Chris E. Finlayson

Microstructured optical fibers as high-pressure microfluidic reactors

Deposition of semiconductors and metals from chemical precursors onto planar substrates is a well-developed science and technology for microelectronics. Optical fibers are an established platform for both communications technology and fundamental research in photonics. Here, we describe a hybrid technology that integrates key aspects of both engineering disciplines, demonstrating the fabrication of tubes, solid nanowires, coaxial heterojunctions, and longitudinally patterned structures composed of metals, single-crystal semiconductors, and polycrystalline elemental or compound semiconductors within microstructured silica optical fibers. Because the optical fibers are constructed and the functional materials are chemically deposited in distinct and independent steps, the full design flexibilities of both platforms can now be exploited simultaneously for fiber-integrated optoelectronic materials and devices.

3D bulk ordering in macroscopic solid opaline films by edge-induced rotational shearing.

A breakthrough in the fi eld of large area photonic structures is reported, based on permanent ordering of solid polymeric fi lms of sub-micrometer spheres by edge rotational-shearing. The resulting high-quality polymer opal thin-fi lms exhibit strikingly intense structural color, as confi rmed by combining a number of spectroscopic approaches. This induced self-assembly on macroscopic length scales represents a step-change away from current surface lithographies, presenting new routes for assembling solid ordered photonic materials. Despite previous reports of shear-ordering in sedimentary colloids in solution, [ 1 , 2 ] no precedents exist for the application of such techniques to these granular solvent-free systems, which allow formation of permanent composite structures in the solid-state. Full 3D ordering of sub-micrometer components into defi ned architectures is a major challenge for bottom-up nanophotonics, nano-electronics, plasmonics and metamaterials. [ 3‐5 ] Even simple structures, such as opaline photonic crystals based on fcc colloidal lattices, have optical properties dominated by defects and cannot be fabricated in any scalable fashion. [ 6‐9 ] Here, we report a signifi cant advance in high-quality polymer opal thin-fi lms exhibiting tunable structural color across visible wavelengths. An edge-induced rotational shearing (EIRS) process produces reproducible highly uniform samples with bulk-ordering of sub-micrometer components, greatly enhancing both the intensity and chromaticity of the observed structural color. The demonstration of reproducible scale-up of these elastomeric synthetic opaline fi lms to industrial length scales makes them very attractive as a route to a wide range of large-area photonics applications, including sensors and coatings [ 10 ] as well as metamaterials when combined with metallic core‐shell particles. The advance reported here is based on a recently developed technique to produce fl exible opals using melting and shearordering under compression of core/shell polymer nanoparticles. [ 11‐13 ] So far, this produces fl exible polymer opals with

Determination of nonlinear refractive index in a Ta2O5 rib waveguide using self-phase modulation

Self-phase modulation has been observed for ultrashort pulses of wavelength 800nm propagating through a 1 cm-long Ta2O5 rib waveguide. The associated nonlinear refractive index n2 was estimated to be 7.23x10-19 m2/W, which is higher than silica glass by more than one order of magnitude. Femtosecond time of flight measurements based on a Kerr shutter configuration show that the group velocity dispersion is small at a wavelength of 800 nm, confirming that dispersion may be neglected in the estimation of n2 so that a simplified theory can be used with good accuracy.

Enhanced Förster energy transfer in organic/inorganic bilayer optical microcavities

We present experimental evidence of enhancement of the Forster energy transfer interaction in bilayer, optical microcavities incorporating CdSe nanocrystals and an organic dye. On optical excitation of the nanocrystal layer, we observe luminescence predominantly at wavelengths associated with emission from the dye, indicating that energy transfer is taking place. No significant energy transfer is observed in the absence of the microcavity, nor in a microcavity where the dye and nanocrystal layers are separated by a 20 nm spacer layer. We estimate that the Forster transfer rate is increased by at least a factor of 10 in the microcavity.

Ordering in stretch-tunable polymeric opal fibers.

We demonstrate the production of high-quality polymer opal fibers in an industrially-scalable process. These fibers exhibit structural color, based on the self-assembly of sub-micron core-shell particles, with a spectrum which is stretch-tunable across the visible region. The internal substructure and ordering of fibers, as inferred from variations in spectral bandwidth, is studied using dark-field microscopy. We employ a granular model to examine flow and shear forces during the extrusion process, and the effects on particle ordering. In both theory and experiment, a concentric zone of the fiber near the exposed surface develops particularly strong structural color. Such elastically-tuned structurally colored fibers are of interest for many applications.

Optical microcavities using highly luminescent films of semiconductor nanocrystals

Colloidally grown CdSe nanocrystals with epitaxial ZnS shells show highly efficient, size-tunable luminescence. We report the incorporation of films of these core-shell nanocrystals into wavelength-scale, high-Q, planar microcavities. Under optical excitation, we find that emission from the nanocrystals couples to the discrete optical modes of the microcavity. The broad free-space emission spectrum of the nanocrystals is modified by the presence of the microcavity, giving a series of sharp emission lines with wavelengths determined by the cavity dimension. Our experiments demonstrate that microcavities with semiconductor emitters can be conveniently fabricated using spin-coating techniques. We find that, at room temperature, the microcavity emission spectrum is independent of excitation intensity for excitation densities up to approximately one electron–hole pair per nanocrystal.

Macromolecular Scaffolding: The Relationship Between Nanoscale Architecture and Function in Multichromophoric Arrays for Organic Electronics

The optimization of the electronic properties of molecular materials based on optically or electrically active organic building blocks requires a fine-tuning of their self-assembly properties at surfaces. Such a fine-tuning can be obtained on a scale up to 10 nm by mastering principles of supramolecular chemistry, i.e., by using suitably designed molecules interacting via pre-programmed noncovalent forces. The control and fine-tuning on a greater length scale is more difficult and challenging. This Research News highlights recent results we obtained on a new class of macromolecules that possess a very rigid backbone and side chains that point away from this backbone. Each side chain contains an organic semiconducting moiety, whose position and electronic interaction with neighboring moieties are dictated by the central macromolecular scaffold. A combined experimental and theoretical approach has made it possible to unravel the physical and chemical properties of this system across multiple length scales. The (opto)electronic properties of the new functional architectures have been explored by constructing prototypes of field-effect transistors and solar cells, thereby providing direct insight into the relationship between architecture and function.

Electrical and Raman characterization of silicon and germanium-filled microstructured optical fibers

Extreme aspect ratio tubes and wires of polycrystalline silicon and germanium have been deposited within silica microstructured optical fibers using high-pressure precursors, demonstrating the potential of a platform technology for the development of in-fiber optoelectronics. Microstructural studies of the deposited material using Raman spectroscopy show effects due to strain between core and cladding and the presence of amorphous and polycrystalline phases for silicon. Germanium, in contrast, is more crystalline and less strained. This in-fiber device geometry is utilized for two- and three-terminal electrical characterization of the key parameters of resistivity and carrier type, mobility and concentration

Infrared emitting PbSe nanocrystals for telecommunications window applications

We demonstrate the colloidal synthesis of PbSe nanocrystal quantum dots, via an organometallic-precursor route, developed from recently reported techniques. This synthesis typically yields a particle size distribution of approximately 5–10%, as may be inferred from the sharp spectral features seen in absorption and from our effective-mass model correlating spectral features to nanocrystal size. An accurate quantitative analysis, using an infrared reference dye, shows these nanocrystals to exhibit infrared photoluminescence from intrinsic quantum-confined states, with high quantum efficiencies of up to 60% in solution. The wavelength of the photoluminescence may also be conveniently size tuned in order to access the 1.3–1.5 µm ‘telecommunications window’. We discuss the significance of this work in the context of future optoelectronic applications.

Long-lived quantum-confined infrared transitions in CdSe nanocrystals

We present quasi-steady-state photoinduced absorption measurements on thin films of CdSe nanocrystals dispersed in a polystyrene matrix. For nanocrystals treated with pyridine we observe an intense, size-dependent absorption peaking in the mid-infrared when the samples are irradiated with visible light. This infrared absorption is associated with a size-dependent bleach in the visible, near the peak of the first excitonic absorption. We attribute the infrared absorption to an intraband electron transition in the quantum dots and measure the lifetime of the absorbing state to be 1.0 ms at 295 K.