Nonlinear Optic Investigations of Nanoscale Interfaces
and Molecular Chromophores

We have employed the utility of hyper-Rayleigh scattering (HRS) to investigate the nonlinear optical properties of a host of systems ranging from molecular chromophores like Ru(DEAS)32+ to colloidal systems such as SiO2 and Au.  These efforts have been partially supported by the Materials Research Center as well as the Army sponsored MURI project at Northwestern.
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    Molecules: Ru(DEAS)32+
    Our studies on Ru(DEAS)32+ (shown below) have been aimed at understanding the symmetry involved in frequency doubling.  The ground state of this structure is of the right architecture to give it octupolar symmetry (D3h), a novel and potentially useful symmetry in NLO device design.  However, using a combination of polarized HRS (see Figure 1) and Stark spectroscopy, we have determined that the excited state charge localized on only one of the three ligands, resulting in an upper state dipole moment.  Thus the overall transition, and thus the mechanism of frequency doubling, appears to be dipolar in nature.  For more information, see ref. 154.
    Figure 1.  When a polarizer is rotated in front of the detector, the ratio of vertically to horizontally scattered light is found to be ca. 2.6.  If the chromophore had been octupolar in nature, the expected ratio would have been 1.5.
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    Nanoscale Materials: Silica Colloids
    In making the transition from molecules to materials, it is important to determine over what length scale a particular experiment is useful.  For example, second harmonic generation (SHG) from interfaces is a powerful tool for the interrogation of surfaces whose physical dimensions are on the order of the wavelength of light (i.e., microns).  For smaller interfaces such as those in SiO2 colloids, which are on the order of 15 nm, we have found HRS to be a particularly useful probe.  We have found that the HRS signal from colloidal silica is highly sensitive to pH (see Figure 2), indicating that the signal is sensitive to protonation/deprotonation of surface (defect) sights.  For more information, see ref. 133.
    Figure 2.  The HRS signal is greatly reduced as the pH is adjusted to pass through the two surface pKa's for colloidal silica.

     
     
     
     
    Nanoscale Materials: Gold Colloids
    Gold colloids have recently become the interest of several research efforts, due largely to their unique optical properties and chemical utility.  We have recently found that 13 nm gold colloidal particles, nominally spherical in shape, have the largest hyperpolarizability (b) per atom yet reported in the literature.  This is surprising given that a truly spherical particle would, by necessity, have a b of zero.  In order to test whether the signal may due to small aggregates, salt was added to a solution of the colloid, causing a color change from red to blue (Figure 3).  The HRS signal responded drastically to this aggregation (Figure 4), showing that the breaking of symmetry plays a large role in the impressive signals observed.  For more information, see ref. 146.  Future work in collaboration with Dan Feldheim at N.C. State will attempt to further ellucidate the mechanism of frequency doubling by studying small aggregates which have been intentionally prepared such that their symmetry is well defined. 
    Figure 3.  As NaCl is added to a solution of gold colloid, the visible spectrum is drastically changed, with an attenuation of the 520 nm peak and the formation of an additional broad feature at longer wavelengths. 
    Figure 4.  The Rayleigh scattering, shown in red, shows that large aggregates are only formed at high salt concentrations (above 0.05 M).  In contrast, the hyper-Rayleigh (blue), shows an enhanced response at low salt concentrations.