Synthesis of Colloidal Photonic Crystals with High Nonlinear Optical Performance: Towards Efficient Second‐Harmonic Generation with Centrosymmetric Structures

Abstract

The achievement of non-centrosymmetry is one of the main challenges in the field of organic materials with second-order nonlinear optical (NLO) properties. The need for highly ordered anisotropic materials has set important limitations on the performance and applicability of some of the nonlinear optical processes. Nevertheless, it has been shown [1] that the necessity of non-centrosymmetry can be avoided by using centrosymmetric colloidal crystalline arrays (CCA) with photonic crystal (PC) properties, for which high NLO performance of the CCA nanospheres surface is critical. We present here an efficient solid-phase based synthesis to covalently bind a large number of molecules with high NLO performance to the surface of CCA nanoparticles. The resulting chemically modified CCAs takes advantage of the nonlinear interaction located at the sphere-water interface and their photonic crystal properties to produce-via second-harmonic generation-light visible to the naked eye.[2] The magnitude of this NLO response is the direct result of the high coverage of nanospheres surface with nonlinear chromophores, achieved with the new synthetic procedure. Furthermore, the broad applicability of this novel methodology, which could be used with the majority of organic and organometallic NLO chromophores described in the literature, suggests a new route to synthesize organic centrosymmetric structures with second-order NLO properties.CCA comprise monodisperse polymeric nanospheres that are surface functionalized with negatively charged groups. When dispersed in a very low ionic strength aqueous media, due to electrostatic repulsions, such nanospheres self-assemble into a crystalline structure.[3] Lattice parameters of these structures depend on the type and the size of the nanospheres, their colloidal volume fraction, the type and the amount of surface charged groups and the ionic strength of the dispersion media, and they are typically on the optical range.[4]Therefore, these materials exhibit a forbidden band for the propagation of light in the neighborhood of those wavelengths that satisfy the Bragg condition, which might be engineered by changing some of the factors mentioned above. This has been used to fabricate optical switches and photonic crystal optical sensors. [5] In recent years, it has also been shown that such materials may be very suitable for any kind of second order nonlinear processes; the large number of interfaces provide a local noncentrosymmetric environment, and the inherent periodicity provides the required phase matching mechanism which has been shown to be effective for second harmonic generation, [1,6,7] and third harmonic generation as well. [8,9] Enhancement of the surface NLO interaction had previously been achieved by coating the nanosphere surface via electrostatic adsorption of a positively charged nonlinear chromophore. [1,10] However, that method is limited to cationic chromophores. Moreover, systems prepared by electrostatic adsorption suffer from poor thermal stability as well as an imbalance ...

Nonlinear optics in spheres: from second harmonic scattering to quasi‐phase matched generation in whispering gallery modes

“…To increase the surface nonlinearity, crystal violet (CV) molecules were chemically bound to carboxylic groups on sphere surfaces [19]. The negatively charged surface sulfate groups were not screened by the positively charged nonlinear molecules and very good quality nonlinear colloidal crystals could be formed.…”

“…This coherence length is given for the case of spherical resonators in Eq. (19) as a function of the polar angle.…”

“…The pattern inscribed was designed from the domain distribution that was determined from the phase matching condition obtained from an equation similar to Eq. (19). In general, patterning a non-flat surface would require xyz scanning in additional to rotational motion of the sphere.…”

Light generation at the anomalous dispersion high energy range of a nonlinear opal film

“…The dye molecule used was Crystal Violet (CV) which was modified to have a hydroxyl binding group. Such hydroxyl group was covalently linked to the functional carboxylic acid groups present at the surface of the spheres [17]. The chemically modified spheres present a high coverage of their surface, estimated to be on the order of 70% [17,18].…”

“…Such hydroxyl group was covalently linked to the functional carboxylic acid groups present at the surface of the spheres [17]. The chemically modified spheres present a high coverage of their surface, estimated to be on the order of 70% [17,18]. After the reaction the sulfate groups remain unaltered preventing coagulation and also helping to maintain stability during the crystallization process of the opal.…”