https://research.manchester.ac.uk/en/publications/huygens-subgridding-for-3-d-frequency-dependent-finite-difference/
Huygens subgridding for 3-D frequency-dependent finite-difference time-domain method - Research...
https://impact.ornl.gov/en/publications/a-new-approach-to-bor-fdtd-subgridding/
A new approach to BOR-FDTD Subgridding - Oak Ridge National Laboratory
a new approachoak ridgebor