Biological Applications of Short Wavelength Microscopy Based on Compact, Laser-Produced Gas-Puff Plasma Source  
[PERSON]  
1 Nuclear Physics Institute, Czech Academy of Sciences, 25068 Rez, Czech Republic; [EMAIL_ADDRESS]  
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[PERSON]  
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[PERSON]  
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2 Institute of Optoelectronics, Military University of Technology, 00-908 Warsaw, Poland; [EMAIL_ADDRESS] (A.B. ); [PERSON]. wavelength sources for microscopy has already been made, especially employing extreme ultraviolet (EUV, \(\lambda\) = 10-120 nm) and soft X-ray (\(\lambda\) = 0.1-10 nm) wavelengths [1], in the so-called \"water window\" spectral range (\(\lambda\) = 2.3-4.4 nm) [3,4], obtaining natural contrast due to the different absorption of carbon and oxygen, limiting such range. High production of energetic EUV and SXR radiation, which represents a fundamental requirement for microscopy, can be obtained by choosing sources and optics capable of reaching a high photon flux [5,6]. Different types of optics were employed in the last 30 years for X-ray imaging experiments, from reflective optics, such as multilayer mirrors [7], Kirkpatrick-Baez mirrors [8], and compound refractive lenses [9] to diffractive optics, such as multilayer Laue lenses [10] and, particularly, Fresnel zone plate optics (FZPs) [11,12]. Moreover, large facilities, such as X-ray free electron lasers (XFELs) [13] and synchrotrons [14,15] were improved, although they require high maintenance costs because of their complexity. In the last two decades, considerable efforts were made to develop compact EUV and SXR sources to be used in compact, cheaper, and user-friendly laboratory systems. Short EUV radiation can be employed to get information about thin samples due to their high absorption by solid thin materials (thicknesses of the order of \(\sim\)100 nm) and by gaseous materials (few millimeters thick) [16]. Moreover, this radiation enables to analyze micro-cracks into nanostructures [17], to visualize the gas flow in two-dimensional (2D) [18] and three-dimensional (3D) mode [19], usually investigated with indirect techniques, such as the interferometry [20], or for high-resolution lithography, reaching sub-7 nm patterning at 13.5 nm [21]. Different types of compact EUV sources currently allow acquiring images with a sub-micrometer resolution. Employing a recombination laser at \(\lambda\) = 18.2 nm, a 700 nm half-pitch spatial resolution was obtained [22]. Radiation from a capillary discharge laser operating at a wavelength of \(\lambda\) = 46.9 nm allowed to obtain a spatial resolution lower than 150 nm [23], down to \(\sim\)50 nm [24], and holographic images down to sub-50 nm [25]. However, the number of acquired images is limited by the relatively short lifetime of the capillaries [26]. A transmission microscope, based on a Ni-like Cd-EUV laser operating at 13.2 nm wavelength, is capable to achieve a sub-38 nm spatial resolution in a picosecond [27]. Gas-plasma sources can be optimized to obtain high-resolution imaging employing different techniques, such as supersonic jets [28], high order harmonic generation (HHG) [29], and laser-plasma sources [30]. Short wavelength microscopy investigations can be extended, employing, also, radiation from the SXR range. Full field X-ray microscopy is typically limited by the low transmission efficiency of the diffractive FZPs employed. This obstacle can be overtaken by developing bright sources, such as spatially coherent plasma-based X-ray lasers (XRLs) [31] or HHG [32], which lead the possibilities to obtain high-resolution images with lensless techniques, such as coherent diffractive imaging (CDI) [33], holography [34], and ptychography [35].