Low‐noise,vibrational phase‐sensitive chemical imaging by balanced detection RIKE

We perform a back‐to‐back comparison between two nonlinear vibrational imaging techniques: stimulated Raman scattering (SRS) and balanced detection Raman‐induced Kerr effect (BD‐RIKE). Using a compact fiber‐based laser system for generation of pump and Stokes signals, we image polymer beads as well as human hepatocytes under the same experimental conditions. We show that BD‐RIKE, despite the slightly lower signal levels, consistently offers an improved signal‐to‐noise ratio with respect to SRS, resulting in significantly higher image quality. Importantly, we observe that such quality is not affected by the static birefringence of the sample, which makes BD‐RIKE a robust and attractive alternative to SRS. We also highlight a unique advantage of the technique, which is its capability to easily access both the real and imaginary parts of the nonlinear susceptibility, thus allowing for vibrational phase imaging. The phase information can be readily obtained from BD‐RIKE with minimal experimental effort and provides an additional chemical selectivity channel for coherent Raman microscopy. Copyright © 2014 John Wiley & Sons, Ltd.     

Design,development and first experiments on the X‐ray imaging beamline at Indus‐2 synchrotron source RRCAT,India

A full‐field hard X‐ray imaging beamline (BL‐4) was designed, developed, installed and commissioned recently at the Indus‐2 synchrotron radiation source at RRCAT, Indore, India. The bending‐magnet beamline is operated in monochromatic and white beam mode. A variety of imaging techniques are implemented such as high‐resolution radiography, propagation‐ and analyzer‐based phase contrast imaging, real‐time imaging, absorption and phase contrast tomography etc. First experiments on propagation‐based phase contrast imaging and micro‐tomography are reported.     

Visualization of mouse spinal cord intramedullary arteries using phase‐ and attenuation‐contrast tomographic imaging

Many spinal cord circulatory disorders present the substantial involvement of small vessel lesions. The central sulcus arteries supply nutrition to a large part of the spinal cord, and, if not detected early, lesions in the spinal cord will cause irreversible damage to the function of this organ. Thus, early detection of these small vessel lesions could potentially facilitate the effective diagnosis and treatment of these diseases. However, the detection of such small vessels is beyond the capability of current imaging techniques. In this study, an imaging method is proposed and the potential of phase‐contrast imaging (PCI)‐ and attenuation‐contrast imaging (ACI)‐based synchrotron radiation for high‐resolution tomography of intramedullary arteries in mouse spinal cord is validated. The three‐dimensional vessel morphology, particularly that of the central sulcus arteries (CSA), detected with these two imaging models was quantitatively analyzed and compared. It was determined that both PCI‐ and ACI‐based synchrotron radiation can be used to visualize the physiological arrangement of the entire intramedullary artery network in the mouse spinal cord in both two dimensions and three dimensions at a high‐resolution scale. Additionally, the two‐dimensional and three‐dimensional vessel morphometric parameter measurements obtained with PCI are similar to the ACI data. Furthermore, PCI allows efficient and direct discrimination of the same branch level of the CSA without contrast agent injection and is expected to provide reliable biological information regarding the intramedullary artery. Compared with ACI, PCI might be a novel imaging method that offers a powerful imaging platform for evaluating pathological changes in small vessels and may also allow better clarification of their role in neurovascular disorders.     

Raman spectroscopy and microscopy of individual cells and cellular components

Raman spectroscopy provides the unique opportunity to nondestructively analyze chemical concentrations in individual cells on the submicrometer length scale without the need for optical labels. This enables the rapid assessment of cellular biochemistry inside living cells, and it allows for their continued analysis. Here, we review recent developments in the analysis of single cells, subcellular compartments, and chemical imaging based on Raman spectroscopy. Spontaneous Raman spectroscopy provides for the full spectral assessment of cellular biochemistry, while coherent Raman techniques, such as coherent anti‐Stokes Raman scattering is primarily used as an imaging tool comparable to confocal fluorescence microscopy. These techniques are complemented by surface‐enhanced Raman spectroscopy, which provides higher sensitivity and local specificity, and also extends the techniques to chemical indicators, i.e. pH sensing. We review the strengths and weaknesses of each technique, demonstrate some of their applications and discuss their potential for future research in cell biology and biomedicine.     

Hard X‐ray phase‐contrast tomography of non‐homogeneous specimens: grating interferometry versus propagation‐based imaging

X‐ray phase‐contrast imaging is an effective approach to drastically increase the contrast and sensitivity of microtomographic techniques. Numerous approaches to depict the real part of the complex‐valued refractive index of a specimen are nowadays available. A comparative study using experimental data from grating‐based interferometry and propagation‐based phase contrast combined with single‐distance phase retrieval applied to a non‐homogeneous sample is presented (acquired at beamline ID19‐ESRF). It is shown that grating‐based interferometry can handle density gradients in a superior manner. The study underlines the complementarity of the two techniques for practical applications.     

Terahertz sensing and imaging based on nanostructured semiconductors and carbon materials

The advantageous properties of terahertz (THz) waves, such as permeability through objects that are opaque for visible light and the energy spectrum in the microelectron‐volt range that are important in materials research, allow their potential use in various applications of sensing and imaging. However, since the THz region is located between the electronic and photonic bands, even the basic components such as detectors and sources have not been fully developed, unlike in other frequency regions. THz technology also has the problem of low imaging resolution, which results from a considerably longer wavelength than that of the visible light. However, the utilization of nanostructured electronic devices has recently opened up new horizons for THz sensing and imaging. This paper provides an overview of the THz detector and imaging techniques and tracks their recent progress. Specifically, two cutting‐edge techniques, namely, frequency‐selective THz‐photon detection and integrated near‐field THz imaging, are discussed in detail. Finally, the studies of superconductors and semiconductors with high‐resolution THz imaging are described.     

Fiber‐based light sources for biomedical applications of coherent anti‐Stokes Raman scattering microscopy

During the past decade coherent anti‐Stokes Raman scattering (CARS) microscopy has evolved to one of the most powerful imaging techniques in the biomedical sciences, enabling the label‐free visualization of the chemical composition of tissue in vivo in real time. While the acquisition of high‐contrast images of single cells up to large tissue sections enables a wide range of medical applications from routine diagnostics to surgical guidance, to date CARS imaging is employed in fundamental research only, essentially because the synchronized multiple wavelength pulsed laser sources required for CARS microscopy are large, expensive and require regular maintenance. Laser sources based on optical fibers can overcome these limitations combining highest efficiency and peak powers with an excellent spatial beam profile and thermal stability. In this review we summarize the different fiber‐based approaches for laser sources dedicated to coherent Raman imaging, in particular active fiber technology and passive fiber‐based frequency conversion processes, i.e. supercontinuum generation, soliton self‐frequency shift and four‐wave mixing. We re‐evaluate the ideal laser parameters for CARS imaging and discuss the suitability of different laser concepts for turn‐key operation required for routine application in clinics.

     

Semiconductor disk lasers for the generation of visible and ultraviolet radiation

Recent developments in semiconductor disk lasers (SDLs) generating visible or ultraviolet light are reviewed. After an introduction on potential applications, we discuss how the combination of vertical‐emitting semiconductor GaAs‐based structures and intra‐cavity nonlinear conversion techniques can be successfully exploited to uniquely meet demands for continuous‐wave radiation in the visible and ultraviolet spectral range. To do so, an overview of the device operating principles and performance is presented highlighting the underlying material considerations, semiconductor structural designs, thermal management techniques and suitable cavity configurations. This summary is completed by a presentation of new developments in the field, with a particular focus on the trends towards miniaturization.     

Photoacoustic microscopy

Photoacoustic microscopy (PAM) is a hybrid in vivo imaging technique that acoustically detects optical contrast via the photoacoustic effect. Unlike pure optical microscopic techniques, PAM takes advantage of the weak acoustic scattering in tissue and thus breaks through the optical diffusion limit (∼1 mm in soft tissue). With its excellent scalability, PAM can provide high‐resolution images at desired maximum imaging depths up to a few millimeters. Compared with backscattering‐based confocal microscopy and optical coherence tomography, PAM provides absorption contrast instead of scattering contrast. Furthermore, PAM can image more molecules, endogenous or exogenous, at their absorbing wavelengths than fluorescence‐based methods, such as wide‐field, confocal, and multi‐photon microscopy. Most importantly, PAM can simultaneously image anatomical, functional, molecular, flow dynamic and metabolic contrasts in vivo. Focusing on state‐of‐the‐art developments in PAM, this Review discusses the key features of PAM implementations and their applications in biomedical studies.     

Recent progress in tissue optical clearing

Tissue optical clearing technique provides a prospective solution for the application of advanced optical methods in life sciences. This paper gives a review of recent developments in tissue optical clearing techniques. The physical, molecular and physiological mechanisms of tissue optical clearing are overviewed and discussed. Various methods for enhancing penetration of optical‐clearing agents into tissue, such as physical methods, chemical‐penetration enhancers and combination of physical and chemical methods are introduced. Combining the tissue optical clearing technique with advanced microscopy image or labeling technique, applications for 3D microstructure of whole tissues such as brain and central nervous system with unprecedented resolution are demonstrated. Moreover, the difference in diffusion and/or clearing ability of selected agents in healthy versus pathological tissues can provide a highly sensitive indicator of the tissue health/pathology condition. Finally, recent advances in optical clearing of soft or hard tissue for in vivo imaging and phototherapy are introduced.     

Synchrotron X‐ray phase contrast imaging of leaf venation in soybean (Glycine max) after exclusion of solar UV (280–400 nm) radiation

The hydraulic efficiency of a leaf depends on its vascular structure as this is responsible for transport activities. To investigate the effect of exclusion of UVAB and UVB radiation from the solar spectrum on the micro‐structure of leaves of soybean (Glycine max, variety JS‐335), a field experiment was conducted using synchrotron‐based phase contrast imaging (PCI). Plants were grown in specially designed UV exclusion chambers, and wrapped with filters that excluded UVB (280–315 nm) or UVAB (280–400 nm), or transmitted all the ambient solar UV (280–400 nm) radiation (filter control). Qualitative observation of high‐resolution X‐ray PCI images obtained at 10 keV has shown the differences in major and minor vein structures of the leaves. The mid‐rib width of the middle leaflet of third trifoliate leaves, for all treatments, were obtained using quantitative image analysis. The width of the mid‐rib of the middle leaflet of third trifoliate leaves of UVB excluded plants was found to be more compared to leaves of filter control plants, which are exposed to ambient UV. The mid‐rib or the main conducting vein transports water and sugars to the whole plant; therefore, mid‐rib enhancement by the exclusion of solar UV radiation possibly implies enhancement in the leaf area which in turn causes an increased rate of photosynthesis.     

Feasibility study of propagation‐based phase‐contrast X‐ray lung imaging on the Imaging and Medical beamline at the Australian Synchrotron

Propagation‐based phase‐contrast X‐ray imaging (PB‐PCXI) using synchrotron radiation has achieved high‐resolution imaging of the lungs of small animals both in real time and in vivo. Current studies are applying such imaging techniques to lung disease models to aid in diagnosis and treatment development. At the Australian Synchrotron, the Imaging and Medical beamline (IMBL) is well equipped for PB‐PCXI, combining high flux and coherence with a beam size sufficient to image large animals, such as sheep, due to a wiggler source and source‐to‐sample distances of over 137 m. This study aimed to measure the capabilities of PB‐PCXI on IMBL for imaging small animal lungs to study lung disease. The feasibility of combining this technique with computed tomography for three‐dimensional imaging and X‐ray velocimetry for studies of airflow and non‐invasive lung function testing was also investigated. Detailed analysis of the role of the effective source size and sample‐to‐detector distance on lung image contrast was undertaken as well as phase retrieval for sample volume analysis. Results showed that PB‐PCXI of lung phantoms and mouse lungs produced high‐contrast images, with successful computed tomography and velocimetry also being carried out, suggesting that live animal lung imaging will also be feasible at the IMBL.     

Optimization of analyzer‐based imaging systems for minimal surface absorbed dose

Analyzer‐based imaging has improved tissue X‐ray imaging beyond what conventional radiography was able to achieve. The extent of the improvement is dependent on the crystal reflection used in the monochromator and analyzer combination, the imaging photon energy, the geometry of the sample and the imaging detector. These many factors determine the ability of the system to distinguish between various bone tissues or soft tissues with a specified statistical certainty between pixels in a counting detector before any image processing. The following discussion will detail changes in the required number of imaging photons and the resulting surface absorbed dose when the imaging variables are altered. The process whereby the optimal imaging parameters to deliver the minimum surface absorbed dose to a sample while obtaining a desired statistical certainty between sample materials for an arbitrary analyzer‐based imaging system will be described. Two‐component samples consisting of bone and soft tissue are discussed as an imaging test case. The two‐component approach will then be generalized for a multiple‐component sample.     

Optical orthogonal frequency division multiplexed transmission using all‐optical discrete Fourier transform

Orthogonal frequency division multiplexing (OFDM) can provide spectrally efficient communication channels because it can utilize carrier orthogonality and various impairment mitigation methods. An optical OFDM signal can be generated electronically to multiplex lower‐rate carriers. In recent advancements, OFDM signals are also shown to be generated and demultiplexed by all‐optical discrete Fourier transform (DFT), overcoming the speed limit of electronics for >Tbps capacity. High‐performance DFT devices, such as arrayed waveguide grating (AWG) or planar lightwave circuit (PLC), are critically required to obtain strong orthogonality for scalable all‐optical OFDM (AO‐OFDM) system implementations. Advanced techniques such as coherent modulation and detection with digital impairment mitigation are also important for long‐reach AO‐OFDM transmissions. More recently, optical superchannel schemes have been introduced utilizing coherent detection for multi‐Tbps AO‐OFDM transmissions. This paper reviews the device and system aspects for the AO‐OFDM technology, including a generalized theoretical model to provide an indepth understanding.     

Phase-contrast X-ray imaging of the gas diffusion layer of fuel cells

The understanding of and in situ observation of the transport and distribution of water in carbon‐paper gas diffusion layers (GDLs) using non‐destructive imaging techniques is critical for achieving high performance in polymer electrolyte fuel cells (PEFCs). To investigate the behavior of water in GDLs of PEFCs, phase‐contrast X‐ray imaging via X‐ray interferometric imaging (XII) and diffraction‐enhanced imaging (DEI) were performed using 35 keV X‐rays. The XII technique is useful for the radiographic imaging of GDLs and in situ observations of water evolution processes in operating PEFCs. DEI provides a way for tomographic imaging of GDLs in PEFCs. Because high‐energy X‐rays are applicable to the imaging of both carbon papers and heavy materials, which make up PEFCs, phase‐contrast X‐ray imaging techniques have proven to be valuable for investigation of GDLs.     

Three‐dimensional imaging of chemical phase transformations at the nanoscale with full‐field transmission X‐ray microscopy

The ability to probe morphology and phase distribution in complex systems at multiple length scales unravels the interplay of nano‐ and micrometer‐scale factors at the origin of macroscopic behavior. While different electron‐ and X‐ray‐based imaging techniques can be combined with spectroscopy at high resolutions, owing to experimental time limitations the resulting fields of view are too small to be representative of a composite sample. Here a new X‐ray imaging set‐up is proposed, combining full‐field transmission X‐ray microscopy (TXM) with X‐ray absorption near‐edge structure (XANES) spectroscopy to follow two‐dimensional and three‐dimensional morphological and chemical changes in large volumes at high resolution (tens of nanometers). TXM XANES imaging offers chemical speciation at the nanoscale in thick samples (>20 µm) with minimal preparation requirements. Further, its high throughput allows the analysis of large areas (up to millimeters) in minutes to a few hours. Proof of concept is provided using battery electrodes, although its versatility will lead to impact in a number of diverse research fields.     

Terahertz spectroscopy and imaging – Modern techniques and applications

Over the past three decades a new spectroscopic technique with unique possibilities has emerged. Based on coherent and time‐resolved detection of the electric field of ultrashort radiation bursts in the far‐infrared, this technique has become known as terahertz time‐domain spectroscopy (THz‐TDS). In this review article the authors describe the technique in its various implementations for static and time‐resolved spectroscopy, and illustrate the performance of the technique with recent examples from solid‐state physics and physical chemistry as well as aqueous chemistry. Examples from other fields of research, where THz spectroscopic techniques have proven to be useful research tools, and the potential for industrial applications of THz spectroscopic and imaging techniques are discussed.     

Laser crystals and ceramics: recent advances

After a brief examination of known insulating laser crystals and the stimulated emission channels of their generating activator ions, this article reviews recent advances in the development of novel lasing crystals and ceramics, as well as inorganic and organic nonlinear‐laser crystals for χ⁽³⁾ and cascaded χ⁽³⁾ ↔ χ⁽²⁾ frequency converters. Several new modern attractive technologies in the physics and techniques of crystalline lasers are also discussed.     

High‐brightness long‐wavelength semiconductor disk lasers

A review on the recent developments in the field of long‐wavelength (λ >1.2μm) high‐brightness optically‐pumped semiconductor disk lasers (OPSDLs) is presented. As thermal effects have such a crucial impact on the laser performance particular emphasis is given to modelling the thermal behaviour and optimisation of the heat‐sinking. Selected OPSDL devices, realized in different III‐V and IV‐VI semiconductor material systems, with corresponding emission wavelengths between 1.2 μm and 5.3 μm are presented. Specific applications in this broad spectral range are addressed and methods to obtain high output power are discussed in terms of the underlying material properties and device operating principles.     

In situ Raman imaging of osteoblastic mineralization

Hydroxyapatite (HA) is synthesized at early stages of bone formation by osteoblasts. Nondestructive observation of early stages of osteoblastic mineralization provides crucial information for biological mechanism of bone formation. Raman microscopy serves as an ideal tool to observe the osteoblastic mineralization process because it shows the chemical information of the sample at a minimally invasive level. In addition, HA is a marker for osteoblastic mineralization, and HA Raman signal is strong enough to identify mineralized spots in osteoblasts. In this research, we visualized the distribution of HA in cultured mouse osteoblasts by Raman imaging and observed the location of the mineralized spots in the culture. We monitored HA Raman signal from osteoblast culture for 3 days after administrating the osteogenic differentiation medium and observed Raman signal associated with HA. We identified mineralized spots of KUSA‐A1 by Raman imaging constructed from the distribution of HA Raman signal. We successfully visualized the distribution of the mineralized spots in the culture of KUSA‐A1. We compared our Raman images with Alizarin red S staining assay, which was a conventional method to evaluate the mineralization process. Raman imaging of the KUSA‐A1 culture visualized the mineralized spots more accurately than Alizarin red S staining assay. Raman imaging of HA serves as a powerful tool to identify the mineralized spots in an in vitro culture of osteogenic lineage cells. Copyright © 2014 John Wiley & Sons, Ltd.