convergence. Over the past decades, numerous studies have aimed to address these issues for BCDI. Challenges include the complex iterative computations across dense wavelength channels, the need for precise spectrum measurements, constraints on non-dispersive specimens, and ensuring solutions converge within the bandlimit.

 

In a recent breakthrough, Professor Shiyuan Liu and his team at Huazhong University of Science and Technology addressed these challenges by developing a numerical coherence-enhanced and superfast-solving monochromatization (CSM) method. Their innovative work advanced BCDI to UDI with unknown probe spectra. The research, titled "Ultra-broadband diffractive imaging with unknown probe spectrum," has been published in Light: Science and Applications.

 

Drawing inspiration from the mono CDI framework, they propose an advancement that expands CDI to ultra-broadband illumination, termed UDI. For the first time, UDI eliminates the need for prior knowledge of the probe spectrum and relaxes constraints on non-dispersive samples. This approach significantly enhances photon efficiency for ultra-broadband computational imaging and effectively addresses the key challenges faced by existing state-of-the-art BCDI frameworks.

 

For in-situ spectral measurement of the UDI system, an ultra-streamlined diffraction-based computational spectral measurement design has been introduced, utilizing coherent mode decomposition from broadband diffraction measurements. This approach represents a significant advancement in broadband computational imaging by enabling the recovery of the compressively sampled spectral profile of the UDI system. It allows for the in-situ recovery of information regarding the spectral characteristics of the imaging system.

 

To restore the coherence of ultra-broadband diffraction patterns, the stabilized biconjugate gradient algorithm (BiCGStab) is proposed for monochromatically solving the decoherence signals of ultra-broadband diffraction data. Using the obtained in-situ spectral measurements, they establish a powerful and superfast monochromatization approach for the UDI system. This innovation not only reconstructs the spectral features of the diffracted radiation but also achieves coherence-enhanced monochromatization of the captured broadband pattern with high efficiency and robustness.

 

This research presents the UDI method as a breakthrough in ultra-broadband diffractive imaging, capable of handling unknown probe spectra while recovering the spectral information of diffracted radiation. It provides CSM reconstructions under ultra-wide spectral illumination, offering nearly fourfold improvement in bandwidth over current mono CDI methods. UDI, for the first time, eliminates the need for prior spectral knowledge and overcomes constraints on non-dispersive specimens, making it ideal for applications across broad wavelength ranges, particularly crucial for applications in the EUV and soft X-ray regions where absorption edge effects are significant.

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