Periodic Reporting for period 3 - 3D-VIEW (Seeing the invisible: Light-based 3D imaging of opaque nanostructures)
Periodo di rendicontazione: 2023-10-01 al 2025-03-31
Nanostructures drive the world around us. Every modern electronic device contains integrated circuits and nano-electronics to provide its functionality. Advances in nanotechnology directly impact society by enabling smartphones, autonomous devices, the internet of things, data storage, and essentially all forms of advanced technology. Fabricating such nanostructures crucially depends on having the tools to make them visible without destroying them. Modern nanodevices often have complex three-dimensional architectures with small features in all dimensions. While imaging methods that achieve nanometer-scale resolution exist, there are currently no compact tools that can look inside 3D nanostructures made out of metals and semiconductors without damaging their delicate internal structure.
This project will address this challenge by developing compact tools to image 3D nanostructures in a non-invasive way. Even though most nanostructures are completely opaque to visible light, we will develop light-based methods, combined with novel computational imaging techniques, to look inside them with unprecedented resolution and contrast. Light-based imaging is unparalleled in speed and versatility, and allows contact-free detection. The intended approaches are to: 1) Use compact laser-produced soft-X-ray sources to image nanostructures with high 3D resolution and element-sensitive contrast; 2) Use laser-induced ultrasound pulses to image complex 3D nanostructures, even through strongly absorbing materials; 3) Employ computational imaging methods to reconstruct high-resolution 3D object images from the resulting complex diffraction signals. This project is set up as a coordinated research program to bring these concepts to reality. This program provides exciting prospects for fundamental science and industrial metrology. It will go beyond the state-of-the-art in nano-imaging, to extend our vision into the complex interior of the smallest structures found in science and technology.
This project will address this challenge by developing compact tools to image 3D nanostructures in a non-invasive way. Even though most nanostructures are completely opaque to visible light, we will develop light-based methods, combined with novel computational imaging techniques, to look inside them with unprecedented resolution and contrast. Light-based imaging is unparalleled in speed and versatility, and allows contact-free detection. The intended approaches are to: 1) Use compact laser-produced soft-X-ray sources to image nanostructures with high 3D resolution and element-sensitive contrast; 2) Use laser-induced ultrasound pulses to image complex 3D nanostructures, even through strongly absorbing materials; 3) Employ computational imaging methods to reconstruct high-resolution 3D object images from the resulting complex diffraction signals. This project is set up as a coordinated research program to bring these concepts to reality. This program provides exciting prospects for fundamental science and industrial metrology. It will go beyond the state-of-the-art in nano-imaging, to extend our vision into the complex interior of the smallest structures found in science and technology.
The project started on October 1st, 2020 and the first junior researchers started on or soon after that date. These are Dr. Mengqi Du (postdoc, started 01/10/2020), Matthijs Velsink (PhD student, started 01/11/2020), and Fengling Zhang (PhD student, started 01/09/2021). With the hiring of Matthias Gouder (PhD student, starting 01/06/2022) all positions have been filled.
On the theme of nano-imaging with short wavelength radiation (Theme 1), good progress has been made in terms of algorithm development, where we have developed the capability for reflection-mode ptychography with automatic angle calibration (paper published), as well as for robust spectrally resolved EUV ptychography. In addition, a beamline has been designed and constructed for EUV refocusing and subsequent ptychographic imaging in reflection and transmission geometries. Furthermore, experiments have been performed on characterizing HHG beams through ptychographic wavefront sensing (PWFS), which are yielding promising results that will aid upcoming nano-imaging studies using such beams. We have used this PWFS concept to explore the properties of high-harmonic generation (HHG) sources in unprecedented detail. As a next step, new multi-wavelength imaging concepts are being tested experimentally and through simulations.
On Theme 2 (nano-imaging with laser-induced ultrasound), the asynchronous optical sampling (ASOPS) beamline has been designed, constructed and tested. By combining this scanning concept with balanced detection, shot-noise-limited detection of photo-acoustic signals has been achieved. A new approach to ASOPS has been developed, which increases the flexibility and efficiency of pump-probe measurements significantly. First tests on the detection of photo-acoustic signals through layers of metal and various oxide materials yield promising results. We are currently implementing an amplified pump laser system at reduced and tunable repetition frequency, to increase the single-pulse signal and enable measurements on samples that cannot handle high average thermal load.
We are currently working on the fabrication of test samples of increasing complexity, to perform first imaging experiments and characterize capabilities. Furthermore, we are actively developing simulation codes to enable detailed analysis of the measured signals, aimed at the retrieval of multilayer structure and in a later stage high-resolution image reconstruction.
On the theme of nano-imaging with short wavelength radiation (Theme 1), good progress has been made in terms of algorithm development, where we have developed the capability for reflection-mode ptychography with automatic angle calibration (paper published), as well as for robust spectrally resolved EUV ptychography. In addition, a beamline has been designed and constructed for EUV refocusing and subsequent ptychographic imaging in reflection and transmission geometries. Furthermore, experiments have been performed on characterizing HHG beams through ptychographic wavefront sensing (PWFS), which are yielding promising results that will aid upcoming nano-imaging studies using such beams. We have used this PWFS concept to explore the properties of high-harmonic generation (HHG) sources in unprecedented detail. As a next step, new multi-wavelength imaging concepts are being tested experimentally and through simulations.
On Theme 2 (nano-imaging with laser-induced ultrasound), the asynchronous optical sampling (ASOPS) beamline has been designed, constructed and tested. By combining this scanning concept with balanced detection, shot-noise-limited detection of photo-acoustic signals has been achieved. A new approach to ASOPS has been developed, which increases the flexibility and efficiency of pump-probe measurements significantly. First tests on the detection of photo-acoustic signals through layers of metal and various oxide materials yield promising results. We are currently implementing an amplified pump laser system at reduced and tunable repetition frequency, to increase the single-pulse signal and enable measurements on samples that cannot handle high average thermal load.
We are currently working on the fabrication of test samples of increasing complexity, to perform first imaging experiments and characterize capabilities. Furthermore, we are actively developing simulation codes to enable detailed analysis of the measured signals, aimed at the retrieval of multilayer structure and in a later stage high-resolution image reconstruction.
We have developed various new methodologies for nanoscale imaging and sensing, including:
- Ptychographic wavefront sensing, enabling high-resolution characterization of the full complex field of light beams at visible and extreme-ultraviolet wavelengths, even for multiple wavelengths in parallel. We have demonstrated characterization of HHG beams with up to nine wavelengths.
- Using this PWFS method, we have studied the HHG process itself, and in particular the wavefront variations between different harmonics, and the transfer of wavefronts from the fundamental driving field to the harmonics. These studies have yielded new insights and provided experimental confirmation of theoretical predictions.
- We have developed modified asynchronous optical sampling (MASOPS), as a new approach to efficient shot-noise-limited pump-probe spectroscopy. With this method, we can reach the limits of sensitivity in any photo-acoustics experiment on planar multilayer samples.
- Ptychographic wavefront sensing, enabling high-resolution characterization of the full complex field of light beams at visible and extreme-ultraviolet wavelengths, even for multiple wavelengths in parallel. We have demonstrated characterization of HHG beams with up to nine wavelengths.
- Using this PWFS method, we have studied the HHG process itself, and in particular the wavefront variations between different harmonics, and the transfer of wavefronts from the fundamental driving field to the harmonics. These studies have yielded new insights and provided experimental confirmation of theoretical predictions.
- We have developed modified asynchronous optical sampling (MASOPS), as a new approach to efficient shot-noise-limited pump-probe spectroscopy. With this method, we can reach the limits of sensitivity in any photo-acoustics experiment on planar multilayer samples.