Optical Diffuse Reflectance Research Articles

Modeling optical fluence and diffuse reflectance distribution in normal and cancerous breast tissues exposed to planar and Gaussian NIR beam shapes using Monte Carlo simulation

Precise knowledge about light propagation in biological tissues is necessary for accurate diagnostics and effective therapies utilizing optical technologies. In the current paper, the Monte Carlo simulation is applied to study light dispersion in normal and cancerous breast after irradiating to different laser beam shapes. Two distinct laser wavelengths (800-1100nm) with planar and Gaussian shapes were employed. The spatially resolved steady-state diffuse reflectance of normal tissue and tumor was investigated using Monte Carlo simulation method via MCML and MCXLAB computations. The diffusion equation was solved to simulate the fluence rate at the tissue surface based on the optical parameter values (i.e., scattering and absorption coefficients). The results confirm differences in diffuse reflectance and optical fluence distribution between the normal and tumor tissues at each wavelength. Tissue optical parameters and the utilized laser beam shape control the distribution of the fluence rate within tissues. Therefore, offering visual representations of these distributions can provide a secure visual route for biological diagnostics.

Monte Carlo simulation of handheld probes to detect non-invasive ductal carcinoma from diffuse optical reflectance signals

To identify the malignancy in human breast, hand-held optical probes are simulated using the Monte Carlo method. These devices were used to scan different tissue equivalent numerical breast phantoms to detect the embedded cancerous tissues in them. The phantoms were simulated with glandular tissues embedded with ductal carcinoma of size 5 mm and 2 mm diameter at various depths. Two optical probes namely dual side detector (DSD) probe and a single side detector (SSD) probe are designed through which one million light photons of wavelengths 600 nm and 800 nm respectively were passed into the numerical phantoms. The photons that were backscattered from the phantoms were received by the detectors in both probes. They were measured as signals and both probes were able to detect the presence of the embedded cancerous tissues at different depths. The results indicate that the probes with a light source at 800 nm could detect deep-seated inhomogeneities. The absolute value of Peak Intensity (|PI|) and Full Width at Half Maximum (FWHM) would be indicative of their location and approximate size of the embedded tissues. The |PI| decreases with the size and depth of the embedded tissue, and the FWHM increases with depth. The results show that the designed probes are capable of detecting small variations in tissues.

Synthesis and characterization of carbon nanotube/copper oxide nanocomposite as an enhanced absorber for solar radiation

In this work, CNTs/CuO nanocomposite was synthesized by chemical deposition method using the alkaline tartrate bath. The CNTs underwent heat and chemical functionalization treatments before starting the deposition process. TEM investigations showed that the untreated CNTs are agglomerated, have long filaments with capped ending, uniform multi-walled graphitic layers of ∼20 nm diameter and 20 μm length. However; the functionalized CNTs were de-agglomerated under sonication and the CNTs long chains were shortened to small segments by breaking of the graphitic structure using the acid oxidizing mixture, besides a de-capping of the CNTs ends took place. The thermal analysis of the CNTs by TGA confirmed the thermal stability up to 600 °C. FESEM investigations showed that; the functionalized CNTs are coated by adhered nano layer composed of CuO and the EDAX analysis confirmed the chemical structure of the deposited cupric oxide. X-ray diffraction analysis confirmed the deposition of cupric oxide phase (CuO) on CNTs. However; a minor phase of cuprous oxide (Cu2O) was detected due to the reduction of the cupric oxide at the surfaces of the CNTs/CuO nanocomposite. For comparing the obtained results; CuO nanosized powder was also prepared by decomposition of copper nitrate trihydrate at 350 °C. The average crystallite size of CuO nanoparticles was estimated as 23–29 nm corresponding to the two main diffracted peaks, while for the CuO/CNTs nanocomposite was ∼9 nm. The optical absorbance and diffuse reflectance of the investigated CNTs, CuO nanoparticles and CuO/CNTs nanocomposites were studied in the UV-VIS-IR region. The absorption edges of both CuO nanoparticles as well as CuO/CNTs nanocomposite were calculated from Kubelka-Monk function of the diffuse reflectance that were found to be 1.48 and 1.53 eV, respectively. The shift of the band gap was discussed by the energy gap confinement effect of nano structured materials. The optical absorbance was increased in both short and long wavelengths for the CuO/CNTs nanocomposite than in the pure CNTs and CuO nanoparticles. Besides, the reflectance of the CuO/CNTs nanocomposite was observed to be smaller than that of both the CuO nanoparticles and CNTs, which gives a rise in utilization of this nanocomposite as solar selective coating than their individual forms.

Synthesis and Characterization of Gadolinium Oxide-Hematite Magnetic Ceramic Nanostructures

Mixed-oxide nanostructures of the type xGd2O3-(1-x)α-Fe2O3 (x=0.1, 0.3, 0.5 and 0.7) were synthesized by mechanochemical activation for ball milling times of 0, 2, 4, 8 and 12 hours. The systems were subsequently analyzed by Mӧssbauer spectroscopy, X-ray powder diffraction (XRPD), magnetic measurements and optical diffuse reflectance spectroscopy. The magnetic hyperfine field was studied as function of ball milling time for all sextets involved and found to be consistent with the formation of a limited solid solution in the systems investigated. The end-product was the gadolinium perovskite, represented by a doublet whose abundance was derived as function of the milling time. The XRPD patterns recorded for the equimolar composition were dominated by the diffraction peaks of GdFeO3 after 12 hours of milling. The hysteresis loops were recorded at 300 and 5 K in an applied magnetic field of 5 T and were interpreted as a superposition of paramagnetic behavior of gadolinium oxide and weak ferromagnetic behavior of hematite and gadolinium perovskite. The Morin transition of hematite was inferred from zero-field-cooling-field-cooling (ZFC-FC) curves measured with a magnetic field of 200 Oe in the 5-300 K temperature range and was found to depend on the ball milling time. Optical diffuse reflectance spectra showed that the compounds were semiconductors with an optical band gap of 2.1 eV.

CaZn(HPO3)2 and Ba2Zn(HPO3)3: novel alkaline-earth zincophosphites with diversified anionic frameworks.

Two novel alkaline-earth zincophosphites, namely CaZn(HPO3)2 and Ba2Zn(HPO3)3, were successfully synthesized under hydrothermal conditions. CaZn(HPO3)2 exhibits a three-dimensional (3D) anionic framework with a 6-connected uni-nodal pcu α-Po primitive cubic topology, constructed by unique Zn2O8 dimers and HPO3 pseudo-tetrahedra, while Ba2Zn(HPO3)3 displays one-dimensional (1D) [Zn(HPO3)3]4- anionic chains. It is worth mentioning that CaZn(HPO3)2 represents the first example of an alkaline-earth zincophosphite compound with a 3D framework structure. Our research also revealed the importance of both alkaline earth cation sizes and Zn/P ratios in anionic open-framework formation. The crystal structures of both compounds were further verified by energy dispersive spectroscopy, IR spectroscopy and Raman spectroscopy. Optical diffuse reflectance spectra, coupled with Tauc's fitting, revealed direct bandgaps with energy values of 4.33 and 4.48 eV for CaZn(HPO3)2 and Ba2Zn(HPO3)3, respectively, which differ from the prediction of theoretical calculations. Density of states calculations were conducted to reveal the origin of the bandgaps and bond interactions. Both compounds exhibited moderate birefringence values. This work may have implications for the design and synthesis of novel metal phosphites with desired properties.

Hydrogenation shrinks the colossal permittivity of Sb/Co co-doped TiO2 nanoparticles-structural and optical investigations

Pristine and (Sb/Co)-codoped TiO2 anatase powders were synthesized by hydrothermal method and some of the samples were hydrogenated. The samples were characterized by the traditional techniques: X-rays, optical diffuse reflection spectroscopy (DRS), and ac-electrical measurements. It was observed that the (Sb/Co) codoping induces the growth of the anatase. The optical absorption investigation was clarified by the generation of an intermediate band in the bandgap of TiO2. The catalysis power of the Co2+ ions to the adsorbed H2-dissociation strongly supports the optical absorption strength. The Sb5+/Co2+ codoping could generate weak colossal permittivity, which was lowered by ∼50% with hydrogenation. Such important results was studied and explained.

Label-Free Optical Spectroscopy for Early Detection of Oral Cancer.

Oral cancer is the 16th most common cancer worldwide. It commonly arises from painless white or red plaques within the oral cavity. Clinical outcome is highly related to the stage when diagnosed. However, early diagnosis is complex owing to the impracticality of biopsying every potentially premalignant intraoral lesion. Therefore, there is a need to develop a non-invasive cost-effective diagnostic technique to differentiate non-malignant and early-stage malignant lesions. Optical spectroscopy may provide an appropriate solution to facilitate early detection of these lesions. It has many advantages over traditional approaches including cost, speed, objectivity, sensitivity, painlessness, and ease-of use in clinical setting for real-time diagnosis. This review consists of a comprehensive overview of optical spectroscopy for oral cancer diagnosis, epidemiology, and recent improvements in this field for diagnostic purposes. It summarizes major developments in label-free optical spectroscopy, including Raman, fluorescence, and diffuse reflectance spectroscopy during recent years. Among the wide range of optical techniques available, we chose these three for this review because they have the ability to provide biochemical information and show great potential for real-time deep-tissue point-based in vivo analysis. This review also highlights the importance of saliva-based potential biomarkers for non-invasive early-stage diagnosis. It concludes with the discussion on the scope of development and future demands from a clinical point of view.

Effects of mechanochemical activation on the structural, magnetic and optical properties of yttrium iron garnet-graphene nanoparticles

Yttrium iron garnet nanoparticles were exposed to mechanochemical activation by high-energy ball milling for 0, 2, 4, 8 and 12 h, with and without graphene nanoparticles. The samples were subsequently characterized by Mössbauer spectroscopy, X-ray powder diffraction (XRPD), magnetic measurements and optical diffuse reflectance spectroscopy. Examination of the quadrupole doublet's abundance as function of ball milling time indicated that graphene slowed down the precipitation of the yttrium iron perovskite (yttrium orthoferrite) phase. The increased linewidth of the doublet showed that the carbon from graphene preferentially entered the lattice of the yttrium orthoferrite. The saturation magnetization decreased with decreasing particle size for prolonged milling due to the occurrence of the antiferromagnetic hematite phase. The enhanced absorption in the infrared region could be associated with the incorporation of carbon from graphene in the lattice of the yttrium orthoferrite. The results are interesting for sensing and microwave applications.

Transition in the preferred orientation of RF sputtered ZnO/Si thin films by thermal annealing: Structural, morphological, and optical characteristics

Polycrystalline ZnO thin films having preferential growth along the polar (002) plane are prepared on Si (111) substrate using RF sputtering technique. A gradual build-up of the semipolar (103) plane with the dominant (002) polar plane of 400–700 °C annealed films is revealed by the Glancing incidence angle X-Ray diffraction (GXRD) pattern. However, a change in preferred orientation from polar (002) to semi-polar (103) and non-polar (100) planes is observed with further annealing in the temperature range 800–1200 °C. This occurs due to an increase in the mobility of adatoms due to annealing which encourages diffusion across grain boundaries and competitive growth among planes leading to the growth of (103) plane. This structural deviation of film from (002) plane to (103) plane leads to the change in surface morphology of annealed films around 800 °C as depicted in Field Emission Scanning Electron Microscopy (FESEM) images. The observed changes in surface morphology of films are further correlated with the variation in optical band gap values and diffuse reflectance spectra. The low reflectance values of the samples represent their anti-reflection property which makes ZnO/Si thin film a conceivable applicant in optoelectronics.

OPTICAL PROPERTIES OF SOLID SOLUTIONS IN THE Ag7SiS5I‒Ag7GeS5I SYSTEM

The present paper is devoted to the comparison of the determination methods of the optical pseudogap values of Ag7(Si1-xGex)S5I (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) solid solutions by means of two optical spectroscopy techniques: optical absorption and diffuse reflection. Optical absorption spectroscopy was performed on thin single crystalline plane-parallel samples. Diffuse reflectance spectra were obtained on samples in the form of microcrystalline powders (~ 10–20 μm). The optical pseudogap values of Ag7(Si1-xGex)S5I solid solutions, determined on the basis of the logarithmic dependence of the absorption coefficient according to Urbach’s tail range and on the basis of the dependence of the Kubelka-Munk function using the Tauc method. It has been established that the pseudogap energies of the Ag7(Si1-xGex)S5I (x = 0; 0.2; 0.4; 0.6; 0.8; 1) solid solutions obtained by the analyzing the diffuse reflectance spectra of microcrystalline powders and spectral dependences of the absorption index for single crystals do not differ by more than 5% Therefore, it is correct to use a non-destructive and simple method of diffuse reflectance spectroscopy, to estimate the values of the optical pseudogap of porous, poly-, micro- and nanocrystalline objects.

Model-based characterization platform of fiber optic extended-wavelength diffuse reflectance spectroscopy for identification of neurovascular bundles.

.Significance: Fiber-optic extended-wavelength diffuse reflectance spectroscopy (EWDRS) using both visible/near-infrared and shortwave-infrared detectors enables improved detection of spectral absorbances arising from lipids, water, and collagen and has demonstrated promise in a variety of applications, including detection of nerves and neurovascular bundles (NVB). Development of future applications of EWDRS for nerve detection could benefit from the use of model-based analyses including Monte Carlo (MC) simulations and evaluation of agreement between model systems and empirical measurements.Aim: The aim of this work is to characterize agreement between EWDRS measurements and simulations and inform future applications of model-based studies of nerve-detecting applications.Approach: A model-based platform consisting of an ex vivo microsurgical nerve dissection model, unique two-layer optical phantoms, and MC model simulations of fiber-optic EWDRS spectroscopic measurements were used to characterize EWDRS and compare agreement across models. In addition, MC simulations of an EWDRS measurement scenario are performed to provide a representative example of future analyses.Results: EWDRS studies performed in the common chicken thigh femoral nerve microsurgical dissection model indicate similar spectral features for classification of NVB versus adjacent tissues as reported in porcine models and human subjects. A comparison of measurements from unique EWDRS issue mimicking optical phantoms and MC simulations indicates high agreement between the two in homogeneous and two-layer optical phantoms, as well as in dissected tissues. Finally, MC simulations of measurement over a simulated NVB indicate the potential of future applications for measurement of nerve plexus.Conclusions: Characterization of agreement between fiber-optic EWDRS measurements and MC simulations demonstrates strong agreement across a variety of tissues and optical phantoms, offering promise for further use to guide the continued development of EWDRS for translational applications.

Utilization of gold nanoparticles for the detection of squamous cell carcinoma of the tongue based on laser-induced fluorescence and diffuse reflectance characteristics: an in vitro study

Squamous cell carcinoma is a very common type of oral cancer that affects the health of people with an unacceptably high mortality rate attributed to the difficulties in detecting the disease at an early stage. Therefore, effective techniques for early diagnosis and effective therapy of oral cancer are necessary. In the present study, we exploit the ability of gold nanoparticles (AuNPs) to undergo coupled surface plasmon resonance when closely spaced to improve diagnosing squamous cell carcinoma of the tongue. The prepared AuNPs are characterized by UV–VIS spectroscopy, dynamic light scattering, Fourier transform infrared spectroscopy, and transmission electron microscopy. The size of the prepared AuNPs is 12 ± 2 nm with narrow size distributions and exhibited high stability with a zeta potential of − 16.5 mV. The light fluorescence of the normal and cancer cells is recorded before and after NP addition using a spectrometer upon excitation by 405-nm laser irradiation. Furthermore, the light reflectance of the examined samples is measured at different laser wavelengths (red to NIR region). The obtained results show that the cancer cells mixed with AuNPs produce a higher fluorescence peak at 489.2 nm than the cancer cells without AuNPs. Moreover, the optical diffuse reflectance analyses reveal that the addition of AuNPs enhances cancer detection especially at the 635-nm irradiation with sensitivity (94%), specificity (87%), and overall accuracy (91%).

Crystal structure, electronic structure, and optical properties of the novel Li4CdGe2S7, a wide-bandgap quaternary sulfide with a polar structure derived from lonsdaleite.

The novel quaternary thiogermanate Li4CdGe2S7 (tetralithium cadmium digermanium heptasulfide) was discovered from a solid-state reaction at 750 °C. Single-crystal X-ray diffraction data were collected and used to solve and refine the structure. Li4CdGe2S7 is a member of the small, but growing, class of I4-II-IV2-VI7 diamond-like materials. The compound adopts the Cu5Si2S7 structure type, which is a derivative of lonsdaleite. Crystallizing in the polar space group Cc, Li4CdGe2S7 contains 14 crystallographically unique ions, all residing on general positions. Like all diamond-like structures, the compound is built of corner-sharing tetrahedral units that create a relatively dense three-dimensional assembly. The title compound is the major phase of the reaction product, as evidenced by powder X-ray diffraction and optical diffuse reflectance spectroscopy. While the compound exhibits a second-harmonic generation (SHG) response comparable to that of the AgGaS2 (AGS) reference material in the IR region, its laser-induced damage threshold (LIDT) is over an order of magnitude greater than AGS for λ= 1.064 µm and τ= 30 ps. Bond valence sums, global instability index, minimum bounding ellipsoid (MBE) analysis, and electronic structure calculations using density functional theory (DFT) were used to further evaluate the crystal structure and electronic structure of the compound and provide a comparison with the analogous I2-II-IV-VI4 diamond-like compound Li2CdGeS4. Li4CdGe2S7 appears to be a better IR nonlinear optical (NLO) candidate than Li2CdGeS4 and one of the most promising contenders to date. The exceptional LIDT is likely due, at least in part, to the wider optical bandgap of ∼3.6 eV.

La4Ga2Se6O3: A Rare-Earth Oxyselenide Built from One-Dimensional Strips.

The oxyselenide La4Ga2Se6O3 was obtained by reaction in NaCl flux. Its monoclinic crystal structure (space group C2/c, a = 21.2832(13) Å, b = 11.6272(7) Å, c = 6.0006(4) Å, β = 106.3430(10)°, Z = 4), which is a new type, consists of strips of edge-sharing OLa4 tetrahedra and zigzag chains of corner-sharing GaSe4 tetrahedra. The separation into distinct structural blocks with mostly ionic vs covalent bonding character was supported by analysis of the electron localization function and crystal orbital bond index. An experimental band gap of 1.8 eV was extracted from optical diffuse reflectance spectra. First-principles calculations suggest that the thermoelectric power factor of this compound would be enhanced by n-doping.

Dispensability of the conventional Tauc’s plot for accurate bandgap determination from UV–vis optical diffuse reflectance data

The use of the conventional Tauc’s method to determine optical bandgap energy from UV–vis spectroscopy data has been overemphasized for decades. However, a misuse of the Tauc’s formula in conjunction with the Kubelka-Munk function to determine the bandgap energy of semiconductors from diffuse reflectance data can lead to erroneous estimates. Particularly, large errors can be associated with computing values of the ordinate using the Tauc-Kubelka-Munk relationship for diffuse reflectance to produce Tauc’s plot. To address this apparent error, the present paper shows that the reflectance versus photon energy plot for bandgap estimation from UV–vis diffuse reflectance data can be employed to obtain reasonable bandgap values, suggesting that the conventional Tauc’s plot is not indispensable for accurate bandgap energy determination. The relatively simple method can be best choice when a researcher is unsure about the accurate computations from diffuse reflectance data which lead to proper Tauc’s plot. Interestingly, the rarely reported (if at all it can be found elsewhere) step-by-step approach to Tauc’s plot from diffuse reflectance data is described in this paper to serve as a simple guide for emerging material scientists. The UV–vis optical diffuse reflectance data for β-Ga2O3, ZnO and TiO2 semiconductor films are used for this study.

Li2Mg2Si2S6 and Li2Mg2Ge2S6: Two nonlinear optical sulfides featuring a unique, polar trigonal structure incorporating ethane‐like anions

AbstractThe new compounds Li2Mg2Si2S6 and Li2Mg2Ge2S6 have been prepared via traditional high‐temperature, solid‐state synthesis. The title compounds crystallize in the polar, noncentrosymmetric, trigonal space group P31m (No. 157) and adopt a new structure type featuring staggered, ethane‐like (T2S6)6− units, where T=Si or Ge. These (T2S6)6− units are nestled within the holes of magnesium‐sulfide “layers” that are created through the edge‐sharing of MgS6 octahedra. The holes found in the lithium‐sulfide “layers”, created by LiS6 edge‐sharing octahedra, remain vacant, containing no (T2S6)6− anionic group. Through the face sharing of the respective MgS6 and LiS6 octahedra, the magnesium‐sulfide and lithium‐sulfide “layers” are stitched together resulting in an overall three‐dimensional structure. The optical bandgaps of Li2Mg2Si2S6 and Li2Mg2Ge2S6 are 3.24 and 3.18 eV, respectively, as estimated from optical diffuse reflectance UV‐Vis‐NIR spectroscopy. The compounds exhibit second harmonic generation responses of ∼0.24×KDP and ∼2.92×α‐quartz for Li2Mg2Si2S6 and ∼0.17×KDP and ∼2.08×α‐quartz for Li2Mg2Ge2S6, using a Nd:YAG laser at 1.064 μm. Electronic structure calculations were performed using density functional theory and the linearized augmented plane‐wave approach within the WIEN2k software package. Examination of the electronic band structures shows that these compounds are indirect bandgap semiconductors.

Two heterometallic silver-iodoplumbates with [Fe(phen)3]2+ complexes: Syntheses, structures, photocurrent responses and theoretical studies

Using in-situ formed [Fe(phen)3]2+ (phen = 1,10-phenanthroline) complexes as templates, we solvothermally obtained and structurally characterized two new heterometallic silver-iodoplumbate hybrids, namely, [Fe(phen)3]AgPb2I7 (1) and [Fe(phen)3]2Ag3Pb2I11 (2). Single crystal X-ray diffraction results show that 1 features the one-dimensional (1D) chain-like anions of [AgPb2I7]2−, while 2 contains the discrete [AgI2] dumbbells and 1D [Ag2Pb2I9]3− anions. Solid state optical diffuse reflectance analyses indicate that the title compounds have the semiconductor behaviors, with the narrow optical band gap values of 1.80 and 1.85 eV, respectively. Under the alternate visible light irradiation, compounds 1 and 2 display interesting photoelectric conversion properties, with the photocurrent densities of approximately 0.10 μA cm−2 for 1 and 0.22 μA cm−2 for 2. In addition, the thermal stabilities, Hirshfeld surface analyses and theoretical calculations of title compounds have also been discussed.

Poly(vinyl pyridine) and Its Quaternized Derivatives: Understanding Their Solvation and Solid State Properties

A series of N-methyl quaternized derivatives of poly(4-vinylpyridine) (PVP) were synthesized in high yields with different degrees of quaternization, obtained by varying the methyl iodide molar ratio and affording products with unexplored optical and solvation properties. The impact of quaternization on the physicochemical properties of the copolymers, and notably the solvation properties, was further studied. The structure of the synthesized polymers and the quaternization degrees were determined by infrared and nuclear magnetic spectroscopies, while their thermal characteristics were studied by differential scanning calorimetry and their thermal stability and degradation by thermogravimetric analysis (TG-DTA). Attention was given to their optical properties, where UV-Vis and diffuse reflectance spectroscopy (DRS) measurements were carried out. The optical band gap of the polymers was calculated and correlated with the degree of quaternization. The study was further orientated towards the solvation properties of the polymers in binary solvent mixtures that strongly depend on the degree of quaternization, enabling a better understanding of the key polymer (solute)-solvent interactions. The assessment of the underlying solvation phenomena was performed in a system of different ratios of DMSO/H2O and the solvatochromic indicator used was Reichardt’s dye. Solvent polarity parameters have a significant effect on the visible spectra of the nitrogen quaternization of PVP studied in this work and a detailed path towards this assessment is presented.

Optical imaging spectroscopy for rapid, primary screening of SARS-CoV-2: a proof of concept

Effective testing is essential to control the coronavirus disease 2019 (COVID-19) transmission. Here we report a-proof-of-concept study on hyperspectral image analysis in the visible and near-infrared range for primary screening at the point-of-care of SARS-CoV-2. We apply spectral feature descriptors, partial least square-discriminant analysis, and artificial intelligence to extract information from optical diffuse reflectance measurements from 5 µL fluid samples at pixel, droplet, and patient levels. We discern preparations of engineered lentiviral particles pseudotyped with the spike protein of the SARS-CoV-2 from those with the G protein of the vesicular stomatitis virus in saline solution and artificial saliva. We report a quantitative analysis of 72 samples of nasopharyngeal exudate in a range of SARS-CoV-2 viral loads, and a descriptive study of another 32 fresh human saliva samples. Sensitivity for classification of exudates was 100% with peak specificity of 87.5% for discernment from PCR-negative but symptomatic cases. Proposed technology is reagent-free, fast, and scalable, and could substantially reduce the number of molecular tests currently required for COVID-19 mass screening strategies even in resource-limited settings.

Toward Federated-Learning-Enabled Visible Light Communication in 6G Systems

Visible light communication (VLC) technology was introduced as a key enabler for the next generation of wireless networks, mainly thanks to its simple and low-cost implementation. However, several challenges prohibit the realization of the full potential of VLC, namely, limited modulation bandwidth, ambient light interference, optical diffuse reflection effects, devices' nonlinearity, and random receiver orientation. On the contrary, centralized machine learning (ML) techniques have demonstrated significant potential in handling different challenges related to wireless communication systems. Specifically, it has been shown that ML algorithms exhibit superior capabilities in handling complicated network tasks, such as channel equalization, estimation and modeling, resources allocation, and opportunistic spectrum access control, to name a few. Nevertheless, concerns pertaining to privacy and communication overhead when sharing raw data of the involved clients with a server constitute major bottlenecks in the implementation of centralized ML techniques. This has led to the emergence of a new distributed ML paradigm, namely federated learning (FL), which can reduce the cost associated with transferring raw data, and preserve privacy by training ML models locally and collaboratively at the clients' side. Hence, it becomes evident that integrating FL into VLC networks can provide ubiquitous and reliable implementation of VLC systems. With this motivation, this is the first in-depth review in the literature on the application of FL in VLC networks. To that end, besides the different architectures and related characteristics of FL, we provide a thorough overview on the main design aspects of FL-based VLC systems. Finally, we also highlight some potential future research directions of FL that are envisioned to substantially enhance the performance and robustness of VLC systems.