Sign convention of diffraction order numbers of Lumerical FDTD/RCWA ?

- January 27, 2024 at 10:22 pm
  
  bailin.zhang
  Subscriber
  
  Hello Everyone,
  Due to there are many sign conventions taken by different computer programs. Some use the dffraction rays propagating to or away from the normal, some use the propagaion direction is forward to positive x axis or negative axis, and even different for propagting to y and x axis direction. Some also use the angle formed with the normal of grating suface which also depends on the sign convnetions about the angle rotation et al.
  Did anyone use the Lumerical FDTD/RCWA for designing gratings or other diffractive elements? If so, could you please describe the sign convention of diffraction orders used in Lumerical FDTD/RCWA. I tried hard to find the sign convention used by Lumerical FDTD/RCWA but couldn't finda any such information. Without kowing this, it is impossible for me trace the exact orders I wanted. Thank you very much !
- February 5, 2024 at 7:49 pm
  
  Guilin Sun
  Ansys Employee
  
  You are right that people use different conventions. Caution should be taken when to interpret the result.
  In Lumerical products, we usually follow the theta, phi convention:
  https://optics.ansys.com/hc/en-us/articles/360034394294-Understanding-direction-unit-vector-coordinates-in-far-field-projections
  A little more:
  usually theta is actually from 0 to 90deg, as it uses the monitor normal as reference. So when light is propagating along +z, the reflection theta angles will be positive as we use cosine (from squareroot operation) to get the angle in degree.