Optics

We have studied the extinction power flow for a dipole in a laser beam, and embedded in a dissipating medium. The power flows along the field lines of the Poynting vector. We have shown that near the particle, the field lines form closed loops, which start and end at the location of the dipole. A closed-form expression for these loops has been derived, and we have shown how the orientation direction of a loop is determined by the permittivities and permeabilities of the host medium and the particle. It is also shown that the spatial extent of these loops is determined by singularities in the flow pattern. It is shown that the extent of the loop structure near the dipole diminishes strongly when there is dissipation in the medium. This is due to the appearance of singularities very close to the particle, which are due to the damping. At greater distances, flow lines run off to the far field or they come in from the far field. Most flow lines change from incoming to outgoing, or vice versa, so they turn around somewhere in the flow field. Singularities, points where the Poynting vector vanishes, appear on the coordinate axes. At these points, field lines split. Off the axes, singularities appear as the centers of vortices. Near a vortex, energy swirls around the singular point. Field lines can come out of the center of a vortex or end there. Full article

We report a deep learning-assisted quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for trace water vapor detection in air. A 1392 nm butterfly-packaged DFB laser is wavelength-modulated at f₀/2, and the QEPAS signal is retrieved by second-harmonic (2f) lock-in demodulation using a commercial quartz tuning fork gas cell. After optimizing the modulation depth to 400 mV, a 1D U-Net denoising network trained with pseudo-clean supervision is applied to the measured 2f traces, yielding an SNR improvement of 2.05× (3.11 dB). Allan deviation analysis indicates a minimum detection limit (MDL) of ~2.21 ppm at an optimum averaging time of ~619 s, corresponding to an ~2.1× improvement compared with the raw output. These results demonstrate that neural-network-based post-processing can improve QEPAS water vapor sensing performance without modifying the optical hardware. Full article

Glare is a critical factor in the design of LED luminaires for street lighting, particularly in environments where pedestrians, cyclists and drivers coexist. Generally, glare assessments are performed for fixed geometries and a single observer, limiting their applicability to real urban environments. This study examines the effect of angular redistribution of the beam on glare and illuminance by introducing the relative angular parameter α into the photometric model and the UGR calculation. A generic LED luminaire is modelled using a cosine-type luminous intensity distribution raised to a power, and the emitting surface is also discretized to evaluate the luminance, solid angle and Guth position index at the patch level. This approach is applied to three distinct observer geometries—pedestrian, cyclist and driver—allowing direct comparison using a unified mathematical formulation. The results show that beam redistribution affects each observer differently, reducing glare for pedestrians while simultaneously increasing it for drivers, whereas cyclists show limited sensitivity to angular changes. Although relative illuminance and UGR show similar monotonic trends, their physical and perceptual interpretation is different. This paper presents a novel tool for the preliminary analysis of trade-offs between visual comfort and luminous efficiency in urban lighting design. Full article

Background: Keloids are fibroproliferative scars with a prominent vascular component, and pulsed dye laser (PDL) is an established treatment, but objective imaging biomarkers of response are lacking. Objective: To evaluate whether dynamic optical coherence tomography (D-OCT) can provide quantitative, depth-resolved monitoring of keloid vascular remodeling under PDL and to explore candidate metrics for hypothesis-generating assessment in future studies. Methods: We conducted a prospective single-case pilot, hypothesis-generating study of a thoracic keloid treated with three sessions of 595 nm PDL, acquiring D-OCT scans at baseline and approximately 30, 60, and 90 days over a standardized 4 × 4 mm region of interest at 0.15, 0.30, and 0.50 mm depths. Primary D-OCT metrics included vascular en-face area, vessel length density, junction density, and mean vessel caliber. Results: The superficial layer (0.15 mm) showed an almost complete collapse of vascular signal (area −88% vs. baseline), the intermediate layer at 0.30 mm exhibited a sustained ~39% reduction in vascular area with parallel decreases in length and caliber at stable branching, and the deep layer at 0.50 mm showed modest area changes with longer but thinner vessels. These depth-resolved changes were consistent with clinical improvement in Vancouver Scar Scale and POSAS scores. Conclusions: D-OCT yielded quantitative, clinically interpretable vascular metrics that align with the expected effects of PDL in this single patient. In this patient, the percentage reduction in vascular area at 0.30 mm by week 8 emerged as a candidate quantitative metric for response monitoring; thresholds in the order of ≥25% could be tested prospectively as hypothesis-generating cut-offs in future controlled and reliability-tested studies, but are not proposed here as validated clinical criteria. Full article

A composite fiber optic sensor based on a misaligned peanut-shaped structure and the single-mode fiber–multimode fiber–single-mode fiber (SMS) structure is proposed for simultaneous strain and temperature measurements. The misaligned peanut-shaped structure is formed by introducing a certain core-offset during fusion splicing. Through a simulation analysis of the sensor, the optical field distribution of the sensor structure under different offset amounts is obtained. The experimental results demonstrate that the sensor achieves a maximum strain sensitivity of −48.21 pm/µε with an offset of 35.61 µm under a strain range of 0–600 µε and a maximum temperature sensitivity of 124.29 pm/°C at a 24.35 µm offset with a temperature range of 35–95 °C. Meanwhile, the sensor with a 35.61 µm offset has two resonance peaks that are selected for simultaneous measurements, with strain sensitivities of −48.21 pm/µε and −47.04 pm/µε and temperature sensitivities of 75.71 pm/°C and 84.29 pm/°C, respectively. Therefore, the simultaneous measurement of the strain and temperature can be achieved through a matrix method, demonstrating that the sensor possesses a dual-parameter sensing capability for the strain and temperature. Full article

In this study, a double-lattice photonic crystal structure was designed to achieve deep ultraviolet lasing without the use of any Distributed Bragg Reflector (DBR), which is called a photonic-crystal surface-emitting laser (PCSEL). The plane wave expansion (PWE) method was used to study the influence of various structural parameters on the resonant wavelength. Utilizing the random forest algorithm, we determined that the importance of the lattice constant to the resonant wavelength is 95.24%. Furthermore, we realized the reverse design of double-lattice photonic crystals from the target wavelength to optimal structural parameters through a radial basis function (RBF) network algorithm. Comparative analysis of the extreme learning machine (ELM) and back propagation (BP) algorithms demonstrated that RBF-based performance was notably superior to the training outcomes of other algorithms. The mean absolute error (MAE) of the lattice constant of the test set in the training results was 0.7610 nm, root mean square error (RMSE) was $1.143\times {10}^{-3}$ nm, and mean absolute relative error (MARE) was $5.489\times {10}^{-3}$. We verified the reliability of the algorithm and designed 13 groups of photonic crystals with different epitaxial structures. The mean square error (MSE) was 0.6188 ${\mathrm{nm}}^2$ compared with that of the plane wave expansion method. This work demonstrates applicability across various wavebands and epitaxial structures in GaN-based devices, providing a novel approach for the rapid iteration of deep ultraviolet PCSELs. Full article

The growing prevalence of multidrug-resistant (MDR) Candida spp. necessitates the development of new antifungal strategies. Photodynamic therapy (PDT), already widely used in the treatment of various oral infections, is based on the synergistic interaction of three key elements: a photosensitizer capable of selectively binding to microbial cells, a light source with the appropriate wavelength, and the presence of molecular oxygen. This interaction results in the production of singlet oxygen and reactive oxygen species, responsible for the selective destruction of microorganisms. In recent years, numerous natural compounds have been explored as potential photosensitizers. Olive oil, a cornerstone of the Mediterranean diet, was recently recognized by the U.S. Food and Drug Administration as a medicinal substance thanks to its soothing, immunomodulatory, and antimicrobial properties, which have also been documented in regard to oral administration. Materials and Methods: The aim of this in vitro study was to evaluate the efficacy of activated olive oil as a novel photosensitizer in PDT against Candida species. Oral MDR clinical isolates of C. albicans, C. krusei, and C. glabrata were analyzed using the Kirby–Bauer method according to EUCAST protocols. Six different experimental conditions were considered for each strain: (i) 100 μL of extra-virgin olive oil (EVOO); (ii) 100 μL of EVOO pre-activated with 3% H₂O₂ (EVOO-H); (iii) 100 μL of EVOO irradiated for 5 min with polarized light (480–3400 nm, 25 W); (iv) 100 μL of EVOO-H subjected to the same polarized light; (v) 100 μL of EVOO irradiated for 5 min with a 660 nm diode laser (100 mW); and (vi) 100 μL of EVOO-H irradiated with the same laser. All plates were incubated at 37 °C for 48 h. Results: The results showed a variable response among the different Candida species. C. glabrata showed sensitivity to all experimental conditions, with a 50% increase in the diameter of the inhibition zone in the presence of polarized light. C. krusei showed no sensitivity under any of the conditions tested. C. albicans showed antifungal activity exclusively when EVOO-H was activated by light. In particular, activation of EVOO and EVOO-H with polarized light resulted in the largest inhibition zones. Conclusions: In conclusion, olive oil, both alone and pre-activated with hydrogen peroxide, can be considered an effective photosensitizer against drug-resistant Candida spp., especially when combined with polarized light. Full article

We investigate the structure of correlation singularities for the Laguerre–Gauss beam of order $n=0$ and $m=2$ in the transverse plane during the propagation of the beam in the beam-wander model. We explicitly derive analytical expressions for the cross-spectral density of the corresponding beam order and the analytic expressions representing the singular behavior. We also verify that the singular points disappear at certain z values and reappear at other z values as shown in the previous numerical study. We investigate the dependence of the absolute value of the complex degree of coherence $\mu$ on the parameter $\delta$ of the beam-wander model during the propagation of the Laguerre–Gauss beam in the corresponding order. The complex degree of coherence depends not only on $\delta$ but also on the relative positions of two transverse observation points ${\mathit{\rho }}_1$ and ${\mathit{\rho }}_2$, as well as on the propagation variable z for the fixed values of the beam waist and the wavelength of the Laguerre–Gauss beam. Experiments on $\mu$ can demonstrate the range of the applicability of the beam-wander model in the turbulent atmosphere. Finally, we examine the orbital angular momentum flux density of the beam and confirm that the general behaviors of the previous studies also hold for $m=2$. Full article

Organic optoelectronic devices receive appreciable attention due to their low cost, ecology, mechanical flexibility, band-gap engineering, brightness, and solution process ability over a broad area. In this study, we designed and studied organic light-emitting diodes (OLEDs) consisting of an assembly of natural dyes, extracted from noble fir leaves (evergreen) and blue hydrangea flowers mixed with poly-methyl methacrylate (PMMA) as light emitters. We experimentally demonstrate the effective conversion of blue light emitted by an inorganic laser/photodiode into longer-wavelength red and green tunable photoluminescence due to the excitation of natural dye–PMMA nanostructures. UV-visible absorption and photoluminescence spectroscopy, ellipsometry, and Fourier transform infrared methods, together with optical microscopy, were performed 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 a narrow full-width at half maximum (FWHM) of 23 nm in the noble fir dye–PMMA layer and a green emission with a FWHM of 45 nm in blue hydrangea dye–PMMA layer were observed. It was revealed that adding monolayer MoS₂ to the nanostructures can significantly enhance the photoluminescence of the natural dye due to a strong correlation between the emission bands of the inorganic–organic emitters and back mirror reflection of the excitation blue light from the monolayer. Based on the investigation of two natural dyes, we demonstrated viable pathways for scalable manufacturing of efficient hybrid OLEDs consisting of assembly of natural-dye polymers through low-cost, purely ecological, and convenient processes. Full article

We report the first observation of room-temperature phosphorescence (RTP) of quinine sulfate (QS) in poly (vinyl alcohol) (PVA) films. Steady-state and time-gated measurements were performed to characterize the phosphorescence spectra, anisotropies, and lifetimes to estimate the phosphorescence properties. The RTP response of organic emitters in polymer matrices is particularly sensitive to ambient humidity and oxygen levels. Hence, to assess the environmental stability of the system, QS-doped PVA films were cast from a single batch and divided into paired specimens, one of which was encapsulated with a pressure-sensitive laminate, while the other one was left non-laminated. Over 14 days under ambient laboratory conditions, the absorbance and fluorescence of both films remained unchanged, whereas the exhibited phosphorescence diverged significantly. The unlaminated film exhibited a progressive loss of afterglow intensity, a noticeable red shift in the phosphorescence spectrum, and a pronounced shortening of the phosphorescence lifetime, while the laminated film retained its initial RTP intensity, spectral profile, and lifetime throughout the entire experiment. Full article

This work presents a compact refractometric system based on In-Line Digital Holography (ILDH) for the non-invasive characterization of transparent media, encompassing both liquids and high-refractive-index optical glasses. The core of the system is a cost-effective, lensless setup in which a 532 nm laser source and a microscope objective generate a divergent spherical wavefront that illuminates a 10 μm aluminum particle. The resulting diffraction pattern, modulated by samples in the optical path, is recorded by a CMOS sensor. The refractive index of the sample is determined by numerically locating the axial position of the particle-reconstructed image, which directly corresponds to the optical path difference introduced by the test medium. The optimal reconstruction plane is objectively located using an autofocus algorithm based on the Kurtosis metric, which identifies the sharpest image. The system successfully characterizes media across a broad refractive index range from 1.33 to 1.78, yielding linear calibration curves for both liquid and solid samples. The instrument achieves an axial reconstruction resolution of 30 μm and a refractive index precision of ±0.01 RIU. This ILDH approach offers a highly portable, cost-effective, and non-contact solution for refractive index measurement, demonstrating significant potential for industrial quality control and high-throughput point-of-care applications. Full article

In this work, we synthesized a lead-free halide double perovskite, Cs₂CuSbCl₆, with high carrier mobility via a one-pot hot-injection method. When combined with a high-resistivity silicon wafer, it forms a Type-II heterojunction structure, and its modulation depth reaches 84% by adjusting the annealing temperature. It demonstrates promising modulation performance at 532 nm. Owing to its strong absorption in the ultraviolet region, Cs₂CuSbCl₆ shows potential for application in ultraviolet-controlled terahertz modulation. Full article

Multiscale optical imaging is expected to address the trade-off between field of view (FOV) and resolution in optical systems. To achieve high resolution imaging with a large FOV, this study employs a double-layer monocentric lens to design the front-stage objective lens and utilizes multiple relay lenses for the secondary system. The design results demonstrate that the RMS value of the image spot size across the full FOV is controlled within 2 μm, and the system’s optical modulation transfer function (MTF) across the full FOV approaches the diffraction limit. Specifically, the MTF values across the full FOV exceed 0.35 at the cutoff frequency of 250 lp/mm. The designed optical system features a simple structure and high imaging quality. When a larger number of secondary relay imaging systems are employed, it is capable of achieving a large FOV with high resolution imaging performance, as required by the optical system. Moreover, it holds significant application potential in wide-area, large-range imaging and related fields. Full article

Object identification (OI) is widely used in fields like autonomous driving, security, robotics, environmental monitoring, and medical diagnostics. OI using infrared (IR) images provides high visibility in low light for all-day operation compared to visible light. However, the low contrast often causes OI failure in complex scenes with similar target and background temperatures. Therefore, there is a stringent requirement to enhance IR image contrast for accurate OI, and it is ideal to develop a fully automatic process for identifying objects in IR images under any lighting condition, especially in photon-deficient conditions. Here, we demonstrate for the first time a highly accurate automatic IR OI process based on the combination of polarization IR imaging and artificial intelligence (AI) vision (Yolov7), which can quickly identify objects with a high discrimination confidence level (DCL, up to 0.96). In addition, we demonstrate that it is possible to achieve accurate IR OI in complex environments, such as photon-deficient, foggy conditions, and opaque-covered objects with a high DCL. Finally, through training the model, we can identify any object. In this paper, we use a UAV as an example to conduct experiments, further expanding the capabilities of this method. Therefore, our method enables broad OI applications with high all-day performance. Full article

Estimating the depth of a fluorescently labeled tumor is beneficial in tumor resection. In this study, we proposed a method for the three-dimensional position estimation of fluorescent tumors using Monte Carlo simulations. A limited proof-of-concept experiment was conducted, and the two-dimensional position of a tumor was estimated by calculating the centroid of the fluorescence distribution, which was obtained by using excitation light to scan the surface of the model. The depth of the tumor was estimated by fitting the analytical equation based on Beer’s law to the diffuse fluorescence profile on the surface of the model. In the estimation of the two-dimensional position, the distance between the embedded and estimated tumor coordinates was 0.71 mm. The estimated tumor depths of 2–6 mm closely matched the embedded depths, with an error rate of approximately 20%. In previous studies, depth estimation was limited to 1–5 mm using visible light, whereas for the simulation used in the present study, the use of longer wavelengths enabled estimation at slightly greater depths. Full article

Mn⁴⁺-activated fluoride phosphors possess outstanding luminescent properties, making them highly suitable for applications in lighting and display technologies. However, the synthesis of such phosphors generally requires the use of large amounts of highly toxic aqueous HF, leading to serious environmental pollution. To eliminate the use of hazardous HF solution, a low-temperature molten salt method employing NH₄HF₂ was developed to synthesize the narrow-band red emitter Cs₂GeF₆: Mn⁴⁺ phosphor. Following the reaction, the product was washed with a dilute H₂O₂ solution to remove residual NH₄HF₂ and other impurities. The phase purity and morphology were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively, and the luminescence properties were examined via photoluminescence (PL) spectroscopy. The obtained phosphors exhibit bright red emission characteristics of Mn⁴⁺ under blue-violet excitation. Among them, Cs₂GeF₆: 0.08 Mn⁴⁺ shows the highest emission intensity, with an internal quantum efficiency (IQE) of 78%. A white light-emitting diode (WLED) fabricated by combining this phosphor with a blue chip and commercial Y₃Al₅O₁₂: Ce³⁺ (YAG) phosphor achieved a high luminous efficacy (LE) of ~146 lm/W, a correlated color temperature (CCT) of ~4396 K, and a color rendering index (Ra) of ~83, alongside excellent operational color stability. Full article

With the rapid advancement of detection technologies, traditional static electromagnetic absorbers increasingly struggle to meet controllable stealth requirements across diverse dynamic environments. To achieve active and controllable modulation of electromagnetic reflection characteristics, this paper proposes a transparent reconfigurable metamaterial absorber based on an origami structure. By adjusting the folding angles of the indium tin oxide (ITO)-polyethylene terephthalate (PET) film, the structure achieves reversible deformation from the vertical state to the horizontal state. This enables continuous modulation of the reflectance from below −10 dB (absorbing state) to nearly 0 dB (reflecting state) within the 4–18.9 GHz frequency range, with a relative bandwidth exceeding 130% and excellent angular stability. The energy loss and current distribution under different states are analyzed, revealing the mechanisms behind broadband absorption and deep modulation. Experimental measurements of the fabricated metamaterial align well with simulation results. Leveraging its flexible structure, reversible modulation capability, and angular stability, this origami-inspired reconfigurable metamaterial demonstrates promising application potential in the fields of adaptive electromagnetic camouflage and stealth protection. Full article

Manipulating complex light fields through highly anisotropic scattering medium (HASM) remains a fundamental challenge due to the intricate underlying physics and broad application potential. We introduce a unified theoretical and experimental framework for generating and controlling arbitrarily polarized curved caustic beams using an extended polarization transfer matrix (EPTM) for the first time, enabling intuitive polarization transformation through HASM. The EPTM is experimentally measured via a four-step phase-shifting technique, and its submatrices are independently modulated with tailored caustic phase profiles. This strategy facilitates the creation of diverse high-resolution caustic beams, including Gaussian and vortex types with tunable energy distribution, polarization states, and vorticity. The achievement of polarization transformation through HASM by our approach offers versatile manipulation over optical field properties such as multiple high-resolution caustic beams, angular momentum flux, and polarization, paving the way for enhanced functionality in advanced optical systems. Full article

In order to further mitigate the channel non-uniformity at the junction between the input slab and the arrayed waveguide grating in traditional AWG structures, we design a highly flexible, structurally adaptive linear auxiliary waveguide. Through systematic parameter scanning utilizing the Particle Swarm Optimization (PSO) algorithm, an optimal set of geometric parameters for the auxiliary waveguide is identified. This optimization strategy achieves a significant reduction in loss non-uniformity by 0.5 dB relative to the conventional AWG configuration, culminating in a final non-uniformity of merely 0.253 dB. This improvement underscores the critical role of advanced structural tuning and algorithmic optimization in enhancing the performance of photonic integrated circuits, particularly in dense wavelength division multiplexing (DWDM) applications for next-generation communication systems such as radio-over-fiber (RoF) architecture-based 6G. The method can provide a scalable and efficient pathway toward high-uniformity, AWG designs without introducing additional fabrication complexity or incurring substantial costs. Full article