Direct evidence of the failure of electric-dipole approximation in second-order nonlinearity of a chiral polymer film

Summary form only given. Magnetic dipole and electric quadrupole contributions to nonlinear processes are usually very weak. However, the relative magnitudes of the various multipole contributions can be influenced by molecular engineering. For example, magnetic contributions are important in the electric-dipole-allowed second-harmonic generation from thin films of chiral polymers (polyisocyanide). However, evidence of magnetic contributions was obtained only from a detailed comparison of several second-harmonic signals; i.e., each signal can be explained separately in the electric-dipole approximation. Here, we show that second harmonic generation from thin films of a chiral polythiophene cannot be explained by the polarised components of the fundamental field.