High-resolution soft x-ray microscopy

Long-range Fourier ptychographic imaging of the dynamic object with a single camera

Fourier ptychographic imaging technology is a new imaging method proposed in recent years. This technology captures multiple low-resolution images, and synthesizes them into a high-resolution image in the Fourier domain by a phase retrieval algorithm, breaking through the diffraction limit of the lens. In the field of macroscopic Fourier ptychographic imaging, most of the existing research generally focus on high-resolution imaging of static objects, and applying Fourier ptychographic imaging technology to dynamic objects is a hot research area now. At present, most of the researches are to use camera arrays combined with multiplexed lighting, deep learning or other algorithms, but the implementation of these methods is complicated or costly. Based on the diffraction theory of Fourier optics, this paper proposes that by expanding and focusing the illumination area, we can apply Fourier ptychographic imaging technology with a single camera to moving objects within a certain range. Theoretical analysis and experiments prove the feasibility of the proposed method. We successfully achieve high-resolution imaging of the dynamic object, increasing the resolution by about 2.5 times. This paper also researches the impact of speckles in the illuminated area on imaging results and proposes a processing method to reduce the impact of speckles.

Nanometer-Resolution Imaging of Living Cells Using Soft X-ray Contact Microscopy

Soft X-ray microscopy is a powerful technique for imaging cells with nanometer resolution in their native state without chemical fixation, staining, or sectioning. The studies performed in several laboratories have demonstrated the potential of applying this technique for imaging the internal structures of intact cells. However, it is currently used mainly on synchrotrons with restricted access. Moreover, the operation of these instruments and the associated sample-preparation protocols require interdisciplinary and highly specialized personnel, limiting their wide application in practice. This is why soft X-ray microscopy is not commonly used in biological laboratories as an imaging tool. Thus, a laboratory-based and user-friendly soft X-ray contact microscope would facilitate the work of biologists. A compact, desk-top laboratory setup for soft X-ray contact microscopy (SXCM) based on a laser-plasma soft X-ray source, which can be used in any biological laboratory, together with several applications for biological imaging, are described. Moreover, the perspectives of the correlation of SXCM with other super-resolution imaging techniques based on the current literature are discussed.

Optimization of a multi-TW few-cycle 1.7-µm source based on Type-I BBO dual-chirped optical parametric amplification

This paper presents the optimization of a dual-chirped optical parametric amplification (DC-OPA) scheme for producing an ultrafast intense infrared (IR) pulse. By employing a total energy of 0.77 J Ti:sapphire pump laser and type-I BBO crystals, an IR pulse energy at the center wavelength of 1.7 µm exceeded 0.1 J using the optimized DC-OPA. By adjusting the injected seed spectrum and prism pair compressor with a gross throughput of over 70%, the 1.7-µm pulse was compressed to 31 fs, which resulted in a peak power of up to 2.3 TW. Based on the demonstration of the BBO type-I DC-OPA, we propose a novel OPA scheme called the "dual pump DC-OPA" for producing a high-energy IR pulse with a two-cycle duration.

Optical coherence tomography (OCT) with 2 nm axial resolution using a compact laser plasma soft X-ray source

We present optical coherence tomography (OCT) with 2 nm axial resolution using broadband soft X-ray radiation (SXR) from a compact laser plasma light source. The laser plasma was formed by the interaction of nanosecond laser pulses with a gaseous target in a double stream gas puff target approach. The source was optimized for efficient SXR emission from the krypton/helium gas puff target in the 2 to 5 nm spectral range, encompassing the entire “water-window” spectral range from 2.3 nm to 4.4 nm wavelength. The coherence parameters of the SXR radiation allowed for the OCT measurements of a bulk multilayer structure with 10 nm period and 40% bottom layer thickness to period ratio, with an axial resolution of about 2 nm and detect multilayer interfaces up to a depth of about 100 nm. The experimental data are in agreement with OCT simulations performed on ideal multilayer structure. In the paper, detailed information about the source, its optimization, the optical system, OCT measurements and the results are presented and discussed.

High-energy infrared femtosecond pulses generated by dual-chirped optical parametric amplification

We demonstrate high-energy infrared femtosecond pulse generation by a dual-chirped optical parametric amplification (DC-OPA) scheme [Opt. Express19, 7190 (2011)]. By employing a 100 mJ pump laser, a signal pulse energy exceeding 20 mJ at a wavelength of 1.4 μm was achieved before dispersion compensation. A total output energy of 33 mJ was recorded. Under a further energy scaling condition, the signal pulse was compressed to an almost transform-limited duration of 27 fs using a fused silica prism compressor. Since the DC-OPA scheme is efficient and energy scalable, design parameters for obtaining 100 mJ level infrared pulses are presented, which are suitable as driver lasers for the energy scaling of high-order harmonic generation with sub-keV photon energy.

A numerical study of resolution and contrast in soft X-ray contact microscopy

We consider the case of soft X-ray contact microscopy using a laser-produced plasma. We model the effects of sample and resist absorption and diffraction as well as the process of isotropic development of the photoresist. Our results indicate that the micrograph resolution depends heavily on the exposure and the sample-to-resist distance. In addition, the contrast of small features depends crucially on the development procedure to the point where information on such features may be destroyed by excessive development. These issues must be kept in mind when interpreting contact microradiographs of high resolution, low contrast objects such as biological structures.

Biological calcium absorption edge imaging using monochromatic synchrotron radiation

Soft X-ray contact absorption edge images of unfixed, unstained biological specimens were made using monochromatic synchrotron radiation. X-ray contact replicas of unfixed, hydrated biological specimens at the nitrogen absorption edge and above and below the CaLIII absorption edge were compared to comparative conventional morphological and elemental high-resolution imaging methods (scanning and transmission electron microscopy, TEM-histochemistry and TEM-X-ray microanalysis). Soft X-ray absorption edge images made above the calcium absorption edge clearly revealed morphological detail and identified regions ladened with calcium as verified by TEM histochemistry of identical spores. Similarly, nitrogen absorption edge images identified residual nitrogenous material in the spore resuspension medium, and non-viable spores with nitrogen loss due to protoplast disaggregation.

Soft X-ray contact imaging of nucleolar chromatin using synchrotron radiation: a comparative scanning and transmission electron microscope study

Contact images (CI) of dehydrated, nucleolar chromatin from amphibian oocytes have been produced by soft X-rays from a synchrotron radiation source. These CI have been compared with the morphology of the original chromatin as seen in scanning and transmission electron microscopes. The quality and informational content of the CI depend very much on certain preparative procedures. The following factors have a marked effect on image quality and need to be carefully controlled: the total X-ray dose, the time and nature of development and the distance of the specimen from the photoresist. The preparation of the chromatin itself, providing that it is critically point dried, is less important. By following a regime of high X-ray dose, sufficient for penetration of the rather thick chromatin rings, and gentle development so that fine detail is not dissolved from the resist surface, it has been possible to obtain images which closely resemble the original chromatin, although the detailed resolution of the CI is not as clear. The smallest biological structures clearly resolved in the CI are ribonucleoprotein granules, which vary in size from 200 to 800 nm. However, by further refinement of preparative conditions it should be possible to improve on the informational content of these images.

Contact microscopy with synchrotron radiation

Soft X-ray contact microscopy with synchrotron radiation offers the biologist, and especially the microscopist, a way to morphologically study specimens that could not be imaged by conventional TEM, STEM, or SEM methods (i.e., hydrated samples, samples easily damaged by an electron beam, electron-dense samples, thick specimens, unstained, low-contrast specimens) at spatial resolutions approaching those of the TEM, with the additional possibility to obtain compositional (elemental) information about the sample as well. Although flash X-ray sources offer faster exposure times, synchrotron radiation provides a highly collimated, intense radiation that can be tuned to select specific discrete ranges of X-ray wavelengths or specific individual wavelengths that optimize imaging or microanalysis of a specific sample. This paper presents an overview of the applications of X-ray contact microscopy to biological research and some current research results using monochromatic synchrotron radiation to image biological samples.

Microholography of Living Organisms

By using intense pulsed coherent x-ray sources that are currently under development, it will be possible to obtain magnified three-dimensional images of elementary biological structures in the living state at precisely defined instants. For optimum contrast, sensitivity, and resolution, the hologram should be made with x-rays tuned to a resonance of nitrogen near 0.3 nanometer. Resolution will then be limited mainly by the hydrodynamic expansion that occurs while the necessary number of photons is being registered. Problems of technique are also briefly discussed.

Improved detail in biological soft X-ray microscopy: study of blood platelets

Improved image quality in soft x-ray contact microscopy can be obtained by examining the resist with transmission rather than scanning electron microscopy. Application of the new technique to air-dried preparations of human blood platelets reveals structures not visible in the same cells with transmission electron microscopy or when the resist is examined by scanning electron microscopy. As seen by the new technique, platelet pseudopods contain a central structure connected to a network in the platelet and dense bodies exhibit a lamellar structure.

Ultrastructure of hydrated proteoglycans using a pulsed plasma source

During the past 10 years, attempts have been made to examine hydrated biological specimens by using wet chambers (at ambient temperature) [1-3] or cold stages (at -30 degrees C and below) during electron microscopic examination. Obtaining sufficient contrast of unstained hydrated biological samples has proven a considerable problem using both of these methods. Many fragile biological specimens, when examined hydrated, frozen or dried, are severely damaged by the electron beam and cannot be imaged by conventional scanning or transmission electron microscopy. In order to increase specimen contrast and eliminate electron beam induced trauma to the specimen, we have developed a wet-cell [4], which when used in concert with a pulsed plasma soft X-ray source, provides high contrast contact replicas of totally hydrated, unstained biological specimens. Although it has been postulated that hydrated unstained samples can be imaged by soft X-ray contact microscopy [5-7], to date there has been little success due to cell movement or degradation of the wet sample during the long exposure period necessary for an adequate imaging dose [8]. With the pulsed plasma source described in this study we have been able to use exposure times of approximately 40-60 ns while maintaining the sample in its hydrated state at atmospheric pressure. The resultant contact replicas exhibit good contrast and better than 30 nm spatial resolution when examined by conventional scanning electron microscopy.

Imaging unstained proteoglycan aggregates by soft x-ray contact microscopy

Soft X-ray contact microscopy is a relatively new form of ultrastructural imaging, having better than 6 nm resolution and being uniquely well suited for the examination of fragile, unstained biological specimens. The biological specimen placed on a layer of photoresist and exposed to soft X-rays (1-10 nm lambda) of a specific wavelength or broad band. After X-ray exposure, the specimen is removed from the photoresist and the latter chemically developed. When the developed replica is examined by high resolution scanning electron microscopy, the fine structure of the original biological specimen is faithfully reproduced. Since the soft X-ray replica is initially formed due to the differential absorption of the incident X-rays by the biological specimen, the resultant contact replica also reveals information about the elemental composition of the sample. This paper presents our application of this new technique for the study of the proteoglycans, the complex polyanionic macromolecules comprising the gel phase in the matrix of mammalian cartilage.

Flash X-ray Microscopy

Soft x-ray contact microscopy, utilizing single-shot exposures of approximately 60 nanoseconds duration in polymethyl methacrylate, has been realized with a resolution of 300 angstroms. The radiation spectrum is intense in the "window" between 23 and 44 angstroms where water is transparent compared to biological materials, and therefore permits viewing of wet samples.

High resolution microchemical analysis using soft X-ray lithographic techniques

High resolution x-ray lithographic studies of cells from chick embryo hearts dried by the CO2 critical point method have been made with soft x-ray radiation of different wavelengths. A marked difference in the relief replica in polymethyl methacrylate (PMMA) resulting from the differential absorption by the dried cells of carbon K alpha radiation at 4.48 nm and broad band synchrotron radiation (SR) with lambda is greater than 1.5 nm demonstrates the potential usefulness of the technique in making high resolution (approximately or equal to 10 nm) chemical identification of the constitutents which make up the various parts of the cell.

Seeing with a New Light: Synchrotron Radiation

The special properties of synchrotron light are leading to a rapid increase in its utilization for both research and technology. At wavelength in the ultraviolet region of the broad spectrum in these beams a number of atomic, molecular, and solid-state spectroscopies are being pursued; soft x-rays are being used for spectroscopy, lithography, microscopy, and topography; at still shorter wavelengths, advantage is taken of scattering properties to probe the structure of matter. Characteristics of synchrotron radiation and of the sources which produce it are described, and some typical investigations and applications are presented to suggest the versatility of these sources.

Transmission microscropy of unmodified biological materials: comparative radiation dosages with electrons and ultrasoft x-ray photons

The minimum radiation dosage in a specimen consistent with transmission microscopy at resolution d and specimen thickness t is calculated for model specimens resembling biological materials in their natural state. The calculations cover 10(4)-10(7) eV electrons and 1.3-90 A photons in a number of microscopy modes. The results indicate that over a considerable part of the (t,d)-plane transmission microscopy on such specimens can be carried out at lower dosage with photons than with electrons. Estimates of the maximum resolutions obtainable with electrons and photons, consistent with structural survival of the specimen, are obtained, as are data on optimal operating conditions for microscopy with the two particles.

Potential operating region for ultrasoft x-ray microscopy of biological materials

Calculations are presented which indicate an extensive suboptical region in the microscopy of biological materials in their natural state which is accessible to ultrasoft x-ray transmission microscopy. Throughout most of the region, radiation dosage levels to the specimen are lower than in electron microscopy.