Ansys Lumerical FDTD (Finite-Difference Time-Domain) is a high-performance electromagnetic simulation tool used to model the interaction of light with sub-wavelength structures. Learning to use it typically follows a structured workflow that transitions from basic geometry setup to advanced data analysis. 1. The Core Simulation Workflow
A standard Lumerical FDTD tutorial starts with five fundamental steps to build a simulation from scratch:
: Verify or add materials (e.g., Silicon, Gold, SiO2) from the built-in material database. Structures
: Define the physical geometry by adding primitives like rectangles, circles, or complex objects from the Object Library Simulation Region
: Add an FDTD solver region to define the computational domain, mesh accuracy, and boundary conditions
(e.g., PML for absorbing boundaries or Periodic for infinite arrays). : Inject light into the system using various types: Plane Wave : For scattering and broadband studies. Mode Source : For injecting specific waveguide or fiber modes. Total Field Scattered Field (TFSF) : Specialized for nanophotonic scattering problems. : Place monitors to record data, such as Power monitors for transmission/reflection or Profile monitors for field visualization. 2. Available Learning Resources
For users seeking structured tutorials, Ansys and partners offer several self-paced paths: Lumerical scripting language - By category - Ansys Optics
This guide provides a foundational workflow for setting up and running a simulation in Ansys Lumerical FDTD , the industry standard for modeling nanophotonic devices. 1. Layout and Material Setup Define Geometry Structures
button to add primitive shapes (rectangles, cylinders) or import GDSII files. Assign Materials : Open the Material Database
to select from pre-defined models like Silicon (Si) or Gold (Au). Ensure the "Mesh Order" is set correctly for overlapping objects. 2. Simulation Region & Meshing FDTD Solver : Add an FDTD simulation region. Set the tab to cover your device. Boundary Conditions : For most photonic chips, use PML (Perfectly Matched Layer) to absorb outgoing waves and prevent reflections. Use Symmetric/Anti-Symmetric boundaries to save memory if your design is periodic. Mesh Settings
: Use a "Mesh Accuracy" of 2 or 3 for initial testing; increase to 4+ for final publication-grade results. 3. Sources and Monitors Add Source : Choose a Plane Wave for bulk materials or a Mode Source for waveguides. Set the wavelength range (e.g., 1.5 for C-band telecommunications). Insert Monitors Frequency-Domain (Power)
: To capture transmission, reflection, and electric/magnetic field profiles ( Time-Domain
: To verify that the fields have decayed before the simulation ends. ResearchGate 4. Running and Analysis Check Layout : Click the button to ensure the mesh and boundaries are valid. Run Simulation : Click the
button. Monitor the "Shutoff Level"; the simulation should reach 10 to the negative 5 power or lower for converged results. Visualize Data : Right-click on your monitors after completion and select (transmission) or (reflection) versus wavelength. For more advanced workflows, you can explore the Ansys Optics Learning Center
for specific examples like grating couplers or metasurfaces. ResearchGate
Mastering Photonic Design: A Comprehensive Lumerical FDTD Tutorial lumerical fdtd tutorial
Ansys Lumerical FDTD (Finite-Difference Time-Domain) is the industry-standard solver for modeling nanophotonic devices, processes, and materials. Whether you are designing a CMOS image sensor, a grating coupler, or a metalens, understanding the fundamentals of FDTD is crucial for moving from theoretical concepts to manufacturable designs.
This tutorial provides a structured walkthrough for setting up, running, and analyzing your first simulation. 1. Understanding the FDTD Method
Before clicking buttons, it is essential to understand what the software is doing. The FDTD method solves Maxwell’s equations in time and space. It divides the simulation volume into a rectangular grid (the Yee Lattice).
Time-Domain: It calculates the E and H fields at each grid point as time progresses.
Broadband Results: Because it is a time-domain solver, a single simulation can provide response data across a wide range of wavelengths via a Fourier Transform. 2. Setting Up Your Layout
The Lumerical CAD environment follows a logical hierarchy. Follow these steps to build your simulation: A. Define Materials
Navigate to the Material Database. Lumerical provides a vast library of sampled data (e.g., Si, SiO2, Ag).
Pro Tip: Always check the "Material Explorer" to ensure the multi-coefficient model (MCM) fits the experimental data accurately over your source bandwidth. B. Geometry Construction
Use the Structures button to add primitives like rectangles, cylinders, or polygons.
Coordinates: Everything is defined relative to the global origin.
Overlap: In Lumerical, the object added later in the objects tree takes precedence if two materials overlap. C. The Simulation Region
Add an FDTD Simulation Region. This is the most critical step. Boundary Conditions:
PML (Perfectly Matched Layer): Absorbs waves without reflection (simulates open space).
Symmetric/Anti-Symmetric: Use these to reduce simulation time by 2x or 4x if your structure and source have symmetry. Periodic: Used for arrays or metasurfaces. 3. Adding Sources and Monitors
To get data, you need to excite the system and record the response. The Source The Core Workflow: A Step-by-Step Tutorial Approach A
For most nanophotonic applications, use a Plane Wave or a Total-Field Scattered-Field (TFSF) source. Define the wavelength range (e.g., 400nm to 700nm).
Ensure the source is placed inside the simulation region but outside any monitors you want to use for "scattered" fields.
Monitors do not affect the simulation; they only record data.
Index Monitor: Use this to verify your geometry is correct before running.
Frequency-Domain Field and Power Monitor: This is the "bread and butter" monitor. It calculates Transmission (T) and Reflection (R).
Movie Monitor: Great for visualizing how light pulses propagate through your device. 4. Convergence Testing: The Key to Accuracy
A common mistake for beginners is trusting the first result. You must perform Convergence Testing to ensure your grid is fine enough. Run the simulation with a coarse mesh (Mesh Accuracy 2).
Refine the mesh (Mesh Accuracy 3 or 4) or add a Mesh Override Region over small features.
Compare results. If the transmission spectrum doesn't change significantly, your simulation has converged. 5. Running the Simulation and Analyzing Data
Click the Run button. Lumerical will partition the task across your CPU cores.
Once finished, enter Analysis Mode (the layout will be locked).
Visualizer: Right-click a monitor to "Visualize" results. You can plot Electric Field intensity or the Poynting vector.
Scripting: Use the Lumerical Script File (.lsf) to automate data extraction. For example, transmission("monitor_name"); will return the fraction of power flowing through that monitor. 6. Common Pitfalls to Avoid
PML Reflections: If your PML is too close to a scattering object, it can cause artificial reflections. Leave at least half a wavelength of "buffer" space.
Simulation Time: Ensure the "Simulation Time" in the FDTD region is long enough for the fields to decay. If the "Autoshutoff" level doesn't reach 10-510 to the negative 5 power , your results may show ripples. 3. Advanced Diagnostics and Optimization
Divergence: If the simulation "blows up," check for overlapping materials with high plasma frequencies or narrow mesh override regions. Conclusion
Lumerical FDTD is a powerhouse for photonic research. By mastering the geometry-source-monitor workflow and prioritizing convergence testing, you can produce high-fidelity simulations that match real-world lab results.
A typical simulation in Lumerical FDTD follows a structured workflow. We will illustrate this using a canonical example: calculating the transmission and reflection spectra of a photonic crystal slab.
Step 1: Defining the Simulation Region. The simulation begins by setting up the FDTD region, a rectangular volume where the field evolution is computed. The user defines its size in the x, y, and z dimensions. Crucially, boundary conditions must be assigned. For an open structure radiating into free space, perfectly matched layers (PML) are applied at the boundaries to absorb outgoing waves without spurious reflections. For periodic structures like gratings or photonic crystals, periodic or Bloch boundary conditions are more appropriate. In our example, we use PML in the vertical (z) direction and periodic boundaries laterally (x, y) to model an infinite slab.
Step 2: Adding Materials and Structures. Lumerical provides a comprehensive material database (e.g., Si, SiO₂, Au, Ag) with wavelength-dependent refractive indices (n, k). Users can also define custom materials using models like Lorentz or Drude for dispersive media. The photonic crystal slab—a layer of silicon with a periodic array of air holes—is constructed using primitive geometric objects (rectangles, cylinders) from the layout editor. Boolean operations and parameter sweeps allow for complex, parameterized designs.
Step 3: Configuring the Source. An excitation source injects light into the simulation. Common choices include:
Step 4: Placing Monitors and Analysis Groups. Monitors record field data. Key types include:
Step 5: Mesh Settings. The FDTD solution's accuracy is governed by the mesh. The default uniform mesh is often insufficient. Users typically employ a conformal mesh that refines near material interfaces. The "mesh override" region allows local refinement in critical areas (e.g., inside the air holes). A standard rule of thumb is a mesh step of at least ( \lambda / 20 ) at the highest frequency of interest. Lumerical also supports a non-uniform mesh to balance speed and accuracy.
Step 6: Running the Simulation and Analyzing Results. After checking for warnings (e.g., insufficient PML thickness, mesh too coarse), the simulation is executed. For 3D problems, this can be memory-intensive. Lumerical leverages parallel computing (multi-core CPU, GPU acceleration). Once completed, results are viewed in the visualizer. We can plot ( T(\lambda) ) and ( R(\lambda) ) versus wavelength, observe the photonic bandgap as a dip in transmission, and visualize the field profile at resonant wavelengths.
Lumerical uses an "Auto-Shutoff" feature to stop the simulation when the energy in the simulation volume drops below a threshold (typically $10^-5$).
Drag the FDTD region from the toolbar.
Monitors record the simulation data.
A. Power Monitor (Transmission)
transmission.B. Profile Monitor (Field Visualization)
field_profile.Hit the green "Run" button. Watch the "FDTD Progress" window. Look for:
Calculate how well a fiber mode couples to your chip. Use the couple function in the script:
E1 = getdata("fiber_monitor","E");
E2 = getdata("chip_monitor","E");
coupling = abs(integrate(E1*conj(E2)))^2 / (norm(E1)^2 * norm(E2)^2);