25. March 2019

Christian Spielmann

Towards lensless imaging of biological samples with table-top XUV sources

Institute of Optics and Quantum Electronics,
Abbe Center of Photonics,
Friedrich Schiller University Jena,

Helmholtz Institute Jena / Jena, Germany

ABSTRACT

Microscopy is one of the most important tools for studying structures and functionalities in organic components. Improving the resolution of images requires small wavelengths and a large numerical aperture of the imaging system. Therefore, extreme ultraviolet light enables high-contrast and high resolution imaging if combined with lensless imaging techniques such as coherent diffraction imaging or ptychography. In this contribution we will review recent work combining table-top light sources in the XUV with lens-less imaging techniques for microcopy of biological samples. To minimize the radiation damage of biological samples we discuss the implementation of ghost imaging microscopy in the XUV.

Summary

Microscopy is the key technology for understanding biological processes and structures. In recent decades, high-resolution microscopy has been demonstrated in a wide range of wavelengths from infrared to visible to X-ray. Excluding modern specialized microscopy approaches such as STED (Stimulated Emission Depletion) or STORM (Stochastic Optical Reconstruction Microscopy), according to Abbes law the resolution with the photon energy for a given numerical aperture (NA) of the imaging optical system. Thus, the shorter the wavelength, the higher the achievable resolution. Moreover, microscopy in different wavelength ranges can provide more information than a simple image, e.g. Raman microscopy enables the collection of information about molecular structures and X-ray microscopy provides data on the electronic structure.

All biological samples contain light-element materials that have absorption edges in the wavelength range between 2 and 30 nm, enabling high-contrast, high-resolution extreme ultraviolet (XUV) imaging. Particularly interesting for the high-contrast imaging of carbon-rich biological samples in an aqueous environment is the so-called “water window” spectral range between the absorption edges of oxygen (2.2 nm) and carbon (4.2 nm). Due to the lack of efficient refractive or reflective optics in the XUV, either lensless or diffractive optical element based approaches are used. Here we report on our recent results in XUV microscopy, which use either lensless techniques such as coherent diffraction imaging (CDI) and its add-on ptychography, or diffractive optical elements such as Fresnel zone plates. For all these high-resolution microscopy approaches, a high photon flux is essential. As laboratory sources, we have used laser-based high-harmonic sources, laser-driven X-ray and laser-driven incoherent plasma sources. We will review the different approaches and discuss their benefits and challenges. An important issue for all biological samples is the radiation damage due to the necessary high flux for high-resolution imaging. This problem can be alleviated if the ghost imaging microscopy in the XUX is implemented to minimize the required flow. In this contribution, we will report on the first steps towards the realization of ghost imaging microscopy with an XUV source based on high harmonic generation.

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