Experiments and Theory of Induced Optical Magnetization.

Abstract

This thesis reports the results of light scattering experiments at moderate optical intensities (~10^8 W/cm^2) in which the magnetic component of light induces magnetic dipolar response of unprecedented intensity by a novel nonlinear mechanism. Both experimentally and theoretically the amplitude of induced magnetization is found to be as large as electric polarization (M=cP) at intensities above ~10^8 W/cm^2 in different materials, greatly exceeding the conventional bounds of the multipole expansion. The transverse nature of the magnetization, its frequency, and its quadratic dependence on incident light intensity are in agreement with an exact theory which identifies the importance of magnetically-induced torque in achieving 2-photon resonance of this ultrafast process.

In this work we report and compare the intensity dependence of cross-polarized scattering in the transparent molecular liquids CCl_4, SiCl_4, SiBr_4, SnCl_4, C_6H_6, C_6D_6, C_6H_5NH_2, and C_6H_5CN and the crystalline solid Gd_3Ga_5O_{12}. Complete radiation patterns of co-polarized and cross-polarized light scattering were recorded as a function of intensity in these homogeneous media and subsequently decomposed into polarized and unpolarized components to provide a more complete picture of scattering dynamics than has been possible in past experiments.

The cross-polarized scattering observed from spherical-top molecules CCl_4, SiCl_4, SiBr_4, and SnCl_4 and crystalline GGG is argued to originate from magnetic dipoles induced by a second-order optical nonlinearity driven jointly by the E and B fields of light. Among the spherical top molecular liquids, SnCl_4 developed more intense magnetic scattering at a fixed intensity than CCl_4, in agreement with the predicted dependence on rotational frequency and damping. Cross-polarized scattering in anisotropic molecules C_6H_6, C_6D_6, C_6H_5NH_2, and C_6H_5CN, on the other hand, is known to originate from optical orientation of permanent electric dipole moments in first-order or differential polarizability in third-order. The importance of rotational dynamics to depolarization in all the liquids studied is outlined and confirmed through observation of an isotopic effect in the scattering from C_6H_6 vs. C_6D_6. Finally, the new nonlinear optical process investigated here provides a method for generating oriented rotations of molecules.