Abstract
Modulating the second-order nonlinear optical susceptibility (χ(2)) of materials at the nanoscale represents an ongoing technological challenge for a variety of integrated frequency conversion and nonlinear nanophotonic applications. Here we exploit the large hyperpolarizability of intermolecular charge transfer states, naturally aligned at an organic semiconductor donor–acceptor (DA) interface, as a means to control the magnitude and sign of χ(2) at the nanoscale. Focusing initially on a single pentacene-C60 DA interface, we confirm that the charge transfer transition is strongly aligned orthogonal to the heterojunction and find that it is responsible for a large interfacial nonlinearity probed via second harmonic generation that is sufficient to achieve d33>10 pm V−1, when incorporated in a non-centrosymmetric DA multilayer stack. Using grating-shadowed oblique-angle deposition to laterally structure the DA interface distribution in such multilayers subsequently enables the demonstration of a χ(2) grating with 280 nm periodicity, which is the shortest reported to date.
Highlights
Modulating the second-order nonlinear optical susceptibility (w(2)) of materials at the nanoscale represents an ongoing technological challenge for a variety of integrated frequency conversion and nonlinear nanophotonic applications
Organic thin films have long been pursued for nonlinear optical (NLO) applications such as frequency conversion[1,2,3], electro-optic modulation[4,5,6], spectroscopy and imaging[7,8] because they combine large and fast nonlinear responses together with versatile processing and integration capabilities[9,10,11,12,13]
Many of these functionalities rely on the secondorder NLO susceptibility, w(2), which in turn depends on high dipole moment charge transfer (CT) transitions that maximize the molecular hyperpolarizability, b
Summary
Results v(2) nonlinearity from intermolecular CT states. The hyperpolarizability associated with a given CT transition can be qualitatively understood from a simple two-level model[24,25], which predicts: bCTð2o; o;. It is important to note, that the oblique-angle deposition process will lead to some asymmetry in the linear index grating (analogous to a blaze due to shape asymmetry or due to the different C60 refractive index on one grating facet), which will contribute to the left/right asymmetry in diffracted SH power To assess this possibility, linear diffraction measurements with low intensity light at the second harmonic wavelength (that is, l 1⁄4 400 nm; blue lines in Fig. 5b,c) were conducted in the same manner. The induced second-order nonlinear polarization from the nonlinear regions is much stronger for right incidence than for left incidence as shown in Fig. 6b, and this in turn leads to the asymmetry in T À 1 diffracted SH power summarized in Fig. 6c, in qualitative agreement with the experimental data
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