We have demonstrated large enhancements of the effective second-order nonlinear susceptibility ( (2) ) of ionic self-assembled multilayer (ISAM) films, causing a film with just 3 bilayers to be optically equivalent to a 700−1000 bilayer film. This was accomplished by using nanosphere lithography to deposit silver nanoparticles on the ISAM film, tuning the geometry of the particles to make their plasmonic resonances overlap the frequency of optical excitation. An enhancement in the efficiency of second harmonic generation (SHG) by as much as 1600 times was observed. Even though this is already a large value, we suggest that further refinements of the techniques are expected to lead to additional enhancements of similar or larger magnitude.Second-order nonlinear optical (NLO) materials are at the heart of telecommunications devices such as electro-optic modulators and optical switches, and in lasers such as highpower green and blue solid-state lasers and optical parametric amplifiers.Conventional NLO materials generally consist of inorganic crystals such as KTP, LiTaO 3 , and LiNbO 3 . 1 While they are quite efficient, high-quality crystals of sufficient size are expensive and difficult to manufacture. Organic NLO materials provide an alternative with the potential to provide high nonlinear susceptibilities in an economical fabrication process. 2 NLO materials based on ionic self-assembled multilayer (ISAM) films are particularly promising because of the ease of tailoring noncentrosymmetric structures and the long-term stability. [3][4][5] These films are made by alternately immersing a substrate in two solutions, containing a polycation and a polyanion, respectively. If the substrate initially carries negative surface charges, dipping in the polycation solution will result in a nanoscale polymer layer selfassembled on the substrate, yielding a positively charged substrate. Subsequent dipping in the polyanion solution results in a second layer of the polyanion formed on top of the first layer. The process can be repeated as many times as desired, building up films to arbitrary thickness with nanoscale precision. ISAM films may have substantial (2) values, comparable to that of lithium niobate. 5 Various methods have been suggested to improve the effective (2) of these films by modifying their composition. 6,7 Here, we demonstrate a new approach to enhance the second-order NLO susceptibility by creating hybrid structures from ISAM films and noble metal nanoparticles. Nanoparticles made from noble metals such as silver or gold have recently attracted considerable attention due to their unusual optical properties which enable light to be controlled in unique new ways. [8][9][10] The interaction of light with the free electrons in such particles gives rise to collective oscillations of the conduction electrons at optical frequencies, known as localized surface plasmons resonances (LSPRs). When excited in this fashion, the particles act as nanoscale antennas, concentrating the electromagnetic (E-M) field into v...

Articles you may be interested inMultilayer films with up to 600 bilayers and 740 nm thickness were fabricated using the alternating deposition of poly͑allylamine hydrochloride͒ and poly͕1-͓p-͑3Ј-carboxy-4Ј-hydroxyphenylazo͒benzenesulfonamido͔-1,2-ethandiyl͖ on glass substrates. Linear relationships for absorbance, thickness, and the square root of the second harmonic intensity versus the number of bilayers demonstrates that the films have long range polar order and optical homogeneity. The deposition conditions ͑i.e., pH of the solutions, solvent quality, deposition and rinsing times͒ are critical variables in fabricating layer-by-layer thick films that exhibit long range polar orientation of chromophores. The ability to fabricate noncentrosymmetric films on the order of a micron thick with bulk second order nonlinear optical responses is crucial because it enables the development of nonlinear optical waveguide devices such as electro-optic modulators.

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By alternating the deposition interaction between covalent and ionic binding, organic self-assembled films are fabricated with electro-optic coefficients of 20 pm/V. The rapidly deposited multilayer films exhibit excellent temporal and thennal stability. 02005 Optical Society ofAmerica OCIS codes: 190.47 10, 310.1860 Organic second order nonlinear optical materials have the potential to yield electro-optic modulators with V" < I V [1] and modulation rates >150 GHz.[2] However, for the case of electro-optic poled polymers, temporal and thermal stability remain as significant challenges. Self-assembly is an attractive alternative, since the assembly forces can be used to create a thermodynamically-stable polar order. Ionic self-assembled multilayer (ISAM) films, for example, spontaneously assemble with a noncentrosymmetric ordering that gives rise to a substantial second order nonlinear optical (NLO) response with exceptional temporal and thermal stability.[3] Typically, polar ISAM films are made from two oppositely-charged polyelectrolytes with an ionic, conjugated NLO chromophore attached as a side-chain to one of the polymers. The second order susceptibility of such a system is diminished due to competing dipole alignment at opposing ends of each polyelectrolyte layer and by randomized chromophore orientation within thicker layers. We have achieved significant enhancements in NLO response by replacing the NLO-active polyelectrolyte with a monomeric chromophore that has reactive functionality and ionic moieties on opposite ends of the molecule.[4] This novel hybrid covalent/ionic self-assembly method yields films in which essentially every chromophore is aligned in the same direction. Films made from this approach have demonstrated electro-optic coefficients of 20 pm/V with no degradation after two years under ambient conditions and 24 hours at 1 50°C.~~~-+ NaO3S SO3Na NN

The development of both "soft" and "hard" fabrication techniques for the patterning of nonlinear photonic devices in ionically selfassembled monolayer ͑ISAM͒ films is reported. A combination of electron beam lithography and reactive ion etching was used to pattern twodimensional holes with a lattice of 710 nm and diameters ranging from 550 to 650 nm. A soft alternative to this fabrication was also demonstrated. Nanoimprint lithography was successfully employed to pattern similar photonic structures with average hole diameters of 490 nm and a lattice spacing of 750 nm, as well as Bragg gratings with a period of 620 nm. Potential impact of this fabrication process on the chemical composition and nonlinear properties of the ISAM films was assessed using Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, and second harmonic generation. The spectroscopy techniques confirmed that the chemical composition and bonding of the ISAM films was not adversely affected by the thermal cycles required for nanoimprinting. Second harmonic generation analysis also confirmed that the nanoimprinting process did not affect the nonlinear properties of the material, PCBS/PAH ISAM films, further indicating the suitability of such materials for the nanoimprinting of nonlinear optical photonic structures.