Straight Waveguide Research Articles

Broadband subwavelength tunable valley edge states induced by fluid filling acoustic metastructure

Topological acoustic insulators demonstrate unusual characteristics in manipulating sound wave, which attract much attention from researchers. However, most of the recent researches are based on passive system, hampering their dispersion tunability. In this paper, a broadband subwavelength tunable fluid filling acoustic topological metastructure is studied. It is composed of perforated cells with tunable water height in the hole, which enables the dispersion of the edge state to be tuned. The inversion symmetry is broken by expanding and shrinking the adjacent holes in the unit cell. Thus, the valley Hall states with opposite Chern number form at the K point in the Brillouin zone. The edge states emerge at the boundary of the different valley Hall phases. The robustness of the edge states is verified by the straight and Z-shaped waveguide. Furthermore, the dispersion of the edge state can be altered continuously by raising and reducing the water height, giving rise to broadband variable topological states, which greatly expands the bandwidth from 40 Hz to 1033 Hz. This work offers a new method to control the topological states and shows great potential for practical application.

Role of unit-cell defects in terahertz topological ring resonators

We demonstrate a terahertz topological ring resonator based on all-dielectric valley photonic crystals. A triangular ring-shaped topological edge is coupled to a straight topological waveguide by suitably merging two valley photonic crystals. The resonant characteristics are investigated for different asymmetry parameters as well as for different domain interfaces (zigzag and bridge type) constituting the ring resonator. In the topological ring resonator, ultra-narrowband high quality (Q)-factor resonances are excited. Then, multiple defect states are intentionally introduced into the domain interfaces, and the influences of defects on the terahertz propagation and the Q-factors are examined for both types of domain interfaces. Numerical simulation studies on terahertz transmission reveal a resonance redshift of ∼ 10 G H z due to the introduction of multiple defect states. Such resonance redshift is attributed to the alteration in the effective refractive indices of the structure. Very high Q-factors within the range of 1240–840 are achieved even in the presence of several unit-cell defects owing to strong field confinements in the domain interface. This research can open up new routes for the development of nonlinear technologies and other advanced on-chip topological devices at THz frequencies.

Modal analysis of arbitrary-oriented ridge waveguides in x-cut lithium niobate thin film

We theoretically and numerically investigate the mode properties of ridge waveguides made on lithium niobate (LN) subwavelength thin film by taking into complete account the anisotropic feature of LN crystal. We analyze the effective refractive index of the quasi-transverse-electrical (q-TE) and quasi-transverse-magnetic (q-TM) modes and their difference in arbitrary-oriented waveguides on x-cut lithium niobate on insulator. Waveguide simulations based on full-vectorial finite element method are performed. The geometrical parameters and directions of the optical axis of the LN material for the ridge waveguides are varied to investigate the single-mode condition, optical power distribution and mode hybridization effect in the straight waveguides. The different trends in optical power distribution in LNOI waveguide between q-TE and q-TM at different crystal optical axis angles will be explained in detail.

Curvature sensor based on femtosecond laser-inscribed straight waveguide in FMF

We propose an ingenious method for higher-order mode excitation in few-mode fiber (FMF), in which a femtosecond laser is utilized to inscribe a straight waveguide in FMF. Based on this method, a new-type in-fiber Mach-Zehnder interferometer (MZI) for bending sensing is proposed. The in-fiber MZI is achieved through femtosecond laser directly writing a straight waveguide in a very short section of FMF between two standard single mode fibers. The MZI is caused by the interference between the fundamental mode and the higher-order mode excited in the modified FMF. When the device is bent in specific four orthogonal orientations, the direction of its spectrum shift is different. The device has a large curvature measurement range of 0–10 m−1. The curvature sensitivity can reach 2 nm/m−1 and the temperature sensitivity is as low as 2.34 pm/°C. Moreover, the device has the superiorities of miniature size, simple manufacturing, high reproducibility, low insertion loss and high spectral contrast.

Simultaneous generation of a broadband MIR and NIR frequency comb in a GaP microring.

Midinfrared (MIR) optical frequency combs are of great significance as broadband coherent light sources used in extensive areas such as coherent communications and molecule detections. Conventional MIR combs are usually restricted in size and power, while most microcombs are focused in the near-infrared (NIR) region because of the limited accessible Q-factor of microrings and the poor performances of available pumps. In this paper, we numerically demonstrate the simultaneous generation of a broadband MIR and NIR comb in a GaP microring with an additive waveguide. The achieved octave-spanning (1890-4050 nm) MIR microcomb at a low pump power of 34 mW can be effectively converted to the second-harmonic NIR comb covering 1120-1520 nm with separate dispersion optimization of the ring cavity and straight waveguide. The proposed system has the advantage of simple structure and low power threshold, which could find potential in highly integrated MIR optical sources and related applications.

Silicon Multimode Waveguide Crossing Based on Anisotropic Subwavelength Gratings

AbstractMultimode waveguide crossings (MWCs) are becoming more and more important as one of the key elements for on‐chip optical routing and cross‐connection. However, it is still very challenging to achieve scalable MWCs with compact footprints, low excess losses (ELs), and low intermode cross‐talks (CTs). This work demonstrates an unprecedented silicon MWC with high performances by using the anisotropy of one‐/two‐dimensional (1D/2D) subwavelength grating (SWG) structures. For the proposed silicon MWC, the 2D‐SWG crossing region at the center has a higher refractive index than the 1D‐SWG regions at both sides for the guided‐modes of TE polarization, due to the anisotropy of the 1D/2D SWGs. As a result, the crossing region can be equivalent to a straight waveguide, and the launched TE modes can transmit through the crossing region with negligible ELs and low intermode CTs. Such an MWC can be scaled very flexibly and easily according to the new principle. The fabricated three‐mode MWC with a footprint of 14.8 × 14.8 µm2 shows ELs of <0.26 dB and intermode CTs of <−20 dB in the wavelength range of 1525–1605 nm.

Valley topological line-defects for Terahertz waveguides and power divider

Topological edge states exhibit low backscattering, unidirectional energy propagation, and robust immunity to sharp bends and defects in photonic topological waveguides. These properties of topological waveguides are expected to be applied in terahertz (THz) technology and expand to functional devices. Here, we introduce a line-defect into valley Hall photonic crystals (VPCs) and discuss the influence of line-defect to edge states of valley Hall photonic topological waveguide. The introduction of line-defect can greatly improve the design freedom of topological waveguides, and is expected to be extended to functional devices. We constructed a straight VPC waveguide and a Z-shaped VPC waveguide to verify the topological valley line-defect edge state and the single-mode transmission of topological line-defect states is excited by a rectangular waveguide with a tapered coupling structure. The VPC waveguide has low bending loss and a working bandwidth (return loss < −10 dB) from 0.4 to 0.422 THz. We propose a topological power divider (TPD) operating at THz frequencies and achieved −3.3 per port equal power divide. Furthermore, this TPD is robust against defects and is promising for on-chip THz integration.

Femtosecond laser micromachining of suspended silica-core liquid-cladding waveguides inside a microfluidic channel

This work addresses the fabrication of straight silica-core liquid-cladding suspended waveguides inside a microfluidic channel through fs-laser micromachining. These structures enable the reconfiguration of the waveguide's mode profile and enhance the evanescent interaction between light and analyte. Further, their geometry resembles a tapered optical fiber with the added advantage of being monolithically integrated within a microfluidic platform. The fabrication process includes an additional post-processing thermal treatment responsible for smoothening the waveguide surface and reshaping it into a circular cross-section. Suspended waveguides with a minimum core diameter of 3.8 µm were fabricated. Their insertion losses can be tuned and are mainly affected by mode mismatch between the coupling and suspended waveguides. The transmission spectrum was studied and it was numerically confirmed that it consists of interference between the guided LP01 mode and uncoupled light and of modal interference between the LP01 and LP02 modes.

Optical Switch Based on Ge2Sb2Se4Te1-Assisted Racetrack Microring

In this work, we have proposed and designed a 1 × 1 optical switch based on the optical phase-change material, Ge2Sb2Se4Te1 (GSST), for GSST-assisted silicon racetrack microring. Its optical power can periodically be exchanged between the straight silicon waveguide and the GSST/Si hybrid racetrack waveguide due to the formed directional coupling structure. By changing GSST from the crystalline state to the amorphous state, the switch shifts from the ON state to the OFF state, and vice versa. With finite-difference time-domain method optimization, the proposed switch shows an extinction ratio of 18 dB at 1547.4 nm. The insert losses at the ON and OFF states are both less than 1 dB. The proposed switch unit has the potential to build an N × N switch matrix.

Interface-Induced Conservation of Momentum Leads to Chiral-Induced Spin Selectivity.

We study the nonequilibrium dynamics of electron transmission from a straight waveguide to a helix with spin-orbit coupling. Transmission is found to be spin-selective and can lead to large spin polarizations of the itinerant electrons. The degree of spin selectivity depends on the width of the interface region, and no polarization is found for single-point couplings. We show that this is due to momentum conservation conditions arising from extended interfaces. We therefore identify interface structure and conservation of momentum as crucial ingredients for chiral-induced spin selectivity, and we confirm that this mechanism is robust against static disorder.

Interlayer Slope Waveguide Coupler for Multilayer Chalcogenide Photonics

The interlayer coupler is one of the critical building blocks for optical interconnect based on multilayer photonic integration to realize light coupling between stacked optical waveguides. However, commonly used coupling strategies, such as evanescent field coupling, usually require a close distance, which could cause undesired interlayer crosstalk. This work presents a novel interlayer slope waveguide coupler based on a multilayer chalcogenide glass photonic platform, enabling light to be directly guided from one layer to another with a large interlayer gap (1 µm), a small footprint (6 × 1 × 0.8 µm3), low propagation loss (0.2 dB at 1520 nm), low device processing temperature, and a high bandwidth, similar to that in a straight waveguide. The proposed interlayer slope waveguide coupler could further promote the development of advanced multilayer integration in 3D optical communications systems.

Narrow-spectrum enhanced multiparameter gas sensor based on Fano resonance in an asymmetric MDM waveguide

In this study, a highly sensitive and narrow–spectrum enhanced multiparameter gas sensor is proposed based on Fano resonance in an asymmetric metal–dielectric–metal (MDM) waveguide. A side-coupled MDM structure comprising an upright rectangular cavity is initially investigated under different gaps. An assumed junction model is used to analyze the physical mechanism of a through coupled system (gap = 0) with an ultralow minimum transmission coefficient and higher refractive index sensitivity compared to other cases. Becausethe transmission spectrum of through-coupled structure is relatively broad, a ring resonator was added on the upper side of straight waveguide to generate an ultranarrow resonant mode as a discrete state that interferes with the previous broad mode serving as a quasicontinuum state to afford Fano resonance. Consequently, its full width at half maximum decreases by 420% from 496 to 11.56 nm. The response rate and detection difficulty of detection of y-asymmetric plasmonic nanostructures used in this study are exceed those of x-asymmetric plasmonic nanostructures. A multiparameter gas sensor can be developed by filling hydrogen- and methane-sensitive films in corresponding cavities of the plasmonic nanostructure. The sensitivities of methane and hydrogen can reach −2.863 and −0.265 nm/%, respectively, after optimizing relevant structural parameters. The proposed method provides an effective approach for designing narrow-spectrum enhanced and multichannel nanophotonic devices.

Observation of topological valley waveguide transport of elastic waves in snowflake plates

In elastic systems, the researchers have observed the topological valley edge states. However, the topological valley edge states can only enable elastic waves to propagate along the narrow interfaces. Here, we obtain the topological valley waveguide states (TVWSs) by constructing the sandwich-like plates. By virtue of the TVWSs, the transport of elastic waves in the wide straight and bent waveguides is achieved. In addition, we realize the splitting effect and converging of elastic waves according to the valley-locking property. Our research may promote the design of innovative devices about energy harvesting, sensing, and information processing.

Design of nanorod-embedded bandstop MIM waveguide filters with high filtering efficiency and sensitivity

A novel band-stop metal–insulator–metal plasmonic waveguide filter with high efficient filtering and high sensitivity is proposed and investigated theoretically. The relationship among the wavelength of the incident wave, the refractive index of the measured material and the transmittance of the incident wave is simulated by finite-difference time-domain (FDTD) and coupled mode theory (CMT) method. The filters which consist of a straight waveguide and two square resonators embedded with silver nanorods have minimum transmittance of 0.04%, 2.2%, 14.9% and 9.0% at the dips. The maximum sensitivity could reach 2860 nm/RIU. The sensitivity of the filter to detect plasma, hemoglobin, glucose and Sylgard184 reaches 2854 nm/RIU, 2852 nm/RIU, 2850 nm/RIU, 2851 nm/RIU, respectively. This waveguide filter may provide a new reference for highly integrated optical circuits and refractive index sensor.

Non-self-adjointness of bent optical waveguide eigenvalue problem

In the literature, the mathematical problem of optical wave propagation in dielectric straight waveguides has been systematically studied as a self-adjoint eigenvalue problem with real eigenvalues. In terms of the underlying physics, such real eigenvalues meant no losses during the wave propagation. However, when the waveguides were bent, experiments showed that the wave propagation became lossy. In this paper, optical wave propagation in dielectric bent waveguides is mathematically analyzed. It is shown that the corresponding eigenvalue problem is a non-self-adjoint eigenvalue problem and has complex-valued eigenvalues. The imaginary part of the eigenvalues is a measure of loss. For large-bend radii, the eigenvalue problem for bent waveguides behaves as an eigenvalue problem for straight waveguides, and the complex-valued eigenvalues approach the real-valued eigenvalues of the straight waveguide problem. By expressing the bent waveguide eigenvalue operator as a sum of the self-adjoint operator and the non-self-adjoint operator, asymptotic behaviour of guided modes and their lossy nature are investigated.

Research on sensing characteristics of diamond multilayer waveguide structure racetrack micro-ring resonator

A racetrack micro-ring resonant cavity sensor based on the diamond's material is proposed in this paper. In the model, the vertical-section of the waveguide adopts a five-layer ridge-type waveguide structure based on CH3NH3PbI3, SiO2 and diamond, i.e. CH3NH3PbI3-SiO2-Diamond-SiO2-CH3NH3PbI3. First, on the basis of the resonant principle and coupled mode theory, this study investigated the light field intensity distribution characteristics of the integral resonant cavity, the longitudinal section of a single straight waveguide, the racetrack micro-ring, and the longitudinal section of the straight waveguide by using the finite element method in COMSOL. The analysis shows that the introduction of CH3NH3PbI3 as an isolation layer can avoid the light scattering loss and leakage loss, and significantly enhance the filtering performance. In addition the sensing characteristics of the Add-drop racetrack resonator were further studied. Results showed that the structure could achieve a quality factor of 105, and the sensitivity could reach 14833.33 dB/RIU. In the detection system with a signal-to-noise ratio of 30 dB, the detection limit was 2.02 × 10−7 RIU. Compared with the traditional All-pass single micro-ring resonator, the proposed structure shows advantages of high tenability, high sensitivity and low detection limit.

Resolvent estimates, wave decay, and resonance-free regions for star-shaped waveguides

Using coordinates $(x,y)\in \mathbb R\times \mathbb R^{d-1}$, we introduce the notion that an unbounded domain in $\mathbb R^d$ is star shaped with respect to $x=\pm \infty$. For such domains, we prove estimates on the resolvent of the Dirichlet Laplacian near the continuous spectrum. When the domain has infinite cylindrical ends, this has consequences for wave decay and resonance-free regions. Our results also cover examples beyond the star-shaped case, including scattering by a strictly convex obstacle inside a straight planar waveguide.

Design of an ultrasharp silicon multimode waveguide bend assisted by multiregional subwavelength grating structures

Sharp multimode waveguide bends are beneficial to improve the integration density of the on-chip mode-division multiplexing (MDM) systems, which can be used to increase the optical information transmission capacity. A method of designing a multimode bending structure based on subwavelength gratings is proposed in this paper, through the mode conversion and matching between the straight and bending waveguide. The proposed bending structure is composed of a 90-deg arc-bend with a radius of 5 μm combined with the shallow-etched grating structures on the top, and two-mode converters on the input and output straight waveguide realized also with the help of nonuniform grating-like structures, achieving a total effective bending radius of 8.25 μm and high transmission efficiencies of the first three TE modes (TE0 − TE2) in a wide wavelength range of 1500 to 1600 nm. At the center wavelength of 1550 nm, the calculated excess losses of the three modes TE0, TE1, and TE2 are 0.21, 0.19, and 0.59 dB, respectively, and the crosstalks for all modes are less than −26.58 dB. More importantly, we add a non-uniform grating converter in the straight waveguide region which pre-deviates the mode fields inside the straight waveguide to help further reduce the size of the bending part to only 5 μm, which will be helpful for large scale integration when a great number of bends are cascaded.

Terahertz Band Communications With Topological Valley Photonic Crystal Waveguide

The sixth generation (6 G) communication standard is expected to include support for very-high data rates (over 100 Gbit/s) and device electronics will require processors with on-chip communications able to support such high bandwidths. Although the terahertz band possesses ample bandwidth, conventional THz waveguides suffer from high bending losses and are sensitive to process defects. The recent revelation of the topological valley photonic crystal (VPC), which exhibits near zero-loss bends, zero back-scattering and zero junction-area, holds much promise for future high speed inter-device communications. Low dispersion in the photonic bandgap region as the number of bends increase is demonstrated through simulation and experiment of the transmission and group delay characteristics. Through comprehensive communications experiments we demonstrate online results below the forward error correction level including an 108-Gbit/s bit rate using multi-level modulation for a 10 mm straight VPC waveguide and a 62.5-Gbit/s bit-rate for a ten sharp bended structure.

Efficient and tunable blue light generation using lithium niobate nonlinear photonics

Thin-film lithium niobate (LN) has recently emerged as a playground for chip-scale nonlinear optics and leads to efficient frequency conversions from near-infrared to near-visible bands. For many nonlinear and quantum photonics applications, it is desirable to operate deep into the visible band within LN's transparency window. However, the strong material dispersion at short wavelengths makes phase-matching difficult, necessitating sub-micrometer scale control of domain structures for efficient phase-matching. Here, we report the operation of thin film LN in the blue wavelength and high fidelity poling of the thin-film LN waveguide to this regime. As a result, quasi-phase matching is realized between IR (871 nm) and blue (435.5 nm) wavelengths in a straight waveguide and prompts strong blue light generation with a conversion efficiency (1040% ± 140%/W). This blue second harmonic generator exhibits stable temperature tunability, which is important for applications that require precise frequency alignment, such as atomic clocks.