Can I calculate non-linear response along non-periodic directions?
At present is not implemented in the code, but in principle it can be done by means of single-point Berry phase (see Phys. Rev. B 58, 1222) or by implementing dipoles in the non-periodic directions.

Is it possible to calculate two-photon absorption?
Two-photon absorption originates from the third order susceptibility, see the scheme below: (Figure from Nonlinear Optics in Signal Processing, published by R.W. Eason,A. Miller).
We calculated it for a simple system (h-BN), the procedure is not so simple but it is explained in this paper: Phys. Rev. B 98, 165126

Is the real-time Bethe-Salpeter available in the code?
Yes, but consider that RT-BSE calculations are quite expesive.

Is it possible to work with polarized system?
We never tested the code on polarized systems, use at your own risk.

What about spin-orbit coupling?
Yes, in principle it works but we never tested it too much.

Is it possible to use polarized light?
Yes, in principle if one modifies the subroutine that describes the external field it is possible to simulate polarized light, but we did not implemented it yet.

Is it possible to calculate electro-absorption?
Yes, it should be possible to calculate electron-absorption and the Franz-Keldish effect. This is shown in the paper of Souza et al. (PRB 69, 085106) that caculated absorption in presence of a constant electric field, but we never tried to do it.

How to cite Lumen?
See here.