Abstract: In this study, we examine the quality of microscale ghost images as a function of the measured histographic signal distribution of the speckle fields from a nonuniform pseudothermal light source. This research shows that the distribution of the detected signal level on each pixel of the camera plays a significant role in improving the contrast-to-noise ratio (CNR) of pseudothermal ghost imaging. To our knowledge, the scaling of CNR with different pixel intensity distributions of the speckle fields is observed for the first time in the field of pseudothermal microscale ghost imaging. The experimental observations are in very good agreement with numerical analysis. Based on these findings, we can predict the settings for light sources that will maximize the CNR of microscale ghost images.
Abstract: Understanding the behaviour of matter under conditions of extreme temperature, pressure, density and electromagnetic fields has profound effects on our understanding of cosmologic objects and the formation of the universe. Lacking direct access to such objects, our interpretation of observed data mainly relies on theoretical models. However, such models, which need to encompass nuclear physics, atomic physics and plasma physics over a huge dynamic range in the dimensions of energy and time, can only provide reliable information if we can benchmark them to experiments under well-defined laboratory conditions. Due to the plethora of effects occurring in this kind of highly excited matter, characterizing isolated dynamics or obtaining direct insight remains challenging. High-density plasmas are turbulent and opaque for radiation below the plasma frequency and allow only near-surface insight into ionization processes with visible wavelengths. Here, the output of a high-harmonic seeded laser-plasma amplifier using eight-fold ionized krypton as the gain medium operating at a 32.8 nm wavelength is ptychographically imaged. A complex-valued wavefront is observed in the extreme ultraviolet (XUV) beam with high resolution. Ab initio spatio-temporal Maxwell-Bloch simulations show excellent agreement with the experimental observations, revealing overionization of krypton in the plasma channel due to nonlinear laser-plasma interactions, successfully validating this four-dimensional multiscale model. This constitutes the first experimental observation of the laser ion abundance reshaping a laser-plasma amplifier. The presented approach shows the possibility of directly modelling light-plasma interactions in extreme conditions, such as those present during the early times of the universe, with direct experimental verification.
Abstract: We report the direct wavefront characterization of an intense ultrafast high-harmonic seeded soft X-ray laser at 32.8 nm wavelength and monitor the exit of the laser plasma amplifier depending on the arrival time of the seed pulses with respect to pump pulses. For the wavefront measurement in phase and intensity, we used high-resolution ptychography. After propagating the wavefront back to the source, we are able to observe the rear end of the plasma amplifier. We compare the characteristics of the seeded soft X-ray Laser to an unseeded one and find an increasing beam stability and lateral coherence important for lensless imaging techniques.
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2020)
Abstract: The growing numerical development of Coherent Diffraction Imaging (CDI) towards ptychography allows for the first time the separate reconstruction of object and the wavefront illuminating it. This work is dedicated to the investigation of further possibilities resulting from the complex reconstruction of object and illumination.
In this thesis, gold structures buried in silicon are reconstructed and examined with regard to their surface morphology in reflection geometry. This completely non-destructive method allows metrology on structures of embedded circuits and otherwise hidden defects.
The increasing demand for easily accessible and compact high-performance light sources around the silicon and water window opens the question regarding their suitability for lensless imaging. In the following chapters a method is introduced which allows an almost complete source analysis by means of a single long time exposed diffraction pattern. The knowledge gained in this way allows an improvement of the source with respect to water window CDI and provides insight into dynamic processes within the source.
The complex-valued reconstruction of the wavefront allows an insight into the plasma and the ionization states prevailing there. The XUV seed pulse of a seeded Soft-X-Ray laser (SXRL), which passes the pumped plasma and changes its properties with respect to the states in the plasma, is reconstructed ptychographically. Adapted Maxwell-Bloch simulations allow by comparison with the measurement to restore the ionization states during the passage of the seed pulse.
Previous experiments showed artifacts during reconstruction, which were directly related to the periodicity of the objects. Simulation of periodic objects of different sizes and with the addition of intentional defects showed a dependence of the reconstruction of the object on the illumination function. Various criteria were derived from this simulation and are presented in this thesis.
Abstract: In this study, the influence of speckle size on contrast-to-noise ratio (CNR) and resolution is examined based on the object dimensions in the macroscopic and microscopic regimes. This research shows that for microscopic samples the conventional scaling laws are no longer effective and the CNR does not counter-propagate in the same manner as the resolution. To our knowledge, a deviation in CNR scaling on speckle size is observed for the first time in the field of microscopic ghost imaging. This result was verified using two different sample shapes. In addition, numerical analysis revealed that the noise of the photodiode is a limiting factor for the CNR. Based on these findings, the conditions for identifying the parameter set that maximizes the CNR and provides high resolution images was defined, which achieving high-quality microscopic ghost images.
Abstract: Imaging of biological specimen is one of the most important tools to investigate structures and functionalities in organic components. Improving the resolution of images into the nanometer range call for short wavelengths light sources and large aperture optics. Subsequently, the use of extreme ultraviolet light in the range of 2 nm to 5 nm provides high contrast and high resolution imaging, if it is combined with lensless imaging techniques. We describe important parameters for high resolution lensless imaging of biological samples and specify the required light source properties. To overcome radiation based damage of biological specimen, we discuss the concept of ghost imaging and describe a possible setup towards biological imaging in the extreme ultraviolet range.
Abstract: We evaluated the capabilities of an intense ultrafast high-harmonic seeded soft X-ray laser at 32.8 nm wavelength regarding single-shot lensless imaging and ptychography. Additionally the wave front at the exit of the laser plasma amplifier is monitored in amplitude and phase using high resolution ptychography and backpropagation techniques.Characterizing the laser plasma amplifier performance depending on the arrival time of the seed pulse with respect to pump pulses provides insight into the light plasma interaction in the soft X-ray range.