Chengzeng Xu

Efficient poling and thermal crosslinking of randomly bonded main‐chain polymers for stable second‐order nonlinearities

We report our second‐harmonic generation study of a new crosslinked main‐chain polymer system for second‐order nonlinear optics (NLO). In this unique polymer system, the rigid second‐order NLO chromophores are randomly bonded in the main chain with flexible links between the rigid chromophores to allow electric poling at or above the glass transition temperature. A thermal crosslinker is added to crosslink the polymer backbone during electric poling. The combination of the flexible chain segments and crosslinking allows us to achieve a high degree of alignment (〈cos3θ〉≊0.34) and long‐term (≳2500 h) temporal stability of the nonlinearities.

Improved poling and thermal stability of sol‐gel nonlinear optical polymers

An aminosulfone dye chromophore was covalently incorporated into a sol‐gel network for second‐order nonlinear optics applications by functionalizing both ends of the chromophore. Covalent incorporation allows for high chromophore loading densities (∼35%) and locking in both ends of the chromophore improves thermal stability. Short term stability of the nonlinearity at 200 °C and long term stability over 500 h at 100 °C was demonstrated. Two different temperature profiles for corona poling were investigated and one found superior to the other. A second harmonic coefficient, d33 of 27 pm/V at the 1064 nm fundamental wavelength was measured.

Improving the thermal stability by anchoring both ends of chromophores in the side‐chain nonlinear optical polymers

We report our study of a new side‐chain second‐order nonlinear optical polymer in which both ends of the chromophores are anchored by thermal crosslinking during electric field poling. A sizable second‐harmonic generation coefficient d33 of 60 pm/V was measured at 1064 nm fundamental wavelength with corona poling. After anchoring both ends of the chromophores, the thermal stability of the poling alignment was greatly improved and the nonlinear optical properties were shown to be stable at both 90 and 125 °C. Channel waveguides were defined in the polymer thin films by reactive ion etching.

Waveguide photonic devices made from thermally crosslinked second-order nonlinear optical polymers

A number of new thermally crosslinkable second-order nonlinear optical polymers have been developed and characterized. These polymers are designed for stable nonlinear optical activities since both ends of the rod-like chromophores are locked into a polymer matrix by covalent bonds. Sizable nonlinearities were measured in these polymers and the thermal stability of both main chain and side chain polymers were improved significantly by thermally induced crosslinking. These thermosetting polymers, with the chromophores either included in the main chain or as side chain pendants, have been used in a number of device applications. Waveguide structures can be defined in these polymers by reactive ion etching or ultraviolet bleaching. Thin film integrated optical devices, such as high frequency electro-optic modulators and birefringent directional couplers have been fabricated with reactive ion etching and photo bleaching methods. Waveguide fabrication techniques for multi-layer structures are discussed in detail.

Recent advances in the synthesis and characterization of nonlinear optical materials: second-order materials

Abstract A major problem in utilizing second-order organic materials for device applications is the requirement for the assembly of these chromophores into a stable, noncentrosymmetric lattice. For certain applications, such as frequency doubling, a periodic noncentrosymmetric lattice may be desirable. In this presentation, we discuss the synthesis of such organic polymeric lattices and evaluation of modulators and doublers fabricated from resulting materials. The general approach is to develop soluble, processible precursor polymers which can be transformed into stable noncentrosymmetric lattices by electric field poling and subsequent photoinduced or thermally induced solid state crosslinking reactions. Photo-processing is utilized to generate periodic noncentrosymmetric lattices for quasi-phase-matched second-harmonic generation.

Measurement of the electro-optic coefficient dispersion in poled polymer materials

A simple and easy method to study the dispersion of electro-optic coefficients of poled second- order nonlinear optical polymer films is presented. Phase grating type electro-optic modulators were used to measure the relative electro-optic effects at different wavelengths. The grating structures were formed by either etching a grating in the nonlinear polymer thin films or defining a transparent electrode on top of a uniform polymer film. When an AC voltage is applied to the electrodes, the diffraction efficiency of the grating is modulated by the small index modulation. The measurement of the electro-optic response can be carried over a large wavelength range, even deep into the absorption band of the material due to the short interaction length of the optical beam and the polymer thin film. The grating modulation method uses a single optical beam in a transmission arrangement which requires little optical alignment and no polarization optics. With a properly designed electrode, the grating modulator can also be used in measuring the electro-optic coefficient, r13, of the poled polymer films. Other measurement applications such as monitoring temporal, thermal, and photo-excitation stability are discussed.

Techniques for ultrastructure synthesis: stabilization of large second-order optical nonlinearities of poled polymers

Ultrastructure synthesis techniques for stabilization of large second order optical nonlinearities of poled polymers are reviewed. These techniques include covalent attachment of chromophores onto polyimide backbones, use of double-end crosslinkable chromophores, crosslinking of main-chain polymers with chromophores as polymer backbone components, and in-situ poling and polymerization of chromophores with multiple functionalities. By using these techniques, second harmonic generation coefficients, measured at 1064 nm wavelength, on the order of 10-7 esu and long term stability of optical nonlinearity at 90 - 125

Second-Order Nonlinear Optical Polymers With Different Chromophore Arrangements

DEGC have been realized.

New crosslinkable polymers with second-order nonlinear optical chromophores in the main chain

Second-order nonlinear optical polymers can be divided into four groups regarding arrangements of chromophore dipoles in polymer backbones, namely, side-chain polymers, random, head-to-tail, and head-to-head Main-chain polymers. A variety of polymers with the aforementioned configurations have been designed and synthesized from functionalized amino-nitro azobenzene chromophores. Poling processes of these polymeric materials have been investigated by in-situ poling and second-harmonic generation detection.

Polymeric Electro‐Optic Modulators: Matereials synthesis and processing

We have developed a new class of main-chain second-order nonlinear optical (NLO) polymers using well-developed condensation polymerization methods. In these polymers, the NLO chromophore dipoles are expected to be randomly arranged along the polymer backbone (i.e., the dipoles can be head-to-tail, head-to-head, or tail-to-tail). For comparison, a side-chain polymer having virtually the same chromophore as a pendant has also been prepared. The effect of variation in polymer structure on the second-order NLO properties and the effect of crosslinking on the stability of poling-induced macroscopic order are studied. Our results demonstrate that the random main-chain, second-order NLO polymers can be efficiently poled, yielding (chi) (2) as high as 300 pm/V. Long-term temporal stability of the poling- induced order (e.g., no significant NLO decay is observed for more than 2000 hours) can be realized by crosslinking the polymer backbone.