9632651 Yablonovitch The analogy between electromagnetic wave propagation in multi-dimensional periodic structures, and electron wave propagation in real crystals, has proven to be a very fruitful one. Initial efforts were motivated by the prospect, of a "photonic bandgap"; a frequency band in 3-dimensional dielectric structures, in which electromagnetic waves are forbidden, irrespective of propagation direction in space. These 3-dimensionally periodic dielectric structures are frequently called "photonic crystals". These can be useful in opto-electronics as electromagnetic micro-cavities for so-called "zero-threshold lasers" and single-mode light emitting diodes. Such structures would exhibit inhibited spontaneous emission, which could lower the power requirements and increase reliability, particularly of optical arrays. Alternately, such structures can show enhanced spontaneous emission which would allow faster modulation speeds for optical interconnects. More and more often, manufacturing consists of making small things. Much of today's technology is the consequence of 2-dimensional patterning on a microscopic scale. In this proposal, the emphasis is on the technology for making valuable 3 dimensional nanostructures, namely photonic crystals. Three dimensional nano-fabrication is an extraordinarily challenging undertaking. This project began in September of 1993. After an intense initial period of equipment acquisition, installation, and testing, of; electron beam writers, optical diagnostic probes, chemically assisted ion beam etchers, and other thin film processing apparatus, the first photonic crystals were made in early 1995. At this point in the project, the initial optical evidence for a 3-dimensional photonic bandgap at optical wavelengths has been detected. The appropriate Figure-of-Merit is the reflectivity within the 3-d forbidden gap. The observed shape of the optical transmission spectrum of the photonic crystal is close to expectations, but thus far the best midgap experimental reflectivity is only ~80%. While this is already sufficient to begin monitoring the effect of the photonic bandgap on spontaneous light emission devices, further improvements will definitely be needed. Improved reflectivity within the photonic bandgap will demand a higher degree of structural precision in the nanostructure. If funded, the work program would include the following: (a) Multiple ion beam etching, in which 3 ion beam directions fire simultaneously, as opposed to the current system of sequential etching steps, from a single ion beam, interrupted by multiple sample rotations. (b) Optical feedback control of the hole diameters during the chemically assisted ion beam etching. (c) 3-d ion beam mask structures will be built and used. (d) If needed, the 3-d thickness of the photon crystal will be built up by stacking nano-structure layers which are separated by epitaxial liftoff, aligned by surface tension techniques, and then rebonded by wafer fusion procedures. (e) 3-d nano-machining technology will be extended to InP, whose electron/hole surface recombination properties are favorable enough to allow the creation of tiny, electrically pumped, light emitting devices. This renewal proposal is being submitted in parallel with a substantially identical one by our collaborator Prof. Axel Scherer of Caltech. Funding would allow continuation of the joint UCLA/Caltech effort to develop the technology for making 3-dimensional photonic crystal structures at the scale of optical wavelengths, and to improve the structural precision and midgap reflectivity of these structures. Such refined photonic crystal structures should lend themselves to opto-electronic technology, and they might be generically useful in optical science. REVISED WORK STATEMENT The overall scope of this NSF proposal will not change, but the amount requested will be diminished to $60K/year respectively (from an original request for $110K/yr.), to allow for th ose functions which will be supported by the Army Research Office through an anticipated MURI contract. The ARO and NSF efforts will divide along the following lines: Extensive equipment development and construction necessary for our ambitious program will be supported through the Army MURI, whereas most of the training of both graduate and undergraduate students to build, use and maintain this equipment will be primarily funded through this NSF grant at UCLA. The complex process which is necessary for defining and evaluating 3-d photonic crystals will provide our students working on this project with an excellent opportunity to become well acquainted with using and maintaining a wide repertoire of advanced fabrication equipment. We propose that much of the process development and optimization as well as student training which will lead to the definition of advanced 3-d photonic crystals will be done under the NSF proposal. On the other hand, the device fabrication and construction of the equipment necessary for this effort will be supported by the Army MURI. Excluded from the NSF project will be more near term mission oriented projects, such as microwave photonic crystals, the inclusion of 2-dimensional photonic crystals as end-mirrors on edge-emitting laser diodes, the definition of deep gratings used to form birefringent computer generated holograms, and the development of high-contrast polarizing beam-splitters (with measured TE/TM contrast ratios in excess of 300:1). Thus, the specific tasks performed for the NSF supported program will be: 1. The design of a multiple ion beam etching system with three ion sources, 2. The development of new three-dimensional masking procedures to make thicker photonic crystals. 3. The development of mechanical stacking procedures using epitaxial liftoff for thick photonic crystal fabrication. 4. The design and modeling of new optoelectronic devices based on photonic crystals. 5. The development of altern ative lithography techniques over large areas, such as through electrochemical self-assembly. All of these tasks will be performed with the help of graduate and undergraduate students, who will be trained in microfabrication conventions, optical measurement techniques, and numerical modeling of optical phenomena in photonic crystals. Graduate students will be directly supported by the grant, whereas undergraduates will be exposed to this state of the art equipment through summer programs and senior thesis and workstudy experience in the laboratory. ***
