O-Pil Kwon

Photochemical stability of nonlinear optical chromophores in polymeric and crystalline materials

We compare the photochemical stability of the nonlinear optical chromophore configurationally locked polyene 2-{3-[2-(4-dimethylaminophenyl)vinyl]-5,5-dimethylcyclohex-2-enylidene} malononitrile (DAT2) embedded in a polymeric matrix and in a single-crystalline configuration. The results show that, under resonant light excitations, the polymeric compound degrades through an indirect process, while the DAT2 crystal follows a slow direct process. We show that chromophores in a crystalline environment exhibit three orders of magnitude better photostability as compared to guest-host polymer composites.

Electro-optic Charon polymeric microring modulators

We propose and demonstrate a new type of electro-optic polymeric microring resonators, where the shape of the transmission spectrum is controlled by losses and phase shifts induced at the asymmetric directional coupler between the cavity and the bus waveguide. The theoretical analysis of such Charon microresonators shows, depending on the coupler design, three different transmission characteristics: normal Lorentzian dips, asymmetric Fano resonances, and Lorentzian peaks. The combination of the active azo-stilbene based polyimide SANDM2 surrounded by the hybrid polymer Ormocomp allowed the first experimental demonstration of electro-optic modulation in Charon microresonators. The low-loss modulators (down to 0.6 dB per round trip), with a radius of 50 microm, were produced by micro-embossing and exhibit either highly asymmetric and steep Fano resonances with large 43-GHz modulation bandwidth or strong resonances with 11-dB extinction ratio. We show that Charon microresonators can lead to 1-V half wave voltage all-polymer micrometer-scale devices with larger tolerances to coupler fabrication limitations and wider modulation bandwidths than classical ring resonators.

Highly ordered thin films of a bis(dithienothiophene) derivative

We report growth as well as structural, optical, and charge-transport properties of highly ordered thin films of a fused thiophene derivative, 6,6′-di-n-hexyl-[2,2′]bi(dithieno[3,2-b:2′,3′-d]thiophenyl) (DH-BDT). DH-BDT was synthesised with the aim of fabricating high quality, molecularly aligned thin films for organic electronic applications. Structural phase transitions are observed upon heating from room temperature, indicating the existence of three different polymorphs, denoted α, β and γ. The transition temperatures are Tα→β = 92 ± 2 °C and Tβ→γ = 140 ± 2 °C. The growth of thin films of DH-BDT can be controlled to afford either the β- or the γ-phase. Thermal annealing leads to the formation of large single crystalline grains with areas as large as 7 × 104 n μm2. The molecules in the γ-phase are cofacially aligned and show horizontally layered thin film growth. The good crystallinity and the large grain size in the γ-phase lead to hole mobilities up to μγ = 0.09 cm2 V−1 s−1, based on the measurement of space-charge-limited currents (SCLC). The β-phase consists of mutually shifted molecules, resulting in a lower hole mobility of 4.4 × 10−5 cm2 V−1 s−1 but improving the relative luminescence quantum yield by 140% relative to that of the γ-phase. Field-effect transistors of DH-BDT in the γ-phase have been fabricated and yield hole mobilities which are of the same order of magnitude as the SCLC mobilities.

High-gain photorefractive reflection gratings in layered photoconductive polymers

Large two-wave mixing gain in reflection grating geometry is obtained in layered organic photorefractive polymers doped with the chromophore piperidinodicyanostyrene. With an applied field of 60 V/μm and for a grating spacing of 0.205 μm, one measures a two-wave mixing gain coefficient of 104u2002cm−1 and a diffraction efficiency of 4.8% in 100-μm-thick samples. Our photorefractive and electrochemical investigations suggest that the excellent material performance in reflection grating geometry is associated with an increased effective number of trapping sites.

Nonlinear optical co-crystal of analogous polyene chromophores with tailored physical properties

A new organic nonlinear optical co-crystal based on analogous configurationally locked polyene chromophores with noncentrosymmetric packing exhibits a large macroscopic second-order nonlinearity with tailored physical properties.

Layered photoconductive polymers: Anisotropic morphology and correlation with photorefractive reflection grating response

We demonstrate that the mesophase morphology of the layered photorefractive polymers has a substantial influence on the photorefractive properties, especially in reflection grating geometries with a minimal grating spacing. The layered morphology of the photoconductive polymers based on poly(p-phenyleneterephthalate) (PPT) with pendent carbazole (CZ) groups can be efficiently controlled by changing their molecular weight. Photorefractive composites based on PPT-CZ polymers with different chromophores, diethylaminodicyanostyrene (DDCST) or piperidinodicyanostyrene (PDCST), show anisotropic morphology induced by the squeezing flow during sample preparation. The contributions of the highest occupied molecular orbital levels of the chromophores and of the degree and anisotropy of the layered crystalline structure to the charge transport and trapping result in a high efficiency of the PDCST composite and a similar response speed in DDCST and PDCST composites in the reflection grating geometry, although of about six times lower photoconductivity in the less-ordered PDCST composite.

Hybrid organic crystal/silicon-on-insulator integrated electro-optic modulators

We demonstrate electro-optic modulation in hybrid organic-crystal/silicon photonic waveguides. The organic material is the newly developed organic crystal OH1 with very high electro-optic figures of merit, n3r = 530 pm/V at 1319 nm, and the processing possibilities considerably improved compared to previous high-nonlinearity organic crystals. We have developed an epitaxial-like solution growth of OH1 on various substrates and fabricated electro-optic modulators with electro-optic functionality either directly in OH1 wire waveguides or in OH1 active cladding of silicon wire waveguides. OH1-based waveguides offer a great potential for high-bandwidth, sub-1-V half-wave voltage, hybrid organic/silicon electro-optic modulators with high electro-optic activity and stability.

Organic electro-optic single crystalline films for integrated optics

We have fabricated organic electro-optic single crystalline thin films on various inorganic substrates. A high refractive index contrast of up to Δn = +0.6 at 1.55 μm with respect to glass substrates and up to Δn = -1.9 at 1.55 μm with respect to silicon substrates has been achieved. The single crystalline films can be grown quasi-epitaxially without lattice matching and also on amorphous substrates providing appropriate interface interactions and solid-liquid phase equilibrium conditions. The thickness of the single-crystalline films can vary between less than 30 nm and above 5000 nm; they are therefore appropriate for optical waveguiding structures, as well as nano-size electro-optic structures needed for future nanophotonics. Several organic electro-optic crystalline materials have been employed using solution or melt-based processing. The techniques are suitable for the fabrication of conventional wire electro-optic waveguides, silicon-organic hybrid electro-optic waveguides, as well as more complex organic-inorganic structures such as single-crystalline electro-optic microring resonators.

Backward beam fanning in organic photorefractive devices

We observe strong beam depletion of about 30% at small incident angles in layered photorefractive polymers at an electric field of −50V∕μm and an intensity of 0.3W∕cm2. At normal incidence this depletion occurs only with negative electric field directions. For positive fields, depletion occurs only in the tilted geometry and is due to forward beam fanning, as previously observed. We attribute the depletion for negative electric fields mainly to the backward beam fanning as a consequence of the relatively high beam coupling efficiency in reflection grating geometries.

High performance reflection gratings in nematiclike photorefractive polymers

Photorefractive materials based on the nematiclike photoconductive polymers PPT-TPA consisting of the rigid backbone poly(p-phenyleneterephthalate), PPT, and pendent photoconducting triphenylamine (TPA) groups were studied in reflection grating geometry and compared to the layered photorefractive composites PPT-CZs with photoconducting carbazole (CZ) groups. The order level of the mesophase structure has a crucial influence on the trapping characteristics of the composites. The nematiclike composites although of lower photoconductivity exhibit one order of magnitude faster response than in the layered composites in the reflection grating regime. This is attributed to the increased trapping rate as a result of the specific mesophase structure.