Electric field poled organic electro-optic materials: state of the art and future prospects.

Optical microresonators have recently attracted growing attention in the photonics community1. Their applications range from quantum electrodynamics to sensors and filtering devices for optical telecommunication systems, where they will probably become an essential building block2. Integration of nonlinear and electro–optical properties in resonators represents a very stimulating challenge, as it would incorporate new and more advanced functionality. Lithium niobate is an excellent candidate material, being an established choice for electro–optic and nonlinear optical applications. Here we report on the first realization of optical microring resonators in submicrometre thin films of lithium niobate. High-index-contrast films are produced by an improved crystal-ion-slicing and bonding technique using benzocyclobutene. The rings have radius R = 100 µm, and their transmission spectrum has been tuned using the electro–optic effect. These results open new possibilities for the use of lithium niobate in chip-scale integrated optical devices and nonlinear optical microcavities.

Electro–optically tunable microring resonators in lithium niobate

Electro–optic (EO) modulators are typically made from inorganic materials such as LiNbO3, but replacing them with organic EO materials, that is, ones with optical properties that change in response to an electric field, could be a promising alternative because they offer large bandwidth, ease of processing and relatively low cost. Here we incorporate a doped, crosslinked organic EO polymer into hybrid polymer/sol–gel waveguide modulator devices with exceptional performance. The half-wave voltages of the resulting Mach–Zehnder (MZ) and phase modulators at 1550 nm are 1 V and 2.5 V, respectively. The unique properties of the sol–gel cladding materials used in the hybrid structure result in a 100% device poling efficiency, leading to respective in-device EO coefficients of 138 pm V–1 and 170 pm V–1 in the MZ and phase modulators. These results are the first to show in-device EO coefficients that are five to six times larger than those of the benchmark inorganic material.

Hybrid polymer/sol–gel waveguide modulators with exceptionally large electro–optic coefficients

Polymeric electro-optic materials have recently been developed that, when fabricated into devices such as Mach-Zehnder interferometers, permit drive ( Vπ) voltages of less than 1 V to be realized at the telecommunications wavelength of 1.3 and 1.55 microns. Operation of polymeric electro-optic modulators to frequencies (bandwidths) of greater than 100 GHz has been demonstrated. The total insertion loss of polymeric electro-optic modulators has been reduced to values as low as 5 dB, which is competitive with values obtained for lithium niobate modulators and is much lower than that obtained for gallium arsenide electro-absorptive modulators. Polymeric electro-optic modulators can be operated for long periods of time at temperatures on the order of 100 °C. Techniques have been developed for seamlessly integrating polymeric electro-optic circuitry with passive low loss optical circuitry (e.g., silica long-haul transmission fiber and medium-range fluoropolymer fibers) and with very large scale integration (VLSI) semiconductor electronics. These advances have created a considerable interest in the commercialization of polymeric electrooptic materials. Polymeric electro-optic materials are now being evaluated for applications such as phased array radar, satellite and fiber telecommunications, cable television (CATV), optical gyroscopes, electronic counter measure (ECM) systems, backplane interconnects for high-speed computers, ultrafast (100 Gbit/s) analog-to-digital (A/D) converters, land mine detection, radio frequency (rf) photonics, and spatial light modulators. This review discusses the structure-function relationships that had to be defined and the synthesis and processing advances achieved before materials appropriate for commercialization could be produced. Topics discussed include the design and synthesis of chromophores that simultaneously exhibit large molecular hyperpolarizability, low optical absorption, processability (e.g., solubility in various processing media), and the prerequisite (thermal, chemical, electrochemical, and photochemical) stability to survive conditions encountered in the fabrication and operation of polymeric electro-optic devices. Chromophore-chromophore intermolecular electrostatic interactions have been shown to be the most serious problem impeding the optimization of electro-optic activity in organic materials. The quantitative theoretical treatment of such interactions by equilibrium and Monte Carlo statistical mechanical methods is discussed. Rules for designing chromophores with shapes leading to maximum obtainable electro-optic activity are discussed and the synthetic realization of such structures is reviewed. The role of electrostatic interactions in influencing the choice of processing conditions is also discussed. A number of processing steps, including spin casting, electric-field poling, lattice hardening, fabrication of buried channel waveguides (including deposition of cladding layers), electrode deposition and connection with electronic circuitry, and integration of active and passive optical circuitry, are required. Each of these steps can affect device performance (e.g., influence optical loss, electro-optic activity, and stability). The systematic optimization of each of these steps is reviewed. Finally, device design and operation are reviewed and speculation on the future of the field is expressed.

Nonlinear Optical Polymeric Materials: From Chromophore Design to Commercial Applications

Monte Carlo statistical mechanical computer simulations of the electric-field poling of second-order nonlinear optical chromophores, characterized by large dipole moments, polarizabilities, and hyperpolarizabilities, are presented. Such theoretical analysis is critical to defining the structure/function relationships that permit maximization of electro-optic activity for π-electron chromophore-containing polymeric materials. Polymeric electro-optic materials may, in turn, be important for high-bandwidth telecommunications, new forms of radar, and high-speed data processing. The experimentally observed maxima in plots of electro-optic activity versus chromophore number density (loading) in polymer matrices are theoretically reproduced, as are the shifts of the maxima to lower loading with increasing chromophore dipole moment. Modification of the chromophore shape to realize the maximum achievable electro-optic activity for a given π-electron structure is discussed, as is the role of polymer electrical perm...

Monte Carlo Statistical Mechanical Simulations of the Competition of Intermolecular Electrostatic and Poling-Field Interactions in Defining Macroscopic Electro-Optic Activity for Organic Chromophore/Polymer Materials†

The authors report the bandpass-modulation characteristics of a ring resonator-based traveling-wave modulator using an electrooptic polymer AJL8/APC. The use of the traveling-wave electrode makes it possible to achieve an efficient modulation around 28 GHz, which is the free spectral range of the resonator, with a 3-dB bandwidth of 7 GHz. The modulation sensitivity is evaluated by defining an equivalent half-wave voltage of ring modulators. It is shown that the modulator has a potential in the microwave and millimeter-wave photonics applications

Ring resonator-based electrooptic polymer traveling-wave modulator

Integrated photonic radio frequency (RF) phase shifters with dc voltage control have been realized using a nested dual Mach-Zehnder modulator configuration in a new nonlinear optical polymer, CLD2-ISX. These modulators have a V/sub /spl pi// of 10.8 V and exhibit excellent frequency performance measured up to 20 GHz. A near linear phase shift exceeding 108/spl deg/ was obtained for a 16-GHz microwave signal by tuning the dc control voltage over a 7.8-V range. It is expected that these integrated polymer phase shifters will find widespread applications in new types of lightweight optically controlled phased array systems.

http://depts.washington.edu/eooptic/publications/files/RF_Phase_Shifter.pdf

Demonstration of a photonically controlled RF phase shifter

High-speed polymer modulators were fabricated using low-Vpi AJL8 chromophore in amorphous polycarbonate, and highly calibrated frequency response measurements were obtained using convenient coplanar-microstrip transitions. These Mach-Zehnder modulators show good frequency response to 50 GHz, with a loss and velocity mismatch-limited Vpi at 50 GHz <12 V. The measurements establish AJL8 as an excellent candidate for future military analog optical links

High-speed AJL8/APC polymer modulator

A number of material properties must be optimized before organic electro-optic materials can be used for practical device applications. These include electro-optic activity, optical transparency, and stability including both thermal and photochemical stability. Exploiting an improved understanding of the structure/function relationships, we have recently prepared materials exhibiting electro-optic coefficients of greater than 50 pm/V and optical loss values of less than 0.7 dB/cm at the telecommunication wavelengths of 1.3 and 1.55 microns. When oxygen is excluded to a reasonable extent, long-term photostability to optical power levels of 20 mW has been observed. Photostability is further improved by addition of scavengers and by lattice hardening. Long-term (greater than 1000 hours) thermal stability of poling-induced electro-optic activity is also observed at elevated temperatures (greater than 80°C) when appropriate lattice hardening is used. The successful improvement of organic electro-optic materials rests upon (1) attention to the design of chromophore structure including design to inhibit unwanted intermolecular electrostatic interactions and to improve chromophore instability and (2) attention to processing conditions including those involved in spin casting, electric field poling, and lattice hardening. A particularly attractive new direction has been the exploitation of dendrimer structures and particularly of multi-chromophore containing dendrimer structures. This approach has permitted the simultaneous improvement of all material properties. Development of new materials has facilitated the fabrication of a number of prototype devices and most recently has permitted investigation of the incorporation of electro-optic materials into photonic bandgap and microresonator structures. The latter are relevant to active wavelength division multiplexing (WDM). Significant quality factors (greater than 10, 000) have been realized for such devices permitting wavelength discrimination at telecommunication wavelengths of 0.01 nm.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400596079

Rational Design of Organic Electro-Optic Materials

An acoustic spectrum analyzer based on electrooptic (EO) polymer integrated optics is presented. The device is used in a scanning heterodyne geometry by zero biasing a Michelson interferometer. It is capable of detecting vibrations from DC to GHz range. The novelty of the work is applying EO polymers to high frequency acoustic sensing applications. EO polymers have been extensively used for communication devices. However, their use in high frequency sensing applications remains unexplored. The sensor presented is designed to analyze thin film structure by utilizing high frequency capabilities of EO polymers. The advantage of this approach over existing methods is in its potential to detect vibrations over 100 GHz with low drive voltages (V/sub /spl pi// less than 1 V), using a device that is completely electrically controlled with no mechanical moving parts. Low frequency tests indicate good agreement between experimental results and theoretical predictions. Acoustic vibrations excited by pulsed Nd-YAG laser are detected to frequencies up to 200 MHz using the device presented.

https://depts.washington.edu/eooptic/publications/files/Acoustic_Spectrum_Analyzer.pdf

W.H. Steier

Papers

Ring resonator-based electrooptic polymer traveling-wave modulator

Demonstration of a photonically controlled RF phase shifter

High-speed AJL8/APC polymer modulator

Rational Design of Organic Electro-Optic Materials

EO polymer-based integrated-optical acoustic spectrum analyzer