All-fibre heterogeneously-integrated frequency comb generation using silicon core fibre.

Originally developed for metrology, optical frequency combs are becoming increasingly pervasive in a wider range of research topics including optical communications, spectroscopy, and radio or microwave signal processing. However, application demands in these fields can be more challenging as they require compact sources with a high tolerance to temperature variations that are capable of delivering flat comb spectra, high power per tone, narrow linewidth and high optical signal-to-noise ratio. This work reports the generation of a flat, high power frequency comb in the telecom band using a 17 mm fully-integrated silicon core fibre as a parametric mixer. Our all-fibre, cavity-free source combines the material benefits of planar waveguide structures with the advantageous properties of fibre platforms to achieve a 30 nm bandwidth comb source containing 143 tones with <3 kHz linewidth, 12 dB flatness, and >30 dB OSNR over the entire spectral region.

Localised structuring of metal-semiconductor cores in silica clad fibres using laser-driven thermal gradients.

The molten core drawing method allows scalable fabrication of novel core fibres with kilometre lengths. With metal and semiconducting components combined in a glass-clad fibre, CO2 laser irradiation was used to write localised structures in the core materials. Thermal gradients in axial and transverse directions allowed the controlled introduction, segregation and chemical reaction of metal components within an initially pure silicon core, and restructuring of heterogeneous material. Gold and tin longitudinal electrode fabrication, segregation of GaSb and Si into parallel layers, and Al doping of a GaSb core were demonstrated. Gold was introduced into Si fibres to purify the core or weld an exposed fibre core to a Si wafer. Ga and Sb introduced from opposite ends of a silicon fibre reacted to form III-V GaSb within the Group IV Si host, as confirmed by structural and chemical analysis and room temperature photoluminescence.

Raman enhanced four-wave mixing in silicon core fibers.

A strong Raman enhancement to the four-wave mixing (FWM) conversion efficiency is obtained in a silicon core fiber (SCF) when pumped with a continuous-wave (CW) source in the telecom band. By tapering the SCFs to alter the core diameter and length, the role of phase-matching on the conversion enhancement is investigated, with a maximum Raman enhancement of ∼15 dB obtained for an SCF with a zero dispersion wavelength close to the pump. Simulations show that by optimizing the tapered waist diameter to overlap the FWM phase-matching with the peak Raman gain, it is possible to obtain large Raman enhanced FWM conversion efficiencies of up to ∼2 dB using modest CW pump powers over wavelengths covering the extended telecom bands.

Semiconductor core fibres: materials science in a bottle.

Novel core fibers have a wide range of applications in optics, as sources, detectors and nonlinear response media. Optoelectronic, and even electronic device applications are now possible, due to the introduction of methods for drawing fibres with a semiconductor core. This review examines progress in the development of glass-clad, crystalline core fibres, with an emphasis on semiconducting cores. The underlying materials science and the importance of post-processing techniques for recrystallization and purification are examined, with achievements and future prospects tied to the phase diagrams of the core materials.

All-optical high-speed modulation of THz transmission through silicon core optical fibers.

High speed optical modulation of THz radiation is of interest for information processing and communications applications. In this paper infrared femtosecond pulses are used to generate free carriers that reduce the THz transmission of silicon based waveguides over a broad spectral range. Up to 96% modulation is observed from 0.5 to 7 THz in an optical fiber with a 210 µm diameter gold-doped silicon core. The observed carrier recombination time of 2.0 ± 0.2 ns makes this material suitable for high speed all-optical signal processing. These results show both enhanced modulation depth and reduced carrier lifetime when compared to the performance of a high resistivity float zone silicon rectangular guide with comparable cross sectional area.

Tapered silicon core fibers with nano-spikes for optical coupling via spliced silica fibers.

Reported here is the fabrication of tapered silicon core fibers possessing a nano-spike input that facilitates their seamless splicing to conventional single mode fibers. A proof-of-concept 30 µm cladding diameter fiber-based device is demonstrated with nano-spike coupling and propagation losses below 4 dB and 2 dB/cm, respectively. Finite-element-method-based simulations show that the nano-spike coupling losses could be reduced to below 1 dB by decreasing the cladding diameters down to 10 µm. Such efficient and robust integration of the silicon core fibers with standard fiber devices will help to overcome significant barriers for all-fiber nonlinear photonics and optoelectronics.

Tapered polysilicon core fibers for nonlinear photonics: erratum.

We correct an error of the nonlinear refractive index used in our original paper.

Laser recrystallization and inscription of compositional microstructures in crystalline SiGe-core fibres.

Glass fibres with silicon cores have emerged as a versatile platform for all-optical processing, sensing and microscale optoelectronic devices. Using SiGe in the core extends the accessible wavelength range and potential optical functionality because the bandgap and optical properties can be tuned by changing the composition. However, silicon and germanium segregate unevenly during non-equilibrium solidification, presenting new fabrication challenges, and requiring detailed studies of the alloy crystallization dynamics in the fibre geometry. We report the fabrication of SiGe-core optical fibres, and the use of CO2 laser irradiation to heat the glass cladding and recrystallize the core, improving optical transmission. We observe the ramifications of the classic models of solidification at the microscale, and demonstrate suppression of constitutional undercooling at high solidification velocities. Tailoring the recrystallization conditions allows formation of long single crystals with uniform composition, as well as fabrication of compositional microstructures, such as gratings, within the fibre core.

Tapered polysilicon core fibers for nonlinear photonics.

We propose and demonstrate a novel approach to obtaining small-core polysilicon waveguides from the silicon fiber platform. The fibers were fabricated via a conventional drawing tower method and, subsequently, tapered down to achieve silicon core diameters of ∼1 µm, the smallest optical cores for this class of fiber to date. Characterization of the material properties have shown that the taper process helps to improve the local crystallinity of the silicon core, resulting in a significant reduction in the material loss. By exploiting the combination of small cores and low losses, these tapered fibers have enabled the first observation of nonlinear transmission within a polycrystalline silicon waveguide of any type. As the fiber drawing method is highly scalable, it opens a route for the development of low-cost and flexible nonlinear silicon photonic systems.

Light trapping in horizontally aligned silicon microwire solar cells.

In this study, we demonstrate a solar cell design based on horizontally aligned microwires fabricated from 99.98% pure silicon via the molten core fiber drawing method. A similar structure consisting of 50 µm diameter close packed wires (≈ 0.97 packing density) on a Lambertian white back-reflector showed 86 % absorption for incident light of wavelengths up to 850 nm. An array with a packing fraction of 0.35 showed an absorption of 58 % over the same range, demonstrating the potential for effective light trapping. Prototype solar cells were fabricated to demonstrate the concept. Horizontal wire cells offer several advantages as they can be flexible, and partially transparent, and absorb light efficiently over a wide range of incident angles.

Properties of transition metal substituted zinc sulfide hexamers and dodecamers.

Density functional theory was used to study the structural and electronic properties of endohedrally- and substitutionally-doped Zn6S6 and Zn12S12 clusters with first-row transition metal atoms. Generally, the lowest energy electronic state of the cluster is that with the maximum multiplicity (Ti and Cr are exceptions). Substitutionally-doped clusters have greater binding energies (per atom) for both cluster sizes, providing an indication that similar doping will be preferred in the bulk material as well. The results are relevant to thin films of doped ZnS in which cluster formation is likely.

Phenylalanine detection using matrix assisted pulsed laser evaporation of molecularly imprinted amphiphilic block copolymer films.

Matrix assisted pulsed laser evaporation was used to deposit molecularly imprinted polymer films of an amphiphilic block copolymer imprinted with an amino acid. This method avoids the need for a common solvent for host and template, and permits fabrication of layers with controlled thicknesses in the nanometer range. Polystyrene-block-polyethylene oxide copolymer and phenylalanine template were co-deposited onto surface plasmon resonance (SPR) sensors from a water/toluene emulsion. FTIR confirmed removal and reintroduction of phenylalanine, and SPR measurements were used for quantitative analysis. A binding ratio of more than 10 was obtained for phenylalanine on imprinted sensors vs. the non-imprinted control surfaces of the same polymer, and a detection limit of 0.5 mM phenylalanine was established. Exposure of sensors to alanine, glutamine, tryptophan, and tyrosine demonstrated that the sensors were highly specific.

Orientation of optically trapped nonspherical birefringent particles.

While the alignment and rotation of microparticles in optical traps have received increased attention recently, one of the earliest examples has been almost totally neglected--the alignment of particles relative to the beam axis, as opposed to about the beam axis. However, since the alignment torques determine how particles align in a trap, they are directly relevant to practical applications. Lysozyme crystals are an ideal model system to study factors determining the orientation of nonspherical birefringent particles in a trap. Both their size and their aspect ratio can be controlled by the growth parameters, and their regular shape makes computational modeling feasible. We show that both external (shape) and internal (birefringence) anisotropy contribute to the alignment torque. Three-dimensionally trapped elongated objects either align with their long axis parallel or perpendicular to the beam axis depending on their size. The shape-dependent torque can exceed the torque due to birefringence, and can align negative uniaxial particles with their optic axis parallel to the electric field, allowing an application of optical torque about the beam axis.

Gold nanoparticle mediated formation of aligned nanotube composite films.

We report on the formation of highly anisotropic nanotube composite materials, made by the attachment of gold nanoparticles to the surface of the single-walled carbon nanotubes, followed by preparation of an aligned composite film by compression in a Langmuir-Blodgett trough. The gold is attached in a one-step sonication procedure. The gold-modified nanotube material forms a stable suspension in toluene and has been characterized by atomic force and scanning force microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. The aligned films have highly anisotropic electrical properties, with a factor of approximately 3000 difference in the conductivity between the aligned and perpendicular directions.

Enhanced nucleation, growth rate, and dopant incorporation in ZnO nanowires.

Pure and Co-doped ZnO nanowire arrays were grown on polished silicon substrates with high rates via an electrochemical technique. A negative potential applied to the substrate not only enhances the nucleation density on polished substrates more than 4 orders of magnitude but also increases the growth rate by 35 times over that obtained in the absence of the potential. Furthermore, incorporation of metallic dopants in ZnO nanowires was demonstrated in the low-temperature process. This fast growth technique provides a route to fabrication of low-cost highly oriented ZnO nanowires on polished substrate for industrial applications.