What's the meaning of wavevector in bloch boundary condition?

When we choose bloch boundary condition in FDTD, we have to set up kx(or ky kz). But what’s the meaning of the wavevectors here? Is it the wavevector of input light? However, the input wavevector is determined by the source. I cannot understand why boundaries need a wavevector! Could someone provide some explanation? Thanks a lot!

The wavevector is used to set up the Bloch BCs as mentioned here
this ensures that the specified wavevector is correct for the given the plane wave source or dipole source on the specified angle. In a simple way, only the wave that satisfies this specified wavevector is correct at the given angle and given frequency. For other frequencies their angles will be different from the specified.


Thank you for your reply! It is helpful for me!