Single-mode–multimode–single-mode (SMS) fiber structures are widely used in optical engineering as compact and low-complexity platforms for refractive-index (RI), temperature, and strain sensing, where spectral features arise from multimode interference (MMI) within the multimode fiber (MMF) section. Despite their practical relevance, predictive modeling remains challenging for large-diameter MMF segments because the number of guided modes can be very large, making the choice of a sufficient modal basis unclear and often computationally prohibitive.

This work presents a numerical framework to quantify how MMF diameter and length govern modal excitation and the resulting transmission spectrum, while enabling a justified reduction in the modal space. For each geometric configuration, MMF eigenmodes and propagation constants are computed, and the propagated field is sampled on transverse planes along the device length. A power-normalized modal decomposition is then performed using overlap integrals between the propagated transverse fields (Ep, Hp) and the MMF eigenmode fields (Em, Hm). This yields complex modal coefficients am(z) and the modal power distribution Pm(z), which are used to rank modes by contribution and to determine a minimal subset that ensures spectral convergence.

The approach provides a physically grounded criterion to select the required number of modes and to assess the impact of higher-order modes on dominant spectral features. The transmission spectrum is interpreted as the accumulated differential phase among the excited modes and their recapture at the output single-mode fiber, offering a practical route toward accurate and computationally efficient simulation of SMS devices for optical sensing and related photonic engineering applications.

Geological structure models are seismic physical models constructed based on geological structures and formations according to specific similarity ratios. In laboratory settings, ultrasonic or laser-ultrasonic techniques are utilized to perform model imaging for field exploration activities. Experiments with seismic physical models have found extensive applications in petroleum and gas exploration. These applications include studying the fundamental laws of wave propagation and seismic response of typical geological structures. The ultrasonic signals transmitted within these complex physical models generally display weak intensities. Therefore, it is necessary to excite high-intensity broadband ultrasonic waves as sources and, at the same time, use high-performance ultrasonic sensors to collect the model echoes.

Traditional piezoelectric transducers have been extensively utilized in the ultrasonic detection and imaging of seismic physical models. In contrast, laser ultrasonic technology possesses the ability to excite ultrasonic fields on the surfaces of objects with various scales and morphologies. These ultrasonic fields possess remarkable characteristics, including a wide bandwidth, multiple modes, and high intensity. In addition, optical fiber ultrasonic sensors are micro-acoustic sensors that employ optical fibers as the sensitive detection elements. Relative to electromagnetic transducers, these sensors demonstrate a superior sensitivity, a broadband frequency response, a compact size, and resistance to electromagnetic interference.

This work mainly presents the sensing mechanism and research status of laser ultrasound transducers and optical fiber ultrasonic sensors developed by our group. In the context of ultrasonic imaging technology for geological structural models, comparative analyses have been carried out on the research progress of conventional electroacoustic transducers, novel fiber-optic ultrasonic sensors, and emerging laser ultrasonic technology, as well as the technological issues and challenges involved.

A group of vinyl copolymers containing electronically isolated phenoxazine as an electron-donating unit and either 2-phenylbenzimidazole or 2-(2-pyridyl)benzimidazole as electron-accepting units was synthesized via a multistep synthetic route. The corresponding vinyl monomers were polymerized by cationic polymerization in solution to obtain amorphous bipolar polymeric materials. The chemical structures and molecular characteristics of the synthesized copolymers were fully characterized by NMR spectroscopy, elemental analysis, and gel permeation chromatography. Thermal and morphological properties were investigated using thermogravimetric analysis and differential scanning calorimetry. All materials exhibited high thermal stability, with initial thermal degradation temperatures in the range of 300–320 °C. Furthermore, the polymers demonstrated very high glass transition temperatures between 151 and 160 °C, indicating the excellent morphological stability of amorphous films, with only a minor dependence on the nature of the aromatic acceptor chromophore. The synthesized copolymers were evaluated as host materials in phosphorescent organic light-emitting diodes (PhOLEDs), using bis(2-phenylpyridine)(acetylacetonato)iridium(III), [Ir(ppy)₂(acac)] as the emissive dopant. Devices based on the copolymer PPxPy1, incorporating phenoxazine and 2-(2-pyridyl)benzimidazole units, exhibited promising electroluminescent performance, characterized by a low turn-on voltage of 3.0 V, a maximum luminance exceeding 2000 cd/m², and a maximum current efficiency of approximately 13.4 cd/A. A similar device employing the copolymer PPxP3, containing phenoxazine and 2-phenylbenzimidazole chromophores, showed the best overall performance, achieving a low turn-on voltage of 3.0 V, a maximum brightness above 2800 cd/m², and a maximum current efficiency of about 13.0 cd/A. These results demonstrate the potential of phenoxazine–benzimidazole-based vinyl copolymers as thermally robust and efficient host materials for phosphorescent OLED applications.

Organic light-emitting diodes (OLEDs) have garnered significant attention due to advantages ranging from innovative product design possibilities to highly efficient and sustainable light sources, creating a demand for new high-performance materials. Addressing this need, new ethylcarbazole-based D-A-D host materials were synthesized using one- or two-step nucleophilic aromatic substitution routes and evaluated for their suitability in OLEDs. Materials had one (CzeCzS, CzeCzM) or two (eCz2S, eCz2M) ethylcarbazole units paired with either sulfonyl (CzeCzS, eCz2S) or methanone (CzeCzM, eCz2M) electron acceptors. Thermal and morphological analyses showed that all materials are highly resistant to degradation, with decomposition temperatures ranging from 393 to 436 °C, and they form stable amorphous films with high T_g values (89–112 °C). Photophysical measurements revealed high triplet state energies (2.71 – 2.90 eV), indicating that these compounds can serve as hosts for both green phosphorescent and yellow TADF emitters. Device studies highlighted that methanone-bridged hosts outperformed phenylsulfonyl analogues due to more balanced charge transport and improved exciton confinement. When applied as host materials to a green phosphorescent emitter, a maximum EQE of 16.5% was achieved. Incorporating these hosts into blended co-host architectures led to notable efficiency enhancements, with external quantum efficiency values surpassing 20%. The best-performing device, based on the eCz2M/B3PyMPM co-host system, achieved an EQE of 20.3%, a luminance efficiency of 72.7 cd/A, and a power efficiency of 95.1 lm/W. Also, for a yellow-emitting TADF OLED prototype, our co-host systems enabled the achievement of an EQE of 10.3%. This work demonstrates that methanone–ethylcarbazole D-A-D structures are strong candidates for next-generation efficient OLED host materials. This work provides fundamental knowledge for developing advanced host materials for both TADF and phosphorescent OLED technologies.

Acknowledgements.

We acknowledge support from the Research Council of Lithuania (grant No. -SLLT-25-2) and support from the Faculty of Chemistry and Chemical Technology of Kaunas University of Technology.

Exploration of the optical nonliniear response in different lanthanide emitters, including photon avalance (PA) luminescence and the emission saturation phenomenon has profound implications in plentiful frontier applications. In the past few years, our studies have reported several universal mechanisms to exhibit giant nonlinear responses in various avalanching emitters and generated record-breaking nonlinear responses up to the 60^th order in high-lying emitting levels for various emitters. By enabling full-spectrum PA luminescence ranging from the visible to near-infrared regions in various emitters, such as Tb³⁺, Eu³⁺, Dy³⁺, Nd³⁺,Tm³⁺, Er³⁺, Ho³⁺ and Yb³⁺, multi-color super-resolution imaging can be realized with a resolution of about 100 nm. Moreover, by further increasing the excitation intensity, the emissions become saturated. By replacing the excitation beam with a doughnut-shaped one, the emission saturation efffect tunes the inner dip of the detected PSF, enabled sub-40 nm super-resolution imaging in 200µm pig kidney slices. Studying these mechanisms opens up exciting avenues for new flexible and high-efficiency PA modulation in multilayer nanostructures, enabling the application of PA in more technologies, such as super-resolution imaging, lithography, and optical detection. In this report, we briefly introduced the mechanism and principles of nonlinear responses in upconversion nanoparticles and highlighted their novel applications in the field of super-resolution imaging.

We developed an efficient inverse-design approach using the Grey Wolf Optimizer (GWO) to synthesize two-dimensional photonic crystal (PC) structures with large photonic band-gap (PBG). Using this method, we identified a new optimal SiC–air PC configuration for transverse magnetic (TM) polarization, achieving a PBG with a Gap-to-Midgap Ratio (GMR) of 34.55%.

Photonic crystals are periodic dielectric structures composed of materials with contrasting refractive indices, enabling the formation of PBGs that prevent electromagnetic wave propagation. The inverse design of PCs is challenging due to the high number of degrees of freedom and the intrinsic complexity of Maxwell’s equations. The GWO is a metaheuristic based on grey wolves’ hunting behavior: pursuit, encirclement, and attack. It offers strong global search capabilities, robustness against local minima, and simpler operators compared to other swarm-based optimizers; therefore, we investigate its suitability for this task.

The python-GWO was first validated with standard benchmark functions and then coupled to a Matlab–FORTRAN Finite Element Method (FEM) solver. The PC unit cell, arranged in a triangular lattice, was discretized into a 10×10 grid (100 bits), each assigned either SiC (n = 2.6) or air. The FEM solver computed dispersion diagrams, and the GMR served as the optimization objective. The optimized TM structure achieved a GMR of 34.55%, and GWO convergence curves helped verify solution saturation by hyperparameter.

In conclusion, the GWO proved effective for fast inverse design of SiC–air PCs, yielding a crystal configuration with significantly improved PBG for the TM polarization.

Introduction: The use of diagnostic methods or devices that are increasingly efficient, accurate, and able to ensure prompt intervention to prevent the progression of aggressive pathologies, such as cancer, is now more fundamental than ever. In this context, to assess the sensing capacity in the presence of tumor cells with a biophotonic platform, based on lossy mode resonance fiber sensors coated with scaffolded nanomaterials and integrated with microfluidic chips, we have preliminarily electrospun three different nanofibrous substrates onto glass planar supports and evaluated their cytocompatibility with a cellular model of human osteosarcoma.

Methods: About 18.000 cells were seeded in complete growth medium onto uncoated or treated glass coverslips in 12-well plates. Cell adhesion, morphology and proliferation were observed after 24 and 48 hours by optical microscopy and several images were captured by using a 10× objective lens magnification after cell washing and medium renewal. A colorimetric cell viability assay was then performed at 48 hours by transferring each coverslip into new multi-well plates and by adding 5% Water-Soluble Tetrazolium 8 (WST-8) in complete medium for 2 hours at 37°C. Cell supernatants were transferred into 96-well plates to quantify the absorbance of formazan at 450 nm using a microplate reader. Formazan is produced via WST-8 reduction by mitochondrial dehydrogenases of viable cells and results in a yield directly proportional to the number of living cells.

Results: Except for a single polymer, all matrices remained unchanged after the incubation with cell culture medium for up to 48 hours. In all electrospun coverslips, cells had properly adhered to the substrates, and their morphology was comparable to the control cells seeded onto uncoated supports. The cells were metabolically active and the viable amount in all treated samples was comparable to relative controls.

Conclusions: These promising results lay the groundwork for the next phase of optimization of the final biophotonic sensing device integrated with microfluidics.

Erbium (Er³⁺)-doped fluoride microstructured fibers based on ZBLYAN glasses, which are recognized for low intrinsic loss in the mid-infrared (MIR) region and strong compatibility with rare-earth ion doping, are fabricated via the stack-and-draw method. This technique enables precise replication of preset air-hole configurations, effectively suppressing parasitic losses in the MIR band and creating a stable gain environment for 2.8 μm lasing, which originates from the ⁴I_11/2→⁴I_13/2 energy level transition of Er³⁺ ions. Using a 976 nm laser diode in an end-pumping configuration, stable 2.8 μm lasing is achieved from an 86 cm-long Er³⁺-doped fluoride microstructured fibers. The system delivers a maximum unsaturated output power of ~34 mW and a slope efficiency of only up to 4.2%-low performance mainly caused by ~4% Fresnel reflection at one fiber end, which provides insufficient photon feedback and induces significant cavity loss. Systematic studies explore the effects of key parameters. Overly short fibers lack sufficient gain accumulation, while overly long ones amplify scattering loss. An increased Er³⁺ concentration enhances gain generation and improves laser efficiency. By contrast, insufficient Er³⁺ concentration fails to provide adequate gain, restricting laser output performance. Resonator reflectivity with an appropriate value facilitates the efficient generation of 2.8 μm lasing. These results confirm Er³⁺-doped fluoride microstructured fibers as promising gain media for 2.8 μm lasers, supporting potential applications in biomedical spectroscopy and environmental gas sensing.

This work is conducted within the framework of the INFN QUANTEP (QUAntum Technologies Experimental Platform) experiment, an initiative aimed at developing a shared experimental platform for the advanced study of optical quantum technologies based on silicon photonics. The project pursues four primary strategic objectives: the design of silicon photonic circuits for quantum computing, the engineering of integrated single-photon sources, the development of integrated single-photon detectors operating at room temperature, and the application of quantum nanomaterials for the realization of integrated devices for polarization control. Specifically, the research presented herein focuses on two crucial aspects for the success of the experiment, adopting a dual-methodological approach. First, specific chip fabrication processes were optimized and completed using dual-beam instrumentation. Leveraging the expertise acquired in Focused Ion Beam (FIB) technology, platinum-assisted deposition was performed to create intra-chip electrical contacts. This technique proved instrumental in effectively operating on complex topologies, such as etched or non-planar areas, which are challenging to process using conventional fabrication techniques. Simultaneously, extensive use was made of FIB technology for the deposition and deterministic positioning of nanostructures on the chip, a key procedure to ensure experimental functionality and enable the modulation of the input optical signal polarization. To achieve this goal, an ad hoc experimental protocol was developed to synergistically integrate different techniques: Electron Beam Lithography (EBL) and subsequent drop-casting, combined with the use of nano-manipulators integrated within the FIB system. The employment of these tools enabled not only the cleaning of the substrate from unwanted residues resulting from the drop-casting process but also the controlled deposition of various nanowires in the vicinity of the waveguide with high spatial precision. This paper details the complete sequence of fabrication steps and the operational strategies implemented to achieve the deterministic positioning of the nanostructures.

Fluorescence and Raman spectroscopy provide complementary optical approaches for detailed analysis of eukaryotic cells, enabling simultaneous investigation of structural and biochemical properties. Fluorescence-based imaging techniques, including wide-field and confocal microscopy, allow high-resolution visualization of cellular morphology, subcellular organelles, and dynamic processes. Specific fluorescent probes and dyes facilitate selective labeling of biomolecules, enhancing the detection of proteins, nucleic acids, and lipids within live or fixed cells. Raman spectroscopy, in contrast, offers label-free chemical characterization by detecting vibrational signatures of molecular bonds, providing insights into cellular composition, metabolic states, and biomolecular interactions. The integration of fluorescence imaging and Raman spectroscopy enables multidimensional analysis, combining spatial, morphological, and molecular information. These optical methods have been applied in studies of cell differentiation, apoptosis, disease-related alterations, and drug response, highlighting their versatility in both basic and applied biological research. Key technical considerations, including signal-to-noise optimization, spectral resolution, and photobleaching effects, are critical for accurate data acquisition and interpretation. Advances in instrumentation, such as confocal Raman microscopy and multimodal platforms, are expanding the capabilities of optical cell analysis, allowing real-time monitoring and high-throughput assessment. Emerging trends include the development of novel fluorescent probes, enhancement of Raman sensitivity, and integration with machine learning for automated cellular characterization. Collectively, fluorescence and Raman spectroscopy represent powerful and complementary tools for quantitative, high-resolution investigation of eukaryotic cells, supporting ongoing research in cellular biology and biomedical applications.