Konstantins Jefimovs

Ptychographic coherent diffractive imaging of weakly scattering specimens

Applying iterative phase retrieval schemes to ptychographic data, i.e. diffraction patterns collected with a localized illumination probe from overlapping regions of a specimen, has enabled the investigation of extended specimens previously inaccessible by other coherent x-ray diffractive imaging methods. While the technique had initially been limited by the requirement of precise knowledge of the illumination function, recent algorithmic developments allow now the simultaneous reconstruction of both the probe and the object. However, these new approaches suffer from an inherent ambiguity, which affects especially the case of weakly scattering specimens. We present new schemes to circumvent this problem and introduce new tools for obtaining information about the scattering behaviour of weak phase objects already during data collection. The new techniques are experimentally demonstrated for a data set taken on Magnetospirillum gryphiswaldense.

Investigation of Plasmon Resonances in Metal Films with Nanohole Arrays for Biosensing Applications

Biosensing with nanoholes is one of the most promising applications of nanoplasmonic devices. The sensor properties, however, are complex due to coupled resonances through propagating and localized surface plasmons. This Full Paper demonstrates experimental and simulation studies on different plasmonic hole systems, namely various patterns of circular holes in gold films. In contrast to most previous work, here, the challenging situation of optically thin films is considered. The refractive-index-sensing properties, such as sensitive locations in the nanostructure and sensitive spectral features, are investigated. The multiple multipole program provides the complete field distribution in the nanostructure for different wavelengths. It is shown that the spectral feature most sensitive to refractive-index changes is the extinction minimum, rather than the maximum. The results are consistent with theory for perfect electrical conductors. The spectral response is investigated for molecular adsorption at different positions inside or outside a hole. Furthermore, the optical properties of nanohole arrays with long-range and short-range order are compared and found to demonstrate remarkable similarities. Our results help to predict the resonance wavelengths of nanoholes with arbitrary patterns, including short-range order. The results presented here are highly important since they extend and challenge several aspects of the current understanding of plasmon resonances in nanohole arrays. These theoretical models, simulation results, and experimental data together help provide the understanding necessary for the development of efficient biomolecular analysis tools based on metallic nanoholes.

Mass spectrometry-based metabolomics of single yeast cells

Single-cell level measurements are necessary to characterize the intrinsic biological variability in a population of cells. In this study, we demonstrate that, with the microarrays for mass spectrometry platform, we are able to observe this variability. We monitor environmentally (2-deoxy-d-glucose) and genetically (ΔPFK2) perturbed Saccharomyces cerevisiae cells at the single-cell, few-cell, and population levels. Correlation plots between metabolites from the glycolytic pathway, as well as with the observed ATP/ADP ratio as a measure of cellular energy charge, give biological insight that is not accessible from population-level metabolomic data.

Advanced thin film technology for ultrahigh resolution X-ray microscopy

Further progress in the spatial resolution of X-ray microscopes is currently impaired by fundamental limitations in the production of X-ray diffractive lenses. Here, we demonstrate how advanced thin film technologies can be applied to boost the fabrication and characterization of ultrahigh resolution X-ray optics. Specifically, Fresnel zone plates were fabricated by combining electron-beam lithography with atomic layer deposition and focused ion beam induced deposition. They were tested in a scanning transmission X-ray microscope at 1.2 keV photon energy using line pair structures of a sample prepared by metal organic vapor phase epitaxy. For the first time in X-ray microscopy, features below 10nm in width were resolved.

High-density micro-arrays for mass spectrometry

Functional high-density micro-arrays for mass spectrometry enable rapid picolitre-volume aliquoting and ultrasensitive analysis of microscale samples, for example, single cells.

Interfacing droplet microfluidics with matrix-assisted laser desorption/ionization mass spectrometry: label-free content analysis of single droplets.

Droplet-based microfluidic systems have become a very powerful tool to miniaturize chemical and biological reactions. However, droplet content analysis remains challenging and relies almost exclusively on optical methods such as fluorescence spectroscopy. Hence, labeling of the analyte is typically required which impedes a more universal applicability of microdroplets. Here we present a novel interface coupling droplet microfluidics and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for label-free content analysis of single droplets. Nanoliter aqueous droplets immersed in perfluorinated oil are created in a microfluidic T-junction, transferred into a capillary, and deposited on a high-density microarray MALDI plate mounted on a motorized xy-stage. The fully automated system is robust and reliable due to two unique features. First, a simple optical droplet detection system is used to synchronize stage movement and exit of droplets from the capillary. Second, the microarray plate contains an array of over 26,000 hydrophilic spots within a hydrophobic coating, each spot acting as a recipient to confine the droplets and to prevent cross-contamination. The MALDI matrix can also be applied using our system by spotting matrix droplets on the microarray in a separate run. To demonstrate the potential of our system, we studied the enzymatic cleavage of angiotensin I by angiotensin converting enzyme and monitored the increasing concentration of the product angiotensin II over time. The interface provides a robust and fully automated method for rapid label-free and information-rich content analysis of single droplets. With the high number of droplets per plate, this method is particularly suitable for high-throughput screening applications.

Optical activity in subwavelength-period arrays of chiral metallic particles

We report on a chirality-induced polarization effect in a planar subwavelength metallic nanograting. We demonstrate that the grating rotates the polarization at normal incidence. Because of the fourfold rotation symmetry, the effect does not depend on the incident beam polarization, but resembles optical activity in isotropic media. We use rigorous diffraction theory to show that polarization effects in the zeroth diffraction order take place in the presence of waveguide resonances with subwavelength-period arrays of chiral metallic particles.

Linear and nonlinear optical responses influenced by broken symmetry in an array of gold nanoparticles

An array of low-symmetry, L-shaped gold nanoparticles is shown to exhibit high sensitivity to the state of incident polarization. Small imperfections in the shape of the actual particles, including asymmetric arm lengths and edge distortions, break the symmetry attributed to an ideal particle. This broken symmetry leads to a large angular displacement of the extinction axes from their expected locations. More significantly, second-harmonic generation experiments reveal significant second-order susceptibility tensor components forbidden to the ideal symmetry.

Chirality arising from small defects in gold nanoparticle arrays

The symmetry of metal nanostructures may be broken by their overall features or small-scale defects. To separate the roles of these two mechanisms in chiral symmetry breaking, we prepare gold nanostructures with chirality occurring on different levels. Linear optical measurements reveal small chiral signatures, whereas the chiral responses from second-harmonic generation are enormous. The responses of all structures are remarkably similar, suggesting that uncontrollable defects play an important role in symmetry breaking.

Ptychographic characterization of the wavefield in the focus of reflective hard X-ray optics

A technique for quantitatively characterizing the complex-valued focal wavefield of arbitrary optics is described and applied to reconstructing the coherent focused beam produced by a reflective/diffractive hard X-ray mirror. This phase-retrieval method, based on ptychography, represents an important advance in X-ray optics characterization because the information obtained and potential resolution far exceeds that accessible to methods of directly probing the focus. Ptychography will therefore be well-suited for characterizing and aligning future nanofocusing X-ray optics.