Ultra-broad Bandwidth Research Articles

High-Performance On-Chip Silicon Beamsplitter Based on Subwavelength Metamaterials for Enhanced Fabrication Tolerance.

Efficient power splitting is a fundamental functionality in silicon photonic integrated circuits, but state-of-the-art power-division architectures are hampered by limited operational bandwidth, high sensitivity to fabrication errors or large footprints. In particular, traditional Y-junction power splitters suffer from fundamental mode losses due to limited fabrication resolution near the junction tip. In order to circumvent this limitation, we propose a new type of high-performance Y-junction power splitter that incorporates subwavelength metamaterials. Full three-dimensional simulations show a fundamental mode excess loss below 0.1 dB in an ultra-broad bandwidth of 300 nm (1400–1700 nm) when optimized for a fabrication resolution of 50 nm, and under 0.3 dB in a 350 nm extended bandwidth (1350–1700 nm) for a 100 nm resolution. Moreover, analysis of fabrication tolerances shows robust operation for the fundamental mode to etching errors up to ±20 nm. A proof-of-concept device provides an initial validation of its operation principle, showing experimental excess losses lower than 0.2 dB in a 195 nm bandwidth for the best-case resolution scenario (i.e., 50 nm).

SIABR: A Structured Intra-Attention Bidirectional Recurrent Deep Learning Method for Ultra-Accurate Terahertz Indoor Localization

High-accuracy localization technology has gained increasing attention in gesture and motion control and many diverse applications. Due to multi-path fading and blockage effects in indoor propagation, 0.1m-level precise localization is still challenging. Promising for 6G wireless communications, the Terahertz (THz) spectrum provides multi-GHz ultra-broad bandwidth. Applying the THz spectrum to indoor localization, the channel state information (CSI) of THz signals, including angle of arrival (AoA), received power, and delay, has unprecedented resolution that can be explored for positioning. In this paper, a Structured Intra-Attention Bidirectional Recurrent (SIABR) deep learning method is proposed to solve the CSI-based three-dimensional (3D) THz indoor localization problem with significantly improved accuracy. As a two-level structure, the features of individual multi-path rays are first analyzed in the recurrent neural network with the attention mechanism at the lower level. Furthermore, the upper-level residual network (ResNet) of the constructed SIABR network extracts hidden information to output the geometric coordinates. Simulation results demonstrate that the 3D localization accuracy in the metric of mean distance error is within 0.25m. The developed SIABR network has very fast convergence and is robust against THz indoor line-of-sight blockage, multi-path fading, channel sparsity and CSI estimation error.

Mid-infrared broadband polarization beam splitter based on GaS photonic crystal fiber with high extinction ratio

We design a mid-infrared polarization beam splitter (PBS) based on GaS chalcogenide dual-core photonic crystal fiber (PCF) with large bandwidth and high polarization extinction ratio (ER). The mode-coupling characteristics between the x- and y-polarized even and odd modes are investigated in the 3.5–4.5 µm range by employing the finite element method (FEM). The effects of PBS structure parameters on the coupling length (CL) and coupling length ratio (CLR) are analyzed. The results reveal that for the optimized structural parameters of a = 0.8 µm, b = 0.23 µm, d1 = 1 µm, d2 = 0.5 µm, e1 = 2.02 µm and e2 = 1.5 µm, an ultrabroad bandwidth of 412 nm (from 3.766 µm to 4.188 µm) with polarization ER better than 20 dB is achieved within an ultrashort length of 45.4 µm. The highest ER of 104 dB at the wavelength of 4 µm is obtained. The proposed PBS will become an excellent candidate for mid-infrared optical communication, optical sensing, optical precision metrology and optical nonlinearity.

Yolk-shell structured Co@SiO2@Void@C nanocomposite with tunable cavity prepared by etching of SiO2 as high-efficiency microwave absorber

Nowadays, high-performance microwave absorption materials with light weight, strong absorbing intensity and wide absorption bandwidth are urgently demanded to solve the electromagnetic pollution issues. In this work, the yolk-shell structured Co@SiO2@Void@C nanocomposites with tunable cavity are obtained by etching SiO2 in the Co@SiO2@C nanoparticles. They exhibit better microwave absorption properties than the unetched counterpart. When the etching time is 6 h, the Co@SiO2@Void@C nanocomposite shows high absorption efficiency with a minimum reflection loss (RL) value of −44.5 dB at 8.8 GHz. Notably, its effective absorption bandwidth (RL < −10 dB) is as wide as 8.0 GHz (9.7–17.7 GHz) at a thin thickness of only 1.7 mm. The excellent microwave absorbing performances are attributed to the abundant heterointerfaces, well-controlled cavity, synergistic effects between magnetic and dielectric loss, and optimal impedance matching. Owing to the characteristics of strong absorbing capacity, ultrabroad absorption bandwidth and thin matching thickness, the yolk-shell structured Co@SiO2@Void@C nanocomposites are promising candidates as highly effective microwave absorbers.

Highly stretchable and self-foaming polyurethane composite skeleton with thermally tunable microwave absorption properties

Stretchable and lightweight polymer composite material possessing tunable microwave absorption (MA) properties under thermal radiations remain a significant challenge. Here, we proposed a facile strategy to fabricate stretchable, magnetic composite skeletons by incorporating the tadpole-like CNTs@Fe3O4 nanoparticles into self-foaming polyurethane (PU) matrix and the electromagnetic responsive of CNTs@Fe3O4/PU composite foams with different CNTs contents under heating−cooling cycle in a temperature range of 253 −333 K were carefully investigated. Enhanced complex permittivity and shifting peak frequency were observed at elevated temperatures. For instance, the 70-CNTs@Fe3O4/PU sample with 15 wt% loading content at 333 K exhibits excellent MA properties including a minimum reflection loss (RLm) of −66.9 dB and ultrabroad effective frequency bandwidth (RL ≤ −20 dB) of 9.98 GHz at the thickness of 1.58−3.37 mm. Meanwhile, great recoverability in terms of RL-f profile was achieved in the process of thermal cooling back to 253 K. Such adjustable MA property was attributed to the well-matched impedance and dramatic attenuation ability, benefiting from the temperature-dependant electrical conductivity, abundant interfacial polarization and interior microcellular structures. Besides, the rising temperature increased the sample elongation and electrical conductivity with a slight sacrifice of maximum tensile strength. This stretchable PU skeleton with a unique assembly of CNTs and Fe3O4 nanoparticles are expected to be promising candidates as smart absorbers for application in the harsh environments.

Endless single-polarization single-mode photonic-crystal planar waveguide arrays with ultra-large mode area and ultra-low transmission loss

In this paper, single-polarization single-mode (SPSM) optical waveguides of ultra-broad bandwidth and very large mode areas are proposed and analyzed based on photonic crystal planar waveguides (PCPWs) and arrays with advanced elliptic features. The multi-unit PCPW arrays have been found particularly effective in achieving ultra-broadband operation with substantially expanded modal areas, as an optimized five-unit PCPW array offers an unprecedentedly broad SPSM spectrum of 4310 nm (from 0.11 to 4.42 μm). In addition, near-zero transmission loss conditions have also been identified for PCPW arrays at various resonant wavelengths. Such very-large-mode-area (VLMA), ultra-broadband, SPSM optical waveguides with “lossless” transmission conditions are highly desirable for basic waveguide research as well as related application fields.

Watt-level and sub-100-fs self-starting mode-locked 2.4-µm Cr:ZnS oscillator enabled by GaSb-SESAMs.

Femtosecond lasers with high peak power at wavelengths above 2 µm are of high interest for generating mid-infrared (mid-IR) broadband coherent light for spectroscopic applications. Cr2+-doped ZnS/ZnSe solid-state lasers are uniquely suited since they provide an ultra-broad bandwidth in combination with watt-level average power. To date, the semiconductor saturable absorber mirror (SESAM) mode-locked Cr:ZnS(e) lasers have been severely limited in power due to the lack of suitable 2.4-µm SESAMs.For the first time, we develop novel high-performance 2.4-µm type-I and type-II SESAMs, and thereby obtain state-of-the-art mode-locking performance. The type-I InGaSb/GaSb SESAM demonstrates a low non-saturable loss (0.8%) and an ultrafast recovery time (1.9 ps).By incorporating this SESAM in a 250-MHz Cr:ZnS laser cavity, we demonstrate fundamental mode-locking at 2.37 µm with 0.8 W average power and 79-fs pulse duration. This corresponds to a peak power of 39 kW, which is the highest so far for any saturable absorber mode-locked Cr:ZnS(e) oscillator.In the same laser cavity, we could also generate 120-fs pulses at a record high average power of 1 W. A comparable laser performance is achieved using type-II InAs/GaSb SESAM as well. These results pave the way towards a new class of high-power femtosecond SESAM mode-locked oscillators operating directly above 2-µm wavelength.

Accurately Engineering 2D/2D/0D Heterojunction In Hierarchical Ti3C2Tx MXene Nanoarchitectures for Electromagnetic Wave Absorption and Shielding.

The accurate heterojunction engineering in MXene-based composites unprecedentedly boosts their electromagnetic (EM) wave absorption and shielding performance. However, the flocculation of MXene caused by abundant termination groups severely restricts the regulation of heterojunction, which hankers for a revolutionary compositing strategy against unmanageable self-aggregation. Herein, electrically neutral coordination compound with large molecular volume is decorated on Ti3C2Tx lamellas to protect them from self-precipitation. A rapid polymerization reaction then controllably assembles them into a hierarchical microsphere composed of superlattice-like 2D/2D polymer/MXene building blocks. In the carbonized Ti3C2Tx/C/MoO2 microspheres, 2D/2D/0D heterojunctions can be precisely tuned to regulate electric/dielectric properties. These heterojunctions simultaneously trigger the intensive interfacial polarization and out-plane electron flowing to exhaust the EM energy as much as possible, confirmed by electron holography. Therefore, our products achieve the first-rate EM wave absorption with an ultrabroad absorption bandwidth of 7.7 GHz at the thickness of 2.5 mm. By altering the heterojunction, the composite acquires excellent EM interference shielding performance with an average shielding effectiveness of 35.9 dB. These accomplishments light a new way to microstructure construction and heterojunction design of MXene-based composites and lay out a profound insight into their EM wave absorption mechanism.

Generation and evolution of different terahertz singular beams from long gas-plasma filaments.

We theoretically and numerically investigate the generation and evolution of different pulsed terahertz (THz) singular beams with an ultrabroad bandwidth (0.1-40 THz) in long gas-plasma filaments induced by a shaped two-color laser field, i.e., a vortex fundamental pulse (ω0) and a Gaussian second harmonic pulse (2ω0). Based on the unidirectional propagation model under group-velocity moving reference frame, the simulating results demonstrate that three different THz singular beams, including the THz necklace beams with a π-stepwise phase profile, the THz angular accelerating vortex beams (AAVBs) with nonlinear phase profile, and the THz vortex beams with linear phase profile, are generated. The THz necklace beams are generated first at millimeter-scale length. Then, with the increase of the filament length, THz AAVBs and THz vortex beams appear in turn almost periodically. Our calculations confirm that all these different THz singular beams result from the coherent superposition of the two collinear THz vortex beams with variable relative amplitudes and conjugated topological charges (TCs), i.e., +2 and -2. These two THz vortex beams could come from the two four-wave mixing (FWM) processes, respectively, i.e., ω0+ω0-2ω0→ωTHz and -(ω0+ω0) + 2ω0→ωTHz. The evolution of the different THz singular beams depends on the combined effect of the pump ω0-2ω0 time delay and the separate, periodical, and helical plasma channels. And the TC sign of the generated THz singular beams can be easily controlled by changing the sign of the ω0-2ω0 time delay. We believe that these results will deepen the understanding of the THz singular beam generation mechanism and orbital angular momentum (OAM) conversion in laser induced gas-filamentation.

Wavemeter Capable of Simultaneously Achieving Ultra-High Resolution and Broad Bandwidth by Using Rayleigh Speckle From Single Mode Fiber

Rayleigh speckle with random jagged pattern resulting from the interference of Rayleigh backscattering lightwave in a single-mode fiber contains the wavelength information of the incident lightwave. We propose an all-fiber wavemeter based on the relation between Rayleigh speckle and wavelength. The system does not suffer from the resolution-bandwidth trade-off relation and simultaneously achieves an ultra-high resolution of 40 attometers and a broad bandwidth of 80 nm covering 1520 to 1600 nm. The wavemeter could enable precise measurement of continuous-wave optical waveforms at time resolution of 100 μs. Combined with a dynamic laser, the system can be regarded as a high-resolution spectroscopy for absorption spectrum analysis. To demonstrate the broad applicability of the technique, we use a laser step-scanned over 80 nm to measure the spectral response of hydrogen cyanide absorption line, and use a continuous-wave laser to measure its nonlinear wavelength scan over 1 nm. The proposed all-fiber wavemeter provides great prospects for ultra-precise spectrum analysis with ultra-broad bandwidth and ultra-high resolution.

Optical parametric amplification by monolayer transition metal dichalcogenides

Optical parametric amplification is a second-order nonlinear process whereby an optical signal is amplified by a pump via the generation of an idler field1. This mechanism is inherently related to spontaneous parametric down-conversion, which currently constitutes the building block for entangled photon pair generation2, a process that is exploited in modern quantum technologies. Here we demonstrate single-pass optical parametric amplification at the ultimate thickness limit; using semiconducting transition metal dichalcogenides3,4, we show that amplification can be attained over propagation through a single atomic layer. Such a second-order nonlinear interaction at the two-dimensional limit bypasses phase-matching requirements5 and achieves ultrabroad amplification bandwidths. In agreement with first-principle calculations, we observe that the amplification process is independent of the in-plane polarization of signal and pump fields. By the use of AA-stacked multilayers, we present a clear pathway towards the scaling of conversion efficiency. Our results pave the way for the development of atom-sized tunable sources of radiation with potential applications in nanophotonics and quantum information technology. Single-pass optical parametric amplification is demonstrated following propagation though an atomically thin semiconducting transition metal dichalcogenide. The demonstration may lead to atom-sized tunable light sources.

High-efficiency suspended three-tip edge coupler for Mid-infrared photonics

A high-efficiency suspended three-tip edge coupler based on a silicon-on-insulator (SOI) platform for fiber-to-chip coupling in Mid-infrared (MIR) region is designed. The device is composed of a three-tip edge coupler and subwavelength grating (SWG) waveguides. The edge coupler is suspended in order to avoid the high absorption loss from the Silicon dioxide (SiO2) in the SOI substrate in MIR. The three tips of the suspended edge coupler increase the coupling between the optical field in the waveguide and the mode field in the single mode fiber and reduce the reflection loss. A low overall coupling loss of 0.19 dB at 4.3μm for transverse-electric (TE) mode polarization can be achieved with the designed 60-μm-long three-tip edge coupler. The device can operate over a broad spectral range of 4.02μm – 4.59μm with extra loss of less than 0.3 dB. Because of its low-loss and ultra-broad optical bandwidth and the simple fabrication process, the proposed device is promising for fiber-to-chip edge coupling in MIR application.

MOF Induces 2D GO to Assemble into 3D Accordion-Like Composites for Tunable and Optimized Microwave Absorption Performance.

Three-dimensional (3D) materials assembled by 2D layered lamella can provide abundant interfaces which are greatly advantageous for high-performance microwave absorbers. Herein, accordion-like CeO2- x /reduced graphene oxide (CeO2- x /RGO) hybrid materials can be successfully synthesized by a solvothermal and hydrothermal method, which are composed of laminated RGO sheets and confined CeO2- x nanoparticles (NPs). The multilayer structure is attributed to the process of Ce-MOF dissolving into NPs, then the NPs combining with graphene oxide (GO) to induce the 2D GO assembled into 3D accordion-like composites. The 3D accordion-like CeO2- x /RGO simultaneously utilizes the insulated CeO2- x and highly conductive RGO to assemble into the laminated structure with moderate electromagnetic parameters. The 3D-laminated lightweight CeO2- x /RGO composite exhibits excellent attenuation ability of an ultrabroad bandwidth (5.84 GHz) and a maximum reflection loss (-50.6 dB) which can be ascribed from the glorious impedance matching, synergistic effect between RGO sheets and the embedded CeO2- x NPs. An off-axis electron holography is carried out to visualize the spatial electrical potential and charge distribution around the CeO2- x /RGO heterojunction, which clarifies the dipole polarization and interfacial polarization. This work enlightens a simple strategy to fabricate an excellent 3D laminated RGO-based microwave absorber.

Collision-Induced Broadband Optical Nonreciprocity.

Optical nonreciprocity is an essential property for a wide range of applications, such as building nonreciprocal optical devices that include isolators and circulators. The realization of optical nonreciprocity relies on breaking the symmetry associated with Lorentz reciprocity, which typically requires stringent conditions such as introducing a strong magnetic field or a high-finesse cavity with nonreciprocal coupling geometry. Here we discover that the collision effect of thermal atoms, which is undesirable for most studies, can induce broadband optical nonreciprocity. By exploiting the thermal atomic collision, we experimentally observe magnet-free and cavity-free optical nonreciprocity, which possesses a high isolation ratio, ultrabroad bandwidth, and low insertion loss simultaneously. The maximum isolation ratio is close to 40dB, while the insertion loss is less than 1dB. The bandwidth for an isolation ratio exceeding 20dB is over 1.2GHz, which is 2 orders of magnitude broader than typical resonance-enhanced optical isolators. Our work paves the way for the realization of high-performance optical nonreciprocal devices and provides opportunities for applications in integrated optics and quantum networks.

Cu2O@nanoporous carbon composites derived from Cu-based MOFs with ultrabroad-bandwidth electromagnetic wave absorbing performance

The application of carbon-based materials in microwave absorption (MA) field are limited due to the impedance mismatch caused by high permittivity and low permeability. In this work, Cu2O@nanoporous carbon (Cu2O@NPC) composites derived from Cu-based MOFs are synthesized using a simple method. Variation of C and Cu2O contents in Cu2O@NPC allows the regulation of permittivity and permeability, resulting in superior ultrabroad-bandwidth absorptivity. The maximum reflection loss (RL) of Cu2O@NPC-4 is up to −31.1 dB at 5.6 GHz, while the effective bandwidth (RL ≤ −10 dB) can reach 7.3 GHz (10.7–18 GHz) with a matching thickness of 1.85 mm. The results of this study open a new opportunity for solving the impedance mismatch of C-based materials and obtain ultrabroad effective bandwidth.

Free‐Space Communications Enabled by Quantum Cascade Lasers

Future generations of wireless communication systems are expected to support orders of magnitude faster data transfer with much lower latency than the currently deployed solutions. Development of wireless transceivers of higher bandwidth, low energy consumption, and small footprint becomes challenging with radio frequency (RF) electronic technologies. Photonics‐assisted technologies show many advantages in generating signals of ultrabroad bandwidth at high carrier frequencies in the millimeter‐wave, terahertz, and IR bands. Among these frequency options, the mid‐IR band has recently attracted great interest for future wireless communication due to its intrinsic merits of low propagation loss and high tolerance of atmospheric perturbations. A promising source for mid‐IR free‐space communications is the semiconductor quantum cascade laser (QCL), which can be directly modulated at a high speed and facilitates monolithic integration for compact transceivers. Herein, the research and development of QCL‐based free‐space communications are reviewed and a recent experimental study of multi‐gigabit transmission with a directly modulated mid‐IR QCL and a commercial off‐the‐shelf IR photodetector is reported on. Up to 4 Gb s−1 transmission of two advanced modulation formats, namely, four‐level pulse amplitude modulation (PAM‐4) and discrete multitone (DMT) modulation, is demonstrated.

Hybrid CARS spectroscopy based on a high-repetition-rate all-PM-fiber laser source

We demonstrate a robust, simple, and compact all polarization-maintaining (PM) fiber laser source with a repetition rate of 79 MHz for broadband coherent anti-Stokes Raman scattering (CARS) spectroscopy based on impulsive excitation and narrowband probing. The careful dispersion management during the generation of pump pulses ensures efficient impulsive excitation, which is verified to cover an ultrabroad bandwidth exceeding 4000 cm–1. The employment of PM fibers enables the laser source to withstand external disturbances. This turn-key configuration can potentially simplify the implementation of many applications of CARS, such as spectroscopic histopathology, weaponized endospore detection, and precise thermometry of gases.

Excellent microwave absorption performance from matched magnetic-dielectric properties of LSMO@RGO Nanocomposites

Lightweight and ultrabroad electromagnetic wave absorption materials are urgently demanded to address electromagnetic (EM) interference or pollution. However, the design of excellent EM wave absorbents still remains a challenge. Herein, La0.7Sr0.3MnO3/reduced graphene oxide (LSMO@RGO) nanocomposites was successfully prepared by a facial two-step method. Three different radios of LSM to RGO (1:2, 1:1, 2:1) and four different addition (3%, 5%, 8%, 10%) for each radio were investigated, and the matched magnetic-dielectric properties of LSMO@RGO nanocomposites endows the excellent microwave absorption performance. The maximum reflection loss of LSMO@RGO nanocomposites reach as strong as −45.71 dB at a 16.47 GHz with a thickness of 2.4 mm with the addition of 5 wt.%. The ultrabroad effective absorption bandwidth of LSMO@RGO nanocomposites also can reach 7.31 GHz with a thickness of 2.8 mm. The excellent microwave absorption performance is mainly originated from the magnetic and dielectric losses as well as well-matched impedance matching. The excellent microwave absorption performance of LSMO@RGO nanocomposites especially for ultrabroad effective absorption bandwidth offer strong potentials for the design of high-performance microwave absorption materials. The material system of LSM and graphene composite was prepared for the first time, the EM absorbing properties were investigated, and this material with simple preparation process, broadband and high absorption is highly expected to be industrialized.

Ultra-broadband microwave absorption by ultra-thin metamaterial with stepped structure induced multi-resonances

Since the electromagnetic (EM) absorbers have wide uses in many military and civilian applications, much effort has been devoted continuously to broadening the absorption bandwidth while making the thickness as thin as possible. In this paper, we present an ultra-wideband metamaterial absorber composed of periodic stepped-structures of magnetic material with a mixture of carbonyl iron powder and resin. The proposed metamaterial absorber features an ultra-thin thickness of only 0.025 wavelength and an ultra-broad operating bandwidth (defined by absorption larger than 90%) from 1.23 to 19 GHz. The ultra-broadband strong absorption is mainly attributed to the giant magnetic loss of the magnetic material, the structure-induced multi-resonances, and the edge diffraction effects of the stepped structures. In addition, good angular performance is observed for all polarizations. Experiments are carried out and are in good agreements with the simulated ones. We also propose a method to further reduce the weight of the structure without affecting much of the overall absorbing performance. The proposed metamaterial absorber has promising prospects in many real-world applications, such as RCS reduction and EM compatibility.

Broadband, wide-angle, and polarization-insensitive enhancement of light absorption in monolayer graphene over whole visible spectrum

By precise control of the resonance positions of four magnetic dipole modes in metamaterials, we numerically exhibit a broadband light absorption enhancement of monolayer graphene in the whole visible region. In the wavelength range from 450 to 800 nm, the minimum and maximum absorption efficiencies of monolayer graphene exceed 40% and 70%, respectively, and the averaged absorption efficiency can reach as high as about 65%, which suggests great potentials in various optoelectronic devices. Thanks to the localization nature of magnetic dipole modes, the light absorption in monolayer graphene under oblique incidence is, on the whole, less dependent on the incidence angle and the polarization direction of light. Besides ultra-broad bandwidth, other good performances of the light absorption enhancement of monolayer graphene, such as wide incidence-angle and polarization insensitivity, are also desired for graphene-based optoelectronic devices in some cases.