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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 ...

Most of the applications based on the special optical properties that photonic crystals (PC) offer to control the interaction between matter and electromagnetic radiation, are based on the periodic structure response at the low energy region of the spectra, where the first order Bragg diffraction takes place. Among those reported applications of PCs some are devoted to the nonlinear (NL) optical processes such as the second harmonic generation (SHG).In the present paper we experimentally show that SHG is also possible in a NL artificial opal, at the high energy range, where the system displays a truly 3D behavior. We have fabricated a nonlinear opal film of monodisperse polystyrene spheres coated with a dye nonlinear molecule. We have measured the SH forward-generated transmitted light upon incidence of a frequency field, propagating close to (111) direction of the crystal. By means of vector KKR method including extinction [1,2], we have determined the phase delay introduced to the specularly reflected and forwardly transmitted electromagnetic fields from our finite photonic structures in that spectral region. We have then calculated the group velocity of the electromagnetic waves from the linear response of the finite crystal which is shown as a function of frequency in Fig.1.a. As seen in Fig.1.b, the observed enhancement of the SHG of light occurs where these phase delays lead to a significant reduction of the group velocity with respect to the average value. In summary, we show that the group velocity slowing-down at the high energy range of 3D photonic nanostructures provides an enhancement mechanism for the nonlinear process. References

We detected the second-order nonlinear response from single isolated spheres comprised from a centrosymmetric material but covered by a layer of a material with strong second-order nonlinear properties and isolated from an ensemble by the optical trapping technique. We show that when large size parameter spheres are used, the measured second-harmonic efficiency deviates strongly from the prediction of the nonlinear Rayleigh scattering theory. Our results are in very good agreement with the predictions from the exact nonlinear Mie scattering theory.

Abstract:We study experimentally and theoretically light propagation and generation at the high energy range of a close-packed fcc photonic crystal of polystyrene spheres coated with a nonlinear material. We observe an enhancement of the second harmonic generation of light that may be explained on the basis of amplification effects arising from propagation at anomalous group velocities. Theoretical calculations are performed to support this assumption. The vector KKR method we use allows us to determine, from the linear response of the crystal, the behavior of the group velocity in our finite photonic structures when losses introduced by absorption or scattering by defects are taken into account assuming a nonzero imaginary part for the dielectric constant. In such structures, we predict large variations of the group velocity for wavelengths on the order or smaller than the lattice constant of the structure, where an anomalous group velocity behavior is associated with the flat bands of the photonic band structure. We find that a direct relation may be established between the group velocity reduction and the enhancement of a light generation processes such as the second harmonic generation we consider. However, frequencies for which the enhancement is found, in the finite photonic crystals we use, do not necessarily coincide with the frequencies of flat high energy bands. ©2009 Optical Society of America