Role of Au nanoparticle aggregation in laser induced anisotropy of ITO transparent substrates

A principal possibility to achieve the photoinduced anisotropy in the Au NP deposited onto the ITO substrate is experimentally shown. The sizes of the Au NP forming the corresponding nanocomposites were 20 nm, 30 nm and 40 nm. As a photoinducing light source we have used two coherent beam originating from the Er:glass laser generating at 1540 nm with frequency repetition 15 Hz as well as its second harmonic doubled frequency signal at 770 nm. The effect is sensitive to the angle between the two laser beams as well as to the Au NP sizes, inter-particle distances and topology connected with their aggregation. The effect shows slow relaxation to the initial state. The optimal conditions are achieved for nanocomposites formed by 30 nm despite the expected 20 nm. This one may be caused by crucial role of the partial aggregation which even changes the effective grain sizes. The contribution of the dipole-dipole as well as quadrupole-dipole interactions to the changes of the anisotropy is discussed. The excitation is far from the resonance which allow to predict that effective role play overlap with nanotrapping levels. So principal role may belongs to surface topology and which is studied using the birefringence directly connected with the anisotropy.