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Inverse Design Validated Optimization of Lead-Free Cs$_3$Cu$_2$Cl$_5$ Visible-Light Microring Resonators Using a Coupled DFT-FDTD Framework

Shoumik Debnath, Sudipta Saha

Abstract

Microring resonators (MRRs) are indispensable for wavelength filtering, sensing, and on-chip signal routing in photonic integrated circuits, yet visible-wavelength implementations using environmentally benign materials remain scarce. We report a numerical design study of add-drop MRRs employing Cs$_3$Cu$_2$Cl$_5$, a lead-free all-inorganic halide with favorable optical properties in the visible spectral range. Wavelength-resolved refractive index (n) and extinction coefficient (k) of Cs$_3$Cu$_2$Cl$_5$, calculated using density functional theory (DFT), are used as direct inputs to three-dimensional finite-difference time-domain (FDTD) simulations. Independent parametric sweeps are performed over ring waveguide width (500-900 nm), coupling gap (150-300 nm), and bend radius (5-20 um). At the balanced operating point of 600 nm ring width, 200 nm gap, and 10 um radius, the device achieves a loaded quality factor Q approx 5386, a free spectral range of 11.3 nm, a drop-port extinction ratio of 32.2 dB, and a finesse of 95.8. The coupling-gap sweep reveals the full transition from over-coupled through critically coupled to under-coupled operation, with the critical point occurring near 200 nm. A pronounced bending-loss threshold is observed between 5 and 10 um, below which all performance metrics degrade rapidly. These results provide the first systematic geometry-performance map for Cs$_3$Cu$_2$Cl$_5$ based microring resonators. Cross-platform validation using Tidy3D reproduces the spectral characteristics of the optimized device, and inverse design of the bus coupling region yields an additional 3 percent improvement in drop-port power transfer.

Inverse Design Validated Optimization of Lead-Free Cs$_3$Cu$_2$Cl$_5$ Visible-Light Microring Resonators Using a Coupled DFT-FDTD Framework

Abstract

Microring resonators (MRRs) are indispensable for wavelength filtering, sensing, and on-chip signal routing in photonic integrated circuits, yet visible-wavelength implementations using environmentally benign materials remain scarce. We report a numerical design study of add-drop MRRs employing CsCuCl, a lead-free all-inorganic halide with favorable optical properties in the visible spectral range. Wavelength-resolved refractive index (n) and extinction coefficient (k) of CsCuCl, calculated using density functional theory (DFT), are used as direct inputs to three-dimensional finite-difference time-domain (FDTD) simulations. Independent parametric sweeps are performed over ring waveguide width (500-900 nm), coupling gap (150-300 nm), and bend radius (5-20 um). At the balanced operating point of 600 nm ring width, 200 nm gap, and 10 um radius, the device achieves a loaded quality factor Q approx 5386, a free spectral range of 11.3 nm, a drop-port extinction ratio of 32.2 dB, and a finesse of 95.8. The coupling-gap sweep reveals the full transition from over-coupled through critically coupled to under-coupled operation, with the critical point occurring near 200 nm. A pronounced bending-loss threshold is observed between 5 and 10 um, below which all performance metrics degrade rapidly. These results provide the first systematic geometry-performance map for CsCuCl based microring resonators. Cross-platform validation using Tidy3D reproduces the spectral characteristics of the optimized device, and inverse design of the bus coupling region yields an additional 3 percent improvement in drop-port power transfer.
Paper Structure (16 sections, 5 equations, 12 figures, 4 tables)

This paper contains 16 sections, 5 equations, 12 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Device design methodology
  3. DFT calculations for optical constants
  4. Device structure
  5. FDTD simulation setup
  6. Performance metrics
  7. Inverse design methodology
  8. Results and discussion
  9. Effect of ring waveguide width
  10. Coupling gap dependence and coupling regime transitions
  11. Radius-dependent performance and bending loss threshold
  12. Design trade-off analysis and optimization guidelines
  13. Mode profile verification
  14. Inverse design optimization and cross-platform validation
  15. Fabrication feasibility
  16. ...and 1 more sections

Figures (12)

  • Figure 1: (a) Unit cell of Cs$_3$Cu$_2$Cl$_5$ ($Pnma$), rendered from Materials Project data (mp-582024) Jain2013. (b) DFT-computed $n$ and $k$ across the visible spectrum. The shaded region indicates the primary operating wavelength range.
  • Figure 2: (a) Plan-view layout of the Cs$_3$Cu$_2$Cl$_5$ add-drop MRR with labeled geometric parameters. (b) Perspective rendering on SiO$_2$ substrate.
  • Figure 3: (a) Through- and drop-port spectra for $W_r = 500$, 600, and 800 nm at fixed $g = 200$ nm and $R = 10$$\mu$m. (b) Drop-port response for all five ring widths.
  • Figure 4: Width dependence of (a) $Q$, (b) drop-port ER, and (c) $\mathcal{F}$. $g = 200$ nm and $R = 10$$\mu$m throughout.
  • Figure 5: (a) Port spectra at $g = 150$, 200, and 250 nm ($W_r = 600$ nm, $R = 10$$\mu$m). (b) Drop-port and (c) through-port transmission for all gap values.
  • ...and 7 more figures