Band structure- implement Bloch boundary condition

hallo all,

How do I implement bloch boundry condition: f(x=0)=f(x=L)*exp(ikL) in the second derivative matrix representation (in a 1D problem) that operates on a vector: x=linespace(0,L,1000)?

where f would be some eigen function of the Laplacian matrix(written here without the bloch boundry condition ):

Lap= (-2*diag(ones(N,1),0) + diag(ones((N-1),1),1) ...

+ diag(ones((N-1),1),-1))/(dx^2);

thanks