Charles Greenlee

Mach–Zehnder interferometry method for decoupling electro-optic and piezoelectric effects in poled polymer films

A Mach–Zehnder interferometer (MZI) is utilized to decouple the electro-optic and piezoelectric tensor effects occurring in a poled polymer film. This method has significant advantages over the commonly used Teng-Man reflection ellipsometry technique by allowing for the independent determination of the Pockel’s coefficients r13 and r33 and the piezoelectric coefficient d33. The r33 value of a guest host polymer that consists of AJLZ53 amorphous polycarbonate was found to be 122.69 pm/V and 123.03 pm/V using the MZI and reflection ellipsometry method, respectively. The r33 data fits well to the dispersion of the second order susceptibility tensor based on the two-level model approximation.

Interdigitated coplanar electrodes for enhanced sensitivity in a photorefractive polymer

Organic photorefractive polymer composites can be made to exhibit near 100% diffraction efficiency and fast writing times, though large external slants are needed to project the applied field onto the grating vector. We show here that the use of interdigitated electrodes on a single plane provides similar performance to these standard devices and geometries but without a external slant angle. This new devices structure also greatly improves the diffraction efficiency and sensitivity compared to less slanted standard devices necessary for some real applications, such as holographic displays, optical coherence imaging, and in-plane switching.

HYBRID SOL-GEL ELECTRO-OPTIC POLYMER MODULATORS: BEATING THE DRIVE VOLTAGE/LOSS TRADEOFF

There has been great progress in the development of electro-optic (EO) polymers with exceptionally high r33 coefficients, with values ranging from 100–400 pm/V now being reported for single layer electro-optic polymer films. While this enables the fabrication of EO modulators with sub-volt operation, it is also necessary to make devices with acceptably low insertion loss (< 6 dB) in order to compete with existing technology. We have developed a solution to the voltage/insertion loss tradeoff in EO polymer modulators by adopting a hybrid geometry that provides for low optical coupling loss, electro-optic polymer limited propagation loss, highly efficient poling, and low cost fabrication. This combination of properties has allowed us to achieve r33 = 170 pm/V in an EO phase modulator. In addition to this proven approach to optimizing the figure of merit, there are several other approaches that can have high impact. The development of low loss EO polymer and cladding materials and waveguides can greatly reduce the insertion loss of EO polymer modulators, through chemical substitution techniques such as selective halogenation, as well as through improved processing to reduce roughness, stress and poling induced losses. Halogenation can be used to reduce the number of C–H bonds, which have well-known stretch and bend vibrational modes whose overtones extend into the optical communications bands at 1550 nm and 1310 nm. While roughness and stress effects are well-understood from work on passive waveguides, the poling process can produce inhomogeneities that lead to increased scattering loss; molecular design can be used to reduce poling induced loss. Another approach is to adopt non-waveguide device formats that are more tolerant of material losses, such as Fabry-Perot etalons. While etalons may not be viable for very high speed applications (i.e., GHz regime), they present entirely new application areas for electro-optic polymers.

Simulation-based optimization of the acoustoelectric hydrophone for mapping an ultrasound beam

Most single element hydrophones depend on a piezoelectric material that converts pressure changes to electricity. These devices, however, can be expensive, susceptible to damage at high pressure, and/or have limited bandwidth and sensitivity. The acousto-electric (AE) hydrophone is based on the AE effect, an interaction between electrical current and acoustic pressure generated when acoustic waves travel through a conducting material. As we have demonstrated previously, an AE hydrophone requires only a conductive material and can be constructed out of common laboratory supplies to generate images of an ultrasound beam pattern consistent with more expensive hydrophones. The sensitivity is controlled by the injected bias current, hydrophone shape, thickness and width. In this report we describe simulations aimed at optimizing the design of the AE hydrophone with experimental validation using new devices composed of a resistive element of indium tin oxide (ITO). Several shapes (e.g., bowtie and dumbbell) and resistivities were considered. The AE hydrophone with a dumbbell configuration achieved the best beam pattern of a 2.25MHz ultrasound transducer with lateral and axial resolutions consistent with images generated from a commercial hydrophone (Onda Inc.). The sensitivity of this device was ~2 nV/Pa. Our simulations and experimental results both indicate that designs using a combination of ITO and gold (ratio of resistivities = ~18) produce the best results. We hope that design optimization will lead to new devices for monitoring ultrasonic fields for biomedical imaging and therapy, including lithotripsy and focused ultrasound surgery.

Design considerations and performance of MEMS acoustoelectric ultrasound detectors

Most single-element hydrophones depend on a piezoelectric material that converts pressure changes to electricity. These devices, however, can be expensive, susceptible to damage at high pressure, and/or have limited bandwidth and sensitivity. We have previously described the acoustoelectric (AE) hydrophone as an inexpensive alternative for mapping an ultrasound beam and monitoring acoustic exposure. The device exploits the AE effect, an interaction between electrical current flowing through a material and a propagating pressure wave. Previous designs required imprecise fabrication methods using common laboratory supplies, making it difficult to control basic features such as shape and size. This study describes a different approach based on microelectromechanical systems (MEMS) processing that allows for much finer control of several design features. In an effort to improve the performance of the AE hydrophone, we combine simulations with bench-top testing to evaluate key design features, such as thickness, shape, and conductivity of the active and passive elements. The devices were evaluated in terms of sensitivity, frequency response, and accuracy for reproducing the beam pattern. Our simulations and experimental results both indicated that designs using a combination of indium tin oxide (ITO) for the active element and gold for the passive electrodes (conductivity ratio = ~20) produced the best result for mapping the beam of a 2.25-MHz ultrasound transducer. Also, the AE hydrophone with a rectangular dumbbell configuration achieved a better beam pattern than other shape configurations. Lateral and axial resolutions were consistent with images generated from a commercial capsule hydrophone. Sensitivity of the best-performing device was 1.52 nV/Pa at 500 kPa using a bias voltage of 20 V. We expect a thicker AE hydrophone closer to half the acoustic wavelength to produce even better sensitivity, while maintaining high spectral bandwidth for characterizing medical ultrasound transducers. AE ultrasound detectors may also be useful for monitoring acoustic exposure during therapy or as receivers for photoacoustic imaging.

Electro-optic polymer spatial light modulator based on a Fabry–Perot interferometer configuration

A spatial light modulator (SLM) based on a Fabry-Perot interferometer configuration has been fabricated and tested. The Fabry-Perot spacer layer is a thin film of the SEO100 electro-optic polymer which serves as the nonlinear medium. Measurement results demonstrate the modulation of multiple pixels operating simultaneously at frequencies ranging from 300 kHz to 800 kHz which is significantly faster than SLMs based on liquid crystal and digital micromirror device technology. An average modulation contrast of 50% for all pixels is achieved with a drive voltage of 70 V(rms) at 100 kHz. Microwave speeds and CMOS compatibility are feasible with improved transmission line and cavity design.

Near-resonance electro-optic activity enhancement and improved modulation performance for polymer based Fabry–Pérot interferometers

Fabry–Perot interferometers (FPIs) are designed to work near and off the absorption resonance of an electro-optic (EO) polymer. The near-resonance EO activity (EO coefficient r13∼25.4pm∕V at ∼1086nm) is enhanced over that off resonance (r13∼9.0pm∕V at ∼1526nm). The full width at half maximum of the FPI transmission band is ∼16nm near resonance compared to ∼22nm off resonance. The modulation performance can, therefore, be improved with a modulation ratio increased to ∼78% near resonance (at 1064nm) from ∼19% off resonance (at 1520nm). The modulation speed is approximately megahertz and can be increased to greater than gigahertz with improved circuit design.

Mach-Zehnder interferometry method for decoupling electro-optic and piezoelectric tensor components in poled polymer films

A Mach-Zehnder interferometer (MZI) is used to decouple the electro-optic (EO) and piezoelectric tensor components for a poled polymer film. In the past those using the MZI method failed to take into account the piezoelectric contribution in the polymer which can lead to erroneous EO coefficient data. The typical poled sample of polymer sandwiched between ITO glass and gold that was developed for the popular Teng-Man reflection ellipsometry method is used, providing for easy comparison with that method. The sample serves as a mirror in one arm of the interferometer with the gold side facing the beam for measuring the piezoelectric modulation and the glass side facing the beam to measure the coupled piezoelectric and EO modulation. Optical biasing in the reference arm allows for the baseline and modulated contrast of the system to be measured from which the tensor components are calculated. This method has the advantage over the reflection ellipsometry method of allowing for the independent determination of the Pockels coefficients r13 and r33 and the piezoelectric coefficient d33. The r33 value of a guest host polymer that consists of AJLZ53/amorphous polycarbonate (APC) was found to be 122.7 pm/V and 123.0 pm/V for the MZI and reflection ellipsometry method respectively. The r33 data fits well to the dispersion of the second order susceptibility tensor χ 333 (2) based on the two-level model approximation. Measurements were done from 100 Hz to 100 kHz with the results showing that at higher frequencies the mechanical effects in the sample are negligible and modulation is almost entirely due to the EO effect, as expected.

Ultrahigh electro-optic coefficient of 170pm/V and low V π of 1V at 1.55μm in hybrid polymer/sol-gel waveguide modulators

We demonstrated the highest electro-optic (EO) coefficient with the highest poling efficiency (~100%) in actual modulator devices. This breakthrough was accomplished with contact poling of a crosslinkable EO polymer with an electrically conductive sol-gel cladding.

Polymers with unprecedented NLO response

Using efficient electro-optical polymer poling in hybrid sol-gel EO modulators we have achieved 0.65V VπMach-Zehnder modulators in a new EO polymer, AJ309, which undergoes thermal crosslinking during the poling process.