Abstract: We review general properties of optical reflection spectra recorded from smooth solid surfaces from the infrared up to the X-ray spectral regions. Emphasis is placed on metal surfaces. Starting from a simple classical oscillator model treatment, general features of normal incidence reflection spectra are derived in a qualitative manner. This rather tutorial approach as relevant for an ideal metal surface is complemented by a broad elaboration of analytical features of realistic reflection spectra. Discussed topics include manageable dispersion formulas, the Kramers-Kronig method, oblique light incidence effects with emphasis on Azzam’s analytical relations between the Fresnel’s coefficients, as well as special spectroscopic configurations involving reflection measurements at grazing light incidence. Among the discussed grazing incidence techniques, emphasis is placed on Infrared Reflection Absorption Spectroscopy IRAS, the Berreman effect, as well as X-ray reflectometry XRR.

Abstract: Einstein’s general theory of relativity predicts that a difference in the gravitational potentials between two locations in a gravitational field would introduce a redshift or blueshift for a light ray travelling from one location to another. However, observations made in our solar system, such as the 21-cm radiation line from Taurus A near occultation by the Sun1, the Pioneer-6 spacecraft experiments2 and the solar limb effect3,4, clearly demonstrate that there exist anomalous redshifts which could not be explained by current theories and models. Here we show that beside the well-known Einstein gravitational redshift which has been verified by various observations, there exists an extra gravitational redshift which is caused by the change of photon’s momentum produced in a curved spacetime. We demonstrate that this type of gravitational redshift is just the super-gravity redshift as has been expected in literature. We derive a mathematical expression of the super-gravity redshift by using a theoretical model: a photon moving in a static Schwarzschild geometry, then apply it to calculate the centre-to-limb redshift variations across the solar disk. It is found that the redshift distribution predicted by the theoretical model agrees well with one of the observed profiles of the solar spectrum lines around 6300 , which demonstrates that the extra gravitational redshift should be the mechanism behind the solar limb effect and other observed anomalous redshifts.

Abstract: Over the past 18 months, we have performed hundreds of temperature characterizations of fiber Bragg gratings inscribed in different germanium-doped silica glass fibers. Under the experimental conditions, the main conclusions, are: the temperature dependence of the “temperature gauge factor” or the normalized temperature sensitivity, KT, was found to be quadratic in the −¬¬50–200 ºC range while it may be considered linear for the −20–100 ºC range; KT values at 20 ºC varies from 5.176 x 10-6 K-1, for a B/Ge co-doped fiber up to 6.724 x 10-6 K-1, for a highly Ge-doped fiber; KT does not depend on the hydrogen-loading process or the gratings coupling strength; KT is essentially independent on wavelength in the 1500–1600 nm range being its value accurately determined with a relative error ~ 0.2%; gratings produced by femtosecond-laser radiation and UV-laser radiation exhibit comparable values of KT. The previous achievements allows, by having knowledge of KT for a single grating, the accurate determination of the temperature dependence of the Bragg wave-length for any other grating inscribed in the same fiber; the presented methodology enables one to determine the “unknown” gratings temperature sensitivity, typically, with an error of 0.01 pm/ºC, being, therefore, very useful in research labs and computer simulations; thus, expressions for the temperature dependence of KT for gratings inscribed in several fibers are given. We have also fully analyzed the potential sources of error in KT determination.

Abstract: The moiré effect has been considered in various objects, such as coplanar layers, hollow shells, and volumetric three-dimensional objects (e.g., parallelepipeds, prisms, cylinders, and LED cubes). However, the moiré effect in refracting objects filled with a transparent substance (such as liquid or glass) has not yet been investigated. We performed a theoretical and experimental study of the moiré effect in rectangular and cylindrical containers with a refractive substance. The formulas for the magnification coefficient and the moiré period in rectangular and cylindrical refracting objects were obtained. Experiments confirm the theory. This study is essential for understanding the physical properties of the moiré effect with refraction. The results can be used to measure the level and refractive index.

Abstract: Plasmonic nanostructures support localized surface plasmon resonances (LSPRs) which exhibit intense light–matter interactions, producing unique optical features such as high near-field enhancements and sharp spectral signatures. Among these, plasmon hybridization (PH) and Fano resonance are two key phenomena that enable tunable spectral responses. In this study, we systematically investigate four representative nanostructures: a simple nanogap dimer (i-type structure), a dolmen structure, a heptamer nanodisk cluster, and a nanoshell particle. We utilize the discrete dipole approximation (DDA) to analyze these structures. The scattering, absorption, and extinction spectra, are calculated, and the near-field electric field vector distributions are visualized to distinguish between hybridized plasmon modes and Fano-type interference. These simulated results reveal that the observation of PH-like characteristics in a dolmen structure exhibiting typical Fano resonance under specific conditions, and conversely, the observation of Fano resonance-like characteristics in the well-known i-structure exhibiting PH, can be comprehended by considering the scattering spectrum and absorption spectrum separately, rather than the extinction spectrum, and by considering the electric field vector rather than the electric field intensity. Finally, the sensitivity of these nanostructures against the ambient medium was evaluated.

Abstract: Organic optoelectronic devices receive appreciable attention due to their low cost, ecology, mechanical flexibility, band-gap engineering, lightness, and solution process ability over a broad area. Here, we designed and studied an organic light-emitting diodes (OLEDs) consisting of assembly of natural dyes extracted from of noble fir leaves (evergreen) and blue hydrangea flowers and mixed with poly-methyl methacrylate (PMMA) as a light emitters. We experimentally demonstrate effective red and green photoluminescence due to the excitation natural dye-PMMA nanostructures by laser/ photodiode blue light. The UV-Visible absorption and photoluminescence spectroscopy, ellipsomentric and Fourier transforms infrared methods together with optical microscopy were achieved for confirming and characterizing the properties of light-emitting diodes based on natural dyes. We highlighted the optical and physical properties of two different natural dyes, and demonstrated how such characteristics can be exploited to make efficient LED devices. A strong pure red emission with narrow full-width at half maxima (FWHM) of 23 nm in the noble fir dye-PMMA layer and a green emission with FWHM of 45 nm in blue hydrangea dye-PMMA layer have been observed. It was revealed that adding the MoS2 monolayer to the nanostructure can significantly enhance the photoluminescence of natural dye due to strong correlation between emission bands of the inorganic-organic emitters and back mirror reflection of the excitation blue light from monolayer. Based on investigation of two natural dyes we demonstrated viable pathways for scalable manufacturing of the efficient OLEDs consisting of assembly of natural dyes -polymer through low-cost, pure ecological and human convenient processes.

Abstract: This article presents the stages of development of an NDIR (Non-dispersive infrared) open-type gas analyzer with a fundamentally new discrete IR radiation generation scheme for measuring the dynamics of water vapor and carbon dioxide concentrations with further application in the instrument base of an ecological and climatic station to implement the eddy covariance method. In addition, the process of selecting components of NDIR gas analyzers, as well as its calibration and conducting experiments in the field as part of an ecological and climatic station is described.

Abstract: Surface-enhanced Raman scattering (SERS) is a highly sensitive analytical technique capable of single-molecule detection, owing to its exceptional chemical specificity, sensitivity, and reproducibility. In this study, we developed a robust SERS platform based on long-range ordered bullseye plasmonic nano-gratings, fabricated via a combination of electron beam lithography and reactive ion etching. The resulting nanostructured arrays—comprising concentric bullseye patterns with tunable period and filling fraction—were uniformly coated with a thin gold film to support strong surface plasmon resonances, generating intense electromagnetic field enhancements across the substrate. Using this platform, we demonstrated quantitative detection of small molecules such as Rhodamine 6G at low concentrations, achieving enhancement factors on the order of 105. Interestingly, we found that the geometric configuration yielding the strongest local electric field did not correspond to the highest SERS enhancement, which we attribute to a mismatch between the field orientation and molecular polarizability alignment. This study provides insights for optimizing plasmonic substrates for sensitive molecular detection.

Abstract: Silica nanoparticles are among the most commonly used materials for functional films. In this work, we develop an effective medium theory for randomly distributed silica nanoparticles with unshelled, shelled, mixed, and hollow spherical structures by incorporating Mie solutions. The proposed effective medium theory is validated against full-wave simulations performed using the finite-element method in three dimensional cases. Our results provide a practical tool for analyzing the optical properties of randomly distributed SNPs with diverse structures, facilitating the design of functional films such as anti-glare coatings.

Abstract: A monolithic all-fiber high-energy chirped pulse amplification (CPA) system with a managed large dispersion is demonstrated. To lower the system’s nonlinearity, two temperature-tuning cascaded chirped fiber Bragg gratings (CFBGs) with a large dispersion of 200 ps/nm are used as the stretcher to stretch the pulse duration to more than 2 ns in the time domain. The main amplifier, a short-length silicate glass fiber with a large mode area and a high gain, increases the energy to 293 μJ at 100 kHz. To compress the large-dispersion chirped pulse into a compact structure, a reflective grating pair with a high density of 1740 lines/mm is used as the compressor. Owing to the high-order dispersion pre-compensation by the CFBGs and the large-sized grating with high diffraction efficiency, a compressed pulse duration of 466 fs with a pulse energy of 250 μJ is obtained, corresponding to a compression efficiency of more than 85%. The well-preserved beam quality with measured M2 value is better than 1.3. To best of our knowledge, this is the highest pulse energy ever achieved in a monolithic fiber femtosecond laser system.

Abstract: This tutorial article provides a detailed mathematical overview of the Optical Bloch equation describing the interaction of the two level atom with the Electromagnetic radi- ation field. The equations have been derived in the weak atom field coupling limit. The notion of the optical Bloch equations has been introduced in a systematic process starting from the interaction of the two level atom with the semiclassical radiation field which cannot completely the idea of the Spontaneous emission process later which was included theoretically by,Weigner-Weisskopf theory of spontaneous emission .In the later part of the article the optical Bloch equations has been derived exactly by treating the radiation field quantum mechanically. The radiation field has been quantized using the free field quantization techniques in the Coloumb Gauge with which the Lindblad Master equation has been derived systematically in the weak coupling limit. The optical Bloch equations has been derived for the resonantly driven two level atom from the Lindblad type master equation and later those equations has been solved in the steady state limit. The article also discuss the analytical aspects of describing the theory of atom matter interaction using full-fledged quantum mechanical approach.

Abstract: A new generation of fluorescent diamond nanoparticles synthesized from hydrocarbons at high pressure appear to be promising for the design of efficient single-photon diamond sources and nanometer-sized optical sensors. A characteristic feature of such nanodiamonds (ND) is the termination of their surface with hydrogen. This hydrogen induces the formation of free holes at the diamond surface, thereby affecting the charge state of nearby fluorescent centers. In this study, the effect of the H-terminated ND surface on negatively charged silicon-vacancy (SiV-) fluorescence as a function of the ND size was investigated. Diamond nanoparticles of various sizes in the 50-300 nm range were analyzed before and after H desorption from their surface. It was shown that a significant increase in SiV- fluorescence (>50%) upon hydrogen removal starts for particles smaller than 100 nm. Based on the measured dependence of the SiV fluorescence intensities on the ND sizes, the effective thickness of the diamond surface layer, within which charge neutralization of SiV- centers occurs under the hydrogen influence, was determined to be 6 nm.

Abstract: A novel optical fiber sensor, based on a composed-type Sagnac loop for gas pressure sensing, has been introduced and experimentally validated. This sensor consists of a centimeter-scale twin-hole and dual-core fiber (THDCF) sandwiched by two segments of polarization-maintaining fibers (PMFs) via splicing. Given that the pure quartz PMF is insensitive to the variations in gas pressure, it is unsuitable for the gas pressure sensing. To improve the sensitivity, a short piece of THDCF is added to the PMF-based Sagnac loop. Theoretical analysis has demonstrated that the presence of THDCF could significantly amplify the impact of air pressure on birefringence. Experimental results reveal that as the ambient gas pressure rises from 0-1.2 MPa, the interference spectrum exhibits an obvious red-shift with a high sensitivity of 8.381 nm/MPa. The sensor’s reliability has undergone repeated verification by increasing and decreasing the pressure. Attributed to its simple structure, easy fabrication, low cost and high sensitivity, the proposed sensor is particularly suited for development in harsh environments.

Abstract: Multiband solar cells offer a promising route to surpass the Shockley-Queisser limit by harnessing sub-bandgap photons through three active energy band transitions. However, realizing their full potential requires overcoming key challenges in material design and device architecture. Here, we propose a novel multiband stacked anti-parallel junction solar cell structure based on highly mismatched alloys (HMAs), in particular dilute GaAsN with ~1-4% N. An anti-parallel junction consists of two semiconductor junctions connected with opposite polarity, enabling bidirectional current control. The structures of the proposed devices are based on dilute GaAsN with anti-parallel junctions, which al-low the elimination of tunneling junctions—a critical yet complex component in conventional multijunction solar cells. Semiconductors with three active energy bands have demonstrated the unique properties of carrier transport through the stacked anti-parallel junctions via tunnel currents. By leveraging highly mismatched alloys with tailored electronic properties, our design enables bidirectional carrier generation through for-ward- and reverse-biased diodes in series, significantly enhancing photocurrent extraction. Through detailed SCAPS-1D simulations, we demonstrate that strategically placed blocking layers prevent carrier recombination at contacts while preserving the three regions of photon absorption in a single multiband semiconductor p/n junction. Remark-ably, our optimized five-stacked anti-parallel junctions structure achieves a maximum theoretical conversion efficiency of 70% under 100 suns illumination, rivaling the performance of state-of-the-art six-junctions III-V solar cells—but without the fabrication complexity of multijunction solar cells associated with tunnel junctions. This work establishes that highly mismatched alloys are a viable platform for high efficiency solar cells with simplified structures.

Abstract: We demonstrate that pulsed laser annealing induces the formation of plasmonic gold nanoparticles in ZnO thin films, with the process monitored in real-time through pulse-by-pulse spectroscopy. Gold nanoparticles (smaller than 5 nm) were initially embedded in sputtered ZnO films on fused silica substrates. ZnO was selected as the matrix material for its inherent antimicrobial properties, which complement the plas-monic sensing functionality. Using 532 nm pulses with energies of 20 mJ, and a diame-ter of 2 mm from a Q-switched Nd:YAG laser, we induced annealing while simultane-ously monitoring transmission spectra via in-situ broadband spectroscopy. A plas-monic resonance dip emerged after approximately 100 laser pulses in the 530–550 nm region, progressively deepening and broadening with continued exposure. The process stabilized after roughly 800 shots, producing larger nanoparticles (40–200 nm diame-ter) with a significant fraction protruding from the ZnO surface. SEM analysis con-firmed substantial particle coarsening. Theoretical modeling supports these observa-tions, correlating spectral evolution with particle size and embedding depth. The pro-truding gold nanoparticles can be functionalized to detect specific biomolecules, of-fering significant advantages for biosensing applications. This approach offers the po-tential to optimize more uniform nanoparticle distributions with pronounced plas-monic resonances while providing superior spatial selectivity and real-time process monitoring compared to conventional thermal annealing.

Abstract: High-resolution optical sensing typically relies on complex, high-finesse interferometers, limiting the scalability and cost-effectiveness of extreme precision metrology. We propose a simple, compact alternative: a metallic-boundary waveguide containing a single point dielectric impurity, operated near its cutoff frequency. This device achieves ultra-high spectral resolution by exploiting Fano resonance, arising from the quantum-optical interference between the waveguide's continuous modes and a quasi-bound state induced by the local impurity. For analytical modeling, we employ the Impurity D Function (IDF), an approach previously confined to quantum mechanical scattering, demonstrating its first application in an integrated optical system. Our analysis shows that the spectral resolution (R) scales powerfully with the geometry, specifically R~(e/w)^-12, where (e/w) is the impurity-to-waveguide ratio. This translates directly into an extremely sensitive strain gauge, with transmission linearity T=1/2+Ry near the 50% working point (y is the mechanical strain). We calculate that for a practical ratio of (e/w)~1%, the device yields a resolution of R~10^20, confirming its potential to measure mechanical strains smaller than 10^-21 using a fundamentally simple, integrated platform.

Abstract: Mie voids have been recently demonstrated as a promising nanophotonic platform for light manipulation and optical sensing. Moreover, the detection volumes of Mie void cavities exceed those of optical nanoantennas, making them appropriate for low-concentration single-molecule fluorescence biosensing. However, the fluorescence enhancement quantification of diffusing molecules in such optical antenna systems has not been addressed. Here, we explore the Mie void ability to enhance single-molecule fluorescence of diffusing fluorophores AF647 with the help of fluorescence correlation spectroscopy. The optimized structure confines 635 nm laser light within a well-defined excitation volume in the Mie void and numerically promotes the excitation gain. We monitor the reduction of the number of molecules, signifying the detection volume reduction in the Mie void and an increase in single-molecule brightness up to 2.8 times. However, we reveal that the observed fluorescence enhancement appears limited owing to the azimuthally symmetric emission direction away from the optical axis when the molecules diffuse in the vicinity of the Mie-void entrance. Altogether, this study demonstrates exploration of Mie-void-based nanoantenna potential for single-molecule fluorescence spectroscopy applications.

Abstract: Neodymium-doped bioactive wollastonite–tricalcium phosphate (W-TCP:Nd) coatings were fabricated by combining dip coating and laser floating zone (LFZ) techniques to in-vestigate the dependence of optical emission on polarization. Structural and spectroscopic analyses were performed on both longitudinal and transversal sections of the coating to study the effects of directional solidification on luminescence and vibrational behavior. Micro-Raman spectroscopy revealed that the coating exhibited sharp, well-defined peaks compared to the W-TCP:Nd glass, confirming its glass-ceramic nature. New Raman modes appeared in the longitudinal section, accompanied by red and blue shifts in some bands relative to the transversal section, suggesting the presence of anisotropic stress and orientation-dependent crystal growth. Optical emission measurements showed that while the 4F3/2→4I11/2 transition near 1060 nm was nearly polarization independent, the 4F3/2→4I9/2 transition around 870–900 nm exhibited strong polarization dependence with notable Stark splitting. The relative intensity and spectral position of the Stark components varied systematically with the rotation of the emission polarization. These findings demonstrate that directional solidification by using LFZ processing technique induces optical anisot-ropy in the coating, with potential applications for polarization-sensitive biophotonic and diagnostic purposes.

Abstract: Exploding or concentrating beams, vortex beams, and cylindrical vector beams, have a precisely shaped transversal amplitude profile such that they produce a continuously concentrating and intensifying focal spot upon focusing as the lens aperture is opened. This effect is the physical manifestation of the mathematical fact that Fresnel diffraction integral predicts an infinite intensity at the focus when the aperture effects are ignored. Here, using a full electromagnetic, nonparaxial focusing model, we show that the singularity in exploding cylindrical vector beams is an artifact of the paraxial approximation. Nevertheless, the exploding or concentrating effect, alien to any other light beam with finite power, keeps going on up to unit numerical aperture, equivalent to infinite aperture radius. This unique feature enables a dynamic control of the focal intensity and spot size down to the sub-wavelength scale using a single light beam, imitating similar control when focusing an ideal plane wave, but requiring a finite amount of power.

Abstract: The study presents a novel approach that integrates laser-induced breakdown spec-troscopy (LIBS) data with machine learning algorithms for the rapid evaluation of coal quality. The developed framework enables quantitative determination of three critical parameters: Ash Content (Aad), Carbon Content (Cd), Sulfur Content (Stad). The experimental implementation utilized an optimized dataset to construct and evaluate the predictive model. The LIBS prototype system enables spectral data acqui-sition under controlled experimental conditions. Data preprocessing is carried out by systematically removing background interference, substrate effects, and saturated signals using adaptive filtering techniques. Characteristic emission peaks correspond-ing to target elements are identified through multivariate analysis, and Partial Least Squares Regression (PLSR) serves as the core algorithm for quantitative analysis. Sys-tematic iterative optimization of multivariate preprocessing parameters and adaptive peak selection strategies yields substantial improvements in both predictive accuracy and computational efficiency, with determination coefficients (R² > 0.90) demonstrat-ed for all target analytes. This enhanced accuracy validates the viability of LIBS as a robust alternative to conventional analytical methods for coal composition analysis. The LIBS demonstrates substantial advantages in coal quality assessment, thereby en-hancing the overall efficiency of both coal extraction and quality evaluation processes.