add target_neff parameter to mode solver

[mdecea]

Summary by Sourcery

New Features:

• Introduce an optional target_neff parameter on the Waveguide model to control the target effective index used by the mode solver. This is very relevant for bent and leaky modes.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Adds an optional target_neff parameter to the Waveguide mode solver API and threads it through to td.ModeSpec, defaulting to the real part of the core refractive index when unspecified.

Sequence diagram for Waveguide target_neff propagation to ModeSpec

sequenceDiagram
    participant WG as Waveguide
    participant Core as CoreMaterial
    participant ModeSpec as td_ModeSpec

    WG->>Core: get_refractive_index()
    Core-->>WG: n_core
    WG->>WG: compute_target_neff()
    note over WG: if self.target_neff is not None
    WG->>ModeSpec: ModeSpec(num_modes, target_neff, bend_radius, bend_axis, num_pml)

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Class diagram for updated Waveguide mode solver API

classDiagram
    class Waveguide {
        +float surface_thickness
        +float surface_k
        +float bend_radius
        +float target_neff
        +int num_modes
        +bool group_index_step
        +Precision precision
        waveguide()
    }

    class ModeSpec {
        +int num_modes
        +float target_neff
        +float bend_radius
        +int bend_axis
        +tuple num_pml
    }

    Waveguide ..> ModeSpec : creates

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File-Level Changes

Change	Details	Files
Expose target_neff as a configurable mode-solver parameter and propagate it into ModeSpec while preserving the previous default behavior.	Extend Waveguide docstring to describe the new target_neff parameter and its default behavior. Add target_neff: float	None = None as a Waveguide model field. Compute a local target_neff value that falls back to n_core.real when the attribute is not set. Pass the computed target_neff into td.ModeSpec in both waveguide() code paths instead of hard-coding n_core.real.

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[sourcery-ai]

Hey - I've left some high level feedback:

• The target_neff selection logic (self.target_neff vs n_core.real) is duplicated in both waveguide implementations; consider extracting this into a small helper/property to avoid repetition and keep behavior consistent if it changes later.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `target_neff` selection logic (`self.target_neff` vs `n_core.real`) is duplicated in both `waveguide` implementations; consider extracting this into a small helper/property to avoid repetition and keep behavior consistent if it changes later.

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[gemini-code-assist]

Code Review

This pull request introduces a target_neff parameter to the Waveguide class in gplugins/tidy3d/modes.py, allowing users to specify a target effective index for the mode solver. This parameter is integrated into the waveguide methods of both Waveguide and WaveguideCoupler. Feedback suggests documenting the new parameter in the WaveguideCoupler docstring for consistency and refactoring the duplicated logic for calculating target_neff into a shared method or property.

[mdecea]

Changed the default material to lossless SiO2. The current SiO2 material causes perfectly good modes to show losses of 50 dB/cm!

[mdecea]

Added functionality to track modes on bend radius sweeps. This makes it so that we are robust to modes changing order as the radius in the mode solver changes.