The Rational Design and Production of Organic Electro-Optic Materials and Devices

نویسنده

  • L. R. Dalton
چکیده

Recently, new theoretical design paradigms have facilitated the production of organic electro-optic materials that have been used to fabricate simple electro-optic devices characterized by halfwave voltages of less than 1 volt and demonstrated frequency response to above 100 GHz. Electrooptic circuitry has been vertically integrated with VLSI electronics and 3-D circuitry has been demonstrated. Several schemes have been introduced for overcoming the problem of mode mismatch leading to high insertion loss demonstrating that per facet losses of less than 1 dB can be achieved. New material design strategies have also resulted in dramatic improvement of auxiliary material properties: Optical loss has been reduced to values as low as 0.2 dB/cm, thermal stability of electro-optic activity has been extended to 250 C, and significant improvements in photostability have been achieved. These properties have not been achieved in the same material and the tradeoffs required are now becoming better defined. In this communication, we review the state-of-the-art of organic electro-optic materials and devices and the design paradigms that have defined that state-of-the-art. We present preliminary data suggesting that a design paradigm shift may lead to even more impressive improvements in the properties of organic electrooptic materials and devices fabricated from those materials. Introduction and Chromophore Synthesis The following steps are required to produce an electro-optic (EO) device based on organic materials: (1) Synthesis of chromophores characterized by large (> 10 esu) molecular first hyperpolarizability (β) dipole moment (μ) products, high thermal (> 300°C) and chemical stability, and good processability (e.g., solubility in spin casting solvents, etc.); (2) processing of these chromophores into acentric (electro-optic active) lattices by introduction of an appropriate host (dilutent) material and by application of an ordering force such as an electric poling field; (3) stabilization of acentric order by lattice hardening chemistry to facilitate subsequent EO circuit and device fabrication; (4) fabrication of buried channel electrooptic waveguides by reactive ion etching (RIE) or photolithographic techniques, deposition of cladding layers and metal drive electrodes; and (5) execution of all remaining steps necessary to achieve integration with optical transmission lines and VLSI drive electronics [1-3]. Organic EO chromophores are dipolar (charge-transfer) molecules consisting of an electron donor, an electron acceptor, and a pielectron bridge providing communication between the donor and acceptor moieties. Control of single/double bond length alternation is critical to maintain pi electron communication between the donor and acceptor and thus to optimization of molecular hyperpolarizability. Bridges containing aromatic and heteraromatic groups yield excellent thermal and chemical stability; however, the larger bond length alternation of these species frequently results in poor molecular hyperpolarizability. The largest hyperpolarizabilities have, to date, been obtained with protected polyene and multiple (including fused) thiophene ring containing bridges. A typical chromophore meeting the criteria of high hyperpolarizability and thermal stability is shown in Figure 1, which also shows the synthetic scheme used to produce the chromophore. In Figure 2, we show electro-optic data recorded for thin films of this chromophore dissolved in amorphous polycarbonate (APC) [4] to form a composite material. Electrically poled APC composite films typically exhibit waveguide loss of 1-1.5 dB/cm at the telecommunication wavelengths (1.3 and 1.55 μm). Such loss values are typically a combination of absorption (from C-H, N-H, or O-H vibrational overtones) and scattering (processing—spin casting, poling, and waveguide fabrication) losses. Reduction of loss to values less than 1 dB/cm requires reduction of proton density in both chromophore and host materials (e.g., by replacement of H with F) and attention to the details of processing to minimize scattering losses [1-3]. The thermal stability (the temperature at which > 95% EO activity is observed after 1000 hours) of poling-induced electro-optic activity of chromophoreAPC composite materials typically lies in the range 60-80°C. Temperature stability can be increased by 20-50°C, or even higher, by lattice hardening chemistry that introduces covalent bonds tethering the chromophore at both ends to a 3-D crosslinked lattice. Stability is also enhanced by increasing the segmental rigidity of the host lattice (an EO chromophore is already a moderately stiff object due to the conjugated pi electron structure). Temperature stability can even be increased to above 200°C; however, more typical values lie in the range 100-125°C. Unfortunately, some attenuation of electro-optic activity is associated with the lattice hardening process, as both poling and lattice hardening are temperature-dependent processes. Figure 1: Synthesis of the CWC chromophore (R = CN) is shown. Removal of the dithiophene bridge gives the commonly used CLD chromophore. Figure 2: Electro-optic activity at 1.3 microns as a function of concentration of CWC in APC. The details of processing (e.g., stepped electric field/temperature poling protocols) necessary to minimize the tradeoff between optimized electrooptic activity and the thermal stability of that activity has been discussed elsewhere [1-3,5,6]. Unless care is exercised lattice hardening can lead to phase separation with resulting dramatic increase in optical loss. Indeed, lattice hardening, which is critical to both thermal and photochemical stability of organic EO devices, is the most demanding aspect of the fabrication of organic EO devices. The photostability of organic EO devices is defined by reactions that chemically disrupt pi electron conjugation by substitution (oxidation) or by bond breaking. Photochemical decomposition is typically dominated by singlet oxygen pathways. As shown in Figure 3, partial exclusion of oxygen results in dramatic improvement in photostability. Figure 3: The photostability of an EO modulator fabricated from CLD/APC for a waveguide power of 20 mW (λ = 1.55 μm). Photostability can also be improved dramatically by lattice hardening and by the incorporation of scavengers. Lattice hardening acts to both decrease oxygen diffusion rates and to promote the recombination of radicals formed by bond breaking. Photochemical stability can also be improved by eliminating points of chemical reactivity in the pi electron structure or by modification of chromophores to sterically protect those sites. Theory and Acentric Order A major problem in the development of organic electro-optic materials is the aggregation (centrosymmetric lattice crystallization) of chromophores at high concentrations. This chromophore intermolecular association both attenuates the maximum achievable electro-optic activity (see Figure 2) and at the highest concentrations leads to optical loss due to light scattering from aggregates. It is driven by chromophore intermolecular electrostatic (dipoledipole) interactions, which compete with dipoleelectric poling field driven acentric ordering. Not only do intermolecular dipole-dipole interactions lead to a maximum in graphs of electro-optic activity versus chromophore loading but the detailed shape of the curves and the position of the maximum O O

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تاریخ انتشار 2002