Low-loss Materials Research Articles

Quest for the porosity-regulated high dielectric and conductive performance of MOF by introducing nanoparticles: Performance evaluation at elevated temperatures

Metal-organic frameworks (MOFs) are among the novel materials that have drawn a lot of attention from the scientific and technological communities. There have been numerous reports of 3D MOFs in the scientific community, but due to distinct structural and electrical properties, scientists are particularly interested in zinc imidazole framework 8 (ZIF-8). Due to its characteristic low dielectric values, ZIF-8 is an excellent choice for low-dielectric applications. However, the incorporation of nanoparticles can increase the dielectric constant of ZIF-8. In this study, ZnO semiconducting nanoparticles are used as a filler of ZIF-8 due to their exceptionally high electron mobility, exciton binding energy, and wide band gap. We hypothesized that the incorporation of semiconducting nanoparticles (ZnO) inside ZIF-8 influences the porosity, pore size distributions, pore morphology, and interconnectivity of ZIF-8 which influences the polarization of pristine ZIF-8 and helps to increase the dielectric properties. ZnO has inherent ionic polarization which not only enhances the dielectric constant but also simultaneously decreases the dielectric loss which is essential for any dielectric materials. The dielectric constant was enhanced by 1.5 times after 5 wt% ZnO doping in ZIF-8 at room temperature and the enhancement was nearly five times at 150 °C. Simultaneously the dielectric loss decreases after ZnO doping compared to pristine ZIF-8. The tailoring of porosity, the number of pores, the presence of defects and oxygen vacancies are responsible for the modulated dielectric properties. The conductivity of ZIF-8 can also be tuned by the nanoparticles incorporation which indicates the larger production of drift electrons inside MOF in the presence of nanoparticles. The detailed morphological characteristics confirm the interaction between ZnO and ZIF-8. The result is a thorough understanding of the intrinsic and extrinsic electrical properties of ZIF-8 composites for future generations of technology. This work aimed to exploit possibilities of MOF towards high dielectric constant and low loss material by simple nanoparticle incorporation.

High-Quality Spherical Silver Alloy Powder for Laser Powder Bed Fusion Using Plasma Rotating Electrode Process.

The plasma rotating electrode process (PREP) is an ideal method for the preparation of metal powders such as nickel-based, titanium-based, and iron-based alloys due to its low material loss and good degree of sphericity. However, the preparation of silver alloy powder by PREP remains challenging. The low hardness of the mould casting silver alloy leads to the bending of the electrode rod when subjected to high-speed rotation during PREP. The mould casting silver electrode rod can only be used in low-speed rotation, which has a negative effect on particle refinement. This study employed continuous casting (CC) to improve the surface hardness of S800 Ag (30.30% higher than mould casting), which enables a high rotation speed of up to 37,000 revolutions per minute, and silver alloy powder with an average sphericity of 0.98 (5.56% higher than gas atomisation) and a sphericity ratio of 97.67% (36.28% higher than gas atomisation) has been successfully prepared. The dense S800 Ag was successfully fabricated by laser powder bed fusion (LPBF), which proved the feasibility of preparing high-quality powder by the "CC + PREP" method. The samples fabricated by LPBF have a Vickers hardness of up to 271.20 HV (3.66 times that of mould casting), leading to a notable enhancement in the strength of S800 Ag. In comparison to GA, the S800 Ag powder prepared by "CC + PREP" exhibits greater sphericity, a higher sphericity ratio and less satellite powder, which lays the foundation for dense LPBF S800 Ag fabrication.

Mechanism of the impact of Ca–Ge co-substitution on the FMR linewidth in BiV-YIG ferrites

Yttrium Iron Garnet (YIG) ferrites, characterized by their superior magnetic and dielectric attributes, are instrumental in advancing the development of microwave devices, particularly in enhancing high-frequency performance, integration, and miniaturization. This paper synthesizes YIG ferrite materials with the composition Bi0.6Y1.4-xCa1+xFe4.5-xV0.5GexO12 (x = 0.10 to 0.50, incrementing by 0.10) through the solid-state sintering method. It delves deeply into the mechanism by which Ca2+-Ge4+ ions substitution regulates the magnetic properties of BiV-YIG. The microstructural and magnetic properties of the samples were systematically tested and analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), vibrating sample magnetometry (VSM), and TE106 perturbation method. At a substitution content of x = 0.50, the substitution of Ge4+ ions for the Fe3+ ions at the d-site transforms the material from ferrimagnetic to paramagnetic, and the saturation magnetization of the sample drops to 132.74 G. By calculating the ferromagnetic resonance (FMR) linewidth using the law of approach to saturation, the study elucidates the negative impact of low saturation magnetization on FMR linewidth. The mechanisms proposed in this paper provide theoretical and experimental bases for reducing the FMR linewidth of high dielectric constant Bi-substituted YIG, paving the way for developing high-dielectric, low-loss YIG ferrite materials for microwave devices.

Enhanced terahertz high-harmonic generation from high-Q quasi-bound states in the continuum empowered by permittivity-broken metasurface

The extraordinary emergence of all-dielectric resonant meta-photonics underpinned by high refractive index and low optical loss materials promises a standout platform for unprecedented manipulation and subwavelength control of light. Recent significant breakthroughs in meta-photonics have to do with the exploration of the non-radiative eigenmodes that lie inside the light cone, called the bound states in the continuum (BICs), which have demonstrated considerable potential to achieve high quality (Q) factors but require ultra-small structure asymmetry, complicating fabrication and hindering precise control of Q factors. Here, we propose a scheme to excite quasi-BICs by breaking symmetry in the permittivity of the comprising meta-atoms where small permittivity perturbation renders the same effect in the conventional geometrically asymmetric design. Empowered by the insignificant permittivity perturbation introduced from chemical doping and carrier injection, arbitrarily small permittivity asymmetry, that is, infinitely high-Q factor, can be precisely and dynamically tailored. As an example of application, we numerically show the THz range, exceptionally high conversion efficiency for both third-harmonic and fifth-harmonic generation from the permittivity-broken metasurface (MS) benefiting from the extreme field confinement at high-Q quasi-BICs resonance. Our results present a feasible and efficient strategy to mitigate the shortcomings of previous photonic platform and shall shed light on the advances of nonlinear all-dielectric THz-MS.

Sb2S3 as a low-loss phase-change material for mid-IR photonics

We study the mid-IR optical properties of Sb2S3 – a phase-change material (PCM) with high potential for mid-IR integrated photonics. We show that Sb2S3 exhibits a high refractive index contrast Δn ≈ 1 between its two phases and an extinction coefficient k below 10−2 at mid-IR wavelengths (up to ∼10 µm). We study the impact of 100 nm Sb2S3 cladding on SiGe-on-Si waveguides and experimentally show that it brings additional propagation loss below 1 dB/cm in the 3.3–3.9 µm wavelength range in both phases. These findings pave the way for new uses of Sb2S3 in mid-IR integrated photonics.

Production of 2 μm-thick 6.5 % Si-Fe cold rolled sheet

Although 6.5% Si-Fe alloys have excellent magnetic properties, they have not been used in practical application because they are mechanically hard and brittle. In this study, we attempted an extremely thin 6.5% Si-Fe to realize a high-frequency, low-loss magnetic material, which is highly demanded for next-generation power electronic circuits. It was found that 6.5% Si-Fe with a thickness of 20 µm prepared by the liquid quenching method could be rolled down to 2 µm under certain conditions. The conditions under which the rollability was achieved were investigated in detail, such as grain structure, grain size, crystal orientation, etc. Since the eddy current loss of the 6.5% Si-Fe alloy with a thickness of 2 µm is expected to be extremely low, this material is very promising as a next-generation magnetic material for high frequency applications.

Exploring localized ENZ resonances and their role in superscattering, wideband invisibility, and tunable scattering

While the role and manifestations of the localized surface plasmon resonances (LSPRs) in anomalous scattering, like superscattering and invisibility, are quite well explored, the existence, appearance, and possible contribution of localized epsilon-near-zero (ENZ) resonances still invoke careful exploration. In this paper, that is done along with a comparison of the resonances of two types in the case of thin-wall cylinders made of lossy and loss-compensated dispersive materials. It is shown that the localized ENZ resonances exist and appear very close to the zero-permittivity regime, i.e., at near-zero but yet negative permittivity that is similar to the ENZ modes in thin planar films. Near- and far-field characteristics of the superscattering modes are investigated. The results indicate that the scattering regimes arising due to LSPRs and localized ENZ resonances are distinguishable in terms of the basic field features inside and around the scatterer and differ in their contribution to the resulting scattering mechanism, e.g., in terms of the occupied frequency and permittivity ranges as well as the sensitivity to the wall thickness variations. When the losses are either weak or tend to zero due to the doping with gain enabling impurities, the sharp peaks of the scattering cross-section that are yielded by the resonances can be said to be embedded into the otherwise wide invisibility range. In the case of lossy material, a wide and continuous invisibility range is shown to appear not only due to a small total volume of the scatterer in the nonresonant regime, but also because high-Q superscattering modes are suppressed by the losses. For numerical demonstration, indium antimonide, a natural lossy material, and a hypothetical, properly doped material with the same real part of the permittivity but lower or zero losses are considered. In the latter case, variations of permittivity with a control parameter can be adjusted in such a way that transitions from one superscattering mode to another can be achieved. In turn, transition from the strong-scattering to the invisibility regime is possible even for the original lossy material. The basic properties of the studied superscattering modes may be replicable in artificial structures comprising natural low-loss materials.

Inverse design of compact nonvolatile reconfigurable silicon photonic devices with phase-change materials

Abstract In the development of silicon photonics, the continued downsizing of photonic integrated circuits will further increase the integration density, which augments the functionality of photonic chips. Compared with the traditional design method, inverse design presents a novel approach for achieving compact photonic devices. However, achieving compact, reconfigurable photonic devices with the inverse design that employs the traditional modulation method exemplified by the thermo-optic effect poses a significant challenge due to the weak modulation capability. Low-loss phase change materials (PCMs) exemplified by Sb2Se3 are a promising candidate for solving this problem benefiting from their high refractive index contrast. In this work, we first developed a robust inverse design method to realize reconfigurable silicon and phase-change materials hybrid photonic devices including mode converter and optical switch. The mode converter exhibits a broadband operation of >100 nm. The optical switch shows an extinction ratio of >25 dB and a multilevel switching of 41 (>5 bits) by simply changing the crystallinity of Sb2Se3. Here, we experimentally demonstrated a Sb2Se3/Si hybrid integrated optical switch for the first time, wherein routing can be switched by the phase transition of the whole Sb2Se3. Our work provides an effective solution for the design of photonic devices that is insensitive to fabrication errors, thereby paving the way for high integration density in future photonic chips.

Microwave dielectric characterization and loss mechanism of biowaste during pyrolysis

The effectiveness of microwave heating of biomass depends on the changes in dielectric properties. This study systematically investigated the microwave dielectric properties and loss mechanism of biowaste at different frequencies (915 and 2,450 MHz) and temperatures (25–800 °C) using the cavity perturbation method. The gas emissions and properties of the biochar produced during pyrolysis were characterised by SEM, XRD, and TG-FTIR. The findings revealed that the dielectric properties and penetration depth of the biomass and biochar changed significantly during microwave pyrolysis. The dielectric constant and dielectric loss coefficient first increased and then decreased during the drying stage, and they decreased continuously with the volatile’s generation in the pyrolysis stage, while the formation and high conductivity of biochar promoted a sharp increase in dielectric properties during the carbonisation stage. The loss tangent angle is 0.01–0.05 in the first two stages, while it increased to 0.10–0.25 in the carbonization stage, demonstrated that the biowaste is a low-loss material. However, the microwave absorption capacity was enhanced for pyrolyzed biochar. The loss mechanism revealed that dipole polarisation primarily contributes to the dielectric loss during the drying and pyrolysis stages, whereas interfacial polarisation plays an important role during the carbonisation stage. The data provided in this paper contribute to the evaluation of microwave penetration depth in biomass and the design of large-scale microwave-based heating systems. Furthermore, this study offers a theoretical basis for modelling and simulating the microwave heating of biomass and achieving efficient energy conversion.

Low-loss hybrid germanium-on-zinc selenide waveguides in the longwave infrared

Abstract The longwave infrared (LWIR) range, which spans from 6 µm to 14 µm, is appealing for sensing due to strong molecular fingerprints in this range. However, the limited availability of low-loss materials that can provide higher-index waveguiding and lower-index cladding in the LWIR range presents challenges for integrated photonics. In this work, we introduce a low-loss germanium-on-zinc selenide (GOZ) platform that could serve as a versatile platform for nanophotonics in the LWIR. By bonding high-quality thin-film germanium (Ge) to a zinc selenide (ZnSe) substrate, we demonstrate transparency from 2 µm to 14 µm and optical losses of just 1 cm−1 at 7.8 µm. Our results demonstrate that hybrid photonic platforms could be invaluable for overcoming the losses of epitaxially grown materials and could enable a wide range of future quantum and nonlinear photonics.

Freeform direct-write and rewritable photonic integrated circuits in phase-change thin films.

Photonic integrated circuits (PICs) with rapid prototyping and reprogramming capabilities promise revolutionary impacts on a plethora of photonic technologies. We report direct-write and rewritable photonic circuits on a low-loss phase-change material (PCM) thin film. Complete end-to-end PICs are directly laser-written in one step without additional fabrication processes, and any part of the circuit can be erased and rewritten, facilitating rapid design modification. We demonstrate the versatility of this technique for diverse applications, including an optical interconnect fabric for reconfigurable networking, a photonic crossbar array for optical computing, and a tunable optical filter for optical signal processing. By combining the programmability of the direct laser writing technique with PCM, our technique unlocks opportunities for programmable photonic networking, computing, and signal processing. Moreover, the rewritable photonic circuits enable rapid prototyping and testing in a convenient and cost-efficient manner, eliminate the need for nanofabrication facilities, and thus promote the proliferation of photonics research and education to a broader community.

Influence of curing time on some properties of GNPs/PDMS nanocomposites

Graphene nanoplatelets/polydimethylsiloxane (GNPs/PDMS) nanocomposites foils are obtained by dispersing nanoplatelets of graphene (GNPs) into polydimethylsiloxane (PDMS) at a given GNPs concentration but at different curing times to study the effects of curing time on some their bulk and surface properties. Their appearance and morphology have been examined by optical and scanning electron microscopies, their wettability and roughness by the sessile drop method and profilometry, their bonds by µRaman and ATR-FTIR spectroscopies, their dielectric response by dielectric spectroscopy. In particular, optical and SEM images and roughness measurements indicate a roughened and hydrophobic surface at the shortest curing time that becomes less roughened, more uniform and less hydrophobic as curing time increases. ATR-FTIR and µRaman spectra reveal the absence of chemical interaction between GNPs and PDMS. The behavior of the PDMS real and imaginary parts of dielectric permittivity without and with GNPs and at increasing curing times seems to define the obtained GNPs/PDMS nanocomposites as low-loss materials. The obtained results show that the curing time significantly affects the filler distribution in the PDMS matrix and consequently the properties of the final nanocomposites.

Evaluation of low-loss alumina material for high-power RF windows

Conventional RF vacuum windows are made of metalized ceramics, hermetically brazed to a pillbox cavity. High-power windows, operating in ultra-high frequency (UHF) band, require the fabrication of ceramic disks with diameters on the order of 9”. Furthermore, a Titanium Nitride (TiN) multipactor suppression coating must be applied to the ceramic surfaces. The large size and complex internal geometry of these windows create challenges in validating the coating in the fully fabricated assembly. This study evaluates a novel low-loss alumina AO479U, provided by Kyocera, and a reactive sputtering process suitable to deposit a 10-20 nm thick TiN coating on a large diameter window. The paper will report the changes in the TiN coating through chemical cleaning and vacuum braze processes using stylus profilometry, optical microscopy, Scanning Electron Microscopy (SEM), and Rutherford Backscattering Spectroscopy (RBS).

Double-Strip Array-Based Metasurfaces with BICs for Terahertz Thin Membrane Detection.

A double-strip array-based metasurface that supports the sharp quasi-bound states in the continuum (quasi-BICs) is demonstrated in terahertz regions. By tuning the structural parameters of metal strips, the conversion of BICs and quasi-BICs is controllable. The simulated results exhibit an achieved maximum Q-factor for quasi-BICs that exceeds 500, corresponding to a bandwidth that is less than 1 GHz. The optical response of quasi-BICs is mainly affected by the properties of substrates. Resonant frequencies decrease linearly with increasing refractive index. The bandwidth of quasi-BICs decreases to 0.9 GHz when n is 2.2. The sharp quasi-BICs are also sensitive to changes in material absorption. Low-loss materials show higher Q-factors. Thus, the selection of a suitable substrate material will be beneficial in achieving resonance with a high Q value. The sensitivity of DSAs for molecules is assessed using a thin membrane layer. The DSAs show high sensitivity, which achieves a frequency shift of 70 GHz when the thickness of the membrane is 10 μm, corresponding to a sensitivity of 87.5 GHz/RIU. This metasurface with sharp quasi-BICs is expected to perform well in THz sensing.

Arbitrary Programming of Racetrack Resonators Using Low-Loss Phase-Change Material Sb2Se3.

The programmable photonic integrated circuit (PIC) is an enabling technology behind optical interconnects and quantum information processing. Conventionally, the programmability of PICs is driven by the thermo-optic effect, free carrier dispersion, or mechanical tuning. These effects afford either high speed or a large extinction ratio, but all require constant power or bias to maintain the states, which is undesirable for programmability with infrequent switching. Recent progress in programmable PICs based on nonvolatile phase-change materials (PCMs) offers an attractive solution to a truly "set-and-forget" switch that requires zero static energy. Here, we report an essential building block of large-scale programmable PICs─a racetrack resonator with independent control of coupling and phase. We changed the resonance extinction ratio (ER) without perturbing the resonance wavelength, leveraging a programmable unit based on a directional coupler and a low-loss PCM Sb2Se3. The unit is only 33-μm-long and has an operating bandwidth over 50 nm, a low insertion loss (∼0.36 dB), high ER (∼15 dB), and excellent fabrication yield of over 1000 cycles endurance across nine switches. The work is a crucial step toward future large-scale energy-efficient programmable PICs.

Optical switching beyond a million cycles of low-loss phase change material Sb2Se3

The development of the next generation of optical phase change technologies for integrated photonic and free-space platforms relies on the availability of materials that can be switched repeatedly over large volumes and with low optical losses. In recent years, the antimony-based chalcogenide phase-change material Sb2Se3 has been identified as particularly promising for a number of applications owing to good optical transparency in the near-infrared part of the spectrum and a high refractive index close to silicon. The crystallization temperature of Sb2Se3 of around 460 K allows switching to be achieved at moderate energies using optical or electrical control signals while providing sufficient data retention time for non-volatile storage. Here, we investigate the parameter space for optical switching of films of Sb2Se3 for a range of film thicknesses relevant to optical applications. By identifying optimal switching conditions, we demonstrate endurance of up to 107 cycles at reversible switching rates of 20 kHz. Our work demonstrates that the combination of intrinsic film parameters with pumping conditions is particularly critical for achieving high endurance in optical phase change applications.

Absorption and birefringence study for reduced optical losses in diamond with high nitrogen-vacancy concentration.

The use of diamond colour centres such as the nitrogen-vacancy (NV) centre is increasingly enabling quantum sensing and computing applications. Novel concepts like cavity coupling and readout, laser-threshold magnetometry and multi-pass geometries allow significantly improved sensitivity and performance via increased signals and strong light fields. Enabling material properties for these techniques and their further improvements are low optical material losses via optical absorption of signal light and low birefringence. Here, we study systematically the behaviour of absorption around 700 nm and birefringence with increasing nitrogen- and NV-doping, as well as their behaviour during NV creation via diamond growth, electron beam irradiation and annealing treatments. Absorption correlates with increased nitrogen doping yet substitutional nitrogen does not seem to be the direct absorber. Birefringence reduces with increasing nitrogen doping. We identify multiple crystal defect concentrations via absorption spectroscopy and their changes during the material processing steps and thus identify potential causes of absorption and birefringence as well as strategies to fabricate chemical vapour deposition diamonds with high NV density yet low absorption and low birefringence. This article is part of the Theo Murphy meeting issue 'Diamond for quantum applications'.

A New Technique for Broadband Matching of Open-Ended Rectangular Waveguide Radiator.

The maximum reflection at an open end of a standard rectangular waveguide is about -10 dB in its operating frequency range. It is often used without matching. For critical applications, it is desirable to further reduce the reflection coefficient. In this paper, a new technique is presented for the broadband impedance matching of an open-ended rectangular waveguide. The proposed technique employs three thin capacitive matching elements placed at proper intervals via a low-loss dielectric material. The capacitance of, and distance between, the matching elements are optimized for broadband impedance matching using a simulation tool. Based on the proposed technique, two design examples are presented for the matching of a WR75 waveguide radiator. A reflection coefficient of less than -16 dB and -20 dB has been achieved over a ratio bandwidth of 2.13:1 and 1.62:1, respectively.

Misalignment Tolerance of Inductive Power Transfer Coupler With Low Loss and High Magnetic Induction Ferromagnetic Materials

Misalignment Tolerance of Inductive Power Transfer Coupler With Low Loss and High Magnetic Induction Ferromagnetic Materials

Polyimide films with ultralow dielectric loss for 5G applications: Influence and mechanism of ester groups in molecular chains

Polyimides are ideal candidates for 5G high-frequency communications due to their good thermal stability, mechanical properties and processability. However, the exploration of how to reduce their high-frequency dissipation factor (Df) remains an urgent and alluring challenge. Inspired by the molecular structure of liquid crystal polyester, which is insensitive to high-frequency electric fields, here we design two series of polyimides derived from diamines with multiple ester groups (AXEB, X = 1, 2 or 3) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) or p-phenylene bis (trimellitate anhydride) (TA2EB). We investigated the effects of ester number on the aggregation structure, water absorption, dielectric properties, heat resistance and mechanical properties of the polyimides and find that the Df of the multiple ester-containing polyimides decreases rapidly with increasing frequency and number of ester groups. Wide-angle X-ray diffraction analysis, polarized optical microscopy observations and molecular dynamics simulations show that the unique properties of these polyimides benefit from the tight packing of the molecular chain and the strong intermolecular interaction force caused by the ester group, as well as the resulting prolonged dipole relaxation time and the greatly reduced water absorption. Therefore, the polyimide films show extremely low Df values of 0.0030 for AXEB-6FDA and 0.0013 for AXEB-TA2EB. Furthermore, AXEB-TA2EB also shows excellent thermal stability, mechanical properties, electrical properties and permittivity. More importantly, their ultralow moisture rate (<0.83 %) ensures that the high-frequency Df remains stable in different humid environments, even after soaking in water. This method can also be extended to other polymers, thus providing a new strategy for the design of low dielectric loss materials.