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Electro-optic (EO) devices are critical to telecommunications, computing, defense, sensing, and transportation technology sectors. Indeed, they will play a central role in the coming technology revolution of chipscale electronicâphotonic integration, which is well recognized as important to future computer technology. Radio-frequency (RF) photonics is also well recognized as an important option for the long-distance delivery of RFâmicrowaveâmillimeter-wave signals. Electro-optic device technology is currently dominated by inorganic materials, such as lithium niobate [which exhibits a useable EO activity of approximately 30 pm/V (picometers/volt) and an intrinsic optical loss of 0.1-0.2 dB/cm]. Unfortunately, there is little possibility for further improvement of such ionic crystalline materials, where EO activity arises as a result of electric-field-induced displacement of ionic charges. Moreover, the mass of these charges limits the response time of the EO effect for such materials, and a substantial velocity mismatch exists between electrical and optical waves propagating in lithium niobate. This velocity mismatch limits both the bandwidth and the drive voltage that can be achieved. In contrast, EO activity in organic Ï-electron materials is determined by applied electric-field-induced electronic charge perturbation, and the lower mass of the coupled electron system leads to very fast (tens of femtoseconds) response times. Moreover, EO activity in organic materials can be systematically improved by modification of the structure and organization of the Ï-electron molecules (called chromophores because they are âcoloredâ due to absorption of visible wavelength light) that make up organic EO materials. A question of major interest to technology sectors dependent on EO devices is: How much and how soon can organic EO materials be improved to enable new and dramatically improved applications? This chapter addresses that concern.
Theoretically inspired design of chromophores with exceptional molecular first hyperpolarizability, nanoscopically engineered to control intermolecular electrostatic interactions for enhanced acentric chromophore order, has produced dramatic improvements in EO activity to values more than 15 times that of lithium niobate. The large EO coefficients of solution-processed organic EO materials permit drive voltages in devices to be reduced to values of less than 1 V. Such voltages are critical to the realization of gain in RFâmicrowaveâmillimeter-wave photonic applications (e.g., amplification of electrical signals in the electrical-optical-electrical signal transduction process). Organic EO materials afford a number of other advantages, including exceptional bandwidth (< 100 GHz), low temperature solution (spin casting) and melt processing (nano-imprint and soft lithography) of thin film devices, adaptability to production of conformai and flexible device structures, and compatibility with a diverse array of materials and material technologies including silicon photonics. This last advantage has permitted integration of organic EO materials with silicon photonic circuitry, resulting in low-power high-bandwidth EO modulation, optical rectification, and all-optical modulation. The last two phenomena relate to the concentration of optical power in the reduced dimensions of silicon photonic circuitry. Another recently demonstrated advantage of organic EO materials relates to terahertz applications, where the combination of large EO coefficients and the absence of attenuation by phonon modes permit development of highly efficient and broad-bandwidth terahertz sources and detectors.
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