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High-resolution microscopy without lenses: a new generation of imaging technology

Periodic Reporting for period 4 - Lensless (High-resolution microscopy without lenses: a new generation of imaging technology)

Reporting period: 2019-11-01 to 2020-04-30

Lensless imaging is an elegant approach to microscopy, in which a sharp image of an object is retrieved by numerical means rather than by actual optical components such as lenses. In lensless imaging, light that has interacted with an object (i.e. a diffraction pattern) is recorded directly, and an image of the object is retrieved through computer algorithms. The main challenge in lensless imaging is that this typically requires knowledge of the electric field of the detected light (both intensity and phase), while optical detectors only record intensity. Nevertheless, through controlled use of imaging conditions and the employed imaging geometry, even intensity-only measurements can allow for a numerical reconstruction of the full electric field of light. If the field is known, aberrations introduced by optical components and alignment imperfections can be mitigated numerically, leading to diffraction-limited high-quality images even when only simple optics, or even no optics at all, are used in the imaging system. The objective of this project is to advance this new technology towards high-impact applications, and to forge a coordinated research program on lensless imaging technology and its applications.
Within the frame of this project, we are developing novel methods that enable the construction of very simple yet highly powerful microscopes and imaging systems, which are not limited by the quality of the optical hardware. We are also working towards methods that allow for imaging through scattering and even optically opaque media, based on the same lensless imaging framework. Since the technology does not require precision optical components, we will be able to extend it to shorter wavelengths. We are working towards applications in EUV metrology and soft-X-ray imaging, such as ultra-high-resolution surface profiling of lithographic wafers. As such, this program provides exciting new prospects for fundamental science, industrial metrology and medical diagnostics alike.
Since the start of the project we have established a mature research team consisting of three PhD students and one postdoc, all working on different aspects of lensless imaging technology. We have developed highly compact lensless microscopes that provide quantitative intensity and phase contrast, using structured illumination schemes to allow robust and accurate numerical phase retrieval. The required algorithms have also been developed in our group. In general, we have developed several algorithms and approaches for phase retrieval and image reconstruction, providing a basis for all sorts of lensless imaging experiments that we envisage. Such algorithms can be translated to different subprojects with only minor modifications, allowing quick translation from proof-of-concept measurements with visible light to more demanding soft-X-ray imaging schemes. Furthermore, we have implemented all-optical methods to detect buried structures underneath optically opaque metal layers. We have designed a microscope system that will enable us to image complex microscopic structures through such opaque layers, and expect to achieve first results very soon. Finally, we have constructed a lensless imaging experiment using coherent extreme-ultraviolet (EUV) radiation produced by high-harmonic generation, which is a compact (table-top) source of coherent radiation at shor wavelengths. In the course of this experiment, we developed methods to perform spatially resolved spectroscopy with EUV radiation, based on Fourier-transform spectroscopy methods (G.S.M. Jansen et al, Optica 3, 1122 (2016)). We have recently extended this method towards high-resolution spectrally resolved imaging without lenses in the EUV range. In the course of this project, we also developed a novel method for broadband wavefront sensing of EUV beams (L. Freisem et al., ArXiv 1712.04234 (2017)), which is a valuable tool for metrology on high-harmonic sources during complex experiments.
We have developed advanced lensless imaging capabilities with both visible and EUV radiation, by developing a combination of both hardware (EUV sources, illumination systems and lensless imaging setups) and software (phase retrieval algorithms based on structured illumination, spatial shearing interferometry with EUV beams). Furthermore, we have developed unique methods for spectrally resolved detection and imaging with EUV radiation, as well as an imaging system that can sense through layers of metal. All in all, we are in a good position for the remainder of the project to demonstrate that lensless imaging can go beyond the capabilities of conventional lens-based methods. We expect to demonstrate spectrally resolved high-resolution EUV imaging on complex materials and imaging inside metal and semiconductor nanostructures. More generally, we expect to show that lensless imaging can provide an integrated framework for advanced imaging and microscopy, going well beyond the current state of the art in terms of contrast, resolution, and field of view.
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