The Dalton Group Research
Modern technology constantly demands new materials exhibiting previously unrealizable properties, and materials processing methods, which yield ever smaller devices and integrated circuits. Not only must new molecules and macromolecules be synthesized to facilitate new technologies, but frequently, molecules must be arranged in unique supramolecular architectures to yield nanoscale and microscale patterned devices and circuits. The Dalton group’s research focuses on the interdisciplinary field of materials chemistry with particular emphasis on high technology electronic, electro-optic, and nonlinear optical materials and emphasis on nanoscale materials and architectural construction techniques. To complement these efforts, Professor Dalton maintains state-of-the-art laboratories in nonlinear optics (including femtosecond laser techniques) and nanoscale imaging (STM, AFM, NSOM).
Dalton’s research in electroactive materials deals with many topics including preparation of materials for fiber optical amplification, sensor protection, all-optical signal processing, high-density optical memories, actuator applications, display applications, etc. However, the best known effort of the Dalton group is in the area of development of polymeric electro-optic materials and processing these materials into sophisticated “opto-chips.” Electro-optic materials are materials for which the index of refraction of the material can be changed by the application of a small electric field. Devices fabricated from such materials can be used for the basic applications of electrical-to-optical signal transduction (conversion), optical switching at nodes of an optical network, and optical beam steering. The first two applications can be thought of as providing the on-ramps and interchanges of the information superhighway. The Dalton research group has produced polymeric electro-optic modulators operating at frequencies above 100 GHz and with drive voltage requirements on the order of one volt.
Research on nanostructures materials includes preparation and application of dendrimers, phase-separating block copolymers, and nano- and microspheres. Recent successes include the development of high-efficiency light harvesting dendrimers (also of use for fiber optical amplification), visible wavelength photonic bandgap materials, new nanoactuators, and templates for the fabrication of inorganic nanostructures.
Background: Over the last two decades, advances in electronics have revolutionized the speed with which we perform computing and communications of all kinds. Three key technologies combined to create a platform that enabled the electronic revolution: semiconductor materials, automated microfabrication of integrated electronic circuits, and integrated electronic circuit design. As a result, the mass manufacturing of low-cost integrated circuits has become possible. But now we are outgrowing the performance of electronics in many applications. Signal propagation and switching speeds in the electronic domain are inherently limited. One area where these limitations are seen clearly is in telecommunications, where bandwidth expansion is desperately needed. To overcome these barriers, we must enter a new computing and communications revolution-this time based on photonics. Photonics plays some crucial and complementing roles to electronics in many application domains. Examples of successful use of photonics can be found in broadband communications, high-capacity information storage, and large screen and portable information display. As demands for information bandwidth increase, information photonics is becoming more and more important in every aspect of today’s technology-driven society. The success of a new technology, however, largely depends on the progress achieved in finding and fabricating new high- performance and cost-effective materials. Recently, as the knowledge base of polymeric materials widened, new functions for polymers have been actively investigated. New and improved polymeric materials were found to show promises in generating, processing, transmitting, detecting, and storing light signals.
Nonlinear optical (NLO) materials, especially organic and polymeric ones, have continued to be at the forefront of research activities since the mid-1980s. High NLO susceptibility, fast response time, low dielectric constants, small dispersion in the index of refraction from dc to optical frequencies, virtually endless possibilities of structural modification, and ease in processability are some of the properties of conjugated organic systems uniquely suited for their applications in photonic devices, such as the frequency converters, high-speed electro-optic (E-O) modulators and switches. In particular, a low half-wave voltage (Vp) of 0.8 V has been achieved recently in a polymer waveguide modulator using highly nonlinear organic chromophores and push-pull poling and driving techniques. This discovery, together with a recent demonstration of exceptional bandwidths (as high as 150 GHz) and ease of integration with very large scale integration (VLSI) semiconductor circuitry and ultra-low-loss passive optical circuitry, have provided a solid foundation for using polymeric materials in next-generation telecommunications and information processing.
It is well-known that the second-order NLO properties originate from non-centrosymmetric alignment of NLO chromophores, either doped as a guest or covalently bonded as side-chains in poled polymers. To obtain device-quality materials, three stringent issues must be addressed:
- Design and synthesis of high mb (m: the chromophore dipole
moment, b: the molecular first hyperpolarizability) chromophores and
realization of large macroscopic E-O activity in the chromophore-incorporated
- Maintenance of long-term temporal stability in the E-O response
of the poled materials in addition to their high intrinsic stability
toward the environment such as heat, light, oxygen, moisture and chemical;
- Minimization of optical loss from design and processing of
materials to fabrication and integration of devices.
Science & Technology Center on Materials
and Devices for Information Technology Research
The Research & Development mission of this project is divided into four central thrusts supported by two core facility activities. Experimental R&D activities are supported by an “end-to-end” theoretical thrust that focuses on quantitative prediction of phenomena ranging from the atomic & molecular level to the performance of integrated systems. Quantum mechanics, so successfully used to predict the properties of molecules and clusters of atoms, is being extended to develop accurate intermolecular (interatomic, interionic) potentials necessary to predict crystallization phenomena and the formation of nano- and microscale ordered structures by self-assembly and “applied force” processing
protocols. The thrust on electro-optic and all-optical devices focuses on the
exploitation of recent advances in the development of materials with very large
second and third order optical nonlinearities for a variety of applications.
The third thrust focuses on development of new light sources, optical amplifiers,
and organic electronics. The fourth thrust deals with the nanofabrication and
microfabrication, by new methodologies, of novel optical and electronic circuit
and device structures and the exploitation of the self-assembly for the fabrication
of nanostructured materials. For example, new two-photon photolithography of
3-D optical circuits will be exploited. Another example is the development
of novel photonic bandgap devices and circuitry. These research thrusts are
supported by Core Facilities in Materials Synthesis (including Scale-Up facilities)
and Device Fabrication.
Realization of new and enhanced materials properties through nanostructural control
Research has focused on the following material areas and research missions: (1) Organic electro-optic materials with the objective of realizing materials characterized by electro-optic coefficients greater than 100 pm/V at telecommunication wavelengths and which pass Telecordia standards. New processing techniques have been developed for fabrication 3-D devices and circuits and for achieving low insertion loss electro-optic devices including active wavelength division multiplexing (WDM) transmitter/receiver systems. The Dalton research group currently serves as the national resource for state-of-the-art electro-optic materials providing materials to government, industry, and academic researchers. (2) Metal core dendrimer materials have been developed for applications as sensors, organic light emitting diodes, and light harvesting solar cell coatings. Materials have also been designed for improved emission properties and photochemical stability both by systematic design of the chelated metal and by design of the surrounding dendrimer structure. (3) Single wall carbon nanotube actuators have been developed and various mechanisms of actuation characterized. These materials were used to fabricate simple optical switches.
Nanoscale Tailoring of Electro-optic Dendrimers for
The main objective of this program is to develop high-performance
organic electro-optic materials that will permit the fabrication of devices
and microresonator) exhibiting low drive voltages, low insertion loss, high
operational bandwidths, and acceptable stability (both thermal and photochemical).
We plan to achieve this goal through:
- Molecular engineering of NLO chromophores through design and synthesis
that will be guided by the bond-order-alternation theory to achieve large
dipole moment (mg) and large molecular first hyperpolarizability (b). We
will employ the highly efficient electron-acceptors and configuration-locked
building blocks. In particular, we will pioneer the concept of “mixed” acceptors.
- We will enhance poling efficiency by control of chromophore shape.
In particular, we will exploit the development of dendrimer nanostructural
engineering to achieve larger electro-optic activity than can be realized
in chromophore/polymer materials. We will endeavor to shorten the time required
to achieve optimized dendrimer structures by developing new condensed matter
theory for quantitatively predicting electro-optic activity that will be
realized for given dendrimer structures and poling conditions.
- We will explore minimization of the poling efficiency/thermal stability
tradeoff that is inherent in the use of crosslinkable dendrimers. We will
develop techniques for in situ monitoring of poling efficiency. We will
also develop an improved version of the constant bias technique for defining
maximum electro-optic activity that can be achieved with a given dendrimer
structure. Comparisons of electro-optic activity for a given chromophore
in dendrimer and polymer structures will be carried out.