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Research

IRG4, Electromagnetic Nanostructures

In IRG 4, Electromagnetic Nanostructures, we combine techniques for making composite materials with periodicity on the order of the wavelength of light with computational modeling tools to explore new physics and new applications. Highlights of this work include planar left-handed optical materials, holey fiber optical waveguides, inverse opal solar cells, field switchable optical fibers, and photorefractive liquid crystal composites.

Recent highlights:

Recent work in IRG4 seeks to design, discover and fabricate fundamentally new optical materials by harnessing powerful theoretical design tools and combining disparate materials into integrated optically active structures.

Integrated Optoelectronics of Holey Fibers. Optical fibers are an established platform for communications technology and fundamental research in photonics. It has thus far not been possible to integrate crystalline semiconductors into optical fibers; such hybrid structures would allow for interaction of semiconductors with waveguided electromagnetic radiation over much longer lengths than can be realized in typical planar device geometries; it would be a major step towards all-fiber opto-electronics. Fabrication of cm-long microwires and nanowires within the narrow capillary pores of microstructured optical fibers (MOFs) seems implausible. We have deposited high-quality polycrystalline and single-crystal semiconductors, metals, and heterostructures within the voids of MOFs via ultrahigh-pressure microfluidic chemical deposition. High-pressure flow, sustained by the mechanical strength of optical fibers, overcomes mass-transport constraints and also enables a strikingly uniform, dense and conformal annular deposition onto the capillary walls, even for pores centimeters long that reduce to less than 10 nm in diameter. The decoupling of fiber design and material deposition provides exceptional design flexibility: these hybrid structures combine the architectural versatility of MOF templates with the compositional complexity of CVD. We have grown the longest semiconductor nanowires produced to date: silicon and germanium wires less than 100 nm in diameter and 30 cm long. The wires can be organized into dense arrays to facilitate cooperative interactions. We have produced in-fiber field effect transistors for proof-of-principle and in-situ materials characterization. The polycrystalline semiconductor cores of these hybrid structures can waveguide and optically modulate 1.55 micron light. Photo-induced deposition generates linear arrays of gold or silicon inside fiber capillaries for full three-dimensional patterning. Gold particles deposited in this manner can trigger Vapor-Liquid-Solid (VLS) growth of single-crystal silicon wires within the fiber pores.

We are currently improving the materials properties to facilitate both fundamental science and device research. The optical losses for our initial polycrystalline silicon wires are already surprisingly low (7 dB/cm). We expect these losses to decrease substantially as the materials are optimized and we investigate single-crystal wires. We are widening the range of materials that can be deposited (currently Si, Ge, GeS₂, Au, Cr, and other metals) to include direct-gap semiconductors useful for e.g. fiber lasers. These structures are also ultrahigh strength extensible containers, since optical fibers are strong, stiff and elastic: One can impose unprecedented uniaxial tensile strains on materials within these fibers. First-principles materials theory calculations suggest that it may be possible to generate large enough uniaxial tension inside a narrow Si-filled fiber pore to stabilize a new silicon crystal structure with three-fold coordination: low-coordination silicon structures have never before been observed. Our unique ability to simultaneously engineer radial, longitudinal and compositional complexity within optical fibers, whose own microstructure can be engineered independently, enables us to fabricate three-dimensional structures of unprecedented complexity within fiber geometries. A paper on this work has recently appeared in Science 311, 1583-1586 (2006).

Negative Index Metamaterials (NIM): We are also designing and demonstrating extremely low-loss negative-index materials with properties that can be scaled to infrared and visible wavelengths. To this end, we employ cascaded arrays of planar etallo-dielectric or all-dielectric frequency selective surfaces (FSSs) designed using a full-wave electro-magnetic modeling code that accounts for the wavelength dependent metallic and dielectric properties and incorporates genetic algorithm (GA) optimization and fabrication design rules to simultaneously optimize desired function (using a full-wave Periodic Moment Method), minimize loss, and maintain ease of fabrication via electron-beam and/or nanoimprint lithography. Our iterative design, fabrication and testing cycle has a strong interplay of theory and experiment. We are scaling our designs for multiband far-IR filters to mid- and near-IR wavelengths using new techniques that we developed for suspending ultrathin (less than 0.5 micron) polyimide films and using e-beam and nanoimprint lithography for patterning the metal elements. The excellent agreement between theory and experiment for these IR FSS filters supports our current activities on planar NIMs, incorporating an inversion algorithm to extract the effective refractive index from the simulation data so that the GA can facilitate the design of metallo-dielectric FSS-NIMs. We are extending the GA designs to multilayers and all-dielectric FSS that push NIM designs into the near-IR and optical, using a full-wave Periodic Finite Element Boundary Integral (PFE-BI) method to accurately and efficiently model inhomogeneous dielectric structures. The low losses in all-dielectric structures will enable us to extend planar FSS design to near IR and visible-wavelength NIMs. In parallel, we are developing highly tunable nonlinear optical materials for incorporation into planar and fiber geometries. We have produced electronically tunable 3D inverse-opal TiO₂ photonic crystals infiltrated with liquid crystal with the widest tuning range to date: >20 nm. We have fabricated and observed giant reversible photorefractive response (10^-2 cm²/watt, among the largest seen) in nematic liquid crystal doped with gold nanowires or CdSe nanocrystals, enabling real-time wave mixing not possible in other liquid crystalline materials. Incorporating tuneable liquid crystals into frequency selective surfaces will enable electro-optical switching, frequency filtering and negative refraction with tuning ranges as wide as 380nm, from the visible to the far infrared.

Widely electrically tunable long-period fiber gratings with ferroelectric polymer cladding. Long-period fiber gratings, which act via interference between core and cladding modes, can function as in-fiber gain equalizers, ban-rejection filters and sensors. If the refractive indices of the core and cladding are matched, and the cladding index is electrically tunable, then the resonant wavelength can be tuned over a very wide range. We have integrated a relaxor ferroelectric P(VDF-TrFE-CFE) terpolymer/ZnS nanocomposite as an index-matched second cladding in a simple, robust, low-power, compact, fast, in-fiber long-period grating with exceptional tunability, ten times larger than any other reported electrically-driven tuning range. Rather than simply attenuate the guided mode, a thin (~100 nm) ITO electrode actually modifies the mode distribution of the cladding mode so as to increase tunability. We plan to sharpen and deepen the resonance by lengthening the grating and optimizing its geometry for our specific nanocomposite second cladding.