Martin Huth

Shapes and vorticities of superfluid helium nanodroplets

X-raying superfluid helium droplets When physicists rotate the superfluid 4He, it develops a regular array of tiny whirlpools, called vortices. The same phenomenon should occur in helium droplets half a micrometer in size, but studying individual droplets is tricky. Gomez et al. used x-ray diffraction to deduce the shape of individual rotating droplets and image the resulting vortex patterns, which confirmed the superfluidity of the droplets. They found that superfluid droplets can host a surprising number of vortices and can rotate faster than normal droplets without disintegrating. Science, this issue p. 906 Vortex lattices inside individual helium droplets are imaged using x-ray diffraction. Helium nanodroplets are considered ideal model systems to explore quantum hydrodynamics in self-contained, isolated superfluids. However, exploring the dynamic properties of individual droplets is experimentally challenging. In this work, we used single-shot femtosecond x-ray coherent diffractive imaging to investigate the rotation of single, isolated superfluid helium-4 droplets containing ~108 to 1011 atoms. The formation of quantum vortex lattices inside the droplets is confirmed by observing characteristic Bragg patterns from xenon clusters trapped in the vortex cores. The vortex densities are up to five orders of magnitude larger than those observed in bulk liquid helium. The droplets exhibit large centrifugal deformations but retain axially symmetric shapes at angular velocities well beyond the stability range of viscous classical droplets.

A pnCCD-based, fast direct single electron imaging camera for TEM and STEM

We report on a new camera that is based on a pnCCD sensor for applications in scanning transmission electron microscopy. Emerging new microscopy techniques demand improved detectors with regards to readout rate, sensitivity and radiation hardness, especially in scanning mode. The pnCCD is a 2D imaging sensor that meets these requirements. Its intrinsic radiation hardness permits direct detection of electrons. The pnCCD is read out at a rate of 1,150 frames per second with an image area of 264 x 264 pixel. In binning or windowing modes, the readout rate is increased almost linearly, for example to 4000 frames per second at 4× binning (264 x 66 pixel). Single electrons with energies from 300 keV down to 5 keV can be distinguished due to the high sensitivity of the detector. Three applications in scanning transmission electron microscopy are highlighted to demonstrate that the pnCCD satisfies experimental requirements, especially fast recording of 2D images. In the first application, 65536 2D diffraction patterns were recorded in 70 s. STEM images corresponding to intensities of various diffraction peaks were reconstructed. For the second application, the microscope was operated in a Lorentz-like mode. Magnetic domains were imaged in an area of 256 x 256 sample points in less than 37 seconds for a total of 65536 images each with 264 x 132 pixels. Due to information provided by the two-dimensional images, not only the amplitude but also the direction of the magnetic field could be determined. In the third application, millisecond images of a semiconductor nanostructure were recorded to determine the lattice strain in the sample. A speed-up in measurement time by a factor of 200 could be achieved compared to a previously used camera system.

Sub-pixel resolution of a pnCCD for X-ray white beam applications

A new approach to achieve sub-pixel spatial resolution in a pnCCD detector with 75 × 75 μm2 pixel size is proposed for X-ray applications in single photon counting mode. The approach considers the energy dependence of the charge cloud created by a single photon and its split probabilities between neighboring pixels of the detector based on a rectangular model for the charge cloud density. For cases where the charge of this cloud becomes distributed over three or four pixels the center position of photon impact can be reconstructed with a precision better than 2 μm. The predicted charge cloud sizes are tested at selected X-ray fluorescence lines emitting energies between 6.4 keV and 17.4 keV and forming charge clouds with size (rms) varying between 8 μm and 10 μm respectively. The 2 μm enhanced spatial resolution of the pnCCD is verified by means of an x-ray transmission experiment throughout an optical grating.

Electron ptychographic phase imaging of light elements in crystalline materials using Wigner distribution deconvolution

Recent development in fast pixelated detector technology has allowed a two dimensional diffraction pattern to be recorded at every probe position of a two dimensional raster scan in a scanning transmission electron microscope (STEM), forming an information-rich four dimensional (4D) dataset. Electron ptychography has been shown to enable efficient coherent phase imaging of weakly scattering objects from a 4D dataset recorded using a focused electron probe, which is optimised for simultaneous incoherent Z-contrast imaging and spectroscopy in STEM. Therefore coherent phase contrast and incoherent Z-contrast imaging modes can be efficiently combined to provide a good sensitivity of both light and heavy elements at atomic resolution. In this work, we explore the application of electron ptychography for atomic resolution imaging of strongly scattering crystalline specimens, and present experiments on imaging crystalline specimens including samples containing defects, under dynamical channelling conditions using an aberration corrected microscope. A ptychographic reconstruction method called Wigner distribution deconvolution (WDD) was implemented. Experimental results and simulation results suggest that ptychography provides a readily interpretable phase image and great sensitivity for imaging light elements at atomic resolution in relatively thin crystalline materials.

4D-STEM Imaging With the pnCCD (S)TEM-Camera

4D-STEM imaging describes a powerful imaging technique where a two-dimensional image is recorded for each probe position of a two-dimensional STEM image. For typical STEM images of 256x256 probe positions, a total of 65,536 2D images needs to be recorded. This amount of data can be recorded with the pnCCD (S)TEM camera in a practical timeframe. This camera uses a direct detecting, radiation hard pnCCD with a minimum readout speed of 1000 full frames per second (fps) [1]. With the pnCCD (S)TEM camera a 4D data cube consisting of 256x256 probe positions with a 264x264 pixel detector image for each probe position can be recorded in less than 70 s. Several measurements have been performed to prove the capability of the camera for 4D-STEM imaging, among them strain analysis, magnetic domain mapping and most recently electron ptychography.

Extending the dynamic range of fully depleted pnCCDs

pnCCDs are a special type of charge coupled devices (CCD) which were originally developed for applications in X-ray astronomy. At X-ray Free Electron Lasers (XFEL) pnCCDs are used as imaging X-ray spectrometers due to their outstanding characteristics like high readout speed, high and homogeneous quantum efficiency, low readout noise, radiation hardness and high pixel charge handling capacity. They can be used both as single-photon counting detectors for X-ray spectroscopy and as integrating detectors for X-ray imaging with count rates up to 104 photons of 1 keV per pixel and frame. However, extremely high photon intensities can result in pixel saturation and charge spilling into neighboring pixels. Because of this charge blooming effect, spatial information is reduced. Based on our research concerning the internal potential distribution we can enhance the pixel full well capacity even more and improve the quality of the image. This paper describes the influence of the operation voltages and space charge distribution of the pnCCD on the electric potential profile by using 2D numerical device simulations. Experimental results with signal injection from an optical laser confirm the simulation models.

Application of a pnCCD for energy-dispersive Laue diffraction with ultra-hard X-rays

In this work the spectroscopic performance of a pnCCD detector in the ultra-hard energy range between 40 and 140 keV is tested by means of an energy-dispersive Laue diffraction experiment on a GaAs crystal. About 100 Bragg peaks were collected in a single-shot exposure of the arbitrarily oriented sample to white synchrotron radiation provided by a wiggler at BESSY II and resolved in a large reciprocal-space volume. The positions and energies of individual Laue spots could be determined with a spatial accuracy of less than one pixel and a relative energy resolution better than 1%. In this way the unit-cell parameters of GaAs were extracted with an accuracy of 0.5%, allowing for a complete indexing of the recorded Laue pattern. Despite the low quantum efficiency of the pnCCD (below 7%), experimental structure factors could be obtained from the three-dimensional data sets, taking into account photoelectric absorption as well as Compton scattering processes inside the detector. The agreement between measured and theoretical kinematical structure factors calculated from the known crystal structure is of the order of 10%. The results of this experiment demonstrate the potential of pnCCD detectors for applications in X-ray structure analysis using the complete energy spectrum of synchrotron radiation.

Development of Fast Pixelated STEM Detector and its Applications using 4-Dimensional Dataset

In scanning transmission electron microscopy (STEM), one can obtain a variety of STEM images such as annular bright-field (ABF), annular dark-field (ADF) and differential phase contrast (DPC) images by changing the shapes of the scintillators of the detectors [1,2]. However, the intensity distribution in the convergent beam electron diffraction (CBED) pattern, which is projected on the plane of scintillator, is not fully utilized using the conventional detectors as they integrate the intensity over the scintillator. Meanwhile, direct electron detectors with fast frame rate and several ten thousand pixels have recently been commercialized and used in scanning electron microscopy [3-6]. Such detectors, when used for recording CBED patterns for each STEM probe position, are called pixelated STEM detectors. With the obtained 4-dimensional (4D) dataset, any type of STEM image can be synthesized in a post or real time processing using a user-defined selection of the integration area.

Analysis of Polymorphs Using Simultaneous X-ray Fluorescence and Diffraction with an Imaging Spectrometer

X-ray fluorescence (XRF) is a widely used method for materials characterization. However, for polymorphs such as Rutile and Anatase, both of which are TiO2, the fluorescence spectra will be nearly identical. Differentiating between these polymorphs requires a second measurement with an X-ray diffractometer. Using two instruments increases the cost of the measurement, and it introduces issues with image registration and sample preparation. Previously, simultaneous XRD-XRF has been done by attaching a silicon drift detector (SDD) to an X-ray diffraction (XRD) system. However, due to the typical geometry and source monochromation of an XRD system, the count rate in the SDD is typically low, making the measurement process slow [1]. Overcoming these limitations requires a detector that is both position sensitive and energy dispersive.

Controlled charge extraction—antiblooming capabilities in pnCCD imaging sensors

Blooming in a CCD occurs when the signal charges accumulating in a pixel exceed the pixel saturation level and spill over into adjacent pixels. They start to spill over the weakest threshold in the electric potential of the pixel structure resulting in a degradation of the spatial information. With antiblooming mechanisms, the spatial resolution of the incoming photons can be preserved, but the intensity information is lost in the overflowing pixels. For imaging experiments, relying on a precise image structure, the preservation of the spatial resolution at the expense of precise intensity information is a workable compromise. In contrast to insulated gate CCDs, notably MOSCCDs, the potential wells of the pixel array of a pnCCD are created by p+n junctions, allowing direct electric access to the pixel structure. This allows to directly drain off charges from the pixels and to define a drain level by applying the appropriate operation voltages. Charge packets from 1 000 to more than one billion signal electrons per readout frame were generated without observing a spillover into adjacent pixels. As soon as the saturation level of the pixel is reached, the excess charge carriers are removed through charge drains exclusively created with the modification of the electric potential of the pnCCD by the operation voltages. No additional antiblooming structures were implemented in the device and the pixel full well capacity of approximately 300 000 electrons in standard operation mode was preserved. A physical model of the antiblooming mechanism of pnCCDs with a pixel size of 75 μ m × 75 μ m was established by two-dimensional numerical device simulations and verified by experiments.