Optically Induced Second Harmonic Generation by Six-wave Mixing:  A Novel Probe of Solute Orientational Dynamics

Abstract

The optically induced generation of second harmonics in isotropic media, which arises through a six-wave mixing interaction, is employed in ultrafast studies of orientational relaxation in solution (4-diethylamino-4′-nitrostilbene in a range of solvents). It is shown that polar order can be induced in the samples by simultaneous irradiation at the fundamental and second harmonic frequencies. Time-resolved experiments in nonpolar solvents show that the polar order (which supports second harmonic generation) decays as a biexponential function of time. The two time constants are linearly dependent on viscosity and fall in the ratio 6:1. This result is shown to be consistent with a simple model of the six-wave mixing interaction incorporating orientational diffusion. The two relaxation components arise through contributions from the first and third moments of the solute orientational distribution function, confirmed by the polarization dependence of the time-resolved signal. Thus, it has been demonstrated that odd moments of the orientational distribution function, required for a complete description of molecular orientation, are accessible through six-wave mixing experiments; such data are not available in four-wave mixing measurements. The measurements have been extended to polar solvents, where the accelerated population relaxation of the solute is apparent. The dynamics at early times in polar solvents are complex, an effect which is explained in terms of a time-dependent molecular hyperpolarizability.

Optically-heterodyne-detected optical Kerr effect (OHD-OKE): Applications in condensed phase dynamics

“…Alternative six-wave mixing experiments have been proposed [73, 77±80]. Resonant six-wave mixing experiments have been demonstrated for studies of solute dynamics [81,82], but the non-resonant multidimensional experiments required to unravel the ground-stat e dynamics of liquids have proved challenging, in particular because the six-wave signal is polluted by contributions from cascaded lower-order signals [83,84]. However, progress in overcoming this problem has recently been reported, and it seems likely that these challenging measurements will provide important new information in the near future [85].…”

Local coherence and the temporal development of second harmonic emission

“…For example, only a weak, incoherent, non-directed, second harmonic signal can emerge from any truly isotropic system-and then only if the constituent particles are non-centrosymmetric [6]. Although a host of exceptions to this symmetry condition have been listed [7], they generally entail conditions resulting in the removal of local isotropy-or, at very high intensities, six-wave interactions [8][9][10][11]. By exploiting the general principle, however, the process of second harmonic generation (SHG) has become a highly valuable tool for studying the surfaces of liquids and other centric systems, since any coherent second harmonic emission generally carries information only from the surface and not the bulk.…”

Symmetries, Conserved Properties, Tensor Representations, and Irreducible Forms in Molecular Quantum Electrodynamics

“…Table 1 lists the structure of decomposition into weights relevant for the most extensively studied kinds of optical interaction; the most prevalent forms of index symmetry are accommodated in tensors up to rank n = 6 (which, though less familiar, arise, for example, for six-wave mixing). [110][111][112][113][114] In this table, the tilde on Q (j) n indicates that index symmetry is taken into account. Other cases of index permutational symmetry are possible, and most can be identified from the distinct partitions of n; although additional possibilities such as T ((λµ)(νπ)) are possible in principle, no experimental implementations of such cases are evident in the optics literature as yet.…”