Justin R. Sparks

Zinc Selenide Optical Fibers

Semiconductor waveguide fabrication for photonics applications is usually performed in a planar geometry. However, over the past decade a new field of semiconductor-based optical fiber devices has emerged. The drawing of soft chalcogenide semiconductor glasses together with low melting point metals allows for meters-long distributed photoconductive detectors, for example.[1,2] Crystalline unary semiconductors (e.g., Si, Ge) have been chemically deposited at high pressure into silica capillaries,[3,4] allowing the optical and electronic properties of these materials to be exploited for applications such as all-fiber optoelectronics.[5-7] In contrast to planar rib and ridge waveguides with rectilinear cross sections that generally give rise to polarization dependence, the cylindrical fiber waveguides have the advantage of a circular, polarization-independent cross section. Furthermore, the fiber pores, and thus the wires deposited in them, are exceptionally smooth[8] with extremely uniform diameter over their entire length. The high-pressure chemical vapor deposition (HPCVD) technique is simple, low cost, and flexible so that it can be modified to fill a range of capillaries with differing core dimensions, while high production rates can be obtained by parallel fabrication of multiple fibers in a single deposition. It can also be extended to fill the large number of micro- and nanoscale pores in microstructured optical fibers (MOFs), providing additional geometrical design flexibility to enhance the potential application base of the fiber devices.[9] Semiconductor fibers fabricated via HPCVD in silica pores also retain the inherent characteristics of silica fibers, including their robustness and compatibility with existing optical fiber infrastructure, thus presenting considerable advantages over fibers based on multicomponent soft glasses.

Low loss silicon fibers for photonics applications

Silicon fibers are fabricated using a high pressure chemical deposition technique to deposit the semiconductor material inside a silica capillary. The silicon is deposited in an amorphous state into pure silica capillaries and can be crystallized to polysilicon after the deposition via a high temperature anneal. Optical transmission measurements of various amorphous and polycrystalline core materials were performed in order to determine their linear losses. Incorporating silicon functionality inside the fiber geometry opens up new possibilities for the next generation of integrated silicon photonics devices.

Silicon p‐i‐n Junction Fibers

Flexible Si p-i-n junction fibers made by high pressure chemical vapor deposition offer new opportunities in textile photovoltaics and optoelectronics, as exemplified by their photovoltaic properties, gigahertz bandwidth for photodetection, and ability to waveguide light.

Extreme electronic bandgap modification in laser-crystallized silicon optical fibres

For decades now, silicon has been the workhorse of the microelectronics revolution and a key enabler of the information age. Owing to its excellent optical properties in the near- and mid-infrared, silicon is now promising to have a similar impact on photonics. The ability to incorporate both optical and electronic functionality in a single material offers the tantalizing prospect of amplifying, modulating and detecting light within a monolithic platform. However, a direct consequence of silicons transparency is that it cannot be used to detect light at telecommunications wavelengths. Here, we report on a laser processing technique developed for our silicon fibre technology through which we can modify the electronic band structure of the semiconductor material as it is crystallized. The unique fibre geometry in which the silicon core is confined within a silica cladding allows large anisotropic stresses to be set into the crystalline material so that the size of the bandgap can be engineered. We demonstrate extreme bandgap reductions from 1.11 eV down to 0.59 eV, enabling optical detection out to 2,100 nm.

Large mode area silicon microstructured fiber with robust dual mode guidance

A silicon microstructured fiber has been designed and fabricated using a pure silica photonic bandgap guiding fiber as a 3D template for materials deposition. The resulting silicon fiber has a micron sized core but with a small core-cladding index contrast so that it only supports two guided modes. It will be shown that by using the microstructured template this fiber exhibits a number of similar guiding properties to the more traditional index guiding air-silica structures. The large mode areas and low optical losses measured for the silicon microstructured fiber demonstrate its potential to be integrated with existing fiber infrastructures.

All-optical modulation using two-photon absorption in silicon core optical fibers

All-optical modulation based on degenerate and non-degenerate two-photon absorption (TPA) is demonstrated within a hydrogenated amorphous silicon core optical fiber. The nonlinear absorption strength is determined by comparing the results of pump-probe experiments with numerical simulations of the coupled propagation equations. Subpicosecond modulation is achieved with an extinction ratio of more than 4 dB at telecommunications wavelengths, indicating the potential for these fibers to find use in high speed signal processing applications.

Mid-infrared transmission properties of amorphous germanium optical fibers

Germanium optical fibers have been fabricated using a high pressure chemical deposition technique to deposit the semiconductor material inside a silica capillary. The amorphous germanium core material has a small percentage of hydrogen that saturates the dangling bonds to reduce absorption loss. Optical transmission measurements were performed to determine the linear losses over a broad mid-infrared wavelength range with the lowest loss recorded at 10.6 µm. The extended transmission range measured in the germanium fibers demonstrates their potential for use in mid-infrared applications.

Confined high-pressure chemical deposition of hydrogenated amorphous silicon

Hydrogenated amorphous silicon (a-Si:H) is one of the most technologically important semiconductors. The challenge in producing it from SiH(4) precursor is to overcome a significant kinetic barrier to decomposition at a low enough temperature to allow for hydrogen incorporation into a deposited film. The use of high precursor concentrations is one possible means to increase reaction rates at low enough temperatures, but in conventional reactors such an approach produces large numbers of homogeneously nucleated particles in the gas phase, rather than the desired heterogeneous deposition on a surface. We report that deposition in confined micro-/nanoreactors overcomes this difficulty, allowing for the use of silane concentrations many orders of magnitude higher than conventionally employed while still realizing well-developed films. a-Si:H micro-/nanowires can be deposited in this way in extreme aspect ratio, small-diameter optical fiber capillary templates. The semiconductor materials deposited have ~0.5 atom% hydrogen with passivated dangling bonds and good electronic properties. They should be suitable for a wide range of photonic and electronic applications such as nonlinear optical fibers and solar cells.

Tapered silicon optical fibers

The tapering of silicon optical fibers is demonstrated using a fusion splicer. The silicon fibers are fabricated using a high pressure chemical deposition technique to deposit an amorphous silicon core inside a silica capillary and the tapering is performed in a separate post-process. Optical and material characterization has revealed a smooth transition region leading to a uniform tapered waist that are both simultaneously crystallized to yield a solid polysilicon core. The strong mode confinement and low taper loss measured in the silicon fibers verifies this tapering approach for the fabrication of structures with nanoscale core dimensions.

Polycrystalline silicon optical fibers with atomically smooth surfaces

We investigate the surface roughness of polycrystalline silicon core optical fibers fabricated using a high-pressure chemical deposition technique. By measuring the optical transmission of two fibers with different core sizes, we will show that scattering from the core-cladding interface has a negligible effect on the losses. A Zemetrics ZeScope three-dimensional optical profiler has been used to directly measure the surface of the core material, confirming a roughness of only ~0.1 nm. The ability to fabricate low-loss polysilicon optical fibers with ultrasmooth cores scalable to submicrometer dimensions should establish their use in a range of nonlinear optical applications.