In this paper, we present some experimental results on X -ray holography, holographic tomography, and a new holographic tomography method called pre-amplified holographic tomography is proposed.
Due to the shorter wavelength and the larger penetration depths, X-rays provide the potential of higher resolution in imaging techniques, and have the ability to image intact, living, hydrated cells w ithout slicing, dehydration, chemical fixation or stain.
Recently, using X-ray source in National Synchrotron Radiation Laboratory in Hefei, we have successfully performed some soft X-ray holography experiments on biological specimen. The specimens used in the experiments was the garlic clove epidermis, we got their X-ray hologram, and then reconstructed them by computer programs, the feature of the cell walls, the nuclei and some cytoplasm were clearly resolved. However, there still exist some problems in realization of practical 3D microscopic imaging due to the near-unity refractive index of the matter. There is no X-ray optics having a sufficient high numerical aperture to achieve a depth resolution that is comparable to the transverse resolution. On the other hand, computer tomography needs a record of hundreds of views of the test object at different angles for high resolution. This is because the number of views required for a densely packed object is equal to the object radius divided by the desired depth resolution. Clearly, it is impractical for a radiation-sensitive biological specimen. Moreover, the X-ray diffraction effect makes projection data blur, this
badly degrades the resolution of the reconstructed image.
In order to observe 3D structure of the biological specimens, McNulty proposed a new method for 3D imaging called "holographic tomography (HT)" in which several holograms of the specimen are recorded from various illumination directions and combined in the reconstruction step. This permits the specimens to be sampled over a wide range of
spatial frequencies to improve the depth resolution. In NSRL, we performed soft X-ray holographic tomography experiments. The specimen was the spider filaments and PM M A as recording medium. By 3D CT reconstruction of the projection data, three dimensional density distribution of the specimen was obtained.
Also, we developed a new X-ray holographic tomography m ethod called pre-amplified holographic tomography. The method permits a digital real-time 3D reconstruction with high-resolution and a simple and compact experimental setup as well.
Some experimental results on soft X-ray microscopy and holography imaging of biological specimens are presented in the paper.
As we know, due to diffraction effects, there exists a resolution limit determined by wavelength λ and numerical aperture NA in conventional optical microscopy. In order to improve resolution, the num erical aperture should be made as large as possible and the wavelength as short as possible. Owing to the shorter wavelength, X-rays provide the potential of higher resolution in X-ray microscopy, holography image and allow for exam ination the interior structures of thicker specimens.
In the experiments, we used synchrotron radiation source in Hefei as light source. Soft X-rays come from a bending magnet in 800 M eV electron storage ring with characteristic wavelength of 2.4 nm. The continuous X-ray spectrums are monochromatized by a zone-plate and a pinhole with 300 m diameter. The experimental set-up is typical contact microscopic system, its main advantage is simplicity and no special optical element is needed.
The specimens used in the experiments of microscopic imaging are the colibacillus, the gingko vascular hundle and the fritillaries ovary karyon. The specimen for holographic imaging is the spider filam ents. The basic structures of plant cells such as the cell walls, the cytoplasm and the karyon especially the joint structures between the cells are observed clearly.
An experimental study on a thick biological specimen that is a whole sporule w ith the thickness of about 30 μm is performed.
In the holographic experiments, the experimental setup is typical Gabor in-line holography. The specimen is placed in line with X-ray source, which provides both the reference w aves and specimen illum ination. The specimen is some spider filament, which adhere to a Si3N4 film. The recording medium is PM M A, which is placed at recording distance of about 400 μm from the specimen. The hologram s were reconstructed by digital method with 300 nm resolutions.
A novel method for recording in-line hologram is proposed which is called X-ray in-line holography with zone-plate magnification in this paper. The magnification factor of the micro zone plate imaging is about 103. The transverse resolution can be 48 nm in this method.
A ne method to record x-ray in-line holograms with quasi- equal-path is proposed in this paper, the key is using a micro zone plate. This method has several advantages over the traditional x-ray in-line holography. First, the requirement of the temporal coherence for the x-ray beam is very low. Second, it reduces the requirement of the resolution of recording media, and last it weakens the disturbance of the twin image. The method can also be used to measure the spatial coherence of x-ray with a single exposure.
It is a general problem in the electron holography that the long-range fields always affect the phase of the reference wave. The electromagnetic field of the specimen, which always spreads outside the specimen, not only is strictly limited within the region crossed by the object wave, but also extends as far as the region crossed by the reference wave, so the phase of the reference wave becomes unknown as well as that of the object wave. The perturbation of the reference wave has far-reaching consequences that a reliable reconstruction of the original object wave is presented since an optical replica of the perturbed reference wave is not available. In this paper, we proposed a method to eliminate the influence of the perturbed reference wave. Based on phases' relationship between the reference wave and the object wave, with digital reconstruction and data process, we can extract the phase of object wave from the phase difference between the object wave and the perturbed wave, which is always obtained in normal reconstruction. An example of one-dimension electromagnetic fields is given to verify this method.
In this paper, aberrations in reconstruction are analyzed, and aberration elimination method are discussed, and the feasibility of direct reconstruction of lensless Fourier transform x-ray hologram with visible light is investigated.
The requirements for the temporal coherence in x-ray holography are theoretically analyzed. The conclusion is that the temporal coherence length required is a quadratic function of the resolution in x-ray holography.
The reconstruction method of phase information "frozen" in an electron hologram is discussed. The experimental results show that the double-exposure method (which does not need laser reconstruction) cannot only obtain directly the phase information, but also gives qualitative and quantitative results. This means also that one can obtain simultaneous information about both the amplitude and phase in an experiment in an electron microscope
A united energy model of free-electron lasers with different magnetic structures is established to calculate the energy transfer rate ofthe free-electron lasers underthe conditions of weak and strong signals, respectively. This model is simple in calculation and clear in physical meaning. It should be used to deal with various problems such as harmonic radiation, parameter variations, and so on. The model is also suitable for three-dimensional problems.
In this paper, a united energy model of the free-electron laser with different magnetic structures has been established to calculate the energy transfer rate of the free-electron laser under the condition of weak signal. This model is simple in its calculations and clear in physical meaning. It should be used to deal with problems such as harmonic radiation, parameter variations, etc.. The model is also suitable for three-dimensional problems.