Ultra-broad Bandwidth Research Articles

Comb-based photonic neural population for parallel and nonlinear processing

It is believed that neural information representation and processing relies on the neural population instead of a single neuron. In neuromorphic photonics, photonic neurons in the form of nonlinear responses have been extensively studied in single devices and temporal nodes. However, to construct a photonic neural population (PNP), the process of scaling up and massive interconnections remain challenging considering the physical complexity and response latency. Here, we propose a comb-based PNP interconnected by carrier coupling with superior scalability. Two unique properties of neural population are theoretically and experimentally demonstrated in the comb-based PNP, including nonlinear response curves and population activities coding. A classification task of three input patterns with dual radio-frequency (RF) tones is successfully implemented in a time-efficient manner, which allows the comb-based PNP to make effective use of the ultra-broad bandwidth of photonics for parallel and nonlinear processing.

Particle swarm optimization of polymer-embedded broadband metasurface reflectors

Many single-layer dielectric metasurfaces studied today offer optimal performance by maximizing the refractive index contrast between a meta-atom and a background material, thereby increasing the field confinement and enhancing the optical resonance. Such architectures typically utilize high-index dielectric meta-atoms contrasted against an air background. Patterning can be done through deposition and growth of high-index materials or etching designs into a high-index layer. Unfortunately, such structures may be easily damaged, are susceptible to moisture accumulation, and cannot be readily incorporated into multilayer designs. To improve upon the mechanical and environmental stability of conventional dielectric-in-air metasurfaces, we show that dielectric metasurfaces in a solid host matrix can still achieve large enough resonances to exhibit high reflectance over a broad spectral band. Particle swarm optimization is used to discover different broadband reflector designs, each with different performance advantages including ultra-wide broadband reflectance and polarization independence. All designs exhibit a reflectance of R ≥ ~99% over the range of at least Δλ/λ0 ≥ ~20% centered at the wavelength λ0 = 1.55 µm. Finally, a decoupled method approximation is employed to predict the spectral response for a multilayer stack of metasurfaces, but with the added benefit of a lower computational cost.

Broadband highly efficient nonlinear optical processes in on-chip integrated lithium niobate microdisk resonators of Q-factor above 108

Microresonators of ultrahigh quality (Q) factors represent a crucial type of photonic devices aiming at ultra-high spectral resolution, ultra-high sensitivity to the environmental perturbations, and efficient nonlinear wavelength conversions at low threshold pump powers. Lithium niobate on insulator (LNOI) microdisks of high Q factors are particularly attractive due to its large second-order nonlinear coefficient and strong electro-optic property. In this letter, we break through the long standing bottleneck in achieving the Q factors of LNOI microresonators beyond 108, which approaches the intrinsic material absorption limit of lithium niobate (LN). The ultra-high Q factors give rise to a rich family of nonlinear optical phenomena from optical parametric oscillation (OPO) to harmonics generation with unprecedented characteristics including ultra-low pump threshold, high wavelength conversion efficiency, and ultra-broad operation bandwidth. Specifically, the threshold of OPO is measured to be only 19.6 μW, and the absolute conversion efficiency observed in the second harmonic generation reaches 23%. The record-breaking performances of the on-chip ultra-high Q LNOI microresonators will have profound implication for both photonic research and industry.

Facile synthesis of CoFe2O4/reduced graphene oxide nanocomposites with adjusting porous morphology for efficient microwave absorption

Developing porous and lightweight carbon-based magnetic composites has been utilized as an effective strategy to fabricate high-performance microwave absorber. Here, CoFe2O4/reduced graphene oxide (CoFe2O4/RGO) nanocomposites with different porous structure have been synthesized using a one-step solvothermal reaction method. The crystallization, micromorphology, magnetic properties, and electromagnetic parameters can be reasonably adjusted by simply controlling the reaction temperature. CoFe2O4/RGO-200 nanocomposites exhibit a strong reflection loss (RL) of −43.0 dB at 14.4 GHz is obtained with a thin thickness of 1.8 mm. Meanwhile, an ultrabroad effective absorption bandwidth reaching 5.6 GHz (12.4–18.0 GHz) is achieved. Moreover, taking advantage of the porous microstructure, magnetic/dielectric cooperation, synergistic effect, and interfacial polarization simultaneously result in the strong absorption capability and impedance matching, which both contributing to the microwave absorption mechanisms. The superior microwave absorption performance and facile preparation method illustrate that the porous CoFe2O4/RGO nanocomposites can be applied as the efficient microwave absorber for large-scale application.

Tunable ultra-broadband terahertz perfect absorber based on vanadium oxide metamaterial

A dynamically tunable ultra-broadband terahertz perfect metamaterial absorber based on vanadium oxide (VO2) is proposed and numerically demonstrated. The excellent absorption bandwidth of greater than 90% absorptance is as wide as 5.10 THz from 3.03 to 8.13 THz under normal incidence. By changing the conductivity of VO2 from 200 S/m to 2×105 S/m, the absorption intensity can be dynamically tuned from 1.47% to 100%. The ultrabroad bandwidth and flexibility are dramatically improved compared with previously reported VO2 based absorbers. The physical mechanism of the ultra-wideband absorption is discussed based on the interference cancellation, impedance matching theory, and field distributions. The influences of structure parameters on perfect absorption are also discussed. In addition, the absorber has the advantages of insensitivity to polarization and incident angle. Such a tunable ultra-broadband absorber may have promising potential in the applications of modulating, cloaking, switching, and imaging technology.

A dual-tunable ultra-broadband terahertz absorber based on graphene and strontium titanate

An electrically and thermally dual-tunable broadband terahertz absorber based on graphene and strontium titanate is designed and analyzed. The results show that by lifting the Fermi energy of graphene, absorption is significantly enhanced, particularly at higher frequencies, resulting in a broader absorption bandwidth. As the temperature of strontium titanate rises, the center absorption frequency shifts to the higher frequency and the bandwidth increases. At a Fermi energy of 1 eV and a temperature of 400 K, the device exhibits an ultra-broad bandwidth of 3.36 THz and remarkable peak absorption exceeding 99%. Moreover, the absorber is insensitive to incident angles, maintaining a stable broad-bandwidth beyond 3.3 THz within a large incident angle of 55° and 50° for TE and TM polarizations, respectively. The physical mechanisms are elucidated by impedance matching theory and electric field analyses. The structure shows great potential in tunable broadband terahertz absorbers and related applications.

Terahertz Wireless Communications for 2030 and Beyond: A Cutting-Edge Frontier

For 6G in 2030 and beyond, key performance metrics long for terabit-per-second, one-tenth of millisecond latency with zero jitter, millimeter-preci-sion sensing and positioning, and seamless connectivity, among others. To meet these demands, the terahertz (0.1–10 THz) band comes into the horizon, with ultra-broad bandwidth and sub-millimeter wavelength, which, however, suffers from distance limitation and power consumption concerns. In this article, four interdisciplinary directions to address the above challenges and unleash the full potential of THz communications are investigated, namely, integrated sensing and communication, ultra-mas-sive MIMO and dynamic hybrid beamforming, intelligent surfaces, and machine/deep learning. Furthermore, open problems as well as integration of these solutions are elaborated. Qualitative and quantitative studies demonstrate the benefits and performance of the proposed solutions.

Synthesis of Nonspherical Hollow Architecture with Magnetic Fe Core and Ni Decorated Tadpole-Like Shell as Ultrabroad Bandwidth Microwave Absorbers.

The advancement of electromagnetic (EM) protection technology promotes the urgent demand for the structural design of electromagnetic functional materials. Here, tadpole-like Fe@SiO2 @C-Ni (FSCN) composites with magnetic core-shell and nonspherical hollow architectures through multiple hydrolysis-polymerization reactions are reported. The Fe core and well-distributed Ni nanoparticles greatly promote the magnetic properties of FSCN and construct a multiscale magnetic coupling network. Meanwhile, the multishell composites consisting of carbon shell with Ni decorated possess an abundance of heterogeneous interfaces, generating effective interfacial polarization and relaxation. The hollow feature and the coordination of magnetic and dielectric capacities offer an optimized impedance balance, providing a fundament for the microwaves propagating into the absorber. Owing to the attractive effects resulted from the deliberate tadpole-like structure design, the FSCN composites ensure an effective EM energy conversion at the high-frequency region, which obtain the strongest reflection loss value of -45.2dB and the extremely broad effective absorption bandwidth of 13.1GHz. This work provides an important solution for magnetic-dielectric nanostructure design for microwave absorption and energy conversion materials.

Broadband microwave absorption performance and theoretical dielectric properties model of hollow porous carbon spheres/expanded polypropylene composite foams

Hollow porous carbon spheres (HPCS)/Expanded polypropylene (EPP) composite foams were prepared by rapid expansion of supercritical solution and steam chest molding. Due to the uniform and continuous foam structure, HPCS/EPP foams can maintain better loss ability at lower permittivity. Excellent impedance matching enables them to achieve ultra-broad absorption bandwidth of over 17 GHz. Furthermore, the high frequency dielectric properties of HPCS/EPP foams with unique dual percolation microstructure were described by the Two Exponent Phenomenological Percolation Equation (TEPPE) in a two-step framework. It was revealed that the fitting values of the complex permittivity of the foams with different expansion ratios are in good agreement with the experimental values in the frequency band from 0.5 to 18 GHz. This work introduces a new approach for predicting and optimizing the electromagnetic properties of ternary foamed composites.

Ultrabroadband suppression of mid-infrared reflection losses of a layered semiconductor by nanopatterning with a focused ion beam

Moth-eye structures are patterned onto gallium selenide surfaces with sub-micrometer precision. In this way, Fresnel reflection losses are suppressed to below one percent within an ultrabroad optical bandwidth from 15 to 65 THz. We tune the geometry by rigorous coupled-wave analysis. Subsequently, ablation with a Ga+ ion beam serves to write optimized structures in areas covering 30 by 30 μm. The benefits are demonstrated via optical rectification of femtosecond laser pulses under tight focusing, resulting in emission of phase-stable transients in the mid-infrared. We analyze the performance of antireflection coating directly in the time domain by ultrabroadband electro-optic sampling.

An ultra-broadband microwave absorber based on hybrid structure of stereo metamaterial and planar metasurface for the S, C, X and Ku bands

Broadband electromagnetic (EM) wave absorbers are persistently desired in the past decades due to their extensive applications in different fields. In this paper, we proposed and demonstrated a metamaterial absorber (MA) based on a hybrid structure of stereo and planar resistive metamaterial elements. In principle, this hybrid MA is a combination of the Dällenbach layer absorber and circuit analog (CA) absorber. By adjusting its dispersion feature, the stereo metamaterial based on standing resistive patch can work as a broadband artificial Dällenbach layer absorber for C, X and Ku bands. Such stereo metamaterial was also utilized as a dispersive dielectric spacer and a metasurface consisting of resistive circle rings was deployed on the top of it to implement an improved CA absorber for S and C bands. By combing them together, the proposed hybrid MA can achieve absorption over 90% in an ultra-broad bandwidth from S to Ku band (2.59–16.80 GHz). Since our design takes full advantage of the physical space, the hybrid MA achieves an actual thickness as only 1.075 times of the ultimate thickness corresponding to its absorption performance. Furthermore, the hybrid MA has comprehensive advantages in terms of stability against oblique incidence, easy processing and light-weight. The MA prototype was fabricated assisted by the silk screen printing process and its ultra-broadband absorption performance was experimentally validated by the measured results. The new concept of hybrid MA and specific design scheme proposed here provide a promising approach for broadband, low-profile and light weight EM absorber.

Multilayer Stairstep Piezoelectric Structure Design for Ultrabroad-Bandwidth Ultrasonic Transducer

In order to obtain an ultrasonic transducer (UT) with ultra-broad bandwidth (BW), a multilayer stairstep piezoelectric structure (MSPS) is designed and fabricated. Based on the theoretical computation, the relationship between the resonant frequency and thickness of the piezoelectric layer is obtained. The designed MSPS includes 15 layers from center to side, and its frequency range is 5-22 MHz. The finite element method (FEM) based PZFlex software is used to predict the performance of UT with the designed MSPS, and the thickness of each layer is optimized according to the simulation results to achieve the ultra-broad-BW UT. The UT with the designed MSPS is fabricated and systematically characterized. The measured results show that the fabricated UT has a center frequency of 10.54 MHz and a -6 dB BW of 121%, which agree well with the simulation results (11.75 MHz and 140%). The MSPS has significant potential in UT applications.

3D Nickel Skeletons as Ultrabroadband Terahertz Absorbers

AbstractRecent advances in terahertz (THz) absorbing materials and technology show futuristic potentials for practical applications in THz radars and telecommunications, stealth and shielding. However, the lack of versatile materials naturally working in this specific electromagnetic wave region with simultaneously featuring high absorption efficiencies, ultrabroad bandwidths, low‐costs, good stabilities, and flexibilities, is impeding the proliferation of real THz disruptive applications. Here a kind of flexible structure material, 3D nickel (Ni) skeleton, fabricated from an electroplating sintering method with irregular pore distribution makes possible the successful realization of a highly absorbing response for ultrabroadband THz waves due to the effective combination of both material and structural absorption mechanisms. 3D Ni skeletons with 90 ppi nonuniform pore‐size ranges enable >99% absorption capabilities in the frequency range of 0.5–2.0 THz independent on both the THz incidence angles and polarizations. Experimental validation of THz shielding implemented on both 100 GHz and 4.3 THz video imaging systems corroborates the highly efficient absorbing with frequency expansibility. Such capabilities are further verified on millimeter‐wave security checkers for 32–36 GHz. This prototypical demonstration lays the foundation for the next‐generation THz absorbing technology, accelerating advanced THz technologies toward practical applications.

Effect of thickness variations of lithium niobate on insulator waveguide on the frequency spectrum of spontaneous parametric down-conversion* *Project supported by the National Key R&D Program of China (GrantNo. 2019YFA0705000), Leading-edge Technology Program of Jiangsu Natural Science Foundation, China (Grant No. BK20192001), and the National Natural Science Foundation of China (Grant Nos. 51890861, 11690031, 11974178, and 11627810).

We study the effect of waveguide thickness variations on the frequency spectrum of spontaneous parametric down-conversion in the periodically-poled lithium niobate on insulator (LNOI) waveguide. We analyze several variation models and our simulation results show that thickness variations in several nanometers can induce distinct effects on the central peak of the spectrum, such as narrowing, broadening, and splitting. We also prove that the effects of positive and negative variations can be canceled and thus lead to a variation-robust feature and an ultra-broad bandwidth. Our study may promote the development of on-chip photon sources in the LNOI platform, as well as opensup a way to engineer photon frequency state.

High-absorption grating-insulator-metal structures.

A thin grating-insulator-metal (GIM) structure consisting of a top metal grating layer on a dielectric layer and a bottom metal layer is proposed, which shows a broadband high absorption at a small thickness. This phenomenon is attributed to the appropriate effective surface permittivity of the top grating layer and the cavity resonance of the middle insulator layer. By optimizing the structural and material parameters, the materials of the GIM structure from top to bottom are Mn, Al2O3, and Mn with thicknesses of 10, 70, and 70 nm, respectively. The structure with these optimum parameters is fabricated and characterized, and an improved performance with absorption exceeding 90% in the visible region is obtained using Mn as the metal layers. The experimental results are in good agreement with the numerical values, depicting an ultrabroad absorption bandwidth. The conclusions presented here could have potential applications in optical devices used for optical displacement detection and visible light absorption.

Efficient generation of UV-enhanced intense supercontinuum in solids: Toward sub-cycle transient

Ultrabroadband generation of white-light continuum spanning from ultraviolet to near-infrared (375–920 nm) is demonstrated by using induced-phase modulation between two-color femtosecond pulses in multiple thin plates. The fundamental wave and its second-harmonic wave from one Ti:sapphire chirped-pulse amplifier are injected into nine 100 μm-thick fused silica plates. When the two pulses are temporally and spatially optimized in the plates, an intense supercontinuum is efficiently achieved by utilizing self-phase modulation and self-steepening together with induced-phase modulation. As a result, using 1 kHz, 1 mJ, and 35 fs pulses at 800 nm as the pump, we demonstrate the generation of 0.6 mJ white-light pulses with an ultra-broad bandwidth supporting 1.6 fs transform-limited pulses corresponding to 0.6-optical-cycle at a central wavelength of 750 nm. Analysis of the calculated spectrum fits the experimental results well, and the pulse is quasi-linearly chirped and compressible. The resulting intense robust supercontinuum could be a promising light source for an ever greater degree of tailored optical waveform coherent control in new parameter spaces.

Achieving broad absorption bandwidth of the Co/carbon absorbers through the high-frequency structure simulator electromagnetic simulation

Confronting with increasing electromagnetic pollution, traditional microwave absorbing materials have their limitations, such as low yield and long preparation periods. At the same time, it is well established that electromagnetic simulation is an effective technology to enhance work efficiency, which can quickly explore the impedance matching conditions and electromagnetic characteristics to improve absorbing performance. Honeycomb structure has been widely used in the field of microwave absorption due to its light weight, high strength and heat resistance. This structure also possesses the advantages of multiple scattering, which is beneficial to microwave attenuation. In this work, Co/carbon (Co/C) particles with effective absorption bandwidth (EAB) (the band corresponding to the reflection loss value less than −10 dB) of 5.8 GHz were synthesized by a simple hydrothermal method. Whereupon through the high-frequency structure simulator (HFSS) electromagnetic simulation, Co/C absorbers with honeycomb structure were macroscopically designed, achieving an ultra-broad effective absorption bandwidth of 11.6 GHz. Herein, the effective combination of electromagnetic simulation and honeycomb structure design greatly improves the absorption bandwidth of Co/C. This work paves the way for designing microwave absorption materials with broad effective absorption bandwidth.

Tantala Kerr nonlinear integrated photonics

Integrated photonics plays a central role in modern science and technology, enabling experiments from nonlinear science to quantum information, ultraprecise measurements and sensing, and advanced applications such as data communication and signal processing. Optical materials with favorable properties are essential for nanofabrication of integrated photonics devices. Here we describe a material for integrated nonlinear photonics, tantalum pentoxide ( T a 2 O 5 , hereafter tantala), which offers low intrinsic material stress, low optical loss, and efficient access to Kerr nonlinear processes. We utilize > 800 n m thick tantala films deposited via ion beam sputtering on oxidized silicon wafers. The tantala films contain a low residual tensile stress of 38 MPa, and they offer a Kerr index of n 2 = 6.2 ± 2.3 × 10 − 19 m 2 / W , which is approximately a factor of 3 higher than silicon nitride. We fabricate integrated nonlinear resonators and waveguides without the cracking challenges that are prevalent in stoichiometric silicon nitride. The tantala resonators feature an optical quality factor up to 3.8 × 10 6 , which enables us to generate ultrabroad bandwidth Kerr soliton frequency combs with low threshold power. Moreover, we characterize tantala waveguides by supercontinuum generation from low-energy, ultrafast seed pulses. These demonstrations characterize future application directions with tantala integrated nonlinear photonics.

Electromagnetic wave absorption performance and electrochemical properties of multifunctional materials: Air@Co@Co3Sn2@SnO2 hollow sphere/reduced graphene oxide composites

Herein, a facile and controllable structural fabrication strategy is presented for the fabrication of multinanoshell Air@metal@intermetallic@oxide-like Air@Co@Co3Sn2@SnO2 hollow spheres with a multinanoshell (HSMNS) structure using a hollow Co sphere template and a simple hydrothermal method. Composites of Air@Co@Co3Sn2@SnO2 HSMNS/reduced graphene oxide (RGO) were obtained, and the electromagnetic wave (EW) absorbing properties of the composites as microwave-absorbing materials as well as the electrochemical properties as the anode electrode materials of lithium-ion batteries (LIBs) were investigated. The obtained results demonstrate that the Air@Co@Co3Sn2@SnO2/RGO composite exhibited remarkable EW absorbing properties with an extremely high reflection loss of −55.49 dB and an ultrabroad effective absorption bandwidth of 5.43 GHz when loaded at only 5 wt% in a paraffin coating owing to the strengthened synergistic effects of the magnetic metal, resistive intermetallic, and dielectric metal oxide shells. In addition, the composite showed a superior performance with long-term cycling stability (960 mAh g−1 after 180 cycles at a current density of 0.1 A g−1) when used as the anode of an LIB. These results indicate that the composite of Air@Co@Co3Sn2@SnO2 HSMNS/RGO, which exhibits light-weight, highly efficient absorbing properties, and long-term cycling stability as the anode electrode of LIBs, has a promising future in EW absorption and electrochemical fields. We hope that this strategy will provide a new method for the construction of complex multinanoshell hollow structural materials in other research fields.

Optical parametric amplification at critical wavelength degeneracy—a proposed approach for 100-PW class femtosecond laser development

At signal critical wavelength degeneracy (CWD), super-broad parametric gain bandwidths (SBGBs) can be obtained in nonlinear crystals. A 280 nm SBGB, for ∼60 small signal peak gain, has been calculated considering a collinear optical parametric chirped pulse amplification (OPCPA) at CWD in a 40 mm long, 58.3% partially deuterated KDP (P-DKDP) crystal, pumped by a frequency doubled nanosecond Nd:glass laser at 1 GW/cm2 pump intensity. This gain bandwidth in the 1 µm spectral range is significantly broader compared to the 150 nm gain bandwidth, calculated under similar conditions, for the broad-band non-collinear (NC) OPCPA in DKDP crystals in the 900 nm spectral range. It can support the amplification of significantly shorter femtosecond pulses, giving rise to a higher peak power at the same amplified pulse energy. The spectral profile of the gain bandwidth can be shaped by slightly adjusting the deuteration ratio (DR) of the P-DKDP crystals. A small-angle non-collinear OPCPA near the CWD can be used for the spatial separation of the signal and idler beams with the same polarization, keeping at the same time an ultra-broad gain bandwidth. A SBGB of 255 nm FWHM has been calculated for a non-collinear OPCPA stage with a 40 mm long P-DKDP crystal, 10.3 mrad signal-pump wavevectors internal angle, 57.8% DR, pumped by a few-ns 527 nm wavelength laser at 1 GW/cm2 pump intensity. The half-meter size P-DKDP crystals, pumped by frequency doubled few-ns multi-kJ Nd:glass lasers, are able to amplify super broad-band nanosecond stretched pulses up to the kJ energy level. They might be appropriate amplifying media for the development of single-beam 100-PW class femtosecond laser systems based on OPCPA at CWD.