Asymmetric dipolar molecules, acting as electric ‘rectifiers’ with a large second-order nonlinear optical (NLO) response (first hyperpolarizability beta), find applications in ultrafast electro-optic switches for optical telecommunication networks and wavelength conversion of lasers [1]. In 3D bulk materials, however, such dipolar molecules tend to align in a pairwise anti-parallel way, thus cancelling each other’s directional properties, such as the NLO response. Parallel alignment is traditionally achieved by electric field poling in a polymer matrix, but long term stability of this alignment remains problematic. Here we show that by encapsulating such elongated dipolar molecules in the 1D internal channel of single-wall carbon nanotubes (SWCNTs), Coulomb interactions naturally favor a polar head-to-tail alignment of the molecular dipoles, leading to a cooperative enhancement of their second-order NLO responses. This principle is demonstrated for a prototypical dipolar molecule encapsulated in various SWCNT samples. The encapsulation is evidenced by extensive wavelength-dependent fluorescence-excitation [2] and resonance Raman [3] experiments on bile-salt solubilized [4] dye-filled SWCNTs, revealing the effect of encapsulation on the vibrational and electronic properties of the SWCNTs and the encapsulated molecules. Wavelength-dependent hyper-Rayleigh scattering [5, 6] (i.e. second harmonic light scattering) experiments effecively show that these organic-SWCNT nanohybrids possess giant NLO responses (beta=68800x10^(-30)esu at resonance; beta_0=9770x10^(-30)esu in the static limit; corresponding to ~70 identically aligned chromophores, depending on the SWCNT diameter range). Their equally giant dipole moment and size promises a good and stable alignment of the nanohybrids in a polymer film, which opens up an entirely new route to the rational design of solution-processible yet stable NLO materials [7].
References [1] E. Goovaerts et al. In Handbook of advanced electronic and photonic Materials and Devices, Vol. 9, Academic Press, San Diego, 127-191 (2001) [2] S. Cambré et al., Luminescence Properties of Individual Empty and Water-Filled Single-Walled Carbon Nanotubes, ACS nano 6, 2649 (2012) [3] W. Wenseleers et al., Effect of Water Filling on the Electronic and Vibrational Resonances of Carbon Nanotubes: Characterizing Tube Opening by Raman Spectroscopy, Advanced Materials 19, 2274 (2007) [4] W. Wenseleers et al., Efficient Isolation and Solubilization of Pristine Single-Walled Nanotubes in Bile Salt Micelles, Advanced Functional Materials 14, 1105 (2004) [5] J. Campo et al., Highly sensitive setup for tunable wavelength hyper-Rayleigh scattering with parallel detection and calibration data for various solvents, Optics Express 17, 4587 (2009) [6] J. Campo et al., Practical Model for First Hyperpolarizability Dispersion Accounting for Both Homogeneous and Inhomogeneous Broadening Effects, Journal of Physical Chemistry Letters 3, 2248 (2012) [7] S. Cambré, J. Campo et al., Asymmetric dyes align inside carbon nanotubes to yield a large nonlinear optical response, Nature Nanotechnology, in press; DOI: 10.1038/nnano.2015.1