Low-loss Materials Research Articles

Terahertz microresonators for material characterisation

Terahertz (THz) technology is rapidly evolving, and the advancement of data and information processing devices is essential. Silicon THz microresonators provide perfect platforms to develop compact and integrated devices that could transform THz technology. Here we present a systematic study on the key figure of merit of silicon THz disc microresonators - the quality factor (Q-factor) - in dependence on the substrate’s resistivity. Our results show that the Q-factor depends linearly on the resistivity and a variation in resistivity from 10 kΩcm to 15 kΩcm changes the Q-factor from 50k to 76k at 0.6 THz. Moreover, we experimentally determine that the silicon material absorption is inversely proportional to the substrate’s resistivity. In general, the presented methodology is ideally suited to precisely measure the material absorption of low-loss materials in the THz domain, which is challenging using conventional THz spectroscopy.

Visible and near-infrared dual band switchable metasurface edge imaging

Optical edge detection at the visible and near infrared (VNIR) wavelengths is deployed widely in many areas. Here we demonstrate numerically transmissive VNIR dual band edge imaging with a switchable metasurface. Tunability is enabled by using a low-loss and reversible phase-change material Sb2S3. The metasurface acts simultaneously as a high-pass spatial filter and a tunable spectral filter, giving the system the freedom to switch between two functions. In Function 1 with amorphous Sb2S3, this metasurface operates in the edge detection mode near 575 nm and blocks near infrared (NIR) transmission. In Function 2 with crystalline Sb2S3, the device images edges near 825 nm and blocks visible light images. The switchable Sb2S3 metasurfaces allow low cross talk edge imaging of a target without complicated optomechanics.

Composition, Roughness, and Topography from Radar Backscatter at Selk Crater, the Dragonfly Landing Site

The Selk crater region is the future landing site of NASA’s Dragonfly mission to Titan. The region was imaged by the Cassini RADAR at incidence angles from 5° to 72° and at various polarization angles. Using this data set, we mapped six terrain units and assembled a backscatter curve for each, providing normalized backscatter cross section (σ 0) as a function of incidence angle. By fitting these backscatter curves with a sum of a quasi-specular and diffuse terms and evaluating three alternative formulations of the first and two for the second, we extracted the best-fit surface effective dielectric constant, rms slope, and scattering albedo. Although the parameters’ absolute values are model dependent, relative values between terrains indicate real variations in surface properties. The results are consistent with the impact exposing and fracturing a low-loss tangent material such as the water-ice bedrock, which is likely also present in the hummocky terrains and to a lesser degree in the plains and interdune regions. The dunes and dark terrains are composed of smooth, uniform material with low dielectric constant (1.5–2.3 median values for all models) compatible with organic sand. A diffuse single-scattering model enabled independent derivation of the dielectric constant from high-incidence observations, leading to low values (<2) over all terrains, indicating a depolarizing (sub)surface. Finally, radarclinometry revealed lateral variations in rim height, which remains below 300 m along the SARTopo profile but reaches up to 600 m at other locations, hinting at a rim less eroded than previously thought.

Modulation of dielectric properties in low-loss polypropylene-based composites at GHz frequencies: theory and experiment

Polymer composites with high dielectric constant and low loss tangent are highly regarded as substrates for modern high-speed electronics. In this work, we analyze the high-frequency dielectric properties of two types of composites based on polypropylene infused with high-dielectric-constant microparticles. Two types of fillers are used: commercial ceramics or titanium oxide (TiO2) with different concentrations. The key observation is that adding the fillers causes an increase of dielectric constants by around 100% (for highest loading) up to 4.2 and 3.4, for micro-ceramics and TiO2 based composites, respectively. Interestingly, for the TiO2 composite, the loss tangent depends on the filler loading volume, whereas the other composite has a slightly increasing tendency, however, being at the level ~ 10–3. To explain the experimental results, a theoretical model determined by microwave reflection and transmission through a representative volume element is proposed, which allows the investigation of the impact of volume ratio, grain shape, aggregation, and size on the loss tangent and permittivity evolution. This approach could be used for modeling other low dielectric loss materials with inclusions.

Design of a broadband dispersion compensated ultra-high nonlinear photonic crystal fiber

A photonic crystal fiber (PCF) with four circular rings of air holes expanded toward the cladding region is proposed. Four circular tiny air hole rings have been used between the air holes in a regular circular PCF to achieve low dispersion and confinement loss. Additionally, the core region is perforated with a rectangular-shaped hole filled with an extremely nonlinear material, gallium phosphide, to achieve the desired level of nonlinearity. We achieved extremely high nonlinearity and birefringence values of 4.6104 W−1 km−1 and 0.078 at the 1.55 µm telecommunication window by doing so. Further, we observed the structure with varying pitch (Ʌ) values and found a significant reduction in dispersion and confinement loss, as well as a decrease in effective material loss. Thus, at 1.55 µm, an ultra-high negative dispersion of − 8000 ps/nm km is achieved, particularly with Ʌ = 1.8 µm, along with extremely low confinement and material losses of 10–9 dB/km and 0.017 cm−1, respectively. Similarly, other critical parameters such as the power fraction, numerical aperture, and effective area have been examined. Hence, owing to these enhanced optical properties, the proposed PCF is capable of effectively compensating for dispersion, generating supercontinuum, and maintaining polarization.

Monolithic Photonic Integrated Circuit Based on Silicon Nitride and Lithium Niobate on Insulator Hybrid Platform

Lithium niobate on insulator (LNOI) has been demonstrated as a promising platform for photonic integrated circuits (PICs), thanks to its excellent properties such as strong electro‐optic effect, low material loss, and wide transparency window. Herein, a monolithic PIC for high‐speed data communication application on a lithium‐niobate‐etchless platform with silicon nitride (Si3N4) as a loading material is proposed and demonstrated. The fabricated PIC consists of four racetrack resonator modulators and a pair of four‐channel mode (de)multiplexers, which shows high data modulation rate of 70 Gbps for single channel and the total data throughput reaches up to 280 Gbps. To the best of knowledge, this is the first demonstration of PIC consisting of high‐speed electro‐optical modulators and (de)multiplexers with such high data capacity on Si3N4‐LNOI hybrid platform, which opens up new avenues for achieving large‐scale monolithic integration on LNOI platform in future.

Transfer characteristics for laser-induced forward transfer of stainless steel particle

Additive manufacturing (AM) holds great development potential in several applications. In particular, the fabrication of metal parts has been rapidly developing as an industry recently. In such applications, a powder feeder with low material loss is required to ensure effective material usage. Thus, the manufacturing process using laser-induced forward transfer (LIFT) is attractive as a candidate for advances in powder feeder technology. We show that stainless steel particle with diameters of 26 to 53 μm can be transferred via the particle LIFT process; the transfer of particles started at a peak fluence of ∼0.05 J cm − 2 (26- and 36-μm diameters) and ∼0.15 J cm − 2 (53-μm diameter), and the particles were fully transferred (i.e., the probability of transfer reaches unity) at a peak fluence of 0.2 (26 μm), 0.4 (39 μm), and 0.5 J cm − 2 (53 μm). The initial velocity, v0, at which the particle leaves the substrate increased proportionally with the increase in peak fluence. When the point of irradiation of the laser beam was displaced from the contact point of the particle on the substrate, the particle was transferred with a transfer angle (i.e., not vertically). These results demonstrate the possibility of transferring metal particles via the LIFT process. To use this technology for AM, the laser power should be adjusted to control the transfer angle of the particle.

Enhancement of radiation-induced EPR signal in bioapatites

Amplification of a dosimetric EPR signal in mineralized biological materials available in limited quantities has been demonstrated in this paper. Powders of irradiated enamel, dentin, and bone tissue were placed into silica ampoules with the outer diameter close to 1.4 mm. To amplify the signal, the dielectric insert in the form of cylinder with the outer radius 2.85 mm, the inner radius 0.75 mm, and the height 1.85 mm made of a high-κ and low-loss ceramic material BaTi4O9 + 8.5% ZnO has been used. It has been shown that maximum signal amplification (about an order of magnitude) has been achieved when the sample is completely inserted into this dielectric. It has been found that the line shape of the dosimetric signal is not distorted, if using the dielectric insert. Decomposition of the amplified EPR spectra allowed us to determine the relative contribution of two types of CO2– radicals to the dosimetric signal, which coincides with the literature data.

Magnetron-sputtered and thermal-evaporated low-loss Sb-Se phase-change films in non-volatile integrated photonics

Chalcogenide phase change materials (PCMs), featuring a large contrast in optical properties between their non-volatile amorphous and crystalline states, have triggered a surge of interest for their applications in ultra-compact photonic integrated circuits with long-term near-zero power consumption. Over the past decade, however, PCM-integrated photonic devices and networks suffered from the huge optical loss of various commonly-used PCMs themselves. In this paper, we focused on the deposition, characterization, and monolithic integration of an emerging low-loss phase change material, Sb2Se3 on a silicon photonic platform. The refractive index contrast between the amorphous and crystalline phase of the evaporated Sb-Se thin film was optimized up to 0.823 while the extinction coefficient remains less than 10−5 measured by ellipsometry. When integrated on a silicon waveguide, the propagation loss introduced by the amorphous thin film is negligibly low. After crystallization, the propagation loss of a magnetron-sputtered Sb-Se patch-covered silicon waveguide is as low as 0.019 dB/µm, while its thermal-evaporated counterpart is below 0.036 dB/µm.

Surface electromagnetic waves in lossy conductive media: tutorial

It is commonly believed that electromagnetic waves cannot propagate in lossy conductive media and that they quickly decay inside such media over short length scales of the order of so-called skin depth. In this tutorial, I demonstrate that this common belief is incorrect if the conductive medium exhibits strong gradients of dielectric permittivity. In fact, surface electromagnetic waves in such gradient-permittivity lossy conductive media may have a propagating character, and the propagation length of such waves may be considerably longer than skin depth. Similar to surface plasmons, the wavelength of these waves may be considerably shorter than the light wavelength in free space, which may enable applications in super-resolution microscopy and nanolithography techniques. However, unlike plasmonics-based nanophotonic devices, which are typically built using a very limited number of low-loss optical materials, the newly found class of surface waves may be supported by a much broader range of lossy media. For example, such materials as graphite and silicon seem to be ideal in UV nanophotonics applications.

Wide-Angle Tunable Critical Coupling in Nanophotonic Optical Coatings with Low-Loss Phase Change Material.

Realizing perfect light absorption in stacked thin films of dielectrics and metals through critical light coupling has recently received intensive research attention. In addition, realizing ultra-thin perfect absorber and tunable perfect absorber in the visible spectrum is essential for novel optoelectronics applications. However, the existing thin film stacks cannot show tunable perfect absorption in a wide-angle range. Here, a tunable perfect absorption from normal incidence to a wide-angle range (0° to 70°) by utilizing a two-layer stack consisting of a high refractive index low-loss dielectric on a high reflecting metal is proposed. This is experimentally demonstrated by depositing a thin layer of low-loss phase change material such as stibnite (Sb2 S3 ) on a thin layer of silver. This structure shows tunable perfect absorption with large spectral tunability in the visible wavelength. Furthermore, the absorption enhancement in 2D materials by transferring monolayer molybdenum disulfide on the stack, which shows 96% light absorption with enhanced photoluminescence, is demonstrated. In addition, the thin film stack can work as a scalable phase modulator offering a maximum phase tunability of ≈140° by changing the structural state of Sb2 S3 from amorphous to crystalline.

Directional Damping of Plasmons at Metal-Semiconductor Interfaces.

ConspectusOver the past decade, it has been shown that surface plasmons can enhance photoelectric conversion in photovoltaics, photocatalysis, and other optoelectronic applications through their plasmonic absorption and damping processes. However, plasmonically enhanced devices have yet to routinely match or exceed the efficiencies of traditional semiconductor devices. The effect of plasmonic losses dissipates the absorbed photoenergy mostly into heat and that has hampered the realization of superior next-generation plasmonic optoelectronic devices. Several approaches are being explored to alleviate this situation, including using gain to compensate for the plasmonic losses, designing and synthesizing alternative low-loss plasmonic materials, and reducing activation barriers in plasmonic devices and physical thicknesses of photoabsorber layers to lower the plasmonic losses. A newly proposed plasmon-induced interfacial charge-transfer transition (PIICTT) mechanism has proven to be effective in minimizing energy loss during interfacial charge transfer. The PIICTT leads to a damping of metallic plasmonics by directly generating excitons at the plasmonic metal/semiconductor heteronanostructures. This novel concept has been proven to overcome some of the limitations of electron-transfer inefficiencies, renewing a focus on surface plasmon damping processes with the goal that the plasmonic excitation energies of metal nanoparticles can be more efficiently transferred to the adjacent semiconductor components in the absence and presence of an effective interlayer of carrier-selective blocking layer (CSBL). Several theoretical and experimental studies have concluded that efficient plasmon-induced ultrafast hot-carrier transfer was observed in plasmonic-metal/semiconductor heteronanostructures. The PIICTT mechanism may well be a general phenomenon at plasmonic metal/semiconductor, metal/molecule, semiconductor/semiconductor, and semiconductor/molecule heterointerfaces. Thus, the PIICTT presents a new opportunity to limit energy loss in plasmonic-metal nanostructures and increase device efficiencies based on plasmonic coupling. The nonradiative damping of surface plasmons can impact the energy flux direction and thereby provide a new process beyond light trapping, focusing, and hot carrier creation.In this Account, we draw much attention to the benefits of interfacial plasmonic coupling, highlighting recent pioneering discoveries in which plasmon-induced interfacial charge- and energy-transfer processes enable the generation of hot charge carriers near the plasmonic-metal/semiconductor interfaces. This process is likely to increase the photoelectric conversion efficiency, constituting "plasmonic enhancement". We also discuss recent advances in the dynamics of surface plasmon relaxation and highlight exciting new possibilities for plasmonic metals and their interactions with strongly attached semiconductors to provide directional energy fluxes. While this new research area comes on the heels of much elaborate research on both metal and semiconductor nanomaterials, it provides a subtle but important refinement in understanding the optoelectronic properties of materials with far-reaching consequences from fundamental interface science to technological applications. We hope that this Account will contribute to a more systematic description of interface-coupled plasmonics, both fundamentally and in terms of applications toward the design of plasmonic heterostructured devices.

Complex Permittivity Measurement of Low-Loss Anisotropic Dielectric Materials at Hundreds of Megahertz

Resonant methods are well known for their accuracy in determining the complex permittivity of materials. However, to guarantee the accuracy, interference modes must be suppressed in the measurement band. In this paper, a multi-mode split rectangular cavity is designed and fabricated to measure low-loss anisotropic dielectric materials in the frequency range from 300 to 1000 MHz. Details of the cavity design are presented. Simulation results for a uniaxially anisotropic material validate both the setup and the processing. Moreover, validation measurements on a sheet of polytetrafluoroethylene (PTFE) are in agreement with the literature data, which indicate that the proposed method in this paper is reliable and accurate for low-loss complex permittivity characterization at hundreds of megahertz.

Compact dual-core liquid crystal photonic crystal fiber polarization splitter for terahertz applications

Compact terahertz dual core photonic crystal fiber (THz-DC-PCF) polarization splitter is presented and analyzed. The dual core is infiltrated with a nematic liquid crystal (NLC) of type E7 that produces high birefringence between the two polarized modes. The suggested design has a TOPAS as a background material with low material loss. The effect of the structure geometrical parameters is investigated to achieve compact polarization splitter. In addition, the impact of the temperature dependence of the NLC in the THz range on the performance of the suggested splitter is also studied. The numerical results show that the THz-DC-PCF has a device length of 3.848 cm with a low crosstalk of −62 dB for the x- and y-polarized modes. Further, large bandwidth of 58 GHz is achieved around the central frequency of 0.6 THz where the crosstalk is better than −10 dB with a good fabrication tolerance. Therefore, the design is less sensitive to the perturbation introduced during the fabrication process with a strong potential for THz applications.

Hot-Corrosion and Particle Erosion Resistance of Co-Based Brazed Alloy Coatings

Tape brazing constitutes a cost-effective alternative surface protection technology for complex-shaped surfaces. The study explores the characteristics of high-temperature brazed coatings using a cobalt-based powder deposited on a stainless-steel substrate in order to protect parts subjected to hot temperatures in a wear-exposed environment. Microstructural imaging corroborated with x-ray diffraction analysis showed a complex phased structure consisting of intermetallic Cr-Ni, C-Co-W Laves type, and chromium carbide phases. The surface properties of the coatings, targeting hot corrosion behavior, erosion, wear resistance, and microhardness, were evaluated. The high-temperature corrosion test was performed for 100 h at 750 °C in a salt mixture consisting of 25 wt.% NaCl + 75 wt.% Na2SO4. The degree of corrosion attack was closely connected with the exposure temperature, and the degradation of the material corresponding to the mechanisms of low-temperature hot corrosion. The erosion tests were carried out using alumina particles at a 90° impingement angle. The results, correlated with the microhardness measurements, have shown that Co-based coatings exhibited approximately 40% lower material loss compared to that of the steel substrate.

Effect of Cationic/Anionic Diffusion Dominated Passive Film Growth on Tribocorrosion

Tribocorrosion behaviours of nickel (Ni) and niobium (Nb) in sodium sulfate (Na2SO4) solution under potentiodynamic and potentiostatic conditions were studied. Under the potentiodynamic condition, the passivation was early broken, accompanied by a sharp increase in frictional coefficient on Nb. The current was more fluctuant, and larger material loss appeared at the higher potential in the potentiostatic condition. However, these phenomena did not occur for Ni, and it even showed lower material loss at the higher potential in the potentiostatic tribocorrosion test. The differences in tribocorrosion behaviour had a close relationship to the passive film growth mechanism, which decided the passive film/metal interface structure. Nb with anionic diffusion dominated mechanism in passive growth would cause the accumulation of oxygen vacancies at the passive film/metal interface. This may weaken the adhesion between the metal and the passive film. However, with the cationic diffusion dominated passive film growth on Ni, cation vacancies concentrated at the passive film/tribo-film interface, and this did not affect the adhesion between metal and passive film. Ni or other passive elements with the cationic diffusion-dominated mechanism in passive film growth were recommended as the alloying element for improving the tribocorrosion resistance of alloys.

Dielectric properties and stratigraphy of regolith in the lunar South Pole-Aitken basin: Observations from the Lunar Penetrating Radar

Aims.We examine data obtained by the Lunar Penetrating Radar (LPR) onboard the Chang'E-4 (CE-4) mission to study the dielectric properties and stratigraphy of lunar regolith on the far side of the Moon.Methods.The data collected from January 2019 to September 2020 were processed to generate a 540 m radargram. The travel velocity of the radar signal and the permittivity of the regolith were deduced from hyperbolas in the radargram. As CE-4 LPR detected distinct planar reflectors, we evaluated the dielectric loss from the maximum penetration depth based on the radar equation. The derived dielectric properties are compared with the measurements of Apollo samples and Chang'E-2 microwave radiometer observations.Results.The results suggest that regolith at the landing site has a permittivity of 2.64–3.85 and a loss tangent of 0.0032–0.0044, indicating that the local regolith is composed of a fine-grained, low-loss material that is much more homogeneous than that found at the Chang’E-3 landing site. The total thickness of weathered material is ~40m, with several regolith layers and a buried crater identified in the reconstructed subsurface structure.Conclusions.These layers clearly record a series of impact events from the adjacent regions. We suggest that the top layer is primarily made up of the ejecta from a large crater 140 km away. In contrast, the material source of other thinner layers comes from nearby smaller craters.

High-performance distributed feedback quantum dot lasers with laterally coupled dielectric gratings

The combination of grating-based frequency-selective optical feedback mechanisms, such as distributed feedback (DFB) or distributed Bragg reflector (DBR) structures, with quantum dot (QD) gain materials is a main approach towards ultrahigh-performance semiconductor lasers for many key novel applications, as either stand-alone sources or on-chip sources in photonic integrated circuits. However, the fabrication of conventional buried Bragg grating structures on GaAs, GaAs/Si, GaSb, and other material platforms has been met with major material regrowth difficulties. We report a novel and universal approach of introducing laterally coupled dielectric Bragg gratings to semiconductor lasers that allows highly controllable, reliable, and strong coupling between the grating and the optical mode. We implement such a grating structure in a low-loss amorphous silicon material alongside GaAs lasers with InAs/GaAs QD gain layers. The resulting DFB laser arrays emit at pre-designed 0.8 THz local area network wavelength division multiplexing frequency intervals in the 1300 nm band with record performance parameters, including sidemode suppression ratios as high as 52.7 dB, continuous-wave output power of 26.6 mW (room temperature) and 6 mW (at 55°C), and ultralow relative intensity noise (RIN) of &lt; − 165 dB / Hz (2.5–20 GHz). The devices are also capable of isolator-free operating under very high external reflection levels of up to − 12.3 dB while maintaining high spectral purity and ultralow RIN qualities. These results validate the novel laterally coupled dielectric grating as a technologically superior and potentially cost-effective approach for fabricating DFB and DBR lasers free of their semiconductor material constraints, which are thus universally applicable across different material platforms and wavelength bands.

Hydrogenated Zinc Oxide as an Alternative Low-Loss Plasmonic Material with Fano Resonance in Near-IR

Hydrogenated Zinc Oxide as an Alternative Low-Loss Plasmonic Material with Fano Resonance in Near-IR

Vacuum ultraviolet nonlinear metalens

Vacuum ultraviolet (VUV) light plays an essential role across science and technology, from molecular spectroscopy to nanolithography and biomedical procedures. Realizing nanoscale devices for VUV light generation and control is critical for next-generation VUV sources and systems, but the scarcity of low-loss VUV materials creates a substantial challenge. We demonstrate a metalens that both generates—by second-harmonic generation—and simultaneously focuses the generated VUV light. The metalens consists of 150-nm-thick zinc oxide (ZnO) nanoresonators that convert 394 nm (~3.15 eV) light into focused 197-nm (~6.29 eV) radiation, producing a spot 1.7 μm in diameter with a 21-fold power density enhancement as compared to the wavefront at the metalens surface. The reported metalens is ultracompact and phase-matching free, allowing substantial streamlining of VUV system design and facilitating more advanced applications. This work provides a useful platform for developing low-loss VUV components and increasing the accessibility of the VUV regime.