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1. Stochastic FDTD Modeling of Propagation Loss due to Random Surface Roughness in Sidewalls of Optical Interconnects

   https://doi.org/10.23919/USNC-URSINRSM51531.2021.9336433  

   Guiana, Brian; Zadehgol, Ata (February 2021, 2021 United States National Committee of URSI National Radio Science Meeting (USNC-URSI NRSM))

   null (Ed.)

   The dielectric waveguide (WG) is an important building block of high-speed and high-bandwidth optical and opto-electronic interconnect networks that operate in the THz frequency regime. At the interface of Si/SiO 2 dielectric waveguides with width above w = 2.5 μm and anisotropic surface roughness, transverse electric (TE) mode surface wave propagation can experience a loss of approximately a = 2 dB/cm; however, propagation losses increase rapidly to near a = 44 dB/cm as the width decreases to w = 500 nm, due to increased interaction of surface waves and sidewall surface roughness that exhibits random distribution inherent to the manufacturing process. Previous works have developed analytic expressions for computing propagation loss in a single dielectric waveguide exhibiting random roughness. More recent works report a = 0.4 dB/cm noting the non-trivial estimation errors in previous theoretical formulations which relied on planar approximations, and highlight the discrepancy in planar approximations vs. the 3-D Volume Current Method. A challenge that remains in the path of designing nanoscale optical interconnects is the dearth of efficient 3-D stochastic computational electromagnetic (CEM) models for multiple tightly coupled optical dielectric waveguides that characterize propagation loss due to random surface roughness in waveguide sidewalls. Through a series of theoretical and numerical experiments developed in the method of finite-difference time-domain (FDTD), we aim to develop stochastic CEM models to quantify propagation loss and facilitate signal & power integrity modeling & simulation of arbitrary configurations of multiple tightly-coupled waveguides, and to gain further insights into loss mechanisms due to random surface roughness in optical interconnects. 

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2. Finite-difference time-domain methods

   https://doi.org/10.1038/s43586-023-00257-4  

   Teixeira, F. L.; Sarris, C.; Zhang, Y.; Na, D.-Y.; Berenger, J.-P.; Su, Y.; Okoniewski, M.; Chew, W. C.; Backman, V.; Simpson, J. J. (December 2023, Nature Reviews Methods Primers)

   The finite-difference time-domain (FDTD) method is a widespread numerical tool for full-wave analysis of electromagnetic fields in complex media and for detailed geometries. Applications of the FDTD method cover a range of time and spatial scales, extending from subatomic to galactic lengths and from classical to quantum physics. Technology areas that benefit from the FDTD method include biomedicine — bioimaging, biophotonics, bioelectronics and biosensors; geophysics — remote sensing, communications, space weather hazards and geolocation; metamaterials — sub-wavelength focusing lenses, electromagnetic cloaks and continuously scanning leaky-wave antennas; optics — diffractive optical elements, photonic bandgap structures, photonic crystal waveguides and ring-resonator devices; plasmonics — plasmonic waveguides and antennas; and quantum applications — quantum devices and quantum radar. This Primer summarizes the main features of the FDTD method, along with key extensions that enable accurate solutions to be obtained for different research questions. Additionally, hardware considerations are discussed, plus examples of how to extract magnitude and phase data, Brillouin diagrams and scattering parameters from the output of an FDTD model. The Primer ends with a discussion of ongoing challenges and opportunities to further enhance the FDTD method for current and future applications. 

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3. Exploiting Ultralow Loss Multimode Waveguides for Broadband Frequency Combs

   https://doi.org/10.1002/lpor.202000353  

   Ji, Xingchen; Jang, Jae K.; Dave, Utsav D.; Corato‐Zanarella, Mateus; Joshi, Chaitanya; Gaeta, Alexander L.; Lipson, Michal (December 2020, Laser & Photonics Reviews)

   Abstract Low propagation loss in high confinement waveguides is critical for chip‐based nonlinear photonics applications. Sophisticated fabrication processes which yield sub‐nm roughness are generally needed to reduce scattering points at the waveguide interfaces to achieve ultralow propagation loss. Here, ultralow propagation loss is shown by shaping the mode using a highly multimode structure to reduce its overlap with the waveguide interfaces, thus relaxing the fabrication processing requirements. Microresonators with intrinsic quality factors (Q) of 31.8 ± 4.4 million are experimentally demonstrated. Although the microresonators support ten transverse modes only the fundamental mode is excited and no higher order modes are observed when using nonlinear adiabatic bends. A record‐low threshold pump power of 73 µW for parametric oscillation is measured and a broadband, almost octave spanning single‐soliton frequency comb without any signatures of higher order modes in the spectrum spanning from 1097 to 2040 nm (126 THz) is generated in the multimode microresonator. This work provides a design method that can be applied to different material platforms to achieve and use ultrahigh‐Qmultimode microresonators. 

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4. Wafer-scale waveguide sidewall roughness scattering loss characterization by image processing

   https://doi.org/10.1364/OE.558186  

   Khurana, Mohit; Delfan, Sahar; Yi, Zhenhuan (May 2025, Optics Express)

   Photonic integrated circuits (PICs) are vital for developing affordable, high-performance optoelectronic devices that can be manufactured at an industrial scale, driving innovation and efficiency in various applications. Optical loss of modes in thin film waveguides and devices is a critical measure of their performance. Thin film growth, lithography, masking, and etching processes are imperfect processes that introduce significant sidewall and top-surface roughness and cause dominating optical losses in waveguides and photonic structures. This roughness, as perturbations couple light from guided to far-field radiation modes, leads to scattering losses that can be estimated from theoretical models. Typically, with UV-based lithography, sidewall roughness is significantly larger than wafer-top surface roughness. Atomic force microscopy (AFM) imaging measurement gives a 3D and high-resolution roughness profile, but the measurement is inconvenient, costly, and unscalable for large-scale PICs and at wafer-scale. Here, we evaluate the sidewall roughness profile based on 2D high-resolution scanning electron microscope (SEM) imaging. We characterized the loss on two homemade nitride and oxide films on 3-inch silicon wafers with 12 waveguide devices on each and correlated the scattering loss estimated from a 2D image-based sidewall profile and theoretical Payne model. The lowest loss of guided fundamental transverse electric (TE₀) mode is found at 0.075 dB/cm at 633 nm across 24 devices, a record at visible wavelength. Our work shows 100% success (edge continuity span exceeding 95% of image width/height) in edge detection in image processing of all images to estimate autocorrelation function and optical mode loss. These demonstrations offer valuable insights into waveguide sidewall roughness and a comparison of experimental and 2D SEM image processing based loss estimations with applications in loss characterization at wafer-scale PICs. 

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5. A 3‐D FDTD Methodology for Modeling the Propagation of VLF Whistler Mode PLHR Waves Through the Ionosphere

   https://doi.org/10.1029/2024JA033273  

   Pedgaonkar, A S; Simpson, J J; Jensen, E A (February 2025, Journal of Geophysical Research: Space Physics)

   Abstract The finite‐difference time‐domain (FDTD) method was previously applied to high‐frequency electromagnetic wave propagation through 250 km of theFregion of the ionosphere. That modeling approach was limited to electromagnetic wave propagation above the critical frequency of the ionospheric plasma, and it did not include the lower ionosphere layers or the top of theF‐region. This paper extends the previous modeling methodology to frequencies below the critical frequency of the plasma and to altitudes encompassing the ionosphere. The following changes to the previous work were required to generate this model: (a) theD,Eand top of theFregions of the ionosphere were added; and (b) the perfectly matched layer absorbing boundary on the top side of the grid was replaced with a collisional plasma to prevent reflections. We apply this model to the study of extremely low frequency (ELF) and very low frequency (VLF) electric power line harmonic radiation (PLHR) through the ionosphere. The model is compared against analytical predictions and applied to PLHR propagation in polar, mid‐latitude and equatorial regions. Also, to further demonstrate the advantages of the grid‐based FDTD method, PLHR propagation through a polar cap patch with inhomogeneities is studied. The presented modeling methodology may be applied to additional scenarios in a straightforward manner and can serve as a useful tool for better tracking and studying electromagnetic wave propagation through the ionosphere at any latitude and in the presence of irregularities of any size and shape. 

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