From the perspective of developing robust mid-infrared (mid-IR) integrated photonic devices, barium-gallium-germanium (BGG) oxide glasses are strong candidates among other mid-IR glasses. Indeed, compared to fluoride, tellurite or chalcogenide glasses, BGG glasses present the highest thermal and chemical stabilities, while transmitting light up to 6 µm. In parallel to this, technological advances in ultrafast direct laser writing (UDLW)-based devices are driving the development of novel photonic glasses. Specifically, there is a need to identify the most efficient mid-infrared transmitting BGG glass compositions for sustaining the UDLW process. In this article, we thoroughly investigate the BGG physicochemical properties through absorption and Raman spectroscopies, refractive index, density, and glass transition temperature measurements in two relevant glass series: one via a Ga3+/Ge4+ ratio fixed to 1 and a barium content varying from 25 to 40 cationic percent, the other via a 2Ba2+/Ga3+ ratio fixed to 1 and a germanium content varying from 20 to 80 cationic percent. In the meantime, we explore the photosensitivity of these glasses under UDLW. Our findings reveal the valuable role of both barium and gallium ions, notably through their concentration, structural stabilization sites and viscosity influence. Finally, we demonstrate the fabrication of an 8.2 cm-long UDLW-induced waveguide with propagation losses of < 0.3 dB.cm-1 at 1550 nm.

We have shown that the optical sign of the uniaxial langasite crystal Ca3TaAl3Si2O14 is positive. We also measured the phase-matching conditions of second-harmonic generation and sum-frequency generation up to 3.5 µm. Simultaneous fitting of these experimental data allowed us to report the Sellmeier equations of the ordinary and extraordinary principal refractive indices, and the magnitude of the nonlinear coefficient.

Given the increasing concerns about global warming, it is undeniable that measuring and controlling carbon dioxide (CO2) levels, a colorless and odorless greenhouse gas, is of great value. In this respect, liquid crystals (LCs) as an anisotropic material hold promise for fabricating such gas sensors. Here, we report a sensitive optical gas sensor for real-time monitoring of CO2 gas, exploiting a textile grid impregnated with LC and diethanolamine (DEA) as a CO2-sensitive material. The sensing mechanism relies on the reorientation of LC molecules upon the interaction of gas analytes with DEA. By tracing optical texture changes and extracting the corresponding intensities, CO2 gas concentrations ranging from 300 to 10,000 ppm were detected. The sensor exhibits a response time of 12 seconds and a recovery time of 7 seconds at 800 ppm. The sensor is simple and cost-effective.

Single-crystalline layers of 12 at.% Tb3+, 5 at.% Gd3+:LiYF4 were grown by the liquid phase epitaxy method on (001) oriented bulk undoped LiYF4 substrates using LiF as a solvent. The growth temperature was 737–740 °C, the growth duration was 15 - 25 min, and the layer thickness was 40–90 µm. The structural, morphological, vibronic and spectroscopic properties of the layers were studied. Tb3+ ions were uniformly distributed in the layers with no diffusion into the substrate. Polarized Raman spectroscopy confirmed the orientation of the layers (growth along the [001] direction). Under excitation in the blue, the layers exhibited intense green emission. For the 5D4 → 7F5 Tb3+ transition, the peak stimulated-emission cross-section is 1.28 × 10−21 cm2 at 542.0 nm for π-polarization. The luminescence lifetime of the 5D4 Tb3+ state is 5.05 ms at room temperature. The crystal-field splitting of Tb3+ multiplets was determined at low temperature. The developed epitaxies are promising for green and yellow waveguide lasers.

Stratified optical films have fascinating optical properties and many important applications. Here we report a theoretical study of stepwise twisted stratified anisotropic optical media. We used Fourier transform to analyze the helical component of the dielectric tensor of the media. We then used the Berreman 4 × 4 method to calculate the reflection spectrum of the media. We discovered that right-handed and left-handed helices could coexist in stepwise twisted layers, which producing a simultaneous reflection of right-handed and left-handed circularly polarized light. This feature can be used to produce films with superior reflection properties, such as high reflectance, broad bandwidth reflection and short wavelength reflection.

A significant amount of control of exciton-polaritons has been achieved over the past decades, including their creation, localization in desired modes, coupling between modes, manipulation by control fields, and detection. As quantum particles maintain coherence (correlations) for some time and interact (causing the evolution of those correlations), exciton-polaritons underlie an emerging field of quantum polaritonics.

In the present work, we report on the preparation of silicate glass containing crystals by means of melting a mixture of YbPO4 xenotime structured crystals and SiO2 nanoparticles. This nanoparticle mixture is used for preparation of large volume core preforms for laser active optical fiber. Temperature dependent sintering and fiber drawing experiments at temperatures up to about 2000 °C were conducted in order to assess the integrity of the crystals in the preform and fiber, respectively. The survival of YbPO4 crystalline particles in silica was investigated by X-ray diffraction (XRD), electron probe microanalysis (EPMA), Raman spectroscopy as well as static and time resolved fluorescence measurements. It was found that the particles withstand the high-temperature steps during the fiber fabrication process. XRD and spectroscopic measurements suggest that the Yb ions are located in a crystalline but also in an amorphous silica-dominated surrounding in the fiber, suggesting the partial decomposition of the crystals during the fiber fabrication.

Fundamental knowledge of scattering in granular compacts is essential to ensure accuracy of spectroscopic measurements and determine material characteristics such as size and shape of scattering objects. Terahertz time-domain spectroscopy (THz-TDS) was employed to investigate the effect of particle size and concentration on scattering in specially fabricated compacts consisting of borosilicate microspheres in a polytetrafluoroethylene (PTFE) matrix. As expected, increasing particle size leads to an increase in overall scattering contribution. Scattering increases linearly at low concentrations, saturates at higher concentrations with a maximum level depending on particle size, and that the onset of saturation is independent of particle size. The effective refractive index becomes sublinear at high particle concentrations and exceeds the linear model at maximum density, which can cause errors in calculations based on it, such as porosity. The observed phenomena are attributed to the change in the fraction of photons propagating ballistically versus being scattered. At low concentrations, photons travel predominately ballistically through the PTFE matrix. At high concentrations, the photons again propagate ballistically through adjacent glass microspheres. In the intermediate regime, photons are predominately scattered.

In this study, we investigate the impact of asymmetry on plasmonic-induced transparency (PIT) in structures with double dark modes. We have identified the ideal structural parameters for the single and double asymmetric dark mode structures, and it has been demonstrated that the usage of these factors improves sensing performance. The suggested structure is made up of a number of parallel black phosphorus nano-ribbons (BPNRs) with similar length, and it has been found that double asymmetric dark-dark systems creates a more stable and consistent transparency window, making it a strong contender to serve as a refractive index sensor. The proposed structure is simple and easy to manufacture, making it a more viable design for refractive index sensors for the terahertz range.

We demonstrate a method for the inverse design of stacked metasurfaces, applicable to any objective that can be expressed via scattering matrices, layering the metasurfaces in far-field approximation. By linking a semi analytical stacking algorithm with a modified genetic algorithm we developed an computationally efficient optimization tool. During optimization, the composition and the number of the layers is adjusted simultaneously and dynamically for continuous and discrete parameters. The method enables us to inversely design layered metasurface stacks in a matter of seconds avoiding thousands of rigorous simulations. We demonstrate the optimization performance of the algorithm for the example of a plasmonic broadband circular polarizer.

Laser-induced damage to the final reflective and diffractive optics limits the total output energy of petawatt laser systems with pulse durations ranging from a few hundred femtoseconds (fs) up to a few tens of picoseconds (ps). In this study, the laser damage to HfO2/SiO2 and Ta2O5/SiO2 multilayer dielectric high-reflectivity (HR) coatings induced by a 1053 nm laser with a pulse width of 8.6 ps was studied to investigate the nano-absorbing precursors in ps regimes. The HfO2/SiO2 HR coating exhibited stronger laser resistance than the Ta2O5/SiO2 HR coating. Flat-bottom pits, pinpoints, and funnel pits were the three typical damage morphologies for the experimental HR coatings. The damage to the HfO2/SiO2 HR coating was primarily dominated by flat-bottom pits, whereas dense pinpoints were the most significant damage for the Ta2O5/SiO2 HR coating. The nano-absorbing precursors introduced by the ion-assisted deposition process were proved to be the damage precursors that trigger pinpoints under a strong electric field intensity (EFI). The nano-absorbing precursors located in the second EFI peak of the SiO2 top layer induced the funnel pits. The funnel pits were expected to be the previous stage of the flat-bottom pits. After they grew along the upward-sloping crack and separated from the interface, the flat-bottom pits were formed. In addition, poor-binding interfaces promoted the formation of flat-bottom pits.

We correct two mistakes in [Opt. Mater. Express 12, 2772 (2022) [CrossRef] ]. These corrections have no influence on the conclusions of the original paper.

The angle-dependent poling properties of 0.5-mm-thick BaMgF4 (BMF) crystals are investigated by means of calligraphically poled, radially oriented domains. We present a fanout periodic poling pattern for quasi-phase matching (QPM) applications, which covers an angular spectrum of about 14°. Within this range there is no angle-dependent degradation of the domain quality. This is the first realization of non-parallel domains for QPM in BMF-crystals.

The increasing demand for high data rates and low power consumption puts silicon photonics at the edge of its capabilities. The heterogeneous integration of optical ferro-electric materials on silicon enhances the functionality of the silicon on insulator (SOI) platform to meet these demands. Lead zirconate titanate (PZT) thin films with a large Pockels coefficient and good optical quality can be directly integrated on SOI waveguides for fast electro-optic modulators. In this work, the relative permittivity and dielectric loss of PZT thin films deposited by chemical solution deposition on SOI substrates are analyzed at high frequencies. We extract εr=1650−2129\varepsilon _r = 1650-2129 and tan (δ) = 0.170 − 0.209 for the PZT thin films in the frequency range 1-67GHz. We show the possibility of achieving bandwidths beyond 60GHz via a Mach-Zehnder modulator with Vπ = 7V, suitable for next generation data communication systems.

Topological defect arrays in liquid crystal is an emerging optical material for smart windows, displays, gratings, and optical vortex generators. Formation of defect arrays is investigated using vertically aligned nematic liquid crystal cells with pad, crossed-strips, and porous electrodes. The location and types of the defects are identified using a polarized optical microscope. The pads and crossed-strips generate alternative radial and hyperbolic defects. Unexpectedly, the holes create dipoles of radial and hyperbolic defects, and the dipoles align in parallel order. The best dense packing of defects is achieved with the 15μm × 15μm unit cell.

We are grateful to Ballato et al. [Opt. Mater. Express 13, 2338 (2023) [CrossRef] ] for their comment on our recently published paper. The optical model and simulation of optical fiber materials are important to design new materials systems and to further improve the fiber laser performance. However, accurate calculation of the non-crystal fiber materials is still challenging, both from the methodology and from the needed calculating resources. The recently published paper [Opt. Mater. Express 13, 935 (2023) [CrossRef] ] has sparked interest, which gives us the opportunity to explain the difference between the modeled data and the well-established experimental results.

Moiré effects arise from stacking periodic structures with a specific geometrical mismatch and promise unique possibilities. However, their full potential for photonic applications has yet to be explored. Here, we investigate the photonic band structure for an atomic stack of strongly coupled linear arrays in the dipolar regime. A moiré parameter θ is used to parameterize a relative lattice constant mismatch between the two arrays that plays the role of a 1D twist angle. The system’s interaction matrix is analytically diagonalized and reveals the presence of localized excitations which strongly enhance the density of optical states in spectral regions that can be controlled via the moiré parameter. We also confirm our findings by numerical simulations of finite systems. Our work provides a better understanding of photonic moiré effects and their potential use in photonic devices such as optical sensors and light traps.

In recent years, metasurfaces have been widely employed in stealth technology, which brings great challenges for radar target detection. In order to address this issue, a novel detection approach for metasurface-stealth-target (MST) based on orbital angular momentum (OAM) vortex wave is proposed in this paper. Compared to a conventional plane wave detection system, the transmitting wave of the proposed approach is OAM-modulated, of which the wavefront is helical along the beam axis. Thus, the differentiated exciting source is introduced to different parts of MST. According to the established scattering model and full-wave simulation, the echo of MST has a strong correlation with OAM order l, which exhibits quite different scattering characteristics from the plane wave illumination. A chessboard metasurface (CM) is taken as an example to be irradiated by an OAM vortex wave of l=±2. The backscattering is significantly boosted compared to plane-wave detection, which is against the stealth ability of CM. This phenomenon is also verified by experiments. The results reveal that OAM detection is a promising approach for MST detection..

We report on a comparative study of the spectroscopic properties and mid-infrared laser performance of five 5 at.% Er3+-doped fluorite-type crystals MF2, including parent compounds CaF2, SrF2, BaF2, and solid-solution (“mixed”) ones (Ca,Sr)F2 and (Sr,Ba)F2. In the M = Ca → Sr → Ba series, the host matrix phonon energy decreases, the absorption and mid-infrared emission spectra of Er3+ become narrower and more structured, and the luminescence lifetimes of the 4I11/2 and 4I13/2 Er3+ manifolds increase. The Er3+ transition probabilities were calculated using the Judd-Ofelt theory. In the “mixed” compounds, the Er3+ ions tend to reside in the larger / heavier cation environment. The low-temperature (12 K) spectroscopy evidences the presence of a single type of clusters at this doping level; the crystal-field splitting for Er3+ ions in clusters was determined. Continuous-wave low-threshold laser operation at ∼2.8 µm (the 4I11/2 → 4I13/2 transition) was achieved with all five Er3+:MF2 crystals. The maximum achieved laser slope efficiency was 37.9% (Er3+:CaF2), 23.5% (Er3+:SrF2) and 17.2% (Er3+:BaF2).

This is an introduction to the special issue on photonics with 2D materials.