Angle-resolved Light Research Articles

In silico full-angle high-dynamic range scattering of microscopic objects exploiting holotomography.

Accurate optical characterization of microscopic objects is crucial in academic research, product development, and clinical diagnosis. We present a method for obtaining full and high-dynamic range, angle-resolved light scattering attributes of microparticles, enabling distinction of variations in both overall morphology and detailed internal structures. This method overcomes previous limitations in observable scattering angles and dynamic range of signals through computationally assisted three-dimensional holotomography. This advancement is significant for particles spanning tens of wavelengths, such as human erythrocytes, which have historically posed measurement challenges due to faint side-scattering signals indicative of their complex interiors. Our technique addresses three key challenges in optical side-scattering analysis: limited observational angular range, reliance on simplified computational models, and low signal-to-noise ratios in both experimental and computational evaluations. We incorporate three-dimensional tomographic complex refractive index data from Fourier-transform light scattering into a tailored finite-difference time-domain simulation space. This approach facilitates precise near-to-far-field transformations. The process yields complete full-angle scattering phase functions, crucial for particles like Plasmodium falciparum-parasitized erythrocytes, predominantly involved in forward scattering. The resultant scattering data exhibit an extreme dynamic range exceeding 100 dB at various incident angles of a He-Ne laser. These findings have the potential to develop point-of-care, cost-effective, and rapid malaria diagnostic tools, inspiring further clinical and research applications in microparticle scattering.

Recovery of angular scattering profiles through a flexible multimode fiber.

Endoscopic angle-resolved light scattering methods have been developed for early cancer detection but they typically require multi-element coherent fiber optic bundles to recover scattering distributions from tissues. Recent work has focused on using a single multimode fiber (MMF) to measure angle resolved scattering but this approach has practical limitations to overcome before clinical translation. Here we address these limitations by proposing an MMF-based endoscope capable of measuring angular scattering patterns suitable for determining structure. Significantly, this approach implements a spectrally resolved detection scheme to reduce speckle and leverages the azimuthal symmetry of the angular scattering patterns to enable measurements that are robust to fiber bending. This results in a unique method that does not require matrix inversion or machine learning to measure a transmitted scattering distribution. The MMF utilized here is 1000 mm in length with a 200 µm core and is demonstrated to recover angular scattering distributions even with bending displacements of up to 30 cm. This advance has a significant impact on the clinical translation of biomedical endoscopic diagnostic techniques that use angular scattering to determine the size of cell nuclei to detect early cancer.

Strong coupling in mechanically flexible free-standing organic membranes.

Strong coupling of a confined optical field to the excitonic or vibronic transitions of a molecular material results in the formation of new hybrid states called polaritons. Such effects have been extensively studied in Fabry-Pèrot microcavity structures where an organic material is placed between two highly reflective mirrors. Recently, theoretical and experimental evidence has suggested that strong coupling can be used to modify chemical reactivity as well as molecular photophysical functionalities. However, the geometry of conventional microcavity structures limits the ability of molecules "encapsulated" in a cavity to interact with their local environment. Here, we fabricate mirrorless organic membranes that utilize the refractive index contrast between the organic active material and its surrounding medium to confine an optical field with Q-factor values up to 33. Using angle-resolved white light reflectivity measurements, we confirm that our structures operate in the strong coupling regime, with Rabi-splitting energies between 60 and 80meV in the different structures studied. The experimental results are matched by transfer matrix and coupled oscillator models that simulate the various polariton states of the free standing membranes. Our work demonstrates that mechanically flexible and easy-to-fabricate free standing membranes can support strong light-matter coupling, making such simple and versatile structures highly promising for a range of polariton applications.

Impact of light polarization on angle-resolved light scattering from gloss scale

The gloss scale is a set of coated paper samples with different surface roughness allowing for appearance variation from high gloss to mat. Nowadays, the gloss scale is widely used as a reference on visual appearance of glossy and mat surfaces. This work provides for angle resolved analysis of light scattering from gloss scales as well as the analysis of the surface topography parameters relevant for the analysis of light-scattering experiments: RMS-roughness σ, auto-correlation function and power spectrum density, correlation length ℓ c and surface slopes. The angle–resolved measurements of light scattered in reflection within the half–space are performed at 4 angles of incidence, while measurements within the incidence plane are made at 17 angles of incidence and under non–polarized, s–and p–polarized illumination. The polarization of incident light has a significant impact on both the specular and the diffused components of scattered light. Brewster scattering angle is especially well observed for black mat surfaces. Qualitative and quantitative analysis of measured scattered light is made with Rayleigh-Rice and Kirchhoff models applied to statistically representative analysis of surface topography.

Shape Discrimination of Individual Aerosol Particles Using Light Scattering

We established an experimental apparatus by combining polarized light scattering and angle-resolved light scattering measurement technology to rapidly identify the shape of an individual aerosol particle. The experimental data of scattered light of Oleic acid, rod-shaped Silicon dioxide, and other particles with typical shape characteristics were analyzed statistically. To better study the relationship between the shape of particles and the properties of scattered light, the partial least squares discriminant analysis (PLS-DA) method was used to analyze the scattered light of aerosol samples based on the size screening of particles, and the shape recognition and classification method of the individual aerosol particle was established based on the analysis of the spectral data after nonlinear processing and grouping by particle size with the area under the receiver operating characteristic curve (AUC) as reference. The experimental results show that the proposed classification method has a good discrimination ability for spherical, rod-shaped, and other non-spherical particles, which can provide more information for atmospheric aerosol measurement, and has application value for traceability and exposure hazard assessment of aerosol particles.

Retrieving refractive index of single spheres using the phase spectrum of light-scattering pattern

We analyzed the behavior of the complex Fourier spectrum of the angle-resolved light scattering pattern (LSP) of a sphere in the framework of the Wentzel–Kramers–Brillouin (WKB) approximation. Specifically, we showed that the phase value at the main peak of the amplitude spectrum almost quadratically depends on the particle refractive index, which was confirmed by numerical simulations using both the WKB approximation and the rigorous Lorenz–Mie theory. Based on these results, we constructed a method for characterizing polystyrene beads using the main peak position and the phase value at this point. We tested the method both on noisy synthetic LSPs and on the real data measured with the scanning flow cytometer. In both cases, the spectral method was consistent with the reference non-linear regression one. The former method leads to comparable errors in retrieved particle characteristics but is 300 times faster than the latter one. The only drawback of the spectral method is a limited operational range of particle characteristics that need to be set a priori due to phase wrapping. Thus, its main application niche is fast and precise characterization of spheres with small variation range of characteristics.

Analysis of very low bacterial counts in small sample volumes using angle-resolved light scattering.

Because of its high sensitivity to even small objects and the quick measurement principle, angle-resolved scattering (ARS) measurements exhibit a promising potential as a rapid analysis tool for bacterial cells at small sample sizes and very low numbers of cells. In this study, investigations on scattered light from various bacterial cell samples revealed applicability down to single cell levels, which is a huge benefit compared to conventional methods that depend on time-consuming cellular growth over several hours or even days. With the proposed setup and data analysis method, it is possible to detect scatter differences among cell types, together with the cell concentration.

Cylindrical Metalens for Generation and Focusing of Free-Electron Radiation.

Metasurfaces constitute a powerful approach to generate and control light by engineering optical material properties at the subwavelength scale. Recently, this concept was applied to manipulate free-electron radiation phenomena, rendering versatile light sources with unique functionalities. In this Letter, we experimentally demonstrate spectral and angular control over coherent light emission by metasurfaces that interact with free-electrons under grazing incidence. Specifically, we study metalenses based on chirped metagratings that simultaneously emit and shape Smith–Purcell radiation in the visible and near-infrared spectral regime. In good agreement with theory, we observe the far-field signatures of strongly convergent and divergent cylindrical radiation wavefronts using in situ hyperspectral angle-resolved light detection in a scanning electron microscope. Furthermore, we theoretically explore simultaneous control over the polarization and wavefront of Smith–Purcell radiation via a split-ring-resonator metasurface, enabling tunable operation by spatially selective mode excitation at nanometer resolution. Our work highlights the potential of merging metasurfaces with free-electron excitations for versatile and highly tunable radiation sources in wide-ranging spectral regimes.

Dual-wavelength angle-resolved light scattering used in the analysis of particles by scanning flow cytometry

For the first time, a scanning flow cytometer (SFC) was applied for the analysis of individual particles using angle-resolved light scattering at two wavelengths simultaneously. The SFC was equipped with three lasers with wavelengths of 405, 444 and 660 nm. The performance of dual-wavelength angle-resolved light-scattering (DWARLS) flow cytometry was demonstrated by the analysis of polymer beads, spherized red blood cells (RBCs) and blood platelets. The main advantage of dual-wavelength flow cytometry relates to an increment in the precision of the inverse light-scattering problem solution. The solution allows one to retrieve the characteristics of individual particles by measuring the light-scattering profiles of these particles. DWARLS has allowed us to measure diameters and refractive indices (RIs) of polystyrene beads with record precision: a median error of 12 nm and a few thousandths for diameter and RI, respectively. Analysis of spherized RBCs with DWARLS flow cytometry has provided record precision in the sizing of spherized RBCs, with a median error of 16 nm for diameter, and an adequate value of the specific refraction increment for hemoglobin. By means of DWARLS, we have formed a numerical criterion for shape separation of blood platelets, which can be described by an oblate spheroid model to increase the precision of the distribution parameters of the platelet fractions.

Measuring bulk and surface acoustic modes in diamond by angle-resolved Brillouin spectroscopy

The acoustic modes of diamond are not only of profound significance for studying its thermal conductivity, mechanical properties, and optical properties, but also play a definite role in the performance of high-frequency and high-power acoustic wave devices. Here, we report on the bulk acoustic waves (BAWs) and surface acoustic waves (SAWs) of single-crystal diamond using angle-resolved Brillouin light scattering (BLS) spectroscopy. We identify two high-speed surface skimming bulk waves (SSBW) with acoustic velocities of 1.277×106 and 1.727×106 cm/s, respectively. Furthermore, we obtain the relationship between the velocity of arbitrary BAWs and that of BAWs propagating along the high-symmetric axis at different incident angles. In the community of diamond-based acoustic studies, our results may provide a valuable reference for fundamental research and device engineering.

Spectral approach to recognize spherical particles among non-spherical ones by angle-resolved light scattering

Most of known light-scattering technologies, which allow one to separate spherical from non-spherical single particles, utilize either analysis of 2D light-scattering pattern or depolarization of light scattered. Both approaches force one to use high-sensitive detectors to provide a suitable signal to noise ratio for two-dimensionless photo matrix or for optical system with crossed polarizers. In this study, we introduce the method for discrimination of spherical and non-spherical single particles. The approach is based on measurement of leading, most intensive, element S11 of light-scattering matrix. To provide maximal signal to noise ratio we specified the light-scattering profile (LSP) in terms of integrated over azimuthal angle S11 as a function of polar scattering angle. The shape-sensitive vector-invariant for individual spherical particles was constructed from the parameters of LSP spectrum. The vector-invariant plays a role of the numerical criterion to identify spherical particles from LSPs. It can be applied to find a sphere with characteristics ranging from 16.5 to 70 and from 0.5 to 7.0 for size and phase-shift parameters respectively (size parameter α = πdn0/λ, where d – sphere diameter, λ – wavelength of the incident light, and n0 – medium refractive index, RI, phase-shift parameter ρ = 2α(m − 1), where relative RI m = n/n0 and n is the sphere RI). These ranges cover all possible characteristics of blood cells within the visible region of wavelengths. The ability of the vector-invariant to recognize spherical cells among non-spherical ones was tested theoretically by LSP databases of optical models of platelets and mature red blood cells. Moreover, experimentally the vector-invariant demonstrated good performance in searching of near-perfect spheres among milk fat globules, isolated nuclei of mononuclear cells, and completely spherized cells in a course of red blood cell lysis.

How Light Passes Through Ordered Macroporous Photonic Crystals Immersed in Solvents

Understanding light transmission through 3D macroporous photonic crystals is essential for linking their optical characteristics to any application, particularly when immersed in solvents. We examined TiO2 and SnO2 IOs in a series of common solvents to fix existing issues in Bragg-Snell theory and the definition of the photonic crystal unit cell and pore infilling. Using a comprehensive angle-resolved light transmission study coupled to microscopy examination of the IO structure, we demonstrate experimentally and theoretically that low fill factors are caused by porous material infilling all interstitial vacancies of the colloidal opal template, instead of complete filling of most interstitial voids. By also including an optical interference condition for inverse opals with a larger effective refractive index than their substrate, the optical data for macroporous photonic crystals are now consistent with microscopy measurements. This analysis provides an accurate correlation between the true response of a macroporous photonic crystal to tits primary properties including the index contrast with a solvent, infilled structure and the unit cell definition, and the directionality of the photonic band gap. As control in functional porous and ordered photonic materials has broadening to include biosensing and energy storage for example, a comprehensive and consistent description of how light passes through ordered macroporous photonic crystal structures is an important requirement.

Fourier-plane investigation of plasmonic bound states in the continuum and molecular emission coupling

Abstract Bound states in the continuum (BICs) or trapped modes can provide an important new avenue for strong light confinement via destructive interference. Dielectric photonic structures have been extensively studied for optical BICs. However, BICs in plasmonic nanostructures have not been explored much yet. Herein, we present a thorough experimental study of plasmonic BICs via Fourier-plane spectroscopy and imaging. Optical mode dispersion in a metal grating covered by a dielectric layer is directly measured in an angle-resolved white light reflection spectrum. Two dielectric layer thicknesses are considered. Both plasmonic and photonics modes are supported in the visible range using a thicker dielectric film; hence, either hybrid or purely plasmonic BICs can be formed. With a thinner dielectric layer, only plasmonic modes are strongly excited and purely plasmonic BICs appear. Our measurements exhibit all features expected for BICs, including a substantial increase in the Q factor. We also demonstrate that the BIC position can be switched from one optical mode branch to the other by tuning a metal grating parameter. Moreover, by mixing luminescent dyes in a dielectric layer, light emission coupling into BICs is investigated. We find that the photoluminescence peak disappears at the BIC condition, which is attributed to the trapping of molecular emission at plasmonic BICs. Therefore, both white light reflection and dye emission measurements in the Fourier plane clearly indicate the formation of trapped modes in plasmonic nanostructures. Our observation implies that plasmonic BICs can enable a highly effective light trapping device despite the simple structure of the device geometry. Plasmonic supercavity design based on the BIC concept may provide many interesting future opportunities for nanolasers, optical sensing, and nonlinear enhancement.

An electronic approach for the automation of angle-resolved spectroscopic measurements.

Angle-resolved light scattering techniques are powerful tools to obtain structural and spectroscopic information on the investigated sample by means of the study of the pattern of the angular distribution of scattered light. In this paper, we show the details of a new electronic system conceived to automate a Raman coherent backscattering setup, in which it is crucial to acquire several spectra at different angles in a wide spectral acquisition range. In this frame, we used this electrical circuit to trigger the signal edges between the charged-coupled device and the motorized nanorotator stage in our setup, carrying out a considerable quantity of measurements only with an initial input given by the operator and minimizing the supervision of the experiment and, therefore, the time invested by the user in it. By means of this system that can be easily integrated in the setup, we can perform distinct type of measurements by using different configurations of the components that make up the experimental setup.

Filling in the gaps: The nature of light transmission through solvent-filled inverse opal photonic crystals

Understanding the nature of light transmission and the photonic bandgap in inverse opal photonic crystals is essential for linking their optical characteristics to any application. This is especially important when these structures are examined in liquids or solvents. We examined TiO2 and SnO2 IOs in a range of common solvents to solve the conflict between Bragg-Snell theory, optical and physical measurements by a comprehensive angle-resolved light transmission study coupled to microscopy examination of the IO structure. Tuning the position of the photonic bandgap and index contrast by solvent infiltration of each inverse opal requires a modification to the Bragg-Snell theory and the photonic crystal unit cell definition. We also demonstrate experimentally and theroetically that low fill factors are caused by less desne material infilling all interstitial vancancies in the opal template to form an IO. By also including an optical interference condition for inverse opals with an effective refractive index greater than its substrate, and an alternative internal refraction angle in the substrate, angle-resolved transmission spectra for inverse opals are now consistent with physical measurements. This work now allows an accurate correlation between the true response of an IO to the index contrast with a solvent, how an IO is infilled, and the directionality and bandwidth of the photonic bandgap. As control in functional photonic materials becomes more prevalent outside of optics and photonics, such as biosensing and energy storage, for example, a comprehensive and consistent correlation between photonic crystals structures and their primary optical signatures is a fundamental requirement for application.

Multimodal Coherent Imaging of Retinal Biomarkers of Alzheimer\u2019s Disease in a Mouse Model

We acquired depth-resolved light scattering measurements from the retinas of triple transgenic Alzheimer’s Disease (3xTg-AD) mice and wild type (WT) age-matched controls using co-registered angle-resolved low-coherence interferometry (a/LCI) and optical coherence tomography (OCT). Angle-resolved light scattering measurements were acquired from the nerve fiber layer, outer plexiform layer, and retinal pigmented epithelium using image guidance and segmented thicknesses provided by co-registered OCT B-scans. Analysis of the OCT images showed a statistically significant thinning of the nerve fiber layer in AD mouse retinas compared to WT controls. The a/LCI scattering measurements provided complementary information that distinguishes AD mice by quantitatively characterizing tissue heterogeneity. The AD mouse retinas demonstrated higher mean and variance in nerve fiber layer light scattering intensity compared to WT controls. Further, the difference in tissue heterogeneity was observed through short-range spatial correlations that show greater slopes at all layers of interest for AD mouse retinas compared to WT controls. A greater slope indicates a faster loss of spatial correlation, suggesting a loss of tissue self-similarity characteristic of heterogeneity consistent with AD pathology. Use of this combined modality introduces unique tissue texture characterization to complement development of future AD biomarker analysis.

Wide-range wavelength and angle resolved light scattering measurement setup.

We present a new version of a light scattering measurement setup, using a high-power supercontinuum laser source, two volume hologram filters, and two low-noise scientific grade cameras. This configuration enables spectral and angle resolved characterization of the light scattered by complex thin-film filters from 400 to 1650 nm. Measurements carried out on specific filters illustrate the performances of the setup.

Spin splitting and strain in epitaxial monolayer WSe2 on graphene

We present the electronic and structural properties of monolayer WSe$_{2}$ grown by pulsed-laser deposition on monolayer graphene (MLG) on SiC. The spin splitting in the WSe$_{2}$ valence band at $\overline{\mathrm{K}}$ was $\Delta_\mathrm{SO}=0.469\pm0.008$ eV by angle-resolved photoemission spectroscopy (ARPES). Synchrotron-based grazing-incidence in-plane X-ray diffraction (XRD) revealed the in-plane lattice constant of monolayer WSe$_{2}$ to be $a_\mathrm{WSe_2}=3.2757\pm0.0008 \mathrm{\r{A}}$. This indicates a lattice compression of -0.19 % from bulk WSe$_{2}$. By using experimentally determined graphene lattice constant ($a_\mathrm{MLG}=2.4575\pm0.0007 \mathrm{\r{A}}$), we found that a 3$\times$3 unit cell of the slightly compressed WSe$_{2}$ is perfectly commensurate with a 4$\times$4 graphene lattice with a mismatch below 0.03 %, which could explain why the monolayer WSe$_{2}$ is compressed on MLG. From XRD and first-principles calculations, however, we conclude that the observed size of strain is negligibly small to account for a discrepancy in $\Delta_\mathrm{SO}$ found between exfoliated and epitaxial monolayers in earlier ARPES. In addition, angle-resolved, ultraviolet and X-ray photoelectron spectroscopy shed light on the band alignment between WSe$_{2}$ and MLG/SiC and indicate electron transfer from graphene to the WSe$_{2}$ monolayer. As further revealed by atomic force microscopy, the WSe$_{2}$ island size depends on the number of carbon layers on top of the SiC substrate. This suggests that the epitaxy of WSe$_{2}$ favors the weak van der Waals interactions with graphene while it is perturbed by the influence of the SiC substrate and its carbon buffer layer.

Influence of different cooling strategies during hard turning of AISI 52100 – part II: characterization of the surface and near surface microstructure morphology

Abstract This two-part paper addresses the impact of seven different cooling conditions on the finishing process as well as the resulting surface and near-surface microstructure morphology of the rolling bearing steel AISI 52100 in martensitic heat treatment state. The applied cooling strategies are liquid nitrogen (LN2-) cooling, CO2-snow cooling at varied mass flows, sub-zero metalworking fluids (MWF) at varied supply temperatures, and dry machining. Part II of the paper concerns the changes in surface roughness, examined by an angle resolved scatter light sensor, micro hardness measurement, microstructure observations, residual stress and phase composition analyses in the near-surface area, caused by the turning process, that is characterized in Part I. The lowest cooling performance, i.e. dry machining, results in low compressive axial residual stresses and a high proportion of retained austenite at the near-surface area. High cooling performance strategies like CO2 cooling or sub-zero MWF show an opposite distribution of the mentioned parameters, wherein the coolants’ lubrication effect is an important aspect. Microstructural differences resulting from the process parameters used in these investigations tend to be less significant. Thus, they do not allow reliable assignment of the microstructure to the cooling strategies used. However, different cooling conditions show an impact on residual stresses, phase distribution and surface topography of the workpieces to varying extents.

International round-robin experiment for angle-resolved light scattering measurement.

An international round-robin experiment has been conducted to test procedures and methods for the measurement of angle-resolved light scattering. ASTM E2387-05 has been used as the main guide, while the experience gained should also contribute to the new ISO standard of angle-resolved scattering currently under development (ISO/WD 19986:2016). Seven laboratories from Europe and the United States measured the angle-resolved scattering from Al/SiO2-coated substrates, transparent substrates, volume diffusors, quasi-volume diffusors, white calibration standards, and grating samples at laser wavelengths in the UV, VIS, and NIR spectra. Results were sent to Fraunhofer IOF, which coordinated the experiments and analyzed the data, while ESA-ESTEC, as the project donor, defined conditions and parameters. Depending mainly on the sample type, overall good to reasonable agreements were observed, with largest deviations at scattering angles very close to the specular beam. Volume diffusor characterization unexpectedly turned out to be challenging. Not all participants provided measurement uncertainty ranges according to the Guide to the Expression of Uncertainty in Measurement; often, a single general scatterometer-related measurement uncertainty value was stated. Although relative instrument measurement uncertainties close to 1% are sometimes claimed, the comparison results did not support these claims for specular scattering samples as mirrors, substrates, or gratings.