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The use of a whispering gallery mode (WGM) resonator with an ultrahigh quality factor Q is promising in highly sensitive, label-free, lab-on-a-chip sensor applications. We investigated a novel method of using the differential frequency of TE and TM modes to reduce the thermal noise baseline, which commonly limits the sensitivity of WGM sensors. We studied the temperature dependence of WGM based sensors and experimentally demonstrated the reduction of temperature fluctuation and thus a significant improvement in the practical sensor detection limit.

A ring-locked, phenyltetraene-based, second-order nonlinear optical chromophore (CLD-1) was synthesized. It shows no drop in decomposition temperature compared to its phenyltriene analogue, while the molecular nonlinearity increases by a factor of 2.25 from 6252 × 10^-48 to 14065 × 10^-48 esu at 1.9 μm. Thin films and waveguide devices were prepared from CLD-1/PMMA and CLD-1/APC (an amorphous polycarbonate) composites to study its electrooptic (EO) activity, optical loss, and photostability. An average EO coefficient of 92 pm/V at 1.06 μm was achieved in a 25 wt % CLD-1/APC composite. Mach−Zender (MZ) modulators fabricated from CLD-1/APC showed good thermal stability, low optical loss, low modulation voltage, and high extinction ratio. The photochemical stability of the CLD-1/APC modulator can be dramatically improved by operating in an inert gas atmosphere. Evidence shows that the predominant photochemical degradation mechanism of the material in air at 1.55 μm is the reaction between oxygen and photoexcited polyene-based chromophores.

This paper presents a class of nonlinear optical (NLO) urethane−urea copolymers, where dimethylsiloxane linkages of different length are incorporated into the polymer main chain. For comparison, a urethane−urea polymer having virtually no siloxane units has also been prepared. Results show that for copolymers with siloxane linkages poled samples have exceptional orientational stability, consistent in part with strong hydrogen-bonding effects. By the introduction of siloxane linkages, low optical loss materials suitable for device-quality applications have been obtained. The low optical loss can be attributed to the relatively lower number density of C−H bonds in the polymer matrix. Simultaneous realization of enhanced temporal stability and lower optical loss has been achieved in a single polymer system.

A high μβ isophorone-derived phenyltetraene chromophore (denoted CLD-5) was synthesized. The chromophore was modified with a hexyl group at the middle of the π-conjugate bridge to improve solubility and processability and was dihydroxy-functionalized for covalent incorporation into various cross-linked PU polymer systems. Its electrical poling and relaxation behavior in PU polymer thin films were studied. First, the chromophore was incorporated into conventional TDI/TEA polyurethane, and an electrooptic (EO) coefficient of 57.6 pm/V at 1.06 μm was obtained, which is 28% higher than that obtained from nonhexylated chromophore (CLD-2) in the same polyurethane system. Two new polyurethane systems, poly[(phenyl isocyanate)-co-formaldehyde] (PPIF)/triethanolamine (TEA) and PPIF/bisphenyl-1,1‘-dimethanol (BPDM), were designed to study the influence of cross-link density and monomer rigidity on electrical field poling of chromophore dipoles and relaxation behavior of poling-induced chromophore alignment. CLD-5/PPIF/TEA polymer has the highest cross-link density (3.07 mmol/g) among all the polyurethanes studied here. It gains 38 °C in thermal stability but loses 50% of EO activity as compared with the CLD-5/TDI/TEA polyurethane material, which has a cross-link density of 1.91 mmol/g. A higher EO coefficient (41 pm/V), lower optical loss of 2.56 dB/cm at 1.3 μm, and the highest dynamic stability (133 °C) were obtained for the CLD-5/PPIF/BPDM polyurethane EO material, which has the lowest cross-link density (1.27 mmol/g) and the most rigid monomers. The results indicate that excessive cross-linking deteriorates electric poling of long chromophores in a cross-linked polymer thin film. Therefore, cross-linking by itself is not necessarily a good approach to achieving high thermal stability of electrical field-induced chromophore alignment in polyurethane systems.

A novel trilinkable thiophene-containing second-order nonlinear optical (NLO) chromophore [(HO)3FTC] was synthesized from a trilinkable donor bridge and a tricyanofuran electron acceptor (TCF). The TCF acceptor was modified with two butyls which greatly increased solubility and processability of the trihydroxy-functionalized chromophore and inhibited strong chromophore−chromophore interaction. A thermal stability study of (HO)3FTC indicates that the free hydroxyl group located close to the cyano acceptor causes the chromophore to decompose at a much lower temperature than FTC chromophores with no free hydroxyl groups. Significantly improved thermal stability of the chromophore in polyurethane film was obtained by masking the free hydroxyl groups with toluene diisocyanate. New polyurethane prepolymer synthetic schemes were designed and studied in detail to improve electric field induced dipole alignment. Enhancement of over 150% in poling efficiency was achieved by reducing the degree of chromophore attachment to polymer backbone before applying an electric poling field through a “guest−host” approach. It was found to be critical to allow isocyanate and hydroxyl cross-linkers to react at higher temperature for a longer time to form a partially cross-linked prepolymer before the −NCO masked trilink chromophore was added. By anchoring chromophores to a three-dimensional cross-linked polyurethane network at three points, the thermal stability of poling-induced electrooptic activity was enhanced by 33 °C.

Isoxazolone (ISX) acceptor based chromophores terminated with hydroxyl functional groups were synthesized, spin cast into optical quality 1−2 μm thick films, poled in the presence of an electric field, and thermoset into polyurethane networks to lock in the poling-induced acentric chromophore order. Alkyl groups were introduced into the chromophore backbone (π-electron bridge) to increase chromophore solubility and to inhibit electrostatic interactions between these “high μβ” chromophores. Thermosetting polyurethanes containing alkyl-substituted chromophores exhibited much (1.7−1.9 times) higher electrooptic coefficients than polymers containing unsubstituted chromophores. Chromophores with hydroxyl groups on only the donor end of the molecule were processed into hardened “themoset” materials, exhibiting a dynamic thermal stability (temperature at which the macroscopic optical nonlinearity is first observed to decrease in a thermal ramping experiment) of poling-induced dipolar order of ~80 °C, while chromophores with hydroxyl groups on both the donor and acceptor ends display a dynamic thermal stability of poling-induced acentric order of ~110 °C. Optical loss in these materials originates from the scattering of incident light by chromophore aggregates, rather than from intrinsic (resonant) optical absorption of the chromophores. Alkyl groups in the modified chromophores serve as “bumpers” to sterically prevent aggregation of chromophores driven by strong, spatially anisotropic, intermolecular electrostatic interactions. Steric inhibition of aggregation serves not only to enhance poling efficiency (and thus, electrooptic coefficients) but also to minimize scattering losses.

It has been commonly assumed that electrostatic interactions between chromophores that exhibit large second hyperpolarizabilities β can be neglected in estimating electro-optic and second-harmonic coefficients, which can be achieved by electric-field poling of chromophore-containing polymers. Macroscopic optical nonlinearity has been assumed to scale as μβ/molecular weight, where μ is the dipole moment. Synthesis of chromophores with μβ values of the order of 10^−44 esu has led to expectations of electro-optic coefficients for organic materials that substantially exceed those of lithium niobate. Expected values have not been easily realized; thus the utility of the above-mentioned scaling factor or chromophore figure of merit has been brought into question. We demonstrate that macroscopic optical nonlinearities are attenuated at high chromophore loading for chromophores characterized by electrostatic interactions that, at close approach distances, exceed thermal energies (kT) and poling energies (μF), where F is the effective electric field.

We report the incorporation of a new trifunctionalized disperse red-type chromophore into a polyurethane thermosetting system. Six polymers with different chromophore loading and NCO/OH ratios were synthesized. This system exhibited very good poling efficiencies, and the optimum chromophore loading density ranged from 30 to 40%. Decreasing chromophore loading caused an attenuation of macroscopic nonlinearity, due to the lowering of chromophore concentration in the polymer; while increasing chromophore loading above 40% also attenuated the nonlinearity, presumably because of chromophore−chromophore electrostatic interactions starting to compete with the electric field−chromophore dipole interactions at high chromophore concentration. Materials with a NCO/OH ratio equal to or greater than unity have much higher temporal stability of dipole alignment than materials with a NCO/OH ratio less than 1. By anchoring the chromophores at both the donor and acceptor ends, the temporal stability of dipole alignment was enhanced significantly. For such materials, the optical loss of unpoled materials with nonstoichiometric NCO/OH ratios was observed to be high; however, this loss could be reduced to less than 1 dB/cm by adjusting the NCO/OH ratio to unity. It is generally believed that poling induced optical loss is a serious problem associated with second-order NLO materials prepared by electric field poling. In our study, we found this type of optical loss could be reduced to only 0.2 dB/cm by careful control of processing conditions.

An electrooptical realization of an optimal phase estimator for phase modulated communication signals is described in this work. The realization uses an electrooptical processor to perform a 1-D convolution in a 2-D space and a computer to complete the calculations. The processor is a realization of the time domain recursive nonlinear filter. This paper describes the nonlinear filter theory, the electrooptical realization, and the performance of the processor.

Following up on our preliminary communication, we present in this paper a novel approach to synthesize processible, cross-linkable, and thermally stable heterocyclic polymers (polyoxazolidones) with nonlinear optical properties. A cross-linkable NLO chromophore was synthesized and transformed into a prepolymer, which was easy to process and cross-link. After being efficiently poled and cured, the optical-quality films exhibited good nonlinearity and thermal stability. The stable, hardened lattice resulted from the formation of the heterocyclic ring. The details of synthesizing the cross-linked (trifunctional) and un-cross-linked (difunctional) heterocyclic polymers are presented. The effects of the processing conditions on the NLO properties are described. d33 coefficients of 77 and 85 pm/V at the 1.06 μm fundamental wavelength for the cross-linked and un-cross-linked polymers were obtained, respectively.