  The spin momentum non-orthogonality in 3D topological insulators leads to
modification of the spin texture and brings in an out-of-plane spin
polarization component. Apart from spin texture, the anomalous thermoelectric
properties of these materials are worth studying. In this paper, we have
pointed out that the off resonant light used to irradiate the surface states,
induces a gap, which becomes momentum dependent due to the presence of
non-orthogonal terms in the Hamiltonian. Importantly, to maintain the off
resonant condition of light, the momentum value should satisfy a bound.
Furthermore, the momentum dependent gap causes a topological transition at
higher value of momentum, which is important to analyse the unusual double peak
structure of the Nernst and electrical conductivities.
