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open source FDTD solver with GPU support

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docs:become [2020/04/22 14:44]
pklapetek
docs:become [2020/04/22 15:28]
pklapetek
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 To test the correctness of the newly implemented code we compared a simple **transmission grating** diffraction pattern to the analytically known solution. ​ To test the correctness of the newly implemented code we compared a simple **transmission grating** diffraction pattern to the analytically known solution. ​
 +Total/​scattered field approach was used to inject the plane wave normally to the plane containing the aperture or multiple apertures. TE mode calculation was used, which should mean the p-polarisation case as requested. Near-to-far field calculation domain was set up to be outside of the plane wave source region, so only transmitted field was propagated to the far-field. Time domain far field calculation was used. Far field data were calculated for wide range of angles for debugging purposes (i.e. not only for the directions of the particular diffraction orders).
 +
 There are at least two possible ways how to treat the grating in FDTD calculation. First of all, we can setup the grating physically in the computation domain, as large as possible, run the calculation and evaluated the far field response. As we are usually interested in an infinite grating response, this would mean an infinite computational domain. Instead, we can use periodic boundary conditions, compute only one motive and evaluate the far field data. This still does not provide the results if the far field data are evaluated only from the single motive, we need to evaluate it from many virtual repetitions to get the impact of some number of motives. We call this There are at least two possible ways how to treat the grating in FDTD calculation. First of all, we can setup the grating physically in the computation domain, as large as possible, run the calculation and evaluated the far field response. As we are usually interested in an infinite grating response, this would mean an infinite computational domain. Instead, we can use periodic boundary conditions, compute only one motive and evaluate the far field data. This still does not provide the results if the far field data are evaluated only from the single motive, we need to evaluate it from many virtual repetitions to get the impact of some number of motives. We call this
 //periodic NFFF// in the next text. //periodic NFFF// in the next text.
 In most of the graphs here we show complete diffraction pattern. In most of the graphs here we show complete diffraction pattern.
-However, if we are interested in the maximum in some diffraction order direction, it is much simpler and it seems that this is the preferably used approach - we calculate the far field value only at the diffraction order maximum. Luckily enough, this value is dependent on the aperture only, which constructs the envelope for the diffraction pattern, so in this case one could work only with a single aperture. However, to construct the diffraction pattern is a good way how to debug the problem.+However, if we are interested in the maximum in some diffraction order direction, it is much simpler and it seems that this is the preferably used approach - we calculate the far field value only at the diffraction order maximum. Luckily enough, this value is dependent on the aperture only, which constructs the envelope for the diffraction pattern, so in this case one could work only with a single aperture. However, to construct the diffraction pattern is a good way how to debug the problem. ​The schematics of 
 +the calculations is here: 
 + 
 +{{:​docs:​schematics.png?​600|}} 
  
 A comparison of the different evaluation methods is shown below, also showing the first A comparison of the different evaluation methods is shown below, also showing the first
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 periodic calculation (based on a single motive) where the far field is evaluated from three periodic calculation (based on a single motive) where the far field is evaluated from three
 and nine virtual repetitions are then compared to the case where the calculation is not periodic and nine virtual repetitions are then compared to the case where the calculation is not periodic
-and three apertures are physically existing in the computational domain. ​The schematics of +and three apertures are physically existing in the computational domain. ​
-the calculations is here: +
- +
-{{:​docs:​schematics.png?​600|}}+
  
 An important message is that An important message is that
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 ---- ----
  
-The transition from transition geometry to reflection geometry was done first with perfect electric conductor as a grating material. The same motive as in the Become grating is used, it +The transition from transition geometry to reflection geometry was done first with perfect electric conductor as a grating material. The same motive as in the Become grating is used and also 
-is only from a different materials for calculation speed and simplicity. ​+the same voxel spacing (5 nm), it is only from a different materials for calculation speed and simplicity. ​All the computational details were same as in the previous example. 
 + 
 Here, we don't have any analytical result to which  Here, we don't have any analytical result to which 
 we could compare. We could therefore only compare the periodic calculation (using field from single motive with periodic boundary conditions and periodic NFFF) and the manual calculation (big model with all the motives). we could compare. We could therefore only compare the periodic calculation (using field from single motive with periodic boundary conditions and periodic NFFF) and the manual calculation (big model with all the motives).
  
 The two models that were compared are shown below. ​ The two models that were compared are shown below. ​
 +{{:​docs:​schematics_reflection_pec.png?​600|}}
  
 The periodic calculation was done the same The periodic calculation was done the same
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 {{:​docs:​validation.png?​800|}} {{:​docs:​validation.png?​800|}}
 +
 +The backup files are {{ :​docs:​2d_reflection_cpml_backup_files.tar.gz |here}}.
  
 ---- ----
  
  
-The **Become benchmark grating** was setup with voxel spacing of 5 nm in every direction. The total computational domain size was 240x100 voxels. The grating was formed by silver, using the PLRC metal handling approach. Periodic boundary conditions were used to introduce the grating periodicity. Total/​scattered field approach was used to inject the plane wave normally to the surface. TE mode calculation was used for this 2D case, which should be the p-polarisation case as requested. Near-to-far field calculation domain was set up to be outside of the plane wave source region, so only reflected and scattered electric field was propagated to the far-field. Time domain far field calculation was used. Far field data were calculated for wide range of angles for debugging purposes (i.e. not only for the directions of the particular diffraction orders).+The **Become benchmark grating** was setup the same way as the above test example 
 +The grating was now formed by silver, using the PLRC metal handling approach. ​ 
 + 
 +Periodic boundary conditions were used to introduce the grating periodicity. Total/​scattered field approach was used to inject the plane wave normally to the surface. TE mode calculation was used for this 2D case, which should be the p-polarisation case as requested. Near-to-far field calculation domain was set up to be outside of the plane wave source region, so only reflected and scattered electric field was propagated to the far-field. Time domain far field calculation was used. Far field data were calculated for wide range of angles for debugging purposes (i.e. not only for the directions of the particular diffraction orders).
 The model setup and a calculation snapshot of the periodic area are shown in the following figure. The model setup and a calculation snapshot of the periodic area are shown in the following figure.
 The far field was evaluated from a fixed number of repetitions of the near-field values, the presented results therefore represent scattering by a finite size grating. The far field value in the direction of the maxima is however not affected by size of grating (number of repetitions),​ only its sharpness is affected. The far field was evaluated from a fixed number of repetitions of the near-field values, the presented results therefore represent scattering by a finite size grating. The far field value in the direction of the maxima is however not affected by size of grating (number of repetitions),​ only its sharpness is affected.
docs/become.txt · Last modified: 2020/04/24 12:27 by pklapetek