Bragg gratings with novel waveguide models fabricated in bulk glass via fs-laser writing and their slow-light effects.

We present the experimental realization of an innovative parallel partially overlapping waveguides (PO-WGs) model grounded in the thermal accumulated regime and fabricated using femtosecond (fs) laser direct-writing within low-iron bulk glass. The 75mm long novel PO-WGs model was made by partially overlapping the shell parts of two core-shell types of waveguides via a back-and-forth single pass fs-laser inscription. The detailed evolution of the PO-WGs model from inception to completion was offered, accompanying by a thorough characterization, which unveils a substantial refractive index (RI) change, on the order of 10-3, alongside low propagation loss (0.2 dB/cm) and distinctive features associated with the single mode and shell-guided light. Notably, the unsaturated performance of PO-WGs model after the primary inscription paves the way for potential applications in the successful creation of two distinctive types of Bragg gratings: first-order dot-Bragg grating and second-order line-Bragg grating. The 75 mm long dot-Bragg grating was written by a periodic dot array with a height of 6 µm atop the PO-WGs, and the birefringence was measured of 1.5 × 10-5 with a 16 pm birefringence-induced wavelength difference. The line-Bragg grating, which was inscribed with dual PO-WGs extending the line grating part to 40 mm in length along its period for increasing the transmission dip, exhibits a pronounced polarization dependence showcasing an effective birefringence of 4.2 × 10-4 at the birefringence-induced wavelength difference of 0.45 nm. We delved into the slow-light effects of the two Bragg gratings thoroughly, which the theoretical analysis revealed an effective group delay of 0.58 ns (group index 2.3) for the dot-Bragg grating. Similarly, the line-Bragg grating exhibited an effective group delay of 0.3 ns (group index 2.3), in good agreement with experimental measurements. These findings underscore the exciting potential of our gratings for creating optical slow-wave structures, particularly for future on-chip applications.

Reduced Graphene Oxide/Barium Ferrite Ceramic Nanocomposite Synergism for High EMI Wave Absorption.

The developed nanocomposite exhibits significantly enhanced shielding performance due to the synergistic effect of high dielectric and magnetic loss materials, which modifies the material's impedance and improves its absorption ability. Different weight percentages (0, 1, 5, 10, 15, 20, and 25 wt %) of thermally treated chemically reduced graphene oxide (TCRGO) were combined with two types of magnets, barium hexaferrite (BF) and magnetite (MAG), using a dry powder compaction technique to produce binary ceramic nanocomposite sheets. The shielding performance of a 1 mm thick compressed nanoceramic sheet over the X-band was evaluated using a vector network analyzer. The 25% TCRGO showed high shielding performance for both BF and MAG, while BF had a total shielding efficiency (SET) that exceeded MAG by 130%. The SET of 25 wt % TCRGO/BF was 52 dB, with a 41 dB absorption shielding efficiency (SEA). Additionally, the effect of different levels of incident electromagnetic wave power (0.001-1000 mW) at various thicknesses (1, 2, and 5 mm) was explored. At 1000 mW, the 5 mm TCRGO/BF had an SET of 99 dB, an SEA of 91 dB, and a reflection shielding efficiency (SER) of 8 dB. The use of BF as a hard magnet paired with TCRGO exhibited excellent and stable electromagnetic shielding performance.

Superior quality chemically reduced graphene oxide for high performance EMI shielding materials.

The chemical reduction process of graphene oxide combined with a mild and controllable thermal treatment under vacuum at 200 °C for 4 hours provided a cost-effective, scalable, and high-yield route for Reduced Graphene Oxide (RGO) industrial production and became a potential candidate for producing electromagnetic interference (EMI) shielding. We investigated graphite, and RGO using l-ascorbic acid and Sodium borohydride before and after thermal treatment by carefully evaluating the chemical and morphological structures. The thermally treated l-ascorbic Acid reduction route (TCRGOL) conductivity was 2.14 × 103 S m-1 and total shielding efficiency (SET) based on mass loadings per area of shielding was 94 dB with about one-tenth less graphite weight and surpassing other graphene reduction mechanisms in the frequency range of 8.2-12.4 GHz, i.e., X-band, at room temperature while being tested using the waveguide line technique. The developed treatment represents valuable progress in the path to chemical reduction using a safe reducing agent and offering superior quality RGO rarely achieved with the top-down technique, providing a high EMI shielding performance.

A Multiband Shared Aperture MIMO Antenna for Millimeter-Wave and Sub-6GHz 5G Applications.

A shared aperture 2-element multiple-input-multiple-output (MIMO) antenna design for 5G standards is presented in this study, one which uses the same radiating structure to cover both the sub-6GHz and millimeter-wave (millimeter-wave) bands. The proposed antenna comprises four concentric pentagonal slots that are uniformly separated from one another. For the sub-6GHz band, the antenna is excited by a single open-end microstrip transmission-line, while a 1 × 8 power divider (PD) connected via a T-junction structure excites the millimeter-wave band. Both the sub-6GHz and mm-wave antennas operate in a MIMO configuration. The proposed antenna design was fabricated on a 120 × 60 mm2 substrate with an edge-to-edge distance of 49 mm. The proposed sub-6GHz antenna covers the following frequency bands: 4-4.5 GHz, 3.1-3.8 GHz, 2.48-2.9 GHz, 1.82-2.14 GHz, and 1.4-1.58 GHz, while the millimeter-wave antenna operates at 28 GHz with at least 500 MHz of bandwidth. A complete antenna analysis is provided via a step-by-step design procedure, an equivalent circuit diagram showing the operation of the shared aperture antenna, and current density analysis at both millimeter-wave and sub-6GHz bands. The proposed antenna design is also characterized in terms of MIMO performance metrics with a good MIMO operation with maximum envelop correlation coefficient value of 0.113. The maximum measured gain and efficiency values obtained were 91% and 8.5 dBi over the entire band of operation. The antenna is backward compatible with 4G bands and also encompasses the sub-6GHz and 28 GHz bands for future 5G wireless communcation systems.

Field Decorrelation and Isolation Improvement in an MIMO Antenna Using an All-Dielectric Device Based on Transformation Electromagnetics.

This work presents a new technique for enhancing the performance of a multiple-input multiple-output (MIMO) antenna by improving its correlation coefficient ρ. A broadband dielectric structure is designed using the transformation electromagnetics (TE) concept to decorrelate the fields of closely placed radiating elements of an MIMO antenna, thereby decreasing ρ and mutual coupling. The desired properties of the broadband dielectric wave tilting structure (DWTS) are determined by using quasi-conformal transformation electromagnetics (QCTE). Next, the permittivity profile of the DWTS is realized by employing air-hole technology, which is based on the effective medium theory, and the DWTS is fabricated using the additive manufacturing (3D printing) technique. The effectiveness of the proposed technique is verified by designing two-element patch-based MIMO antenna prototypes operating at 3 GHz, 5 GHz, and 7 GHz, respectively. The proposed technique helped to reduce the correlation coefficient ρ in the range of 37% to 99% in the respective operating bandwidth of each MIMO antenna, thereby, in each case, improving the isolation between antenna elements by better than 3 dB, which is an excellent performance.