Search Results (1,007)

Spaceborne spectrometers are key optical payloads for global and regional greenhouse gas (GHGs) monitoring. With the increasing demands for high-precision and high-efficiency monitoring, spectrometers are required to provide a wide swath, high spatial resolution, and high spectral resolution. However, existing spaceborne grating spectrometers still face a trade-off between swath width and spatial resolution. To address this issue, this paper presents the optical design and fabrication of an immersed-grating spectrometer for GHG monitoring. The proposed spectrometer achieves a swath width of 100 km and a spatial resolution of 3 km × 3 km while providing high spectral resolution. It operates in four channels centered at 0.76, 1.61, 2.06, and 2.30 μm, covering the O₂-A band and the main absorption bands of CO₂ and CH₄, with corresponding spectral resolutions of 0.04, 0.07, 0.09, and 0.10 nm, respectively. The four channels share a common slit, which reduces system volume and inter-channel spatial registration errors. Immersed gratings are used as the core dispersive elements, enabling high spectral resolution in a compact optical configuration. To correct the smile and anamorphic beam compression induced by high-angular-dispersion immersed gratings, a prism-based simultaneous correction method is proposed. Based on this method, the initial parameters of the dispersion module are determined, and the optical design of the spectrometer is completed. Large-sized immersed gratings with high groove density are precisely fabricated using holographic lithography and ion-beam etching, after which the spectrometer is aligned and tested. The test MTF at the Nyquist frequency of the spatial dimension exceeds 0.72, indicating good imaging quality. The test spectral resolution of the four channels is all better than the design value, and the maximum smile and trapezoidal distortion are both within one pixel. This spectrometer provides an effective technical solution for achieving wide-swath, high-spatial-resolution, and high-spectral-resolution GHG monitoring under constraints imposed by detector size, signal-to-noise ratio, and payload size and mass. Full article

Ultraviolet remote sensing systems generally encounter the technical limitation of insufficient effective signal energy. Optical systems featuring a lightweight and compact layout are emerging as the mainstream research and development trend in this field. Varied-line-space (VLS) gratings can simultaneously achieve effective aberration correction and beam focusing in ultraviolet spectral imaging systems, enabling fewer system components and a simplified optical layout. On this basis, the modulation mechanisms of the groove distribution of planar VLS gratings for aberration correction and dispersion manipulation in the non-vacuum ultraviolet (non-VUV) band are thoroughly investigated. We elaborate on the theories concerning the line density function of VLS gratings based on phase distribution, and implement global optimization for the parameters of holographic gratings. The overall optical performance of the grating system is evaluated via ray tracing, which verifies the capability of VLS gratings to improve spectral resolution. We further perform optical design, device fabrication, and experimental validation using VLS gratings with a central groove density of 300 lp/mm. A spectral resolution of 0.345 nm is finally realized at the central wavelength of 300 nm. This work not only enriches the fundamental theories of VLS grating systems but also demonstrates that VLS gratings can significantly boost the aberration correction performance of ultraviolet spectrometers while adopting only a small number of optical elements. The theoretical conclusions are validated by measured data, and a complementary research framework integrating theoretical analysis and experimental testing is established. This study offers novel design ideas for hyperspectral and high-spatial-resolution spectral imaging systems. Full article

Metasurfaces enable versatile wavefront control, but passive designs struggle with multichannel dynamic switching, while active approaches often introduce complexity and require external power. Here, we propose a rotation-driven multifunctional metasurface holographic encryption scheme based on a cascaded architecture of two single-layer dielectric metasurfaces. By mechanically rotating one metasurface relative to the other, dynamic switching of holographic images across multiple predefined focal planes is achieved without any external energy supply. The encryption information is encoded into a multidimensional key space, defined by three independently controllable physical dimensions: rotation angle (4 states), incident polarization (3 states), and imaging distance (3 states), offering up to 36 theoretical key combinations. These parameters constitute distinct and independently controllable dimensions within the key space, substantially enhancing resistance to unauthorized access. As a proof-of-concept demonstration, full-wave simulations confirm faithful reconstruction of four independent images under four representative key combinations at a fixed operating frequency. This passive, mechanically reconfigurable approach offers a practical and secure pathway for three-dimensional dynamic displays and holographic encryption, with obvious advantages in simplicity, cost, and integrability over active tuning methods. Full article

This paper focuses on the significant potential of specific optical non-invasive methods, such as digital holographic microscopy, digital speckle pattern interferometry, and digital holographic interferometry, as scientific and technological tools for retrieving physical and biomechanical parameters embedded in the optical phase of laser-illuminated biomedical samples. These techniques take advantage of the laser speckle phenomena observed when non-specular surfaces are illuminated, enabling whole-field measurements and reconstruction of 3D images. Their versatility in implementation and application has led to advances in various fields of research and has broadened our understanding in both the basic and applied sciences. In clinical environments, the aforementioned quantitative optical studies are particularly valuable for understanding the behavior of biological samples, as they allow precise characterization of deformations, displacements, stress, strain, refractive index, and morphological features. Applications presented span from soft to hard tissues at both micro- and macro-scales, with results obtained from vocal cords, skin tissues, melanoma cells, and teeth. Furthermore, this overview provides a general perspective of some current speckle-based approaches and their growing relevance in biomedical research. Full article

Since the invention of lasers, coherent imaging has been widely employed in digital holographic microscopy. Improving the resolution of the image field remains a key challenge for achieving high-precision measurements. However, due to the high coherence of the laser, the resolution of the wavefront at the image plane depends not only on the radius of curvature of the illumination wavefront, but also on the observation position and direction. Existing theoretical approaches, which provide only approximate calculations of the amplitude distribution of the image field, are insufficient for practical applications. In this study, a theoretical framework for calculating the complex wavefield at the image plane is established, and analytical expressions describing the spectral distribution as functions of observation position and direction are derived. The proposed theory is experimentally validated using digital holographic microscopy. The results show good agreement between theory and experiment, demonstrating that the proposed approach accurately characterizes the spectral and resolution variations in the image field. These findings provide a solid theoretical foundation for the optimal design of digital holographic microscopy systems and illumination wavefields. Full article

Based on the Rényi entropy, Rényi holographic dark energy has been proposed to explain the current accelerated expansion of the universe. In this paper, we analyze holographic inflation and slow-roll inflation within the framework of Rényi holographic dark energy (RHDE) using ACT DR6. Our results show that holographic inflation is ruled out by the data, while slow-roll inflation with power-law potentials for $n={\displaystyle \frac{1}{2}}$ and $n={\displaystyle \frac{1}{3}}$ is viable for a suitable choice of N and C. We also analyze the inflationary attractor and confirm its existence. In addition, we compute the primordial power spectrum and find it falls well within the observational bounds. Thus, slow-roll inflation is favored in RHDE, but holographic inflation is not. Full article

Robotic assistive devices, such as exoskeletons, are increasingly employed in walking rehabilitation. Therefore, the measurement of both movement kinematics and cognitive workload is important to understand this human–robot interaction in real-world contexts. To address this need this study presents the validation of a framework integrating inertial motion capture (Xsens) and eye-tracking sensor (Pupil Neon) within a Mixed Reality (Meta Quest 3) architecture. We developed an overground dual-task paradigm in which holographic numbers appear in the user’s peripheral vision. This setup actively stimulates visuospatial attention while quantifying kinematic and cognitive output. To validate the framework, the protocol has been tested on 30 healthy subjects across repeated exoskeleton training sessions. Statistical analyses revealed that the Coefficient of Multiple Correlation (CMC) and Spectral Arc Length (SPARC), calculated on the shank angular velocity, together with the Step Length Variability, exhibited significant time effects (p < 0.01), mapping the transition toward automated gait. Concurrently, pupillometric data demonstrated a measurable reduction in neurocognitive demand; specifically, the Task-Evoked Pupillary Response (TEPR) decreased significantly across progressive training sessions (p < 0.05). With this work, we validated a measurement protocol that aims to provide a novel methodology for objectively evaluating motor and cognitive adaptation in wearable assistive devices. Full article

The Gibbs Paradox (concerning the entropy of mixing and entropic extensivity) was explored in depth by Edwin Jaynes (1992). We take up Jaynes’ treatment, considering the special cases for which entropy is (approximately) extensive, and the general case in which it is not. We also explore the Holographic Principle which (strictly speaking) excludes the extensivity of entropy. The formalism of Quantitative Geometrical Thermodynamics shows that, being isomorphic to energy, it is entropy production (not entropy) that is extensive. As a corollary, Shannon information is also not extensive, although information production is extensive. Full article

We investigatewhether a finite local information capacity of spacetime can account for the gravitational phenomena commonly attributed to cold dark matter. Starting from a covariant effective-field-theory description, we modelcoarse-grained entropy deposition as a dynamical scalar field $S\left(x\right)$ whose stress–energy tensor contributes to structure formation. The macroscopic action contains a single dimensionless coupling λ multiplying the canonical kinetic term, ensuring ghost-free dynamics and conservation of the associated stress–energy tensor. In a slow-roll regime, defined by a covariant source term $\Gamma \equiv \ddot{S}+3H\dot{S}=0$, where H is the Hubble parameter and overdot denotes derivative with respect to cosmic time, and $|\ddot{S}|\ll H|\dot{S}|$, the entropy sector behaves as pressureless dust at background and in linear order. Implemented in a modified Cosmic Linear Anisotropy Solving System (CLASS) Boltzmann solver, the entropy component fits Planck satellite 2018 cosmic microwave background (CMB) data, baryon acoustic oscillation (BAO) measurements, and the Pantheon + Type Ia supernova sample for $0.5\lesssim \lambda \lesssim 2$, while preserving the linear growth factor to within 0.2% over Euclid space telescope scales. To regulate ultraviolet contributions, we introduce a holographically motivated prescription in which gravitationally active entropy deposition is confined to causal two-surfaces, yielding a $\rho \propto r^{-2}$ halo envelope with a finite-density core determined by local entropy saturation. Fixing the flux scale $\mathcal{A}$ from astrophysical entropy budgets reproduces Milky-Way-mass halos without introducing fine-tuned length scales. Pilot N-body simulations that evolve the entropy field on a staggered grid reproduce the halo mass function down to ${10}^{10.5}{M}_{\odot }$, mitigate the cusp–core and missing-satellite tensions, and remain consistent with cluster lensing constraints. On linear scales, the model predicts percent-level, scale-dependent deviations in the lensing convergence and matter power spectra, testable by Euclid space telescope, the Roman Space Telescope High Latitude Survey, and the CMB-S4 experiment. Full article

Current terahertz (THz) metasurfaces are often constrained by fixed operational states, lacking the flexibility to switch dynamically between transmission and reflection modes. To address this limitation, we propose a tunable coded metasurface based on the photo-adjustable conductivity of silicon, enabling seamless mode switching and versatile wavefront manipulation. By leveraging the photo-induced dielectric-to-metallic transition, the device functions as a high-efficiency transmission-type polarization converter under zero pump fluence, transforming incident X-polarized waves into Y-polarized waves across a broad frequency range of 0.85–1.5 THz, with a polarization conversion ratio (PCR) exceeding 99%. Upon excitation by 800 nm near-infrared laser pulses, the metasurface transitions to reflection mode, where it simultaneously achieves linear polarization conversion and generates dual-channel orbital angular momentum (OAM) beams through a phase-coding strategy integrated with Fourier convolution. Furthermore, by employing the Gerchberg–Saxton (GS) algorithm to optimize the phase profile, holographic reconstruction is realized in the far field. This design integrates diverse manipulation capabilities into a single, dynamically controllable platform, offering a promising technological approach for THz information processing and integrated photonic systems. Full article

In this study, we employed one of the green synthesis methods utilizing water extracts prepared from solid industrial wastes of Rosa damascena Mill. (RD) and Oriental variety tobacco (Nicotiana tabacum)-mixed stems and leaves (O) as a natural reducing agent for PdCl₂ to obtain environmentally friendly Pd nanoparticles (PdNPs). Transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDX) in TEM were applied to determine the morphology, microstructure, phase, and elemental composition of PdNPs synthesized. The concentration of PdNPs in the suspensions was quantified by inductively coupled plasma optical emission spectroscopy (ICP-OES), which is essential for their intended application. Furthermore, the synthesized PdNPs were incorporated as dopant into a polymer matrix (PAZO) developed for optical applications. As will be demonstrated, doping PAZO with specific concentrations (0.1, 0.2, 0.25, 0.3, 0.4, 0.5, and 1 wt. %) of green PdNPs enhances the maximal value of the photoinduced birefringence by more than 50%. This improvement enables more efficient inscription of polarization-selective holographic optical elements in the resulting photoanisotropic nanocomposite materials with nearly 25% higher diffraction efficiency. Using a digital polarization holographic setup, the spatial modulation of polarization was recorded on thin nanocomposite films of the azopolymer PAZO, doped with certain concentrations of the green PdNPs. Full article

The detection and degradation analysis of subsurface microcracks in mural paintings remain challenging due to their inhomogeneous multilayered structure and complex deterioration mechanisms. In this study, we propose a multimodal stepwise method for three-dimensional characterization and cross-scale degradation analysis by integrating digital holography (DH), infrared thermography (IRT), acoustic excitation (AE), and molecular dynamics (MD) simulations. In the first step, an adjustable field-of-view (FOV) digital holographic system is developed to capture subsurface deformation under acoustic excitation, enabling high-resolution planar characterization of subsurface microcracks. Infrared thermography is then employed to estimate crack depth through an inverse thermal model, achieving full three-dimensional reconstruction of crack geometry. Based on the reconstructed structures, MD simulations are conducted to investigate the evolution of stress, bond breaking, and crack propagation under varying temperature and humidity conditions, with particular emphasis on water molecule migration and chemically induced degradation. The results demonstrate that environmental factors promote stress concentration and material embrittlement at crack tips, leading to secondary microcrack formation and progressive deterioration. Experimental aging tests show strong agreement with simulation results, validating the proposed methodology. This work establishes a unified “characterization–simulation–validation” paradigm, providing new insights into the mechanisms of mural degradation and offering a robust framework for non-destructive evaluation and preventive conservation of multilayer cultural heritage materials. Full article

The pervasive contamination of aquatic environments by microplastic particles necessitates the development of rapid, cost-effective and field-deployable detection methodologies to complement established but laboratory-bound spectroscopic techniques such as Fourier-transform infrared and Raman microscopy. The demand for field-suitable methods with a broad accessibility comes from researchers themselves. In this review we systematically examine recent advances in optical methods for microplastics identification with a particular emphasis on birefringence as a key diagnostic feature of partially crystalline synthetic polymers. In particular, we analyze three complementary technological directions: liquid crystal-based sensors that exploit orientational order disruptions at interfaces for label-free microplastics detection; polarization holographic imaging combined with machine learning for high-throughput particle classification; and on-chip polarization light microscopy enabling compact and portable analyzing systems. Liquid crystal platforms demonstrate exceptional sensitivity to submicron particles and enable real-time visualization of microplastics aggregation at aqueous interfaces, though they currently lack polymer-specific chemical identification. Conversely, smart polarization holography integrated with Stokes polarimetry and deep learning algorithms achieves over 90% accuracy in distinguishing microplastics from natural particles while processing up to 10,000 particles per minute. Emerging on-chip polarized light microscopy offers a pathway toward miniaturized, low-cost devices suitable for field applications. By synthesizing insights from foundational studies, this review identifies convergent interdisciplinary trends—particularly the integration of artificial intelligence with multimodal optical imaging—and outlines persistent challenges including standardization, interference from natural organic matter, and the transition from laboratory prototypes to robust field-deployable instruments. The systematization of birefringence-based approaches aims to guide future research towards integrated monitoring systems capable of addressing water quality concerns. Full article

In situ marine particle-field observation by parallel phase-shifting digital holography (PPSDH) produces long image sequences under real deployment conditions, but exhaustive full-frame reconstruction and segmentation are computationally expensive when many frames are low contrast and particle-like targets occupy sparse regions. This paper presents a training-free two-stage selective-processing workflow for a 9521-frame coastal South China Sea PPSDH campaign. Stage 1 uses an amplitude-derived contrast metric as a campaign-specific pruning rule to form a retained-frame pool, and Stage 2 combines coarse reconstruction, candidate filtering, valid-field gating, and ROI merging for ROI-restricted reconstruction and segmentation. Stage 1 retained 6970 frames, corresponding to 73.2% of the full sequence. On a balanced 120-frame benchmark, Stage 2 achieved a spatial-support reduction ratio of 49.9% ± 12.0%, and the complete workflow provided a 5.66-fold end-to-end speedup relative to a matched full-frame baseline. The efficiency gain was accompanied by a measurable fidelity cost, with a baseline-matched correspondence rate of 0.612 and a count-based yield gap of 0.287, mainly associated with small or weak targets within the selected ROI support. These results show that the proposed workflow can support computation-aware review of real marine PPSDH particle fields by efficiently prioritizing informative frames and particle-like regions for downstream visual assessment. Full article

Accurate axial localization of microparticles is a key requirement in in-line digital holography (ILDH), particularly under noisy conditions and for weakly scattered objects. This work presents experimental and simulated benchmarking of three widely used focus metrics: maximum intensity, complex amplitude, and Kurtosis. Experimental holograms of microparticles with different diameters were recorded using a compact ILDH system, while simulated holograms of a 10 µm particle were generated. Numerical reconstruction was performed using a Fresnel convolution approach with FFT-based propagation over a range of axial distances. The performance of each focus metric was evaluated based on peak definition, robustness to coherent noise, and consistency across particle sizes and configurations. The results show that both maximum intensity and Kurtosis provide consistent and reliable axial localization, with very similar behavior across all cases. In contrast, the complex amplitude metric is more sensitive to noise and exhibits larger fluctuations in the axial response. These results indicate that simple intensity-based metrics can achieve accurate localization under moderate signal-to-noise conditions, while higher-order statistical metrics improve robustness in more challenging scenarios. This work provides practical guidelines for selecting autofocus criteria in ILDH systems for particle imaging and holographic metrology. Full article