Abstract: Carbon dots (CDs) have enormous potential in optical sensing applications because of their remarkable physicochemical properties. Benefiting from high specific surface area, rich active sites, bright photoluminescence, high photostability, and biocompatibility, CDs have been widely used as functional layers in optical fiber sensors, resulting in notable improvements in sensitivity, response speed, and environmental stability. This review describes recent advances in CD-integrated optical fiber sensors, with a focus on CD synthesis techniques and their integration with optical fibers for the sensing of diverse analytes, such as heavy metal ions, biomarkers, and dyes. CD-integrated fiber sensors exhibit significantly enhanced detection performance in terms of sensitivity, selectivity, repeatability, response time, and recovery time, compared to their non-CD counterparts. Finally, current challenges and future perspectives are discussed. This review aims to provide valuable insights for the design and development of novel CD-integrated optical fiber platforms for sensing chemically and biologically relevant analytes.

Abstract: The effect of confinement of fluorophores (Rhodamine 6G) in nano-cavities of porous 3D sculptured coatings made by glancing angle deposition (GLAD) was investigated by fluorescence-lifetime imaging microscopy (FLIM). Shortening of fluorescence/photoluminescence lifetime by ~ 10% was observed from the dye-permeated (in liquid) structure; however, there was no rotational hindrance of dye molecules. When dried, a strong rotational hindrance 89% was observed for the orientation along the ordinary optical axis (fast-axis), and the hindrance was smaller than 57% for the extraordinary direction (fast axis). Light intensity distribution inside the nano-structure with a form-birefringence was numerically modelled using plane wave illumination and a dipole source. Nanoscale localisation of light intensity due to dipole nature I ~ 1/radius^6 and boundary conditions for E-field allows efficient energy deposition inside the region of lower refractive index (nanogaps).

Abstract: This paper outlines a simple theoretical argument for the possibility of a quantum lin-ear Voigt effect at low temperatures in certain media in the optical regime. An unlikely starting point for the ensuing argument arises out of a long-established hydrodynamic Lorentz field-modified classical dispersion theory whose Voigt component of the opti-cal conductivity, when subjected to the Uncertainty Principle, results in a modified form in the quantum region. In contrast to its classical, second-order counterpart, this quantum Voigt conductivity is shown to have a (modular) linear field dependence

Abstract: Multiphoton endomicroscopy (MPEM) has recently become a key development in optical biomedical diagnostics, providing histologically relevant in vivo images that are eliminating both the need for tissue damage during biopsy sampling and the need for dye injections. Due to its ability to visualize structures at the epithelial, extracellular matrix, and subcellular levels, MPEM offers a promising diagnostic method for precancerous conditions and early forms of gastrointestinal (GI) cancer. The high specificity of multiphoton signals—the two-photon fluorescence response of endogenous fluorophores (NADH, FAD), the second-harmonic generation signal from collagen, and others—makes this method a promising alternative to both traditional histology and confocal endoscopy, enabling real-time assessment of metabolic status, intestinal epithelial cell status, and stromal remodeling. Despite the promising prospects of multiphoton microscopy, its practical implementation is progressing extremely slowly. The main factors here include the difficulty of delivering ultrashort pulses with high peak power, which is necessary for multiphoton excitation (MPE), and obtaining these pulses at the required wavelengths to activate the autofluorescence mechanism. One of the most promising solutions is the use of specialized multimode optical fibers that can both induce the beam self-cleaning (BSC), which allows for the formation of a stable beam profile close to the fundamental mode, and significantly broaden the optical spectrum, which can ultimately cover the entire region of interest. This review presents the biophysical foundations of multiphoton microscopy of GI tissue, existing endoscopic architectures for MPE, and analyzes the potential for using novel nonlinear effects in multimode optical fibers, such as the BSC effect and supercontinuum generation. It is concluded that the use of optical fibers in which the listed effects are realized in the tracts of multiphoton endomicroscopes can become a key step in the creation of a new generation of high-resolution instruments for the early detection of malignant neoplasms of the GI tract.

Abstract: Colloidal PbSe quantum dots are promising candidates as saturable absorbers for ultrafast fiber lasers, but their performance is often limited by surface-related defects and chemical instability, leading to aggregation under optical pumping. In this study, we present a freestanding PbSe/PbS quantum dot-polystyrene composite saturable absorber film, with PbS overcoating on PbSe to enhance surface passivation and oxidation resistance. The composite exhibits a saturation intensity of 5.76 kW·cm-2, a modulation depth of 33%, and an optical damage threshold of 13.6 mJ·cm-2. When integrated into a bidirectionally pumped erbium-doped fiber laser in the anomalous-dispersion regime, the device demonstrates self-starting soliton mode locking at an ultralow pump threshold of 6 mW, generating 1.06 ps pulses with a radio-frequency signal-to-noise ratio of approximately 65 dB. The spectra remain stable over a six-month period, showing excellent environmental and operational durability. These findings confirm that PbSe/PbS quantum dots in a polymer matrix offer a robust, low-threshold saturable absorber platform for ultrafast fiber lasers.

Abstract: Optical precision measurement is fundamental to space technology and physics. For over a century, the “ray-of-light” paradigm and the Equivalence Principle have underpinned both theoretical and applied optics. However, recent theoretical and experimental work demonstrates that these paradigms are fundamentally flawed when applied to photon-level phenomena. This manuscript synthesizes a trilogy of research—spanning theoretical falsification, experimental confirmation, and practical application—to show that photons do not inherit the velocity vector of their source, and that the Equivalence Principle does not hold for photon propagation. We introduce the Real Velocity Measuring Device (RVMD), a novel instrument enabling direct measurement of real velocity vectors in real space. The potential implications for spacecraft navigation and metrology (including our planet) are profound, necessitating a paradigm shift in optical science.

Abstract: Quantum Encryption in Phase Space (QEPS) is a physical-layer encryption framework that harnesses the quantum-mechanical properties of coherent states to secure optical communications against both classical and quantum computational threats. By applying randomized phase shifts, displacements, or their dynamic combinations—implemented as unitary transformations in phase space—QEPS disrupts the phase reference essential for coherent detection, establishing a phase synchronization barrier. This review synthesizes the theoretical foundations, security mechanisms, and experimental progress of the QEPS framework, encompassing its three principal variants: the round-trip Quantum Public Key Envelope (QPKE) protocol—a public-key-like scheme built upon phase randomization (QEPS-p), the symmetric phase-only QEPS-p, and the displacement-based QEPS-d. Experimental validations demonstrate that authorized users achieve bit-error rates (BER) below the forward-error-correction threshold, whereas eavesdroppers are confined to BER near 50%, equivalent to random guessing—all while utilizing standard coherent optical transceivers at data rates up to 200 Gb/s over 80 km of fiber. We further examine QEPS’s robustness to channel impairments, its seamless compatibility with existing digital signal processing (DSP) pipelines, and its distinctive position within the post-quantum cryptography landscape. Finally, we outline key challenges and future research directions toward deploying QEPS as a practical, quantum-resistant security layer for next-generation optical networks.

Abstract: We present a high-dimensional quantum key distribution protocol by using N-qudits quantum light states, that is, product states with N photons, each of them in a quantum superposition of dimension d which provides a high dimension d^N and accordingly a very high security. We present the implementation of this protocol in different types of optical fibers where the mentioned states undergo perturbations under propagation in optical fibers; such perturbations can be notably reduced in a passive (autocompensation) or active way and importantly the N-qubits present a great robustness against such optical perturbations. Likewise, quantum states also undergo attenuation, that is, some photons are lost under propagation in the optical fibers and then effective N′ (< N)-qudits are obtained which also are used to generate secret keys. In fact, the detection of states combines standard projective measurements along with photon coincidences. Besides, we analyze the security of this high-dimensional protocol under an intercept and resend attack realized by Eve, and the resulting secure key rates are calculated showing a significative increasing with the dimension provided by the number N of photons.

Abstract: Optical amplification and spatial multiplexing technologies have important applications in quantum communication, quantum networks, and optical information processing. In this paper, based on the non-reciprocal amplification of a pair of co-propagating conjugate four-wave mixing (FWM) signals induced by a one-way pump field in a double-Λ-type hot atomic system, we demonstrate a spatially multiplexed multiple FWM processes by introducing a counter-propagating collinear pump field. This configuration enables simultaneous amplification of bidirectional four-channel FWM signals. Furthermore, when the injected signal and pump beams are modulated to Laguerre-Gaussian beams carrying different optical orbital angular momentum (OAM), the OAM of the pump beam is transferred to each amplified field. Through the tilted lens method, we experimentally demonstrate that the OAM of the amplified signal light remains identical to that of the original injected signal light. In contrast, the OAM of the other three newly generated FWM fields are governed by the angular momentum conservation law of their respective FWM processes, which enables the precise manipulation of the OAM for the other generated amplified fields. Theoretical analysis of the dynamical transport equation for the density operator in light-matter interaction processes fully corroborates the experimental results. These findings establish a robust framework for developing OAM-compatible optical non-reciprocal devices based on complex structured light.

Abstract: This work presents an ultra-compact three-way power splitter designed for photonic integrated circuits using topology optimization driven by a custom-developed genetic algorithm. The proposed approach enables global shape reconfiguration within a confined footprint of only 1.88 λ² (λ = 1550 nm), while maintaining high transmission uniformity and minimal mode mismatch. Nearly equal power splitting is achieved with output arms separated by approximately 90°. After gradient-based refinement, the splitter reaches a total transmission efficiency of 90.6%, with only 3.75% reflection and 5.65% radiation losses. This paper constitutes the first reported demonstration of sharp angle three-way power splitting within a sub-2 λ² footprint in a low index contrast (εᵣ ≈ 4.0) platform (such as Si₃N₄-on-SiO₂) through a single jointly optimized junction region. A minimum feature size of 125 nm ensures full compatibility with standard lithography and current fabrication techniques. This approach therefore offers a robust and fabrication-friendly solution for next generation high density power-divider systems.

Abstract: In freeform optical metrology, wavefront fitting over non-circular apertures is hindered by the loss of Zernike polynomial orthogonality and severe sampling grid distortion inherent in standard conformal mappings. To address the resulting numerical instability and fitting bias, we propose a unified framework curve shortening flow (CSF)-guided progressive quasi-conformal mapping (CSF-QCM), which integrates geometric boundary evolution with topology-aware parameterization. CSF-QCM first smooths complex boundaries via curve-shortening flow, then solves a sparse Laplacian system for harmonic interior coordinates, thereby establishing a stable diffeomorphism between physical and canonical domains. For doubly connected apertures, it preserves topology by computing the conformal modulus via Dirichlet energy minimization and simultaneously mapping both boundaries. Benchmarked against state-of-the-art methods (e.g., Fornberg, Schwarz-Christoffel and Ricci flow) on representative irregular apertures, CSF-QCM suppresses area distortion and restores discrete orthogonality of the Zernike basis, reducing the Gram matrix condition number from >900 to < 8. This enables high-precision reconstruction with RMS residuals as low as $3\times10^{-3}\lambda$ and up to 92\% lower fitting errors than baselines. The framework provides a unified, computationally efficient, and numerically stable solution for wavefront reconstruction in complex off-axis and freeform optical systems.

Abstract:

This work introduces a symmetrical smart-pixel-based bidirectional optical convolutional neural network (Sym-SPBOCNN) that unifies forward and backward propagation within a single geometrically symmetric free-space architecture. By pairing two identical SPOCNN modules side by side, the system allows both propagation directions to share the same lens arrangement, pixel geometry, and imaging distances, thereby eliminating the asymmetric optical paths and complex alignments required in earlier SPBOCNN designs. Each smart-pixel light modulator (SPLM) integrates a photodetector, electronic processor, memory, and light emitter, enabling nanosecond-scale weight updates and high-bandwidth optical modulation. Optical analysis confirms that the symmetric configuration preserves high-definition imaging while supporting kernel sizes up to 46×46 under typical lens-array constraints. Performance estimates show that a single 4K-resolution SPLM layer achieves over 4.1×10¹⁴ MAC/s, and a pipelined ten-layer configuration exceeds 4.1×10¹⁵ MAC/s without additional overhead. The architecture also inherits the reciprocity of SPBOCNNs, enabling efficient optical inference while substantially reducing the complexity of optical design, fabrication, and alignment. These characteristics establish the Sym-SPBOCNN as a compact and scalable bidirectional optical processor suitable for future hybrid electro-optical AI hardware.

Abstract: Surface plasmon resonance (SPR) sensors based on nanostructured metasurfaces offer enhanced sensitivity through engineered electromagnetic responses. In this study, we present an analytical–numerical investigation of the plasmonic behavior of gold nanopillar (Au-NP) and nanohole (Au-NH) arrays under both p- and s-polarized illumination, employing the Effective Medium Theory (EMT) in combination with the Transfer Matrix Method (TMM). This framework provides a consistent and computationally efficient description of the macroscopic optical response of multilayer plasmonic systems. For p-polarization, the nanostructure geometry strongly modulates the real and imaginary parts of the effective permittivity, with nanoholes supporting stronger SPR coupling and reduced optical losses compared to nanopillars. Under s-polarization, the effective permittivity remains largely invariant, driven mainly by filling fraction. The analysis reveals that polarization-dependent effects arise from variations in boundary-condition coupling rather than distinct localized resonances, aligning with classical plasmonic theory. Benchmarking against analytical dispersion relations and published experimental data for Au/BK7 systems shows close agreement within ±2°, confirming the physical consistency of EMT–TMM predictions. No full-wave simulations or experiments are presented; all results derive from analytical-numerical modeling. Rather than proposing new excitation mechanisms, this study provides a validated theoretical framework for understanding how polarization and nanostructural filling fraction jointly modulate SPR coupling in thin-film metasurfaces. The results offer a foundation for rational design and optimization of plasmonic coatings and SPR sensors with tunable surface sensitivity.

Abstract: We simulate the propagation of a W states through an optical fiber in the presence of mode coupling. We illustrate the propagating quantum state graphically on a group of higher order Poincaré spheres. At the fiber output we show how to recover the input quantum state using the simulated quantum state information displayed on the multiple spheres. The geometry of these states is an SU(N) quantum geometry. Applications include higher dimensional quantum communications, quantum cryptography, and quantum networks, and longer-term quantum optical computing.

Abstract: The moiré effect is a physical phenomenon in periodic (or nearly periodic) structures. A straightforward approach does not enable us to fully understand this complex phenomenon and describe it in all its details. Therefore, modeling of the effect is often necessary. The combined simulation incorporates both physical and computer simulations. Computer tools for simulating the moiré effect in parallel layers and volumetric displays are presented, along with methods for replacing original microscopic objects with their macroscopic equivalents, thereby facilitating the development of a physical model. (It resembles an aerodynamic model of an aircraft or vehicle.) The combined simulation was made for 3D displays, cylindrical structures (single- or double-layered nanoparticles), and volumetric 3D structures. The results can be applied to nanoparticles, crystallography, and the improvement of the visual quality of 3D displays.

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.