Abstract
The imaging of hydrated biological samples - especially in the energy window of 284-540 eV, where water does not obscure the signal of soft organic matter and biologically relevant elements - is of tremendous interest for life sciences. Free-electron lasers can provide highly intense and coherent pulses, which allow single pulse imaging to overcome resolution limits set by radiation damage. One current challenge is to match both the desired energy and the intensity of the light source. We present the first images of dehydrated biological material acquired with 3rd harmonic radiation from FLASH by digital in-line zone plate holography as one step towards the vision of imaging hydrated biological material with photons in the water window. We also demonstrate the first application of ultrathin molecular sheets as suitable substrates for future free-electron laser experiments with biological samples in the form of a rat fibroblast cell and marine biofouling bacteria Cobetia marina.
Highlights
The most recent step in the development of highly brilliant and coherent X-ray light sources is the evolution from 3rd generation synchrotrons to free-electron lasers (FELs) [1,2]
For FEL experiments direct encoding of the phase may be a useful option as a hologram could be used as an initial guess for a hybrid imaging technique like Fresnel coherent diffractive imaging (FCDI) in order to „seed‟ a successful and fast converging high resolution reconstruction even if the data has been acquired under difficult conditions – e.g. a partially coherent source
Since higher temporal coherence can be achieved by narrower filtering of the FEL radiation, the attempt of pushing the coherence-limited resolution further up is accompanied by a reduction of photon flux, leading to a drop in the signal-to-noise ratio (SNR) and subsequently in resolution
Summary
The most recent step in the development of highly brilliant and coherent X-ray light sources is the evolution from 3rd generation synchrotrons to free-electron lasers (FELs) [1,2]. The above FELs which are based on the self-amplified spontaneous emission (SASE) principle [10] have one key property: they provide ultrashort coherent light pulses, with pulse lengths down to a few femtoseconds These ultrashort X-ray pulses open the door to high resolution imaging with intense radiation at short wavelengths with a single pulse. The ultrashort pulses allow the evasion of conventional resolution limitations set by radiation damage [11] The availability of these sources stimulates the development of coherent imaging techniques that are capable of tapping the full potential of these 4th generation light sources. At FLASH, recent CXDI data in the 3rd harmonic at a photon energy of 462 eV was recorded, but the phase retrieval was hampered due to the limited coherence length in this case [20,21]. An overview of current experiments on soft X-ray imaging at FLASH can be found in [30]
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