Giant Optical Nonlinearity from Carbon Nanotubes Filled with 1D Arrays of Dipolar Molecules
Authors: Cambré Sofie , Campo Jochen , Beirnaert Charlie , Verlackt Christof C. , Wenseleers Wim 
Published: February 2020

Over the past decades, the large 2nd order nonlinear optical (NLO) response (hyperpolarizability β) achievable in dipolar organic chromophores has attracted great interest due to a wide range of potential applications: for example, in optical telecommunication networks as ultrafast photonic switches and electro-optic modulators, and also as optical frequency convertors.[1] Unfortunately, due to their large dipole moments, such molecules tend to align in a pairwise anti-parallel way when incorporated in a (two- or) three-dimensional bulk material such as a crystal, thus cancelling each other’s NLO responses at the macroscopic level. A macroscopically polar alignment is typically achieved by aligning the dipolar molecules in a heated polymer film, followed by cooling below the glass-transition temperature to ‘freeze’ the molecular orientation. This however intrinsically results in a thermodynamically unstable state, and long term degradation remains an issue.
In this presentation, we will show for the first time that by encapsulating such organic NLO molecules in single-wall carbon nanotubes (SWCNTs), thus creating a one-dimensional (1D) molecular array, Coulomb interactions naturally favor a polar head-to-tail alignment of the dipolar chromophores, resulting in a coherent addition of their 2nd order NLO responses. Thanks to our previous work on the solubilization of carbon nanotubes using bile salt surfactants (cholates),[2] the nanohybrids can be dispersed as well-isolated individual tubes, and optically characterized with high spectral resolution. Similar to the filling with water,[3,4] the filling with these more complex organic molecules is first characterized by extensive wavelength-dependent fluorescence-excitation and resonance Raman experiments, studying in detail the effect of the encapsulated molecules on the electronic and vibrational properties of the SWCNTs. Moreover, the chromophoric nature of these organic molecules also allows for the effect of the encapsulation on the organic molecules themselves to be studied. Efficient energy transfer from the encapsulated molecules to the SWCNTs is observed.
The nonlinear optical response of the nanohybrids is characterized using a unique setup for wavelength-dependent hyper-Rayleigh scattering (i.e. second harmonic light scattering),[5,6] indeed revealing a giant hyperpolarizability, indicating a coherent addition of NLO responses over domains of tens of perfectly aligned molecules. Their equally giant total dipole moment and size promises an easy and stable alignment of the nanohybrids, opening an entirely new route towards the rational design of solution processable yet stable NLO materials.
[1] E. Goovaerts, W. Wenseleers, M.H. Garcia, G.H. Cross, “Design and Characterisation of Organic and Organometallic Molecules for Second Order Nonlinear Optics”, In: Handbook of advanced electronic and photonic Materials and Devices, Vol. 9: Nonlinear optical materials, Academic Press, San Diego, 127-191 (2001).
[2] W. Wenseleers, et al., Adv. Funct. Mater.
14, 1105 (2004).
[3] W. Wenseleers, S. Cambré, J. Culin, A. Bouwen, E. Goovaerts, Adv. Mater.
19, 2274 (2007).
[4] S. Cambré, W. Wenseleers, Angew. Chem.
50, 2764 (2011).
[5] J. Campo, F. Desmet, W. Wenseleers, E. Goovaerts, Optics Express
17, 4587 (2009).
[6] J. Campo, W. Wenseleers, J.M. Hales, N.S. Makarov, J.W. Perry, J. Phys. Chem. Lett.
3, 2248 (2012).