Slot Waveguide Research Articles

Slotted waveguide antenna design for maritime radar system

Waveguide structures have got popularity because of its extensive application in radar system of naval ships and aircrafts. Waveguide models provide high probability of small target detection and reduce rate of false target detection. There are a large number of studies on the waveguide slotted in the wide wall. Researches concerning the narrow wall of the waveguide are much less known. An edge slotted waveguide antenna array based on semicircular end of inclined slots radiating waveguide is proposed. Length of the inclined slot is extended to the adjacent broad wall with semicircular cutting. This extended length increases the resonant length and hence higher gain is obtained. Semicircular cutting at the end of the slot reduces cross-polarization component hence side lobe level obtained are low. Narrow wall inclined slotted waveguide is analyzed and designed to operate in X-band. The radiating slots are etched and rotated alternatively on the broadened top plate with semicircular cutting into the adjacent walls. This technique deletes the radial component of the propagating wave and adds the axial component of the propagating wave. Semicircular cutting increases the resonant length and enhances the gain of the antenna. Designed waveguide structure provides high gain, and cross-polarization component is minimized. Gain of 26 dB is obtained from the simulation results obtained in HFSS (High frequency Software Simulation) and side lobe level obtained is around 20 dB while hardware design provides the gain of 24.5 dB measured on VNA (Vector Network Analyzer) keeping the side lobe level minimum

Design of a Novel Dual-band Dual-circularly Polarized Waveguide Slot Array Antenna

A novel dual-band dual-circularly polarized waveguide slot array antenna is designed. The antenna opens two orthogonal slots with different lengths on the standard waveguide and is excited at two frequencies. The electric fields of TE10 and TE01 modes are obtained. To achieve the purpose of dual-band dual-circular polarization. Two orthogonal slots can not only make the antenna elements have slow-wave properties and delay phase, but also make the antenna elements in different frequency bands have the same virtual short-circuit wall, which simplifies the difficulty of design and processing. The antenna can achieve an equidistant array and become an array antenna. Through simulation verification, the S11<-10 dB in the two frequency bands from 11 GHz to 11.3 GHz and from 13.1 GHz to 13.8 GHz. In this bandwidth, the axial ratio bandwidth is less than 3 dB, respectively. The isolation between the two ends is better than 12 dB, and it has left and right circular polarization. The problem of the deterioration of the axial ratio of the circular polarization of the antenna caused by the different phases between the waveguide slot antennas is solved.

Metasurface-Based Beam Scanning Array With In-Band Co-Polarized Scattered Field Shaping

In this article, a novel array of radiation and scattering function-integrated metasurface antenna (RSIMA) elements is proposed, simulated, and measured for radiation beam scanning and two examples of in-band co-polarized (IBCP) scattered field shaping. The RSIMA consists of a slot-connected metasurface (SCMS) subarray of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times $ </tex-math></inline-formula> 3 unit cells and a substrate integrated slot waveguide (SISW). When illuminated by plane waves, each SCMS unit cell can independently obtain a wide range of reflection phases by tuning structural parameters. When excited by the coaxial port of SISW, the SCMS subarray will also act as a radiation carrier of RSIMA. Moreover, the layout change of SCMS subarray will not significantly affect the radiation performance of RSIMA. Based on the principle of phase compensation and the phase superposition method, the phase distribution for two examples of scattered field shaping can be designed. In simulations and experiments, the array of 12 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times12$ </tex-math></inline-formula> RSIMAs radiates an IBCP beam scanning over ±31.5° in the E-plane and shapes the IBCP scattered fields into a deflected vortex beam (DVB) and a deflected high-gain multi-beam (DHMB). It is worth mentioning that the SCMS layout change between two examples of scattered field shaping does not significantly affect the radiation performance of the RSIMA array.

Raman mediated solitonic pulse compression

A comprehensive analysis of Raman mediated solitonic compression of pulses inside a silicon nanocrystal embedded slotted photonic crystal waveguide (SPCW) is presented. The ultrahigh Raman gain coefficient of the silicon nanocrystal material, being fortified further by the intense optical confinement inside the SPCW, essentially outweighs the contribution of the Kerr in the self-phase modulation phenomenon that is responsible for the solitonic propagation of a pulse. This work particularly emphasizes the contribution of Raman nonlinearity in the soliton dynamics, which however has been neglected conventionally, leading to an unprecedented miniaturization in both the operating power and footprint. Spatiotemporal evolution of pulses inside the SPCW has been calculated using the nonlinear Schrödinger equation considering the slow-light regime. A compression factor as high as 44.3 is obtained using a 30 µm long SPCW with a peak input power of 8 mW at 1550 nm.

Simulation of a High-Performance Polarization Beam Splitter Assisted by Two-Dimensional Metamaterials.

It is challenging to simultaneously consider device dimension, polarization extinction ratio (PER), insertion loss (IL), and operable bandwidth (BW) to design a polarization beam splitter (PBS) that is extensively used in photonic integrated circuits. The function of a PBS is to separate polarizations of light, doubling the transmission bandwidth in optical communication systems. In this work, we report a high-performance PBS comprising two-dimensional subwavelength grating metamaterials (2D SWGMs) between slot waveguides. The 2D SWGMs exhibited biaxial permittivity by tailoring the material anisotropy. The proposed PBS showed PERs of 26.8 and 26.4 dB for TE and TM modes, respectively, and ILs of ~0.25 dB for both modes, with an unprecedented small footprint of 1.35 μm × 2.75 μm working at the wavelength λ = 1550 nm. Moreover, the present structure attained satisfactory PERs of >20 dB and ILs of <0.5 dB within an ultrabroad BW of 200 nm.

Study on Surface Roughness of Sidewall When Micro-Milling LF21 Waveguide Slits

The surface quality of the sidewall in waveguide antennae is important, especially surface roughness, which directly affects the electrical performance of the slotted waveguide antenna. Micro-milling is a potentially effective processing technique for the antenna. However, surface roughness has been difficult to guarantee within a reasonable accuracy range. In this study, orthogonal experiments of micro-milling LF21 waveguide slits were conducted. The results of the range analysis mainly sorted the factors that affected the surface roughness and also helped to determine how surface roughness could be kept at a minimum. The surface roughness was predicted by using the group method of data handling (GMDH). The importance of the applied GMDH was that it continuously adjusted the network structure according to the potential relationship between cutting parameters and the corresponding surface roughness, which helped determine the model most optimally fitted to the experimental data. This research can be used as a reference for selecting cutting parameters in micro-milling LF21.

Generation of Airy beams in Smith-Purcell radiation.

The metasurface has recently emerged as a powerful platform to engineer wave packets of free electron radiation at the mesoscale. Here, we propose that Airy beams can be generated when moving electrons interact with bianisotropic metasurfaces. By changing the intrinsic coupling strength, full amplitude coverage and 0-to-π phase switching of Smith-Purcell radiation can be realized from the meta-atoms. This unusual property shifts the wave front of the assembled Airy beam toward a parabolic trajectory. Experimental implementation displays that evanescent fields bounded at slotted waveguides can be coupled into Airy beams via Smith-Purcell radiation from a designed bianisotropic metasurface. Our method and design strategy offer an alternative route toward free-electron lasers with diffraction-free, self-accelerating, and self-healing beam properties.

Low power optical phase shifter using liquid crystal actuation on a silicon photonics platform

Low-power and compact phase shifters are crucial for large photonic circuits, both to cope with variability and to create programmable waveguide circuits scaling to thousands of tuning elements. This work demonstrates a liquid crystal phase shifter where there is a lateral silicon electrode "rail" on one side of the waveguide core. Using this architecture, a strong quasi-static electric field E actuation can be applied over the gap, which is filled with liquid crystal cladding material, with modest voltages. Because the mode is largely confined in the waveguide, optical losses are limited, compared to earlier experiments with slot waveguides. The liquid crystal is deposited locally on three different device variations using inkjet printing. The local deposition avoids unwanted interference of the liquid crystal with other optical components such as grating couplers. Measurements show similar trends as simulations of the liquid crystal orientations. For one device with a length of 50 µm, a phase shift of almost 0.9π is shown at 10 VRMS. We also discuss the challenges with this first demonstration of this phase shifter geometry using a silicon side-rail as an electrode.

Design, Analysis, and Optimization of a Plasmonic Slot Waveguide for Mid-Infrared Gas Sensing.

In this work, we investigated the optimization of a plasmonic slot waveguide (PSWG) in the mid-IR region particularly for a representative wavelength of 4.26 µm, which is the absorption line of CO2 and thus particularly relevant for applications. We analysed the mode features associated with metal-dielectric-metal (MDM), dielectric-metal-dielectric (DMD), and truncated metal film (TMF) structures with respect to the considered PSWG. Subsequently, the mode features of the PSWG were considered based on what we outlined for MDM, DMD, and TMF structures. Furthermore, as confinement factor and propagation length are two crucial parameters for absorption sensing applications, we optimized the PSWG based on a figure of merit (FOM) defined as the product of the aforementioned quantities. To characterize the propagation length, the imaginary part of the effective mode index of a guided mode was considered, leading to a dimensionless FOM. Finally, we investigated the PSWG also for other wavelengths and identified particularly attractive wavelengths and geometries maximizing the FOM.

Analytical prediction for quasi-TE mode in silicon nanowire optical rectangular waveguide

Theoretical and numerical mode estimation performed on silicon nanowire optical rectangular waveguide (SNORW) is presented for on-chip communication in photonic integrated circuits. The propagation behavior of electric and magnetic field is investigated, where zeroth order mode is found dominating inside the nanoslot region of SNORW, for the circularly symmetric quasi-TE mode to propagate. This SNORW structure supports hybrid mode, which derives its behavioral root from the rectangular waveguide and functional root from the slot waveguide. In periodic silicon nanowire-based waveguide, it is found that the envelope of mode field intensity closely matches with rectangular waveguide, and the guiding properties closely match with slot waveguide. The type of mode is analyzed by full-vectorial finite element method (FEM) and the analytical expression is derived using effective index method. Analytical expressions are used to express Quasi-TE mode in terms of material profile and waveguide physical parameters. The results obtained for SNORW are in S, C and L wavelength bands and are compared with the earlier reported work on slot waveguide, and the field intensity obtained with the theoretical equations is also compared with that of FEM results.

Study on mechanism of sidewall roughness scattering in slot optical waveguides by FDTD simulation

The slot optical waveguide is different from the conventional solid-core optical waveguide in both structure and features and has many unique and important applications. However, serious sidewall roughness scattering (SRS) loss limits the performance of the devices based on it. So far, the mechanism of the SRS in the slot waveguide is still not clear, and it seems not reasonable to analyze it directly by the Bragg diffraction condition. In this paper, through finite-difference time-domain simulation, we demonstrate that the regimes of the SRS and backscattering in the slot waveguide are the same as those of the solid-core waveguide, and the conditions of the diffraction radiation caused by the outer and the inner sidewalls are the same. The rationality of these conclusions are explained by taking the slot waveguide as a whole. The scattering regimes revealed and verified in this paper will benefit the suppression of the SRS loss and the utilization of the slot waveguide.

A Low-Profile Hollow Waveguide Slot Array Antenna with Full-Corporate Feeding Network at K-Band

A hollow waveguide slot array antenna with a total of 338x106.5x28.9 mm3 is proposed as a sensor for radar applications at K-band. The proposed antenna consists of a radiating slot plate with 8x32 slots, a middle plate with 64 cavities and apertures, and a full-corporate-feeding network. The antenna is fabricated with these three parts in which the sub-array feeding technique is employed. The proposed antenna is designed for millimeter-wave short-range radar applications. For this purpose, the antenna radiates and achieves reasonable gain values between 23.72–25.30 GHz frequencies with 6.45% bandwidth. The measurement directivity is 32.1 dBi, and the realized gain of the antenna is measured as 30.83 dBi at 24.5 GHz center frequency with 74.8% total efficiency. The antenna achieves high gain and efficiency with straightforward fabrication. Although there are some fabrication errors and connector losses, antenna measurements are obtained sufficiently consistent with simulations.

Triple-Band Dual-Polarized SIW Cavity-Backed Differential Slot Antenna

A differential triple-band dual-polarized cavity backed substrate integrated waveguide (SIW) slot antenna is presented in this letter. The hybrid TE130, TE320, and TE140 modes of the square SIW cavity are excited by differential feed for a triple-band realization. The proposed antenna exhibits dual-polarized radiations at 9.06 GHz, 11.86 GHz and 14.23 GHz. An isolation better than 40 dB in all the three frequency bands between the differential ports is obtained. The proposed structure exhibits the differential gain of 5.4 dBi, 10.7 dBi and 9.5 dBi at 9.06 GHz, 11.86 GHz and 14.23 GHz, respectively. To validate the design approach, the proposed antenna is fabricated and its characteristics are measured.

High-Performance All-Optical Modulator Based on Graphene-HBN Heterostructures

Graphene has emerged as an ultrafast photonic material for on-chip all-optical modulation. However, its atomic thickness limits its interaction with guided optical modes, which results in a high switching energy per bit or low modulation efficiencies. Nonetheless, it is possible to enhance the interaction of guided light with graphene by nanophotonic means. Herein, we present a practical design of an all-optical modulator that is based on graphene and hexagonal boron nitride (hBN) heterostructures that are hybrid integrated into silicon slot waveguides. Using this device, a high extinction ratio (ER) of 7.3 dB, an ultralow insertion loss (IL) of <0.6 dB, and energy-efficient switching (<0.33 pJ/bit) are attainable for a 20{\mu}m long modulator with double layer graphene. In addition, the device performs ultrafast switching with a recovery time of <600 fs, and could potentially be employed as a high-performance all-optical modulator with an ultra-high bandwidth in the hundreds of GHz. Moreover, the modulation efficiency of the device is further enhanced by stacking additional layers of graphene-hBN heterostructures, while theoretically maintaining an ultrafast response. The proposed device exhibits highly promising performance metrics, which are expected to serve the needs of next-generation photonic computing systems.

Meta-Surface Cavity-Based Waveguide Slot Array for Dual-Circularly Polarized Dual Beam

We introduce a meta-surface cavity into the waveguide slot array design and demonstrate the perfect generation of dual-circularly polarized (CP) dual-beams, where the cavity consists of meta-surface coated dielectric substrate surrounded by metallic vias and metal walls. When integrated with the rectangular waveguide, the meta-surface cavity functions as the leaky wave passage, capable of coupling the linearly polarized incidence from the feed into the cavity and releasing the resonant mode with birefringent radiations of right-hand and left-hand CP waves. Our design, performing the beam splitting and polarization conversion simultaneously as a waveguide slot array, should pave the way for building up more advanced meta-surface-based architectures for the manipulations of electromagnetic fields in the transmission regime.

High-Gain and Low-Loss Dual-Polarized Antenna Array With Reduced Sidelobe Level Based on Gap Waveguide at 28 GHz

In this letter, a 16×16-element dual-polarized antenna array with a reduced sidelobe level is proposed at 28 GHz for fifth-generation (5G) communication systems. The dual linear polarization operation is carried out through cross-shaped waveguide slots and multilayer feeding networks. Low-loss gap waveguide technology and Taylor distribution are applied in the feeding networks. A ridge unequal power divider with flexible power ratio and phase difference is devised and applied in the gap waveguide feeding network to reduce the complexity of unequal power structural design. A prototype of the proposed dual-polarized antenna array is manufactured by milling technology. Measured results show that the realized gains are higher than 28 dBi over the operating frequency band of 27.4–28.6 GHz. Moreover, the sidelobe level is lower than −18.5 dB for the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i> -direction polarization and −19.5 dB for the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> -direction polarization.

Thermo-optically tunable slot waveguide-based dual mode-splitting resonators with enhanced sharp lineshapes.

Slot waveguide plays an essential role in achieving high-performance on-chip photonic sensors and nonlinear devices. Ideally, slot waveguide features a large evanescent field ratio and strong electric field intensity in the slot, leading to a high waveguide sensitivity. Unfortunately, the microring resonator (MRR) based on the slot waveguide suffers the less steep spectral slope due to the low quality factor induced by the huge optical propagation loss of the slot waveguide. In this work, a novel dual mode-splitting resonator based on the slot waveguide is proposed and demonstrated to steepen the slope of lineshapes. The device is implemented by two racetrack resonators based on a slot waveguide and a feedback waveguide to introduce coherent optical mode interference, which could induce mode-splitting resonance (MR) with sharp asymmetry line shape and large extinction ratio (ER). The proposed device is fabricated by the standard complementary metal-oxide-semiconductor (CMOS) technologies on silicon-on-insulator (SOI) platform, and the characterization results show dual MRs with an ER of 45.0 dB and a slope rate (SR) of 58.3 dB/nm, exhibiting a much steeper lineshape than that of the conventional MRR with slot waveguide. And the resonance can be tuned efficiently by applying various voltages of the TiN microheater. Investigations in dual MRs devices promote many potential applications in the field of optical switching, optical modulating, and on-chip optical sensing.

Purcell-enhanced dipolar interactions in nanostructures

Strong light-induced interactions between atoms are known to cause nonlinearities at a few-photon level, which are crucial for applications in quantum information processing. Compared to free space, the scattering and the light-induced dipolar interaction of atoms can be enhanced by a dielectric environment. For this Purcell effect, either a cavity or a waveguide can be used. Here, we combine the high densities achievable in thermal atomic vapors with an efficient coupling to a slot waveguide. In contrast to free-space interactions, atoms aligned within the slot exhibit repulsive interactions that are further enhanced by a factor of 8 due to the Purcell effect. The corresponding blueshift of the transition frequency of atoms arranged in the essentially one-dimensional geometry vanishes above the saturation, providing a controllable nonlinearity at the few-photon level. The experimental results are in good agreement with Monte Carlo simulations that include the dielectric environment, dipolar interactions, and motional effects. The results pave the way towards a robust scalable platform for quantum nonlinear optics and all-optical quantum information processing at room temperature.

Designing an optical filter based on subwavelength grating slot waveguide embedded with phase-change material

Abstract In the past decades, photonic integrated circuits have been considered to overcome the bandwidth limitations of electronics circuits. However, photonic devices have yet to offer the same reconfigurability and programmability available in their electronics counterpart. This challenge can be addressed by integrating phase-change materials such as Ge2Sb2Te5 (GST) into photonic devices. We inserted a periodic arrangement of GST in a subwavelength grating (SWG) slot waveguide in order to have an optical filter with Bragg grating. The non-volatile and reversible phase-transition of GST, embedded in the silicon SWG slot waveguide, enables us to design a reconfigurable notch filter without static power consumption. The three-dimensional finite-difference time-domain (3D FDTD) simulation confirms that it is possible to create a 6.1 nm blueshift at the middle of the passband only by changing and controlling the phase transition of the GST. The spectral characteristics and their dependences on the geometrical parameters are investigated. The comparison of our filter with a conventional slot waveguide clearly indicates that the SWG slot waveguide provides a larger wavelength shift upon phase-transition of GST. Moreover, the SWG structure greatly improves the extinction ratio of the notch filter.

Numerical analysis of various plasmonic MIM/MDM slot waveguide structures

Numerical analysis of various plasmonic MIM/MDM slot waveguide structures