Bringing Electro-optics to the Marketplace

P. Carey Reid

The enormous strides in electronics in the last fifty years have driven the information and communication revolution that still goes full-blast all around us. But scientific experts now agree that the next phase will be the age of the photon. According to Ron Dagani of Chemical and Engineering News, “Photons can carry information faster, more efficiently, and over longer distances, with less signal degradation, than electrons.”

Associate Professor of Chemistry Alex Jen has emerged as a central player in a rapidly escalating series of research coups and technological-application breakthroughs as more and more devices based on electro-optics begin to enter the marketplace. At the heart of the drama lie recent dramatic improvements in materials’ stability and optical nonlinearity, the results of a steady toppling of formidable obstacles to photon-based communication applications.

Decades ago, the first applications, known as fiber optics, emerged with the use of certain inorganic crystals to manipulate light; the cost of high-quality single crystals and the difficulty of integrating them in electronic devices, however, have reduced their attractiveness. In the past ten years, scientists and engineers have turned instead to organic polymers to control light with electricity because they have proven to be more efficient and less expensive to produce. They are also very compatible with current semiconductor processes and equipment components.

Other properties have also made polymer-based optoelectronics an exciting prospect. The second-order nonlinearity of polymers allows a quadrupling of the information a normal laser can store on an optical disc; additionally, applied voltages yield remarkable flexibility in light path transmission and information encoding. The key to achieving this kind of optical nonlinearity is the design of chromophores with enhanced second-order nonlinear optical (NLO) properties and then the incorporation of these chromophores into polymer matrices.

Practical problems arose early on regarding the matrices. For one thing, the polymer devices have to operate efficiently at relatively high use temperatures (80–100ºC). The preparation of electro-optic polymers begins with the synthesis of an NLO-active chromophore. This is a conjugated molecule with an electron-donating group on one end and an electron-accepting group on the other. The chromophore’s second-order nonlinearity arises from an external field causing the molecule’s electric-charge separation. The resulting asymmetric charge distribution has a marked effect on how the chromophore interacts with light. Sizable amounts of chromophore must be added to achieve large nonlinearities, and the dipole moments of the chromophores must be aligned so that the materials have no center of symmetry. To achieve this character, the matrix materials are heated to the point where the NLO molecules can move about freely; they are then poled by an electric field and locked into place in subsequent cooling.

In laboratory tests, however, heating in use has led to decay of the polymer’s optical nonlinearity due to the relaxation of the pole-induced polar order. This problem might now have been solved by the use of polyimides whose glass-transition temperature is well above a device’s operating temperature. Bonding the chromophore to the polymer at more than one site (cross-linking) has yielded some success as well. Finally, several cross-linking schemes have been applied to anchor the wandering chromophores. The most dramatic success in this area was announced last year by a research group at IBM who devised a polyimide family that resisted chemical degradation at 350ºC for several hours. Ironically, the need for integrity at high temperatures is partially dictated by electronics-manufacturing methods where optics and electronics are customarily soldered to the same chip, a process that heats the materials to 250ºC. Unfortunately, while solving the heat problem, the IBM chromophore molecule has produced disappointing optical qualities.

A U.S.-French team based at the California Institute of Technology has had greater optical success by concentrating on chromophores configured around a powerful heterocyclic acceptor. Unfortunately, with the greater optical acuity has come a greater vulnerability to heat-induced degradations. The seesawing character of progress ruled the field until chemistry and engineering professor Larry R. Dalton and his coworkers at the University of Southern California developed a family of electro-optic polymers in which the aligned chromophores are anchored at both ends to polymer chains. Dalton has collaborated with two electrical-engineering professors at the University of California, Los Angeles, to produce an advanced polymer-based prototype modulator that can convert the equivalent of fifteen million simultaneous telephone conversations from electronic to optical form. Already, TACAN Corp. of Carlsbad, California, is collaborating with the USC-UCLA researchers to commercialize the modulators.

Dr. Alex Jen joined the college in 1997. He received his B.S. from National Tsing University in Taiwan and his Ph.D. in organic chemistry from the University of Pennsylvania. He has amassed a considerable amount of expertise in the areas of organic/polymer synthesis and several new types of physical phenomena in organic functional polymers. Before coming to the college, Jen was vice president of materials at ROITech’s optical materials division in Monmouth Junction, New Jersey. This U.S. firm has already brought high-temperature electro-optic polymers and devices to market. Last November, ROITech announced its polymeric NLO materials system, Optimer, which possesses high thermal stability and high electro-optic activity. Jen’s fabrication process includes several steps that subject the new device to temperatures of 350ºC for periods of thirty minutes or more before chromophore poling is effected; thus far, the material has remained thermally and chemically stable throughout.

The Optimer materials system exploits a molecular configuration wherein an L-shaped chromophore is bonded to a polyimide backbone, resulting in good poled stability for short periods at 250ºC and longer periods at over 100ºC with low optical loss and solid mechanical properties. ROITech products based on Optimer have found eager buyers. Last November, the firm introduced a high-speed electro-optic switch called HEOS. When an electric pulse is applied to HEOS’s electro-optic polymer, the material’s refractive index is altered, directing the light to another channel. Jen asserts that this device will allow the design of optoelectronic integrated circuits and molecules for use in telecommunications, cable TV, computers, and instrumentation. Several military and industrial customers have already purchased Optimer-based products.

Jen and his coworkers have been laboring for years to develop Optimer, with help from other academic collaborators along the way. The project began in 1988 at the Monmouth Junction facility of EniChem America, where Jen established the firm’s NLO materials group. In early 1995, the group became a part of ROITech, with which it had been collaborating. Further work entailed incorporating newly developed chromophores into polymers using side-chain aromatic polyimides simultaneously researched by groups at IBM and the University of Chicago. The research was hampered, however, by the drawbacks of the three groups’ synthetic approaches: chemical production methods proved too tedious for practical purposes and, worse, too harsh for most chromophores to survive.

Jen’s group at ROITech, which includes Tian-An Chen and Yongming Cai, solved these problems by developing a facile, generally applicable, two-step approach for making NLO side-chain aromatic polyimides. The critical second step involves a room-temperature condensation method that avoids the drawbacks completely. The new method allows a wide variety of polyimide backbones to be prepared with a large choice of chromophores. Additionally, the polymers prepared using the improved two-step process have been found to possess excellent solubility and processibility.

At present, Jen and his colleagues are working on another materials system, this one based on rigid-rod polyquinolines; to date, it is still short of the device stage. Polyquinolines are ideal materials for devices because they are tough, easily produced in low-optical-loss thin films, and stable up to 500 and 600ºC. The obstacle the group is currently confronting is a respectable — but, to Jen, less than acceptable — performance level caused by an initial drop of r value when the materials system is used in devices. The team has now prepared polyquinolines containing covalently bonded chromophores that have exhibited enhanced mechanical properties and higher stability. Jen has strong hopes that the new materials will be superior to existing polyimide systems for devices.

The entire electro-optic field is blossoming rapidly, with new devices appearing in laboratories and even in the marketplace every day. A working prototype now exists for a high-voltage sensor based on electro-optic polymers that can measure a greater range of voltages than can current sensors based on an inorganic crystal. For the layperson, this breakthrough could translate into lower power bills. With greater efficiency in power use, the negative impact of fuel use on the environment will be reduced.

What lies ahead for Jen and his fellow electro-optic pioneers is the mythic goal of “all-optical information processing,” wherein electronics is wholly pushed to the sidelines. This so-called third-order nonlinearity will enable a light to be switched by a second beam of light instead of an electronic pulse. For the college’s Alex Jen, that’s where the intellectual excitement now lies.

Alex Jen is an associate professor of chemistry. His research is funded by the Air Force Office of Scientific Research, the Office of Naval Research, and the Department of Defense.