Reflection Bandwidth Research Articles

Enhanced broadband reflection properties of PSCLCs films supported by electrospun nanofiber networks loaded with Cs0.33WO3 nanoparticles

ABSTRACT The introduction of nanoparticles into cholesteric liquid crystals has long provided inspiration for innovative device materials. A novel material design strategy was demonstrated to achieve a broadband reflection of polymer-stabilised cholesteric liquid crystals (PSCLCs) based on Cs0.33WO3 nanoparticles and the polymer nanonetworks in the PSCLCs. Through the adjustment of nanoparticle and chiral dopant (C6M) concentrations and meticulous control of polymerisation conditions, an appropriate pitch gradient distribution was achieved in the polymer-stabilised cholesteric liquid crystals (PSCLCs). This was attained by striking a delicate balance between the polymerisation speed and chiral dopant diffusion rate, resulting in optimal PSCLCs optical properties. Modification of Cs0.33WO3 nanoparticles by oleic acid was successfully accomplished as confirmed by IR spectroscopy. SEM and EDS analyses further demonstrated the distribution of nanoparticles in the spun fibre film. Moreover, POM observations showed that liquid crystals with nanofiber networks maintained the planar texture and temperature stability. The prepared broadband reflective film opens an avenue towards developing the reflection bandwidth in the PSCLCs, essential for intelligent windows and infrared shielding devices.

N, O-Doped Walnut-Like Porous Carbon Composite Microspheres Loaded with Fe/Co Nanoparticles for Adjustable Electromagnetic Wave Absorption.

This study addresses the challenge of designing simple and environmentally friendly methods for the preparation of effective electromagnetic wave (EMW) absorbing materials with tailored microstructures and multi-component regulation. N, O doped walnut-like porous carbon composite microspheres loaded with FeCo nanoparticles (WPCM/Fe-Co) are synthesized through high-temperature carbonization combined with soap-free emulsion polymerization and hydrothermal methods, avoiding the use of toxic solvents and complex conditions. The incorporation of magnetic components enhances magnetic loss, complementing dielectric loss to optimize EMW attenuation. The unique walnut-like morphology further improves impedance matching. The proportions of Fe and Co components can be adjusted to regulate the material's reflection loss, thickness, and bandwidth, allowing for fine-tuning of absorption performance. At a low filling ratio (16.7%), the optimal WPCM/Fe-Co composites exhibit a minimum reflection loss (RLmin) of -48.34dB (10.33GHz, 3.0mm) and an overall effective absorbing bandwidth (EAB) covering the entire C bands, X bands, and Ku bands. This work introduces a novel approach to composition regulation and presents a green synthesis method for magnetic carbon composite absorbers with high-performance EMW absorption at low loading.

MWCNT decorated Magnetic/Ceramic/Polymer (Fe2O3/Fe3O4/SiO2/PANI) nanocomposite with enhanced microwave absorption at an entire X and Ku frequency bands

Two new magnetic/ceramic/carbon and magnetic/ceramic/polymer/carbon nanocomposites were created by decorating the surface of MWCNT with magnetic/ceramic (Fe2O3/Fe3O4/SiO2 NPs) and magnetic/ceramic/polymer (Fe2O3/Fe3O4/SiO2/PANI). First, in order to have a narrow size distribution and high saturation magnetization, Fe2O3/Fe3O4 NPs were produced successively under ideal conditions. Subsequently, Fe2O3/Fe3O4 NPs were produced by coating them with a thin layer of SiO2. Fe2O3/Fe3O4/SiO2/MWCNT nanocomposite was created by applying Fe2O3/Fe3O4/SiO2/NPs at a specified weight ratio (1/2) to the MWCNT surface. Subsequently, Fe2O3/Fe3O4/SiO2 NPs were utilized as an initiator to polymerize the aniline; as a result, Fe2O3/Fe3O4/SiO2/PANI NPs were created and the polyaniline grew at their surface. Ultimately, Fe2O3/Fe3O4/SiO2/PANI NPs were added to MWCNT at a weight ratio of 1/2, resulting in the synthesis of a Fe2O3/Fe3O4/SiO2/PANI/MWCNT nanocomposite. XRD, SEM, and TEM pictures verified the effective synthesis of Fe2O3/Fe3O4/SiO2/MWCNT and Fe2O3/Fe3O4/SiO2/PANI/MWCNT nanocomposites. By evaluating their EM characteristics, the microwave absorption properties of these composites were characterized. The findings shown that for the Fe2O3/Fe3O4/SiO2/MWCNT nanocomposite with an effective bandwidth of 5.7 GHz, the minimum reflection loss reached −18.5 dB. For the Fe2O3/Fe3O4/SiO2/PANI/MWCNT nanocomposite, there was a considerable improvement in both minimum reflection loss and bandwidth in comparison with similar composites. The Fe2O3/Fe3O4/SiO2/PANI/MWCNT demonstrated a minimum reflection loss of −87dB over whole frequency range (8–18 GHz) with an effective bandwidth.

The Impact of Al3+ ion substitution on microwave absorption of hexaferrite compound Ba0.6Sr0.4Fe12−xAlxO19 (x = 0.25, 0.50, 0.75, and 1.00)

This paper discusses the impact of Al3+ ion substitution on Fe3+ ions in the hexaferrite compound Ba0.6Sr0.4Fe12−xAlxO19 on the reflection loss value and bandwidth in absorbing microwaves. Hexaferrite Ba0.6Sr0.4Fe12−xAlxO19 (x = 0.25, 0.50, 0.75, and 1.00) was synthesized by high-energy ball milling (HEBM) for forty hours, followed by sintering at 1000 °C in the air for five hours. Then, the crystal structure, formed phase, surface morphology, magnetization, and ability to absorb microwaves in the X-band region were successively characterized. All samples (x = 0.25, 0.50, 0.75, and 1.00) had a single phase with a hexagonal crystal structure. The particle size remained heterogeneous, ranging from 200 to 300 nm, with a platelet-like shape. The maximum amount of magnetic energy that could be absorbed by the material (Ms) increases from x = 0.25 to 0.75 and then decreases slightly for x = 1.00. The maximum value of reflection loss (RLmax) decreases as x increases. For x = 0.25, RLmax is − 16.17 dB (f = 11.06 GHz) with a bandwidth of 3.56 GHz, whereas for x = 1.0, RLmax is −13.94 dB (f = 11.14 GHz) with a bandwidth of 1.68 GHz.

Chirped fiber Bragg grating enabled mode-dependent gain equalization of 2-mode EDFA

We report a few-mode chirped fiber Bragg grating (FM-CFBG) enabled mode-dependent gain (MDG) equalizer for the all-fiber FM-EDFA. Relying on the mode-dependent reflectivity of the FM-CFBG, the MDG arising in an all-fiber 2-mode EDFA can be effectively mitigated. With the LP01 pump light injection, the average gain is up to 17 dB while the MDG can be equalized to be less than 1.26 dB within the reflection bandwidth of FM-CFBG. The MDG equalizer based on a FM-CFBG is equipped with several merits of compact size, low insertion loss, and wavelength scalability, which is meaningful for long-haul MDM transmission.

An ultra-wideband plasmonic reflector based on local resonant bandgap and Bragg bandgap

Unlike previous reports that utilized periodic modulation of insulation layer thickness or dielectrics, we propose the use of split ring resonators (SRRs) and their arrays to modulate the propagation characteristics of MIM plasmon waveguides. Due to the strong resonance backscattering of SRR, resonance transmission valleys appear in the transmission spectra of MIM waveguides. Changing the size of SRR can achieve continuously adjustable positions of resonance transmission valleys. The introduction of SRRs periodic arrays will result in two bands (bandgaps) with transmission minimum in the transmission spectra. Combining the dependence of the two bandgaps on the array period and the field distribution, the two bandgaps are Bragg bandgaps and local resonance bandgaps, respectively. By modulating the local resonance bandgap, the Bragg bandgap can be tuned. The introduction of local bandgap increases the degree of freedom to modulate the transmission characteristics of MIM waveguides. Combining local resonant bandgap and Bragg bandgap can significantly increase the reflection bandwidth, achieve broadband filtering, and facilitate the miniaturization of waveguide devices.

Near-Infrared Reflective Polymer Films Based on UV-327-Doped Zinc Oxide Nanoparticles.

We prepared cholesteric liquid crystal (CLC) films with broadband reflective properties by admixing organic dye UV-327 into inorganic zinc oxide nanoparticles (ZnO NPs), utilizing the principle of pitch distribution from a large to a small gradient along the film thickness direction, leading to broadband reflection. ZnO NPs are poorly dispersed and easy to gather, but they do not decompose easily. The addition of UV-327 makes up for the above shortcomings. UV-327 is an organic compound with good compatibility and dispersion with liquid crystal systems. Therefore, we used the method of mixing two UV-absorbing dyes (UV-327 and ZnO NPs) to obtain CLC films. UV-absorbing dyes (UV-327 and ZnO NPs) made the liquid crystal films form a UV intensity gradient in the direction of thickness, prompting the polymerizable monomers to polymerize faster on the stronger side of the light, leading to the relative diffusion of chiral molecules and polymerizable monomers, forming the concentration gradient of chiral molecules in the direction of thickness. The pitch has a gradient distribution as the chiral concentration varies. Then, anchored by the polymer network, the pitch gradient distribution no longer changes. Broadened reflective bandwidth can reach up to 881 nm. Furthermore, the film covers the near-infrared wavelength band well, which can be applied to future smart windows or laser shielding for medical and military applications. It is also believed that this achievement will optimize the preparation technology of broadband reflective CLC films in the future.

An optically transparent coplanar L-probe fed antenna based on capacitively coupled array of patches

An optically transparent antenna fed by a coplanar L-probe is proposed in this paper. Indium-doped tin oxide (ITO) with a sheet resistance of 3.5 Ω/sq etched on a soda-lime glass substrate is used as radiation array and ground to achieve high optical transmittance. TM 10 and TM 20 modes are excited simultaneously to expand work bandwidth. The fabricated antenna achieves a –10 dB reflection bandwidth of 27.5% from 4.7 to 6.2 GHz with the overall size of 42 mm × 42 mm × 3.3 mm. Different from many antennas composed of ITO, the proposed antenna exhibits directional radiation patterns with the maximum realized gain of 4.6 dBi and the maximum efficiency of 61%. Meanwhile, the optical transmittance of the antenna can reach above 58%.

Reflective Full‐Color Structural Colors with High Brightness and High Purity Based on Ge2Sb2Se4Te1 Chalcogenide Film

AbstractThis study proposes a general strategy for constructing an asymmetric Fabry–Perot structure based on an ultra‐thin composite absorber, Ni/Ge2Sb2Se4Te1, to produce reflective full‐color structural colors with high brightness and purity. The composite absorber effectively enhances the strong interference effect of thin film, which can significantly reduce the reflection bandwidth of the target band and the reflectivity of the non‐target band. Under the premise of optimizing the five‐layer base structure, full‐color structural colors can be tuned by only changing the thickness of the LaTiO3 layer. Using the ion‐assisted electron beam evaporation technique, a simple and efficient film deposition process, six color devices (namely red, orange, yellow, green, blue, and purple) are successfully prepared with reflection peaks exceeding 90%. The chromaticity coordinates of the proposed high‐purity red, green, and blue (RGB) samples are (0.554, 0.339), (0.280, 0.600), and (0.164, 0.075), respectively. These coordinates are fairly close to the standard RGB color coordinates used in liquid crystal displays. This device has a simple structure with a novel material combination and a low production cost, which makes it feasible for mass production using just one coating run process. It has excellent application potential in various fields such as micro‐nano displays, anti‐counterfeiting measures, reflective color filters, and decorations.

Design of 2.45GHz Circularly Polarized Tag Antenna

This paper presents the design and development of a circularly polarized (CP) tag antenna to overcome the polarization mismatch between the reader antenna and the tag antenna in RFID systems. This CP antenna is designed at 2.4 GHz for radio frequency identification (RFID) applications. It consists of a simple square loop loaded with an open gap, two feeding strips, and a matching stub. The antenna is fabricated on an FR4 substrate with a dielectric constant of 4.4, a dielectric loss tangent of 0.02, and a thickness of 0.8mm. In order to achieve conjugate matching between the circular tag antenna and the tag chip, the technique of loading a matching short line on the tag chip was used. Additionally, the radiation of circular polarization (CP) was achieved by placing an open gap and two feeding strips on the square loop. Through electromagnetic simulation analysis, the antenna performance is evaluated. The reflection coefficient (S11) at 2.45 GHz is measured to be - 17.16db, indicating good impedance matching. The 3 dB axial ratio bandwidth is 0.19 GHz (7.7%), covering the frequency range of 2.34 GHz to 2.53 GHz. The 10db power reflection bandwidth is approximately 8.9% (2.33 GHz to 2.55 GHz), ensuring broad operational bandwidth. The antenna also exhibits a favorable antenna gain of 2.29dBi. This study successfully demonstrates the design and performance evaluation of a CP tag antenna for RFID applications

Reflection performance of the Au/Cr asymmetrical grating under electric and magnetic fields

Symmetric grating is difficult to meet the requirements of optical communication, optical sensing, and solar cells, as the strictly integrating performance of the transmission or reflection efficiency, polarization, incident angle, and bandwidth. The asymmetrical grating is introduced to improve the performance of the optical devices. Here, the polarization properties, and spatial displacement of the P- and S- laser beams reflected from the Au/Cr asymmetrical grating under electric and magnetic fields are studied. The Goos-Hänchen (GH) shifts of the P-laser beam show dumbbell distribution. The GH shifts of the S-laser beam can be affected by the external direct current and magnetic field. The maximum GH and Imbert-Fedorov (IF) shifts of the P-laser beam are 15.75 µm and 35.10 µm, respectively. The maximum GH and IF shifts of the S-laser beam are 88.40 µm and 121.00 µm, respectively. The Au/Cr asymmetrical grating can be applied in polarization optics, magnetic sensors, electric sensors, and light manipulation.

A Review of Developments in Polymer Stabilized Liquid Crystals.

Polymer-stabilized liquid crystals (PSLCs) are multi-functional materials consisting of polymer networks in a continuous phase of liquid crystals (LCs), of which polymer networks provide anchoring energy to align the LCs. A number of improvements are detailed, including polymer-stabilized nematic liquid crystals (PSNLCs), polymer-stabilized cholesteric liquid crystals (PSCLCs), polymer-stabilized blue phase liquid crystals (PSBPLCs), polymer-stabilized smectic liquid crystals (PSSLCs), polymer-stabilized ferroelectric liquid crystals (PSFLCs), and polymer-stabilized antiferroelectric liquid crystals (PSAFLCs) in this review. Polymer stabilization has achieved multiple functionalities for LCs; in smart windows, a sufficiently strong electric field allows the LCs to reorient and enables switching from a scattering (transparent) state to a transparent (scattering) state. For broadband reflectors, the reflection bandwidth of LCs is manually tuned by electric fields, light, magnetic fields, or temperature. PSBPLCs open a new way for next-generation displays, spatial light modulators, sensors, lasers, lenses, and photonics applications. Polymer networks in PSFLCs or PSAFLCs enhance their grayscale memories utilized in flexible displays and energy-saving smart cards. At the end, the remaining challenges and research opportunities of PSLCs are discussed.

Low-Profile Broadband Dual-Polarized Dipole Antenna for Base Station Applications

A low-profile broadband dual-polarized antenna is investigated for base station applications. It consists of two orthogonal dipoles, fork-shaped feeding lines, an artificial magnetic conductor (AMC), and parasitic strips. By utilizing the Brillouin dispersion diagram, the AMC is designed as the antenna reflector. It has a wide in-phase reflection bandwidth of 54.7% (1.54–2.70 GHz) and a surface-wave bound range of 0–2.65 GHz. This design effectively reduces the antenna profile by over 50% compared to traditional antennas without an AMC. For demonstration, a prototype is fabricated for 2G/3G/LTE base station applications. Good agreement between the simulations and measurements is observed. The measured −10-dB impedance bandwidth of our antenna is 55.4% (1.58–2.79 GHz), with a stable gain of 9.5 dBi and a high isolation of more than 30 dB across the impedance passband. As a result, this antenna is an excellent candidate for miniaturized base station antenna applications.

Performance improvement of yellow flip-chip mini-LEDs via full-angle distributed Bragg reflector

Miniaturized light-emitting diodes (mini-LEDs) have gained much attention for liquid crystal displays back-light and self-emissive displays. However, the unsatisfactory performance of InGaN long-wavelength mini-LEDs hinders their practical applications. Herein, we develop a full-angle distributed Bragg reflector (DBR) and investigate its impact on the performance of yellow flip-chip mini-LEDs. The full-angle DBR consists of different stacks of Ti3O5/SiO2 layers that are optimized for multiple central wavelengths. Compared to conventional single-DBR stacks, such full-angle DBR enables wider reflectance bandwidth and less dependence of reflectance on the light incident angle from 0° to 50°. Specifically, full-angle DBR features high reflectance beyond 99% from 447 to 580 nm at an incident angle of 50°. Full-angle DBR is found to enhance the electric field intensity at the top of devices, indicating that light extraction efficiency is promoted. As a result, yellow flip-chip mini-LEDs with full-angle DBR show higher light output power than those without DBR at the same currents. Our work paves a way to improve the performance of small-size emitters with long wavelength.

Metasurface-Covered Planar Endfire Antenna With Vertical Polarization

A trapped wave-based solution to design compact planar vertically-polarized (VP) endfire antennas is proposed in this paper. By placing a four-element series-fed magnetic dipole array with an inter spacing of half wavelength within a dual-layer metasurface and loading shorting pin arrays at two edges of the metasurface, the VP radiation is realized in a wide impedance band. In addition, the antenna profile can be further reduced by half according to the electric field distribution within the metasurface. Finally, a VP endfire antenna with a reflection bandwidth of 4.3 ~ 5.4 GHz, a peak gain of 9.8 dBi and a low total profile of 0.065λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the free-space wavelength at the center operation frequency) is designed for the validation. The proposed antenna can be used for some wireless communication systems on or near the ground plane, such as the car roof-mounted terminal for vehicle-to-roadside unit/base station communications.

Tuning the Reflection Bandwidth of Polarization Volume Gratings by Guest Material Penetration

Polarization volume gratings (PVGs) based on polymerized cholesteric liquid crystal (CLC) films can potentially be applied as optical combiners in augmented reality (AR) smart glasses. The reflection bandwidth of PVGs is determined by the helical pitch and birefringence of the CLC, which must be tuned to match the desired color for reflection, but tuning the material properties is challenging as the materials must also be polymerizable. Here, we report a postprocessing method based on guest material penetration for the tuning of the reflection bandwidth of CLC polymer films. Solution coating of a high-birefringence guest liquid crystal on a CLC film modifies the birefringence as well as elongates the helical pitch. Penetration of a high-birefringence 5CT liquid crystal was found to cause a birefringence increase of the CLC film by approximately 18%.

Zero-field bandwidth stable, reflection bandwidth controllable polymer- stabilised cholesteric liquid crystals enabled by dynamic covalent boronic ester bonds

ABSTRACT Here, an electrically controllable reflection bandwidth and zero-field bandwidth stable PSCLC is judiciously designed and prepared through integrating dynamic covalent boronic ester bonds into the main-chain polymer network. The modulating effect of dynamic covalent bonding on polymer networks can transform the polymer networks from an elastically deformed network to a plastically deformed network by changing the parameters, such as specific wavelengths of light or temperature. Based on the plastic deformation of the network, the zero-field broadband reflection stable and multiple adjustable reflection bandgaps are realised. This work provides new insights into smart and energy-efficient windows.

Preparation of Cholesteric Polymer Networks with Narrow-Bandwidth Reflection and Memory Effect

A polymer network with a memory effect based on a polymer-stabilized narrow-bandwidth cholesteric liquid crystal (CLC) was prepared using the washing-out/refill method. The effects of different polymerization conditions on the reflection properties of CLC films were investigated. Meanwhile, the selective reflection property and narrow-bandwidth reflection memory effect of the polymer network were proved, and the response mechanism was provided. Furthermore, different materials from liquid crystals, with an anisotropic refractive index, to toluene, with an isotropic refractive index, were refilled to polymer scaffolds with helical structures, which originated from the periodic arrangement of CLCs. It was confirmed that the reflection bandwidth of these films can be dramatically narrowed by the reduced birefringence (Δn) of the refilled materials. The narrowest bandwidth of 22.5 nm refilling toluene with an isotropic refractive index (Δn = 0) was obtained. These results may provide a novel idea for flexible reflective displays, color filters, printing, and colored cladding of a variety of objects.

Geometrical Optics Analysis of Diffraction in Patterned Cholesteric Liquid Crystals

A geometrical optics method is developed to analyze the diffraction in patterned cholesteric liquid crystals. We extend the equation for the center wavelength of selective reflection in a conventional cholesteric liquid crystal to the wavelength edges of the diffraction in patterned cholesteric liquid crystals. This method can be used to determine the reflection bandwidth and its wavelength edges of patterned cholesteric liquid crystals which depends on a slant angle and an incident angle. Furthermore, we also investigate a difference in the diffraction between planar and slanted patterned cholesteric liquid crystals using the geometrical optics method.

Wideband Circularly Polarized Reflectarray Antenna Using Rotational Symmetrical Crossed Dipoles

A wideband circularly polarized (CP) reflectarray (RA) antenna using rotational symmetrical crossed dipoles is presented. This is the first time investigating coupled crossed dipoles as the unit cell for wideband reflection bandwidth in the CP RA design. Equivalent circuit analysis shows that when a CP wave impinges on the designed unit cell, two series resonances and two parallel resonances are simultaneously excited on the crossed dipoles. Owing to these four different resonances, the CP reflection bandwidth is greatly improved and elaborately adjusted by controlling the coupling between the crossed dipoles. The CP reflection bandwidth ratio of the unit cell is enhanced up to 2:1 for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S_x</i> <-15 dB with a thin thickness of 0.12λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> . Based on this unit cell, a wideband -20° collimated CP RA antenna with a circular aperture of 316 unit cells was designed, fabricated, and measured for final performance verification. The measured results show that a wide CP bandwidth of 7.6-15.9 GHz is achieved with the axial ratio <3 dB. In addition, the measured 3dB gain bandwidth is better than 43.7% with the peak realized gain of 26.3 dBic and the maximum aperture efficiency of 58.3%.