Sub-wavelength coherent imaging of periodic samples using a 13.5 nm tabletop high harmonic light source (2403.19766v1)

Abstract: Coherent diffractive imaging is unique as the only route for achieving diffraction-limited spatial resolution in the extreme ultraviolet and X-ray regions, limited only by the wavelength of the light. Recently, advances in coherent short wavelength light sources, coupled with progress in algorithm development, have significantly enhanced the power of x-ray imaging. However, to date, high-fidelity diffraction imaging of periodic objects has been a challenge because the scattered light is concentrated in isolated peaks. Here, we use tabletop 13.5nm high harmonic beams to make two significant advances. First we demonstrate high-quality imaging of an extended, nearly-periodic sample for the first time. Second, we achieve sub-wavelength spatial resolution (12.6nm) imaging at short wavelengths, also for the first time. The key to both advances is a novel technique called modulus enforced probe, which enables robust, quantitative, reconstructions of periodic objects. This work is important for imaging next generation nano-engineered devices.

Sub-wavelength Imaging of Periodic Samples via High Harmonic Light Sources

The paper presents a significant advancement in coherent diffractive imaging (CDI) techniques applied to nearly-periodic samples using a 13.5 nm tabletop high harmonic generation (HHG) light source. The paper introduces a novel method termed the Modulus Enforced Probe (MEP), enabling robust, quantitative reconstructions of periodic samples and achieving sub-wavelength spatial resolution imaging, reported at 12.6 nm for the first time. This breakthrough facilitates high fidelity imaging of extended samples, opening new possibilities for next-generation nanotechnology applications.

Methodology and Findings

In conventional CDI, the phase of the scattered light is computationally retrieved via iterative phase retrieval algorithms, allowing for diffraction-limited spatial resolution without the need for costly and imperfect X-ray optics. This paper leverages the MEP technique as a constraint within an extended ptychographic iterative engine (ePIE) algorithm, providing precise initialization for the illumination beam. The MEP strategy minimizes reconstruction artifacts due to cross-talk between the sample and the illumination, enhancing convergence speed and robustness against noise. It relies on collecting a single image of the un-scattered direct beam, thus ensuring quantitative amplitude and phase information retrieval.

The experimental setup utilizes bright, phase-matched HHG beams produced by an ultrafast laser focused through a Helium-filled waveguide, rejecting residual laser light with filtering optics and selecting the single harmonic order at 13.5 nm. The sample, a zone plate, is scanned under the EUV beam to gather diffraction patterns. By integrating the MEP constraint, the method achieved a record wavelength-to-resolution ratio of 0.9λ, validating sub-wavelength imaging capabilities limited only by the numerical aperture.

The paper presents reconstructed intensity images of the samples with and without the MEP constraint. The implementation of MEP results in high-fidelity imaging, demonstrating a significant enhancement over traditional ptychography which suffers from artifacts when applied to periodic patterns. The reported 12.6 nm resolution surpasses prior limits where resolution was restricted to 1.3×λ under optimal conditions.

Implications and Future Prospects

The results have profound implications for imaging nano-engineered devices, supporting advancements in EUV lithography, nanoelectronics, and data storage. MEP increases the reliability of CDI techniques for materials science, allowing detailed exploration of chemical and elemental specificity in the EUV and soft X-ray spectral regions. Furthermore, this research suggests potential for imaging at even shorter wavelengths, achievable with advanced HHG sources or large-scale synchrotron facilities, ideal for atomic-scale investigations.

This paper lays groundwork for further exploration into multiplexed wavelength imaging, undersampling, and analysis of thick samples. As ptychography evolves to handle complex and extended samples, the addition of MEP provides a strategic advantage in solving for multiple variables in CDI algorithms, likely improving convergence and stability across numerous applications.

In conclusion, the paper's contribution to enhancing CDI methodology represents a crucial step forward in the field, with potential consequences for both theoretical understanding and practical applications in advanced imaging technologies.