Spatial Resolution Depth Research Articles

Phase-Sensitive Vibrational Sum and Difference Frequency-Generation Spectroscopy Enabling Nanometer-Depth Profiling at Interfaces

The unique physical and chemical properties of interfaces are governed by a finite depth that describes the transition from the topmost atomic layer to the properties of the bulk material. Thus, understanding the physical nature of interfaces requires detailed insight into the different structures, chemical compositions, and physical processes that form this interfacial region. Such insight has traditionally been difficult to obtain from experiments, as it requires a combination of structural and chemical sensitivity with spatial depth resolution on the nanometer scale. In this contribution, we present a vibrational spectroscopic approach that can overcome these limitations. By combining phase-sensitive sum and difference frequency-generation (SFG and DFG, respectively) spectroscopy and by selectively determining different nonlinear interaction pathways, we can extract precise depth information and correlate these to specific vibrationally resonant modes of interfacial species. We detail the mathematical framework behind this approach and demonstrate the performance of this technique in two sets of experiments on selected model samples. An analysis of the results shows an almost perfect match between experiment and theory, confirming the practicability of the proposed concept under realistic experimental conditions. Furthermore, in measurements with self-assembled monolayers of different chain lengths, we analyze the spatial accuracy of the technique and find that the precision can even reach the sub-nanometer regime. We also discuss the implications and the information content of such depth-sensitive measurements and show that the concept is very general and goes beyond the analysis of the depth profiles. The presented SFG/DFG technique offers new perspectives for spectroscopic investigations of interfaces in various material systems by providing access to fundamental observables that have so far been inaccessible by experiments. Here, we set the theoretical and experimental basis for such future investigations.

Aberration correction in stimulated emission depletion microscopy to increase imaging depth in living brain tissue.

.Significance: Stimulated emission depletion (STED) microscopy enables nanoscale imaging of live samples, but it requires a specific spatial beam shaping that is highly sensitive to optical aberrations, limiting its depth penetration. Therefore, there is a need for methods to reduce optical aberrations and improve the spatial resolution of STED microscopy inside thick biological tissue.Aim: The aim of our work was to develop and validate a method based on adaptive optics to achieve an a priori correction of spherical aberrations as a function of imaging depth.Approach: We first measured the aberrations in a phantom sample of gold and fluorescent nanoparticles suspended in an agarose gel with a refractive index closely matching living brain tissue. We then used a spatial light modulator to apply corrective phase shifts and validate this calibration approach by imaging neurons in living brain slices.Results: After quantifying the spatial resolution in depth in phantom samples, we demonstrated that the corrections can substantially increase image quality in living brain slices. Specifically, we could measure structures as small as 80 nm at a depth of inside the biological tissue and obtain a 60% signal increase after correction.Conclusion: We propose a simple and robust approach to calibrate and compensate the distortions of the STED beam profile introduced by spherical aberrations with increasing imaging depth and demonstrated that this method offers significant improvements in microscopy performance for nanoscale cellular imaging in live tissue.

A low-noise solid-state nanopore platform based on a highly insulating substrate.

A solid-state nanopore platform with a low noise level and sufficient sensitivity to discriminate single-strand DNA (ssDNA) homopolymers of poly-A40 and poly-T40 using ionic current blockade sensing is proposed and demonstrated. The key features of this platform are (a) highly insulating dielectric substrates that are used to mitigate the effect of parasitic capacitance elements, which decrease the ionic current RMS noise level to sub-10 pA and (b) ultra-thin silicon nitride membranes with a physical thickness of 5 nm (an effective thickness of 2.4 nm estimated from the ionic current) are used to maximize the signal-to-noise ratio and the spatial depth resolution. The utilization of an ultra-thin membrane and a nanopore diameter as small as 1.5 nm allow the successful discrimination of 40 nucleotide ssDNA poly-A40 and poly-T40. Overall, we demonstrate that this platform overcomes several critical limitations of solid-state nanopores and opens the door to a wide range of applications in single-molecule-based detection and analysis.

Spatial resolution in depth for time-resolved diffuse optical tomography using short source-detector separations.

Diffuse optical tomography for medical applications can require probes with small dimensions involving short source-detector separations. Even though this configuration is seen at first as a constraint due to the challenge of depth sensitivity, we show here that it can potentially be an asset for spatial resolution in depth. By comparing two fiber optic probes on a test object, we first show with simulations that short source-detector separations improve the spatial resolution down to a limit depth. We then confirm these results in an experimental study with a state-of-the-art setup involving a fast-gated single-photon avalanche diode allowing maximum depth sensitivity. We conclude that short source-detector separations are an option to consider for the design of probes so as to improve image quality for diffuse optical tomography in reflectance.

A comparative study of detectors for relative dose measurements in kilovoltage small beams

Introduction The XRAD225Cx is a small animal radiotherapy device using a medium energy beam (225 kVp) and small circular fields. In addition to the half-value layers and the absolute dose rate, the commissioning of this equipment requires relative dose measurements such as percentage depth dose (PDD), Output Factor (OF) and Tissue Maximum Ratio (TMR). The aim of this study was to compare four detectors to determine the optimal conditions to perform these relative measurements. Materials and methods RW3 material is known not to be waterequivalent at medium energy for absolute dose measurements. However the impact for relative dose measurements is negligible. To study the influence of the detector, four dosimeters (an IBA SFD diode, a PTW PinPoint 31014 microchamber, EBT3 films and a PTW-23342 plane-parallel chamber) were compared for PDDs, OFs and TMRs in water and/or RW3 depending on the dosimeter sealing. Measurements were performed in small fields (20, 15, 10, 8, 5 and 2.5 mm in diameter). OFs, PDDs, and TMRs were also computed with the GATE Monte Carlo model of this device. Results Regardless of detectors, simulated and measured OFs showed no differences down to a diameter beam of 8 mm. For the twosmallestbeams(5 and 2.5 mm),ionizationchambers yielded large discrepancies compared to SFD and EBT3 measurements and simulations. This is due to the size of the sensitive volume of chambers compared to beam diameter. For PDDs and TMRs, measurement accuracy depends on spatial resolution in depth of the detector. Therefore, Pin-Point chamber was not used. Plane ionization chamber, SFD diode and film measurements were closed to Monte Carlo computed results. SFD diode isknownto be energy-dependent, howevermeasurementswere satisfactory whatever the field diameter at 225 kVp. Conclusions For relative dose measurements such as PDD, OF and TMR, all studied detectors may be used down to a beam diameter of 8 mm. In smaller beams (5 and 2.5 mm), measurements should be performed with the SFD diode or EBT3 films. In particular for such small beams, EBT3 appeared to be very suitable.

Single-view volumetric PIV via high-resolution scanning, isotropic voxel restructuring and 3D least-squares matching (3D-LSM)

Scanning PIV as introduced by Brücker (1995 Exp. Fluids 19 255–63, 1996a Appl. Sci. Res. 56 157–79) has been successfully applied in the last 20 years to different flow problems where the frame rate was sufficient to ensure a ‘frozen’ field condition. The limited number of parallel planes however leads typically to an under-sampling in the scan direction in depth; therefore, the spatial resolution in depth is typically considerably lower than the spatial resolution in the plane of the laser sheet (depth resolution = scan shift Δz ≫ pixel unit in object space). In addition, a partial volume averaging effect due to the thickness of the light sheet must be taken into account. Herein, the method is further developed using a high-resolution scanning in combination with a Gaussian regression technique to achieve an isotropic representation of the tracer particles in a voxel-based volume reconstruction with cuboidal voxels. This eliminates the partial volume averaging effect due to light sheet thickness and leads to comparable spatial resolution of the particle field reconstructions in x-, y- and z-axes. In addition, advantage of voxel-based processing with estimations of translation, rotation and shear/strain is taken by using a 3D least-squares matching method, well suited for reconstruction of grey-level pattern fields. The method is discussed in this paper and used to investigate the ring vortex instability at Re = 2500 within a measurement volume of roughly 75 × 75 × 50 mm3 with a spatial resolution of 100 µm/voxel (750 × 750 × 500 voxel elements). The volume has been scanned with a number of 100 light sheets and scan rates of 10 kHz. The results show the growth of the Tsai–Widnall azimuthal instabilities accompanied with a precession of the axis of the vortex ring. Prior to breakdown, secondary instabilities evolve along the core with streamwise oriented striations. The front stagnation point's streamwise distance to the core starts to decrease while the rear stagnation point distance remains constant which indicates that the front part of the ring is at first losing its mass during breakdown.

Emerging applications for OCT in the head and neck

Objectives: To describe the current and promising new applications of Optical Coherence Tomography (OCT) as a helpful tool when imaging the different sites in the head and neck. We used the OCT Niris system, which is the first commercially available OCT device for applications outside the field of ophthalmology. Methods: OCT images were obtained of normal, benign, premalignant and malignant lesions in different areas of the head and neck. The OCT imaging system has a tissue penetration depth of approximately 1-2mm, a scanning range of 2mm and a spatial depth resolution of approximately 10-20μm. Imaging was performed using a flexible probe in two different settings, the outpatient clinic and the operating room. Results: High-resolution cross-sectional images from the larynx were obtained with the patient awake, without the need for general anesthesia, under direct visualization with a flexible fiberoptic endoscope. The OCT probe was inserted through the nasal cavity and placed in slight contact with the laryngeal tissue. In the ears, cholesteatoma was differentiated from inflamed middle ear mucosa by the different hyperintensity. In the neck, normal as well as different pathologies of the thyroid were identified. Conclusions: This system is non invasive and easy to incorporate into the operating room setting as well as the outpatient clinic. It requires minimal set-up and only one person is required to operate the system. OCT has the distinctive capability to obtain highresolution images, and the microanatomy of different sites can be observed. OCT technology has the potential to offer a quick, efficient and reliable imaging method to help the surgeon not only in the operating room but also in the clinical setting to guide surgical biopsies and aid in clinical decision making of different head and neck pathologies, especially those arising form the larynx.

DESENVOLVIMENTO DE METODOLOGIA PARA AVALIAÇÃO DE AÇOS ESFEROIDIZADOS. PARTE 1: DETERMINAÇÃO DO GRAU DE DESCARBONETAÇÃO

In this work, simple methodology was developed for the quantitative evaluation of the microstructure of spheroidized steels. In this first part, the decarburization of these steels was analyzed and, sequentially, the determination of the spheroidization degree will be analized, as the available industrial norms are restricted to the comparison with perlitic micrographic standards, in the case of the decarburization degree and with qualitative standards, in the case of the spheroidization degree. The AISI 1095 steel was studied in the hot-rolled and spheroidized-annealed condition. Using stereologic methods and scanning electron microscopy it is possible to quantify, from the volumetric fraction of carbides, decarburization profiles of spheroidized products with a spatial depth resolution of circa 5 µm.

Surface characterization and depth profile analysis of glasses by r.f. GDOES

AbstractThe characterization of surfaces, interfaces and interphases in glasses is analytically challenging and may benefit from recent developments in radiofrequency glow discharge optical emission spectroscopy (r.f. GDOES) applied to non‐conductive materials. Thus, the main thrust of this study is to evaluate the potential of r.f. GDOES for the characterization of glass surfaces, physical heterogeneities such as discrete layers and continuous interphases and the distribution of elements in glass materials. Model glasses with increasing structural and chemical complexity thus were analysed using a Jobin Yvon GDOES system (5000 RF): the glasses included a multilayered glass (triple‐coated glass), two photoreactive glasses (with potential chemical diffusion gradients) and acid‐leached glass with an alteration layer. Concurrently, surface characterization of the glasses was conducted using scanning electron microscopy or transmission electron microscopy with energy‐dispersive x‐ray imaging to assess the chemical and physical gradients in the materials and to investigate the glow discharge sputtering process. From this the capability of r.f. GDOES for elemental depth profiling in glass materials is established. However, although the sputtering rates for glass are greatly reduced compared with conductive materials (∼2 nm s−1 compared with 50–150 nm s−1 for metals), r.f. GDOES offers good spatial depth resolution (nm) and elemental sensitivity for several applications, such as the characterization of interfaces or interphases and the determination of chemical gradients in glasses. Furthermore, the technique distinguishes submicrometre discrete layers in the coated glass, and the diffusion of elements within glass matrices may be monitored. Copyright © 2003 John Wiley & Sons, Ltd.

Direct measurement of proton straggling in GaAlAs for nuclear profiling

We present the first direct measurement of proton energy-loss straggling in Ga1−xAlxAs/GaAs, enabling us to take maximum advantage of the 27Al (p, γ) 28Si nuclear reactions as a powerful nondestructive technique for measuring Al profiles in these structures. Results were obtained using samples produced by molecular beam epitaxy and fabricated to have step-function Al concentration distributions to prescribed depths. The exact straggling width was obtained by a least-square comparison of the experimental spectra with curves calculated using a parameterized straggling distribution. With these results, profiling measurements can now be made giving the Al concentration fall-off at the GaAl/GaAs interface with a spatial depth resolution of about 4% and epilayer thickness determinations to about 2%. We have also observed in one sample a nonabrupt transition at the GaAlAs/GaAs interface, due to differences in substrate surface preparation procedures prior to growth of the GaAlAs layer.