Tatiana Latychevskaia

Solution to the twin image problem in holography.

While holography truly constitutes an ingenious concept, ever since its invention by Gabor it has been troubled by the so-called twin-image problem limiting the information that can be obtained from a holographic record. For symmetry reasons there are always two images appearing in the reconstruction of a hologram and the unwanted out of focus twin-image obscures the object. Here we show a universal method of reconstructing a hologram completely free of twin-image disturbances while no assumptions about absorbing or phase shifting properties of the object need to be imposed. Thus, truthful amplitude and phase distributions are retrieved.

Graphene as a transparent conductive support for studying biological molecules by transmission electron microscopy

We demonstrate the application of graphene as a support for imaging individual biological molecules in transmission electron microscope (TEM). A simple procedure to produce free-standing graphene membranes has been designed. Such membranes are extremely robust and can support practically any submicrometer object. Tobacco mosaic virus has been deposited on graphene samples and observed in a TEM. High contrast has been achieved even though no staining has been applied.

Terahertz in-line digital holography of dragonfly hindwing: amplitude and phase reconstruction at enhanced resolution by extrapolation

We report here on terahertz (THz) digital holography on a biological specimen. A continuous-wave (CW) THz in-line holographic setup was built based on a 2.52 THz CO(2) pumped THz laser and a pyroelectric array detector. We introduced novel statistical method of obtaining true intensity values for the pyroelectric array detectors pixels. Absorption and phase-shifting images of a dragonflys hindwing were reconstructed simultaneously from single in-line hologram. Furthermore, we applied phase retrieval routines to eliminate twin image and enhanced the resolution of the reconstructions by hologram extrapolation beyond the detector area. The finest observed features are 35 μm width cross veins.

Simultaneous reconstruction of phase and amplitude contrast from a single holographic record

We present a reconstruction technique for simultaneous retrieval of absorption and phase shifting properties of an object recorded by in-line holography. The routine is experimentally tested by applying it to optical holograms of a pure phase respectively a pure amplitude object of micrometer dimensions that has been machined into a glass-plate using a focused ion beam. The method has also been applied to previously published electron holograms of single DNA molecules.

Terahertz in-line digital holography of human hepatocellular carcinoma tissue

Terahertz waves provide a better contrast in imaging soft biomedical tissues than X-rays, and unlike X-rays, they cause no ionisation damage, making them a good option for biomedical imaging. Terahertz absorption imaging has conventionally been used for cancer diagnosis. However, the absorption properties of a cancerous sample are influenced by two opposing factors: an increase in absorption due to a higher degree of hydration and a decrease in absorption due to structural changes. It is therefore difficult to diagnose cancer from an absorption image. Phase imaging can thus be critical for diagnostics. We demonstrate imaging of the absorption and phase-shift distributions of 3.2 mm × 2.3 mm × 30-μm-thick human hepatocellular carcinoma tissue by continuous-wave terahertz digital in-line holography. The acquisition time of a few seconds for a single in-line hologram is much shorter than that of other terahertz diagnostic techniques, and future detectors will allow acquisition of meaningful holograms without sample dehydration. The resolution of the reconstructions was enhanced by sub-pixel shifting and extrapolation. Another advantage of this technique is its relaxed minimal sample size limitation. The fibrosis indicated in the phase distribution demonstrates the potential of terahertz holographic imaging to obtain a more objective, early diagnosis of cancer.

Nondestructive imaging of individual biomolecules

Radiation damage is considered to be the major problem that still prevents imaging an individual biological molecule for structural analysis. So far, all known mapping techniques using sufficient short wavelength radiation, be it x rays or high energy electrons, circumvent this problem by averaging over many molecules. Averaging, however, leaves conformational details uncovered. Even the anticipated use of ultrashort but extremely bright x-ray bursts of a free electron laser shall afford averaging over 10{6} molecules to arrive at atomic resolution. Here, we present direct experimental evidence for nondestructive imaging of individual DNA molecules. In fact, we show that DNA withstands coherent low energy electron radiation with deBroglie wavelength in the Angstrom regime despite a vast dose of 10{8} electrons/nm{2} accumulated over more than one hour.

Practical algorithms for simulation and reconstruction of digital in-line holograms

Here we present practical methods for simulation and reconstruction of in-line digital holograms recorded with plane and spherical waves. The algorithms described here are applicable to holographic imaging of an object exhibiting absorption as well as phase-shifting properties. Optimal parameters, related to distances, sampling rate, and other factors for successful simulation and reconstruction of holograms are evaluated and criteria for the achievable resolution are worked out. Moreover, we show that the numerical procedures for the reconstruction of holograms recorded with plane and spherical waves are identical under certain conditions. Experimental examples of holograms and their reconstructions are also discussed.

Low-energy electron transmission imaging of clusters on free-standing graphene

We investigated the utility of free-standing graphene as a transparent sample carrier for imaging nanometer-sized objects by means of low-energy electron holography. The sample preparation for obtaining contamination-free graphene as well as the experimental setup and findings are discussed. For incoming electrons with 66 eV kinetic energy, graphene exhibits 27% opacity per layer. Hence, electron holograms of nanometer-sized objects adsorbed on free-standing graphene can be recorded and numerically reconstructed to reveal the objects shapes and distribution. Furthermore, a Moire effect has been observed with free-standing graphene multi-layers.

Depth-resolved holographic reconstructions by three-dimensional deconvolution

Methods of three-dimensional deconvolution with a point-spread function as frequently employed in optical microscopy to reconstruct true three-dimensional distribution of objects are extended to holographic reconstructions. Two such schemes have been developed and are discussed: an instant deconvolution using the Wiener filter as well as an iterative deconvolution routine. The instant 3d-deconvolution can be applied to restore the positions of volume-spread objects such as small particles. The iterative deconvolution can be applied to restore the distribution of complex and extended objects. Simulated and experimental examples are presented and demonstrate artifact and noise free three-dimensional reconstructions from a single two-dimensional holographic record.

Absorption Spectroscopy on Single Molecules in Solids

Absorption signals of single terrylene molecules in n-hexadecane and naphthalene crystals were recorded at liquid-helium temperatures. The method is based upon rf Stark effect modulation in the megahertz range. The electric rf field strength was applied by means of interdigitating electrodes with 18 μm spacing. Signal-to-noise ratios better than 10 were obtained with approximately 300 ms integration time. The measured line shapes depend on the relative contributions of the linear and the quadratic Stark shift.