Waveguide Design Research Articles

Design and FDTD simulation of a remote-controllable visible-light plasmonic waveguide and refractive-index-sensitive switch

Design and FDTD simulation of a remote-controllable visible-light plasmonic waveguide and refractive-index-sensitive switch

Subwavelength-modulated silicon photonics for low-energy free-electron-photon interactions

We investigate silicon waveguides with subwavelength-scale modulation for applications in free-electron-photon interactions. The modulation enables velocity matching and efficient interactions between low-energy electrons and co-propagating photons. Specifically, we design a subwavelength-grating (SWG) waveguide for interactions between 23-keV free electrons and ≈1500-nm photons. The SWG waveguide and electron system exhibit a coupling coefficient of |gQu| = 0.23, and as we corroborate with time-domain, particle-in-cell simulations, the system operates as a backward-wave oscillator. Overall, our results show that modulated waveguides could open the door to strong, extended interactions between photons and low-energy (10-keV-scale) electrons, like those typically present in scanning electron microscopes. Additionally, our SWG waveguide design suggests that periodic waveguides could offer intriguing dispersion engineering opportunities for tailoring these interactions.

Analytical formulae for design of one-dimensional sonic crystals with smooth geometry based on symbolic regression

Even though locally periodic structures have been studied for more than three decades, the known analytical expressions relating the waveguide geometry and the acoustic transmission are limited to a few special cases. Having an access to numerical model is a great opportunity for data-driven discovery. Our choice of cubic splines to parametrize the waveguide unit cell geometry offers enough variability for waveguide design. Using Webster equation for unit cell and Floquet-Bloch theory for periodic structures, a dataset of numerical solutions was prepared. Employing the methods of physics-informed machine learning, we have extracted analytical formulae relating the waveguide geometry and the corresponding dispersion relation or directly the bandgap widths. The results contribute to the overall readability of the system and enable a deeper understanding of the underlying principles. Specifically, it allows for assessing the influence of the waveguide geometry, offering more efficient alternative to computationally demanding numerical optimization.

In Situ Multiphysical Metrology for Photonic Wire Bonding by Two-Photon Polymerization.

Femtosecond laser two-photon polymerization (TPP) technology, known for its high precision and its ability to fabricate arbitrary 3D structures, has been widely applied in the production of various micro/nano optical devices, achieving significant advancements, particularly in the field of photonic wire bonding (PWB) for optical interconnects. Currently, research on optimizing both the optical loss and production reliability of polymeric photonic wires is still in its early stages. One of the key challenges is that inadequate metrology methods cannot meet the demand for multiphysical measurements in practical scenarios. This study utilizes novel in situ scanning electron microscopy (SEM) to monitor the working PWBs fabricated by TPP technology at the microscale. Optical and mechanical measurements are made simultaneously to evaluate the production qualities and to study the multiphysical coupling effects of PWBs. The results reveal that photonic wires with larger local curvature radii are more prone to plastic failure, while those with smaller local curvature radii recover elastically. Furthermore, larger cross-sectional dimensions contribute dominantly to the improved mechanical robustness. The optical-loss deterioration of the elastically deformed photonic wire is only temporary, and can be fully recovered when the load is removed. After further optimization based on the results of multiphysical metrology, the PWBs fabricated in this work achieve a minimum insertion loss of 0.6 dB. In this study, the multiphysical analysis of PWBs carried out by in situ SEM metrology offers a novel perspective for optimizing the design and performance of microscale polymeric waveguides, which could potentially promote the mass production reliability of TPP technology in the field of chip-level optical interconnection.

Monolithic dispersion engineered mid-infrared quantum cascade laser frequency comb

The high-power quantum cascade laser (QCL) frequency comb capable of room temperature operation is of great interest to high-precision measurement and low-noise molecular spectroscopy. While a significant amount of research is devoted to the longwave spectral range, shortwave 3–5 μm QCL combs are still relatively underdeveloped due to the excessive material dispersion. In this work, we propose a monolithic integrated multimode waveguide scheme for effective dispersion engineering and high-power-efficiency operation. Over watt-level output power at room temperature with a wall plug efficiency of 7% and robust dispersion reduction is achieved from a quantum cascade laser frequency comb at a wavelength approximately 4.6 μm. Narrow beatnote linewidth less than 1 kHz and clear dual-comb multiheterodyne comb lines manifest the coherent phase relation among the comb modes which is crucial to fast molecular spectroscopy. This monolithic dispersion engineered waveguide design is also compatible to an efficient active–passive optical coupling scheme and would open up a new research playground for ring comb and on-chip dual-comb spectroscopy.

Impact of modal gain and waveguide design on two-state lasing in quantum well-dot lasers.

We study the current-controlled lasing switching from the ground state (GS) to the excited state (ES) transition in broad-area (stripe width 100 µm) InGaAs/GaAs quantum well-dot (QWD) and quantum well (QW) lasers. In the lasers with one QWD layer and a 0.45 µm-thick GaAs waveguide, pure GS lasing takes place up to an injection current as high as 8 A (40 kA/cm2). In contrast, in QW lasers with a similar design, ES lasing emerges already at 3 A (15 kA/cm2). The ES lasing in the QWD lasers is observed only in the devices with a waveguide thickness of 0.78 µm that supports a 2nd order transverse mode at the wavelength of the ES transition. Increasing the modal gain in the lasers with 0.78 µm-thick waveguide by using two QWD layers in the active region suppresses the ES lasing.

Design of waveguide with double layer diffractive optical elements for augmented reality displays

We investigated a diffraction optical waveguide structure with a double layer coupling configuration. This double layer-coupled diffraction optical waveguide structure modulates light information through wavefront modulation for propagation within the optical waveguide and then reproduces the light information through further wavefront modulation, thus achieving optical information transmission. Analysis indicates that the theoretical maximum field of view can reach 90° × 90°. With an actual field of view set to 53° × 53°, an entrance pupil size of 3.2 × 3.2 mm², an eye relief of 10 mm, and 50 pupil expansion, the system’s total energy transmission efficiency is 37%, with field of view uniformity at 91% and uniformity within the eye movement range reaching 97%.

Design and Analysis of Ridge and Sub-Wavelength Grating Waveguide Based Micro Ring-Resonator Sensor

The subwavelength grating microring resonator based filter shows great promise in optical sensing and communication. However, it spectrum nature makes it unsuitable for tracking or extracting a single wavelength from a broadband light source. In this paper, we used side-mode suppression, based on an angular grating- subwavelength microring resonator which offer high flexibility in wavelength design within the silicon-on-insulator (SOI) waveguide. By periodically arranging silicon waveguide pieces in three different widths, we can set the effective index along the inner sidewall of subwavelength microring resonator. thereby can achieve the precise wavelength selection. We examine the effects of various key parameters on the center wavelength, side-mode suppressed ratio, and quality factor of this filter.The proposed structure's isalso analysed for coupling strength between the bus and subwavelength grating ring resonator. The wavelength selection, compact size, compatibility with other similar nature waveguide-based devices and design flexibility highlight its significant potential in compact optical sensing and optical communication

Suppression of the Petermann noise factor in a chiral optical mode converter without encircling an exceptional point

The engineering of exceptional points (EP) in gain-loss-assisted optical systems has recently garnered considerable attention for the development of unconventional photonic components. Moreover, the substantial noise associated with EPs poses a significant challenge to realizing their full potential in applications. There is a gap in the exploration of asymmetric mode conversion phenomena in the close proximity of the dynamical encirclement around an EP. In response, we propose the design of a fabrication-feasible dual-mode optical planar waveguide. Non-Hermiticity, in terms of a customized gain-loss profile, is introduced to modulate the interaction between the coupled modes. Contrary to previous assumptions, we establish that the dynamical encirclement of an EP is not a prerequisite for achieving asymmetric mode conversion. Furthermore, we calculate the Petermann factor (K) near the EP and demonstrate a significant two orders of magnitude reduction in the Petermann noise factor when we do not dynamically encircle the EP. Hence, we propose a new scheme to reduce the inherent noise.

Enhancing stimulated Brillouin scattering in suspended silicon waveguides through subwavelength nanostructuration [Invited

Brillouin optomechanics is playing a key role in the development of groundbreaking devices and novel functionalities in integrated silicon photonics, such as narrow linewidth filtering and lasers, tunable frequency, non-reciprocity, etc. Most silicon-based optomechanical waveguides, which use anchoring arms or perforated slabs to ensure mechanical stability and operate for transverse-electric polarized light, face challenges with acoustic mode leakage into the lateral Si slab, limiting the photon-phonon overlap and the Brillouin gain. Here, we propose new waveguide designs based on subwavelength nanostructuration to tailor near-infrared photons and GHz phonons and maximize the Brillouin gain. We introduce six different geometries suitable for both membrane or fully suspended configurations (i.e., without transversal arms anchoring the core to the Si slab). Our three-dimensional optomechanical simulations predict that subwavelength silicon membranes with strip, slot, and SWG slot core waveguides achieve gains up to 12257 W-1m-1 at mechanical frequencies of 12-13 GHz. Moreover, suspended silicon waveguides with SWG slots achieve a high gain of 43542 W-1m-1 at 4.45 GHz, with the ability to adjust the mechanical frequency from 4 to 9 GHz. Further enhancements in the Brillouin gain are studied by integrating side arms to amplify the moving boundaries effect in the suspended SWG slot waveguides and leveraging the slow light regime, which can significantly increase the Brillouin gain up to 17 × 106 W-1m-1 for a mechanical mode at 11.18 GHz.

Selecting robust silicon photonic designs after Bayesian optimization without extra simulations

Optimization methods are frequently exploited in the design of silicon photonic devices. In this paper, we demonstrate that pushing the objective function to its minimum during optimization often results in devices that gradually become more sensitive to perturbations of design variables. The dominant strategy of selecting the design with the smallest objective function can lead to fabrication failure or yield loss due to manufacturing process variations. To address this issue, we propose an intuitive selection criterion that can identify designs not only possessing small objective functions but that are also robust to variations. Our simulation results on the Y-splitter, direction coupler, and bent waveguide designs demonstrate that the proposed method can achieve 2x higher coverage of robust designs with almost negligible run time, compared to the two baseline methods.

Programmable multi-wavelength distributed feedback laser array integrated in a liquid crystal polymer waveguide.

Liquid crystal (LC) distributed feedback (DFB) lasers hold significant potential for integrated photonics applications. However, limitations in wavelength spacing for wavelength switching, device size, and compatibility with other technologies have impeded advancements of the LC DFB laser in integration and responsiveness. Herein, we propose a thin-film multi-wavelength DFB laser array utilizing high-resolution patterned programmable nematic LC polymers, enabling rapid switching with high-resolution wavelength spacing between wavelength division multiplexing channels while maintaining a stable single longitudinal mode (SLM) for each laser. The underlying physical mechanism involves modulating the effective refractive index of the DFB laser by varying the LC molecules' orientation angles between adjacent regions of the LC grating to achieve wavelength modulation. Additionally, a specialized LC waveguide design connects the DFB lasers, facilitating wavelength modulation as well as straight-line and bending propagation of the laser. Furthermore, the laser array demonstrates a relatively low energy threshold, facilitating its applications in high-integration scenarios.

Design and Optimization of InAs Waveguide- Integrated Photodetectors on Silicon via Heteroepitaxial Integration for Mid- Infrared Silicon Photonics

Design and Optimization of InAs Waveguide- Integrated Photodetectors on Silicon via Heteroepitaxial Integration for Mid- Infrared Silicon Photonics

Silicon nitride directional coupler-based polarization beam splitter utilizing shallow ridge waveguides for improved fabrication tolerance

This study presents the design and fabrication of a silicon nitride (Si3N4) asymmetrical directional coupler (DC)-based polarization beam splitter (PBS) utilizing a shallow ridge waveguide structure. The use of Si3N4 with a moderate refractive index contrast and the design of a shallow ridge waveguide enables the DC structure to have a wider coupling gap of approximately 600 nm, which can be readily achieved with standard I-line lithography. The simulation results indicate that the polarization splitting is highly efficient with minimal insertion loss. This is corroborated by the experimental measurements, which show consistent broadband performance. The fabricated polarization beam splitter (PBS) exhibits a high polarization extinction ratio (PER) of over 20 dB for both transverse electric (TE) and transverse magnetic (TM) modes across a broad bandwidth of 120 nm (1500-1620 nm). This work demonstrates the potential of shallow ridge Si3N4 waveguides to enhance the fabrication tolerance and performance of integrated photonic devices.

First-principal analysis and design of TaPO5-based waveguides for photonic circuitry

With the rapid development of telecommunication systems, supercomputers, and large data centers, there is a growing need for advanced photonic materials to overcome the limitations of traditional photonic circuitry. This paper introduces monoclinic TaPO5, an indirect-bandgap semiconductor material, as a compact platform for photonic integrated circuits. TaPO5, with a bandgap of 2.148 eV, is analyzed for its structural, vibrational, thermodynamic, and optical properties using pseudo-potential plane waves within the density functional theory (DFT) framework. Our analysis reveals that TaPO5's thermodynamic properties, such as entropy, internal energy, and heat capacity, increase with temperature, while Helmholtz's free energy decreases. The materials demonstrate favorable optical characteristics, including a refractive index (n) of 2.11, an extinction coefficient (k) of 1.2 × 10⁻⁵, and a dielectric function with a real part of 4.45 and an imaginary part of 5.1 × 10⁻⁵ at a wavelength of 1.55 μm. TaPO5 exhibits promising performance metrics for photonic applications. Specifically, a proposed 1 × 2 multimode interference (MMI) splitter demonstrates 0.03 dB excess loss, a compact footprint of 4 × 18 μm2, and a 50:50 splitting ratio at 1.55 μm. These findings indicate that TaPO5 is a viable material for next-generation photonic integrated circuits, offering theoretical and practical advantages for various applications in photonic technology.

Design of long-range hybrid plasmonic waveguides with graphene-based electrical tuning of propagation length

Design of long-range hybrid plasmonic waveguides with graphene-based electrical tuning of propagation length

Purcell gain equalized zero-mode waveguide

We refresh the design of zero-mode waveguides (ZMWs) by introducing metamaterials that makes the zeroth order resonant mode existence. Of particular importance, the resulting electromagnetic field exhibits nearly constant distribution but not a trivial solution of Maxwell’s equation, showing great advantage to equalize the excitation rate of molecules throughout the waveguides. A closed form expression for the wave impedance is derived which is verified by the finite-difference time-domain simulations. Benefitted from the cavity Purcell effect which is lacking in existing ZMWs, fluorescence amplification and lifetime reduction are simultaneously enhanced. A practical design where the excitation volume reduced down to sub-zeptoliter and the fluorescence lifetime shortened to picosecond scale is illustrated. This result makes single molecule real time (SMRT) sensing of biochemical reactions at biophysically relevant concentrations (~ μM) possible, combining off-the-shelf ultrafast lasers.

Design of an Ultrawideband Air-Filled Grounded Coplanar Waveguide for Near-Field Probe Calibration

Design of an Ultrawideband Air-Filled Grounded Coplanar Waveguide for Near-Field Probe Calibration

Design and Performance Analysis of Strip Photonic Waveguide with Coating Layer for Multimode Propagation

Introduction: Photonic devices play a pivotal role in the realm of high-speed data communication due to their inherent capability to expedite the transfer of information. Historically, research efforts in this domain have predominantly concentrated on investigating the fundamental mode propagation within photonic waveguides. Methods: This study diverges from the conventional approach by delving into the untapped potential of higher-order modes in addition to the fundamental mode of propagation. The exploration of these higher-order modes opens up new possibilities for optimizing and enhancing the performance of photonic devices in high-speed data communication scenarios. As a distinctive aspect of this study, various coating materials were scrutinized for their impact on both fundamental and higher-order mode propagation. The materials under examination included AlN (aluminum nitride), Germanium, and Silicon. These materials were chosen based on their unique optical properties and suitability for influencing different modes of light propagation. The findings from the study reveal that applying a coating of germanium demonstrates advantageous characteristics, particularly in terms of reduced signal loss, even when considering higher-order modes of propagation within photonic devices. Results: In this context, the results indicate that germanium-coated waveguides exhibit notably low propagation losses, with measurements as minimal as 0.25 dB/cm. This low level of loss is particularly noteworthy, especially when the waveguide has a width of 550 nm and is coated with a thickness of 50 nm. The dimensions and coating specifications play a crucial role in determining the efficiency of light transmission within the waveguide. Conclusion: The fact that the propagation loss is substantially low under these conditions suggests that the germanium-coated waveguide, even when considering higher-order modes of light propagation, can effectively maintain the integrity of the optical signal.

On the role of geometric phase in the dynamics of elastic waveguides.

The geometric phase provides important mathematical insights to understand the fundamental nature and evolution of the dynamic response in a wide spectrum of systems ranging from quantum to classical mechanics. While the concept of geometric phase, which is an additional phase factor occurring in dynamical systems, holds the same meaning across different fields of application, its use and interpretation can acquire important nuances specific to the system of interest. In recent years, the development of quantum topological materials and its extension to classical mechanical systems have renewed the interest in the concept of geometric phase. This review revisits the concept of geometric phase and discusses, by means of either established or original results, its critical role in the design and dynamic behaviour of elastic waveguides. Concepts of differential geometry and topology are put forward to provide a theoretical understanding of the geometric phase and its connection to the physical properties of the system. Then, the concept of geometric phase is applied to different types of elastic waveguides to explain how either topologically trivial or non-trivial behaviour can emerge based on the geometric features of the waveguide. This article is part of the theme issue 'Current developments in elastic and acoustic metamaterials science (Part 2)'.