Depth-resolved Information Research Articles

A-scan fluorescence microscopy for rapid cross-sectional imaging

This paper presents a microscopy technique that can perform snapshot depth resolved optical imaging in the same manner as A-scan in ultrasound imaging and optical coherence tomography. In this technique, a laser line along the axial dimension is used to illuminate a sample to create a fluorescent line object. By transforming the line object along the axial dimension (Z) to a ring image on the lateral dimensions (X-Y) using a full cone mirror, common optics can be used to relay and acquire the ring image precisely. Then, by converting half of the ring image back to a line image using a half cone mirror, the opening side of the half cone mirror allows the line image, which contains the full depth resolved information of the line object, to be taken in one snapshot. This eliminates the requirement of axial scanning in traditional depth resolved imaging techniques such as confocal microscopy to obtain the same information. The technique is demonstrated by imaging fluorescent microspheres of different diameters. This technique offers a simple alternative to traditional depth resolved imaging techniques such as confocal microscopy and light sheet microscopy. It is particularly useful in imaging samples with multiple layers in which multiple A-scans or a few B-scans are sufficient to represent the entire sample.

Utilizing quantitative phase microscopy to localize fluorescence in three dimensions via the transport of intensity equation.

We demonstrate the use of a novel, to the best of our knowledge, localization algorithm for digitally refocusing fluorescence images from a three-dimensional cell culture. Simultaneous phase and fluorescence intensity images are collected through a multimodal system that combines digital holography via quantitative phase microscopy (QPM) and fluorescence microscopy. Defocused fluorescence images are localized to a specific z-plane within the three-dimensional (3D) matrix using the transport of intensity equation (TIE) and depth-resolved information derived from the QPM measurements. This technique is applied to cells stained with different fluorescent tags suspended in 3D collagen hydrogel cultures. Experimental findings demonstrate the localization of defocused images, facilitating the analysis and comparison of cells within the hydrogel matrix. This method holds promise for comprehensive cellular imaging of fluorescence labeling in three-dimensional environments, enabling detailed investigations into cellular behavior and interactions.

Harnessing ion beam erosion engineering for controlled self-assembly and tunable magnetic anisotropy in epitaxial films

The engineering of the surface morphology and the structure of the thin film is one of the essential technological assets for regulating the physical properties and functionalities of thin film-based devices. This study presents an easy and handy approach to tailor the surface structure of epitaxial thin films utilizing low-energy ion beam. Here, we investigate the evolution of the surface structure and magnetic anisotropy (MA) in epitaxial Fe/MgO (001) model systems subjected to multiple cycles of ion beam erosion (IBE) after thin film growth. The growth of Fe film occurs in the form of three–dimensional islands and exhibits intrinsic biaxial MA. Following a few cycles of IBE, an induced uniaxial magnetic anisotropy leads to a split in the hysteresis loop, and the film displays almost uniaxial magnetic switching behavior. More distinctly, we present a clear and conclusive evidence of (2 × 2) reconstruction of the Fe surface due to the atomic rearrangement by IBE. Furthermore, 57Fe isotope sensitive nuclear resonance scattering measurement provides insight into the depth-resolved magnetic information due to the modified surface topography. We also demonstrate that thermal annealing can reversibly tune the surface reconstruction and induced UMA. The feasibility of the IBE technique by adequately selecting IBE parameters for surface structure modification has been highlighted apart from conventional tailoring of the morphology for the tuning of UMA and introduces a new dimension to our understanding of self-assembled surface morphology evolution by IBE.

Depth-resolved elemental and molecular analysis of polymeric multilayers with EI-MS and ICP-OES using chemometric evaluation

Polymeric composite materials with layered structures have increasingly gained importance as, for specific applications, the required properties cannot be achieved with one single material; thus, for example, multi-layer polymers or special surface coatings are employed.None of the conventionally available techniques enables a comprehensive characterization of composite materials, including depth-resolved information about their organic and elemental constituents.In this work, a previously introduced approach enabling the direct characterization of the organic and inorganic constituents of solid polymer samples was further developed and optimized for depth profiling analysis of stacked layer polymers. For this purpose, the ablation chamber’s washout behavior and transfer line’s transport efficiency were improved. Chemometric methods were applied for the data analysis to gain greater insight and enable more precise differentiation, comparison and classification.The work investigates the feasibility of the developed approach by analyzing a multi-layer stacked polymer sample created from nail polishes and using cluster analysis on the data collected with the bimodal detection. The layers of the sample could be differentiated by their organic and elemental constituents’ fingerprints, achieving a depth resolution of 7 µm.

Artificial neural network for enhancing signal-to-noise ratio and contrast in photothermal optical coherence tomography

Optical coherence tomography (OCT) is a medical imaging method that generates micron-resolution 3D volumetric images of tissues in-vivo. Photothermal (PT)-OCT is a functional extension of OCT with the potential to provide depth-resolved molecular information complementary to the OCT structural images. PT-OCT typically requires long acquisition times to measure small fluctuations in the OCT phase signal. Here, we use machine learning with a neural network to infer the amplitude of the photothermal phase modulation from a short signal trace, trained in a supervised fashion with the ground truth signal obtained by conventional reconstruction of the PT-OCT signal from a longer acquisition trace. Results from phantom and tissue studies show that the developed network improves signal to noise ratio (SNR) and contrast, enabling PT-OCT imaging with short acquisition times and without any hardware modification to the PT-OCT system. The developed network removes one of the key barriers in translation of PT-OCT (i.e., long acquisition time) to the clinic.

Application of Time-of-flight Secondary Ion Mass Spectrometry in Lithium-ion Batteries

Abstract: Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is becoming a powerful tool in the Lithium-Ion Batteries (LIBs) field due to its excellent resolution and sensitivity, as well as its ability to provide spectrally and depth-resolved information. The perspective comprehensively delves into the application of ToF-SIMS in two major areas of LIBs research. Firstly, the article elucidates how ToF-SIMS has been instrumental in deciphering the Solid Electrolyte Interphase (SEI) composition and analyzing electrolyte aging. The insights gleaned from such studies have paved the way for enhancing the longevity and safety of LIBs. Secondly, we explore the role of ToF-SIMS in scrutinizing the distribution of interface reactions, which are critical for understanding charge and discharge mechanisms. The analysis aids in optimizing the interface properties, thereby improving battery performance. Such detections are paramount in ensuring the safety and operational stability of batteries. Overall, the integration of ToF-SIMS in LIBs research offers a promising avenue for the development of advanced and safer energy storage systems.

Improving the characterization of red coloring matter from prehistoric cave art by means of laboratory confocal XRF depth profiling combined with synchrotron XRF imaging

Chemical in situ study of red coloring matter in cave art is challenging because characteristic trace elements can be present both in the matter and in the wall support, and the latter has a quite heterogeneous composition.In this study, a stalactite presenting red iron oxide coloring matter from La Garma cave, Northern Spain, has been analyzed with complementary techniques. The aims are, on the one hand, confirming for the first time the feasibility of confocal XRF (CXRF) depth-resolved scans to successfully separate the complex stratigraphy of red prehistoric coloring matter on a calcitic support. On the other hand, finding differentiation criteria and improving the characterization of this red iron-based coloring matter would help understand and virtually separate it from its support, as well as improve future in situ analyses in cave sites.CXRF depth-resolved scans were performed using the LouX3D device available in our laboratory. With this technique, we can precisely determine the chemical depth composition of an object, layer by layer, with a spatial resolution in the micrometer scale. These measurements were combined with a synchrotron induced micro-X-ray fluorescence (SR-µXRF) chemical mapping performed at the PUMA beamline, SOLEIL synchrotron. These highly sensitive measurements not available with any other non-destructive lab-based technique allow seeing a greater number of trace and minor elements associated with Fe. Complementing them with the direct depth-resolved information given by CXRF, it was possible to find differentiation criteria distinguishing the wall support and the coloring matter, finely characterize this red iron-based matter, and confirm the feasibility as well as the advantages of applying CXRF for the non-invasive technical study of prehistoric cave figures.

Resolving buried interfaces with low energy ion scattering

We investigate the use of low energy ion scattering (LEIS) to characterize buried interfaces of ultrathin films. LEIS spectra contain depth-resolved information in the so-called subsurface signal. However, the exact correlation between the subsurface signal and the sample’s depth composition is still unknown. For this reason, LEIS spectra so far only provided qualitative information about buried interfaces. In this study, we investigate nm-thin films of Si-on-W and Si-on-Mo, where we compare simulated data to LEIS spectra. We present a method to extract depth-sensitive compositional changes—resolving buried interfaces—from LEIS spectra for the first few nanometers of a thin-film sample. In the case of Si-on-Mo, the simulation of the LEIS subsurface signal allows obtaining a quantitative measurement of the interface profile that matches the value determined using the LEIS layer growth profile method with an accuracy of 0.1 nm. These results pave the way to further extend the use of LEIS for the characterization of features buried inside the first few nanometers of a sample.

Chemical Influence of Carbon Interface Layers in Metal/Oxide Resistive Switches.

Thin layers introduced between a metal electrode and a solid electrolyte can significantly alter the transport of mass and charge at the interfaces and influence the rate of electrode reactions. C films embedded in functional materials can change the chemical properties of the host, thereby altering the functionality of the whole device. Using X-ray spectroscopies, here we demonstrate that the chemical and electronic structures in a representative redox-based resistive switching (RS) system, Ta2O5/Ta, can be tuned by inserting a graphene or ultrathin amorphous C layer. The results of the orbitalwise analyses of synchrotron Ta L3-edge, C K-edge, and O K-edge X-ray absorption spectroscopy showed that the C layers between Ta2O5 and Ta are significantly oxidized to form COx and, at the same time, oxidize the Ta layers with different degrees of oxidation depending on the distance: full oxidation at the nearest 5 nm Ta and partial oxidation in the next 15 nm Ta. The depth-resolved information on the electronic structure for each layer further revealed a significant modification of the band alignments due to C insertion. Full oxidation of the Ta metal near the C interlayer suggests that the oxygen-vacancy-related valence change memory mechanism for the RS can be suppressed, thereby changing the RS functionalities fundamentally. The knowledge on the origin of C-enhanced surfaces can be applied to other metal/oxide interfaces and used for the advanced design of memristive devices.

Nanometer-scale depth-resolved hard x-ray absorption spectroscopy based on the detection of energy-loss Auger electrons with low energies

X-ray absorption spectroscopy (XAS) provides information on the chemical state and atomic structure of a target element even without long-range periodicity. A depth-resolved surface-sensitive XAS method for K-edge using hard x rays is proposed herein. The principle of this method is based on the selective detection of low-energy electrons using an electron analyzer. The detected electrons originate from the LMM Auger electrons that cascade from the KLL Auger process caused by K-shell absorption and lose the energy corresponding to their travel distance in a solid. The analysis depth was confirmed to be in the nanometer range using a GaN substrate with a thin oxide film of the defined thickness. In addition, depth-resolved information regarding the local atomic structure, including the interatomic distance and crystallinity, was obtained via the Fourier transformation of the spectral oscillations. Because the proposed technique does not require a change in the detection angle, it is also expected to be used for particles and samples with uneven surfaces.

Soil-plant interactions modulated water availability of Swiss forests during the 2015 and 2018 droughts.

Central Europe has been experiencing unprecedented droughts during the last decades, stressing the decrease in tree water availability. However, the assessment of physiological drought stress is challenging, and feedback between soil and vegetation is often omitted because of scarce belowground data. Here we aimed to model Swiss forests' water availability during the 2015 and 2018 droughts by implementing the mechanistic soil‐vegetation‐atmosphere‐transport (SVAT) model LWF‐Brook90 taking advantage of regionalized depth‐resolved soil information. We calibrated the model against soil matric potential data measured from 2014 to 2018 at 44 sites along a Swiss climatic and edaphic drought gradient. Swiss forest soils' storage capacity of plant‐available water ranged from 53 mm to 341 mm, with a median of 137 ± 42 mm down to the mean potential rooting depth of 1.2 m. Topsoil was the primary water source. However, trees switched to deeper soil water sources during drought. This effect was less pronounced for coniferous trees with a shallower rooting system than for deciduous trees, which resulted in a higher reduction of actual transpiration (transpiration deficit) in coniferous trees. Across Switzerland, forest trees reduced the transpiration by 23% (compared to potential transpiration) in 2015 and 2018, maintaining annual actual transpiration comparable to other years. Together with lower evaporative fluxes, the Swiss forests did not amplify the blue water deficit. The 2018 drought, characterized by a higher and more persistent transpiration deficit than in 2015, triggered widespread early wilting across Swiss forests that was better predicted by the SVAT‐derived mean soil matric potential in the rooting zone than by climatic predictors. Such feedback‐driven quantification of ecosystem water fluxes in the soil–plant‐atmosphere continuum will be crucial to predicting physiological drought stress under future climate extremes.

A Shipborne Photon-Counting Lidar for Depth-Resolved Ocean Observation

Depth-resolved information is essential for ocean research. For this study, we developed a shipborne photon-counting lidar for depth-resolved oceanic plankton observation. A pulsed fiber laser with frequency doubling to 532 nm acts as a light source, generating a single pulse at the micro-joule level with a pulse width of less than 1 ns. The receiver is capable of simultaneously detecting the elastic signal at two orthogonal polarization states, the Raman scattering from seawater, and the fluorescence signal from chlorophyll A. The data acquisition system utilizes the photon-counting technique to record each photon event, after which the backscattering signal intensity can be recovered by counting photons from multiple pulses. Benefitting from the immunity of this statistical detection method to the ringing effect of the detector and amplifier circuit, high-sensitivity and high-linearity backscatter signal measurements are realized. In this paper, we analyze and correct the after-pulse phenomenon of high-linearity signals through experiments and theoretical simulations. Through the after-pulse correction, the lidar attenuation coefficient retrieved from the corrected signal are in good agreement with the diffuse attenuation coefficients calculated from the in situ instrument, indicating the potential of this shipborne photon-counting lidar for ocean observation applications.

Photothermal Radar Shearography: A Novel Transient-Based Speckle Pattern Interferometry for Depth-Tomographic Inspection

As an increasingly recognized optical interferometric technique for nondestructive testing and evaluation, shearography has attracted extensive interest in various industrial applications. However, it suffers from the ignorance of the whole process of dynamic surface deformation and the difficulty in determining the depth-resolved information of inhomogeneities. In this regard, a novel speckle pattern interferometric modality named photothermal radar shearography is proposed. Unlike the differential mode of conventional shearography, the present article focuses on the dynamic surface displacement field channel technique with more comprehensive understanding of the subsurface structural information. With the utilization of frequency modulated photothermal excitation, the depth-distributed information of subsurface structures and inhomogeneities is encoded into the induced dynamic surface deformation. Using Hilbert transform and least-square method solved by discrete cosine transform, the time-domain interference signal of each pixel from the recorded speckle patterns sequence is demodulated and unwrapped to obtain the transient full-field shearographic phase distribution which indicates the dynamic surface deformation. In the meanwhile, incrementally delayed cross-correlation matched filtering allows for the localization of axial energy distribution to generate depth-selective structural display for tomography. This proposed transient-based interferometric methodology thus enables depth-tomographic profiles and three-dimensional visualization of subsurface anomalies for the first time, which significantly improves the superiority and attractiveness of shearography in providing insight into the status, performance and reliability of industry.

Depth-resolved magnetization profile of MgO/CoFeB/W perpendicular half magnetic tunnel junctions

In this work, we used the soft X-ray resonant magnetic reflectivity to study the depth-resolved out-of-plane (oop) magnetization profile of a CoFeB/MgO sample with W/Ta cap layer after annealing at 400°C. It is a powerful technique to probe buried magnetic interfaces of ultra-thin films by combining the depth-resolved information of X-ray reflectivity with the species selectivity of X-ray magnetic circular dichroism. It allowed us to resolve the oop magnetization within a 1.36 nm thick CoFeB layer by the measurement of angle-dependent specular reflectivity at large scattering angles (up to 80°). We determined a graded magnetic distribution for both Fe and Co with a 20% increase at the interface with MgO, decreasing slightly over a thickness of 0.7 nm from MgO before it rapidly decreases to 50% at the interface with W. After applying a non-saturating magnetic field in the plane of the sample, we also quantified a similar magnetization profile with an inclined moment configuration. This indicates that the magnetization gradient is a robust property of the CoFeB layer in the studied sample.

A combination of ion beam sputtering and in situ x-ray diffraction as a method for depth-resolved phase analysis using nitrogen-implanted austenitic stainless steel as an example

In situ x-ray diffraction (XRD) during ion implantation or thin film deposition is a powerful method to follow the time evolution of diffusion and phase transition processes in thin films, even as the depth resolution is still dominated by the information depth of the x rays. However, in the case of sputter etching with energetic ions at moderate temperatures, where no diffusion or phase transformation processes are active, this limitation is no longer of concern. Here, thin surface layers which are removed by sputtering can be identified with a depth resolution of 25 nm or better—while information from the substrate—despite overlayers of several micrometers—is accessible. However, considerable mathematical operations are necessary to convert the time series of diffractograms measured by XRD into a depth series. In this paper, a method is highlighted describing which depth-resolved properties of thin films can be accessed using such in situ measurements during ion beam sputtering in the model system austenitic stainless steel + nitrogen: (i) the influence of concentration gradients on the peak shape and peak width for conventional XRD scans in Bragg–Brentano geometry is determined; (ii) correlations between the local nitrogen concentration and the local lattice expansion can be established; and (iii) the evolution of the scattering intensity with depth becomes accessible, thus depth-resolved information on defect densities or grain size (normal to the surface) can be extracted without resorting to transmission or scanning electron microscopy.

The current clinical role of optical coherence tomography angiography in neuro-ophthalmological diseases.

After the revolutionary effect of optical coherence tomography (OCT) on ophthalmology practice, recent OCT-based technology OCT angiography (OCT-A) also has rapidly gained a wide clinical acceptance. OCT-A is a noninvasive, depth-resolved imaging tool for the evaluation of retinal vascular changes. Since its introduction, the understanding of retinal vascular diseases, pacychoroid spectrum diseases, and other diseases have been enriched in many ways. More importantly, OCT-A provides depth-resolved information that has never before been available. The whole spectrum of neuro-ophthalmological diseases shows consistent peripapillary and macular capillary changes with structural and functional correlation. The superficial and deeper retinal and choroidal vasculatures are affected depending on the nature of the disease process. Therefore, OCT-A play an important role in the diagnosis and management of optic nerve-related diseases as well. In this review, we summarized existing literature on the use of OCT-A in neuro-ophthalmological diseases such as arteritic anterior ischemic neuropathy, nonarteritic anterior ischemic neuropathy, papillitis, papilledema, multiple sclerosis. Currently, OCT-A has an important position as a useful, noninvasive tool in the evaluation of neuro-ophthalmologic diseases; however, OCT-A has several limitations regarding its technical capabilities in challenging neuro-ophthalmic cases. With the improvement in the technical capacity of OCT-A, it will have a more important place in the diagnosis and follow-up of neuro-ophthalmological diseases in future.

High resolution depth profiling using near-total-reflection hard x-ray photoelectron spectroscopy

By adjusting the incidence angle of incoming x rays near the critical angle of x-ray total reflection, photoelectron intensity is strongly modulated due to the variation of x-ray penetration depth. Photoelectron spectroscopy combined with near-total reflection exhibits tunable surface sensitivity, providing depth-resolved information. In this Review, we first describe the experimental setup and specific data analysis process. We then review three different examples that show the broad application of this method. The emphasis is on its applications correlated to oxide heterostructures, especially quantitative depth analyses of compositions and electronic states. In the last part, we discuss the limitations of this technique, mostly in terms of the range of samples that can be studied.

Quantitative label-free optical technique to analyze the ultrastructure changes and spatiotemporal relationship of enamel induced by Msx2 deletion.

New advances in the molecular mechanism of enamel mineralization reveal the practical significance of regenerative medicine in clinical transformation. Muscle segment homeobox 2 (MSX2), a transcription factor, is recently reported to be closely associated with the amelogenesis imperfecta (AI). To elucidate the biomineralization framework of AI enamel, herein, Msx2 gene mutant mice are investigated by dual-mode noninvasive spectroscopic analytical techniques for the first time. Optical coherence tomography (OCT) records the depth-resolved structural information of mice teeth, where a dramatic decrease in enamel thickness and quality occurred in Msx2 deficient (Msx2-/- ) enamel. And it has the advantages of fast, noninvasive and low cost. Raman spectroscopy, a powerful molecular fingerprint tool, further witnesses an imbalance of inorganic and organic contents in Msx2-/- enamel. In addition, abnormal expression of MSX2 also influences the spatial distribution of phosphate in enamel according to the Raman spectral imaging. Therefore, OCT integrated with Raman spectroscopy provides the quantitative label-free optical parameters of both the physical structure and chemical component in mice enamel, which strengthens the understanding of the biomineralization process underlying the Msx2-related amelogenesis imperfect.

Polarization state tracing method to map local birefringent properties in samples using polarization sensitive optical coherence tomography.

We propose a method that utilizes the trajectory of output polarization states on the Poincaré sphere to derive depth-resolved birefringent information within samples using a fiber-based polarization sensitive optical coherence tomography. The apparent (or intermediate) optic axis and the local phase retardation are first obtained by fitting a plane to the adjacent output polarization states along depths in the Poincare sphere. A sequence of 3D rotation operation determined by the local birefringent property of the upper layers is then applied to the apparent axis to finally determine the local optic axis. This method requires only one input polarization state and is compatible with both free-space and fiber-based PSOCT systems, simplifying the imaging system setup. The theoretical framework is presented to derive the local phase retardation and optic axis from the output polarization states and then demonstrated by mapping local birefringent information of the mouse thigh tissue in vitro.

Macroscopic fluorescence lifetime topography enhanced via spatial frequency domain imaging.

We report on a macroscopic fluorescence lifetime imaging (MFLI) topography computational framework based around machine learning with the main goal of retrieving the depth of fluorescent inclusions deeply seated in bio-tissues. This approach leverages the depth-resolved information inherent to time-resolved fluorescence data sets coupled with the retrieval of in situ optical properties as obtained via spatial frequency domain imaging (SFDI). Specifically, a Siamese network architecture is proposed with optical properties (OPs) and time-resolved fluorescence decays as input followed by simultaneous retrieval of lifetime maps and depth profiles. We validate our approach using comprehensive in silico data sets as well as with a phantom experiment. Overall, our results demonstrate that our approach can retrieve the depth of fluorescence inclusions, especially when coupled with optical properties estimation, with high accuracy. We expect the presented computational approach to find great utility in applications such as optical-guided surgery.