Asymmetric dye molecules have unusual optical and electronic properties. For instance, they show a strong second-order nonlinear optical (NLO) response that has attracted great interest for potential applications in electro-optic modulators for optical telecommunications and in wavelength conversion of lasers. However, the strong Coulombic interaction between the large dipole moments of these molecules favours a pairwise antiparallel alignment that cancels out the NLO response when incorporated into bulk materials. Here, we show that by including an elongated dipolar dye (p,p'-dimethylaminonitrostilbene, DANS, a prototypical asymmetric dye with a strong NLO response) inside single-walled carbon nanotubes (SWCNTs), an ideal head-to-tail alignment in which all electric dipoles point in the same sense is naturally created. We have applied this concept to synthesize solution-processible DANS-filled SWCNTs that show an extremely large total dipole moment and static hyperpolarizability (β0 = 9,800 × 10(-30) e.s.u.), resulting from the coherent alignment of arrays of ∼70 DANS molecules.

Chemical control of the endohedral volume of single-wall carbon nanotubes (SWCNTs) via liquid-phase filling is established to be a facile strategy to controllably modify properties of SWCNTs in manners significant for processing and proposed applications.

A very sensitive experimental setup for accurate wavelength-dependent hyper-Rayleigh scattering (HRS) measurements of the molecular first hyperpolarizability beta in the broad fundamental wavelength range of 600 to 1800 nm is presented. The setup makes use of a stable continuously tunable picosecond optical parametric amplifier with kilohertz repetition rate. To correct for multi-photon fluorescence, a small spectral range around the second harmonic wavelength is detected in parallel using a spectrograph coupled to an intensified charge-coupled device. Reliable calibration against the pure solvent is possible over the full accessible spectral range. An extensive set of wavelength-dependent HRS calibration data for a wide range of solvents is presented, and very accurate measurements of the beta dispersion of the well-known nonlinear optical chromophore Disperse Red 1 are demonstrated.

The wavelength-dependent molecular first hyperpolarizability β of the zwitterionic nonlinear optical (NLO) chromophore picolinium quinodimethane (PQDM) is determined by hyper-Rayleigh scattering (HRS) and used to test and improve theoretical β dispersion models. Experimental HRS data are obtained over a very wide fundamental wavelength range (780−1730 nm), spanning the entire range of two-photon resonance with the intramolecular charge-transfer (ICT) transition and reaching the onset of a higher energy resonance. Reliable calibration against the pure solvent (dimethylformamide, DMF, and DMF-d 7 at the longest wavelengths) was achieved over the full spectral range as a result of the high sensitivity of the HRS setup. Extremely high resonant β values are obtained (up to 4560 × 10-30 esu at 1360 nm) and also away from resonance β remains very large (1210 × 10-30 esu at 1730 nm). The two-photon resonance with the ICT band shows a pronounced red shift (∼33 nm in second-harmonic wavelength) relative to the absorption maximum. The various two-level β dispersion models available in the literature are considered, and some important improvements are introduced. Furthermore, a vibronic model including a single vibrational mode and incorporating inhomogeneous broadening is developed and contrasted to the other extreme of a continuum of vibronic lines without inhomogeneous broadening. The red shift of the β maximum can be largely explained by either an improved inhomogeneous broadening model or by vibronic coupling. However, the vibronic models are physically more realistic and lead to a better description of the observed β dispersion. In general, models with more inhomogeneous broadening result in a narrower β resonance, whereas incorporating more homogeneous broadening yields a broader resonance. Hence, the derived static electronic hyperpolarizability β0 depends very critically on the precise modeling of the broadening mechanisms. Upper and lower bounds to the true β0 are estimated from the two limiting vibronic models.