Todd D. Day

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.

Ultrafast optical control using the Kerr nonlinearity in hydrogenated amorphous silicon microcylindrical resonators

Microresonators are ideal systems for probing nonlinear phenomena at low thresholds due to their small mode volumes and high quality (Q) factors. As such, they have found use both for fundamental studies of light-matter interactions as well as for applications in areas ranging from telecommunications to medicine. In particular, semiconductor-based resonators with large Kerr nonlinearities have great potential for high speed, low power all-optical processing. Here we present experiments to characterize the size of the Kerr induced resonance wavelength shifting in a hydrogenated amorphous silicon resonator and demonstrate its potential for ultrafast all-optical modulation and switching. Large wavelength shifts are observed for low pump powers due to the high nonlinearity of the amorphous silicon material and the strong mode confinement in the microcylindrical resonator. The threshold energy for switching is less than a picojoule, representing a significant step towards advantageous low power silicon-based photonic technologies.

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.

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.

Ultrafast wavelength conversion via cross-phase modulation in hydrogenated amorphous silicon optical fibers

We present a characterization of the spectral modulation and wavelength shifting induced via cross-phase modulation (XPM) in a hydrogenated amorphous silicon (a-Si:H) core optical fiber. Pump-probe experiments using picosecond and femtosecond signal pulses are shown to be in good agreement with numerical simulations of the coupled nonlinear propagation equations. The large 10nm red-shifts obtained with the femtosecond probe pulses are attributed to the high Kerr nonlinearity of the a-Si:H material. Extinction ratios as high as 12 dB are measured for the conversion process at telecommunications wavelengths, indicating the potential for high-speed nonlinear optical control in a-Si:H fibers and waveguides.

Four-wave mixing and octave-spanning supercontinuum generation in a small core hydrogenated amorphous silicon fiber pumped in the mid-infrared

An octave-spanning supercontinuum is generated in a hydrogenated amorphous silicon core fiber when pumped in the mid-infrared regime. The broadband wavelength conversion which extends from the edge of the telecommunications band into the mid-infrared (1.64-3.37 μm) is generated by four-wave mixing (FWM) and subsequent pulse break-up, facilitated by the high material nonlinear figure of merit and the anomalous dispersion of the relatively small 1.7 μm diameter core fiber. The FWM sidebands and corresponding supercontinuum can be tuned through the pump parameters, and show good agreement with the predicted phase-matching curves for the fiber.

Nonlinear transmission properties of hydrogenated amorphous silicon core fibers towards the mid-infrared regime.

The nonlinear transmission properties of hydrogenated amorphous silicon (a-Si:H) core fibers are characterized from the near-infrared up to the edge of the mid-infrared regime. The results show that this material exhibits linear losses on the order of a few dB/cm, or less, over the entire wavelength range, decreasing down to a value of 0.29 dB/cm at 2.7μm, and negligible nonlinear losses beyond the two-photon absorption (TPA) edge ~ 1.7μm. By measuring the dispersion of the nonlinear Kerr and TPA parameters we have found that the nonlinear figure of merit (FOM(NL)) increases dramatically over this region, with FOM(NL) > 20 around 2μm and above. This characterization demonstrates the potential for a-Si:H fibers and waveguides to find use in nonlinear applications extending beyond telecoms and into the mid-infrared regime.

Selective Semiconductor Filling of Microstructured Optical Fibers

A novel selective filling technique has been developed for the patterning of semiconductor materials in microstructured optical fibers (MOFs) based on waveguide curing of epoxy filled capillary holes. The technique is compatible with the high pressures required for the semiconductor deposition and allows for quick and convenient selective filling, or coating, of complex designs in a range of MOF hole sizes and spacings. A variety of semiconductor filled MOFs have been demonstrated including the patterning of different materials within selected holes. The ability to selectively fill MOFs with multiple semiconductor materials is a step towards developing arrays of both passive and active all-fiber optoelectronic devices.

High Pressure Chemical Vapor Deposition of Hydrogenated Amorphous Silicon Films and Solar Cells

Thin films of hydrogenated amorphous silicon can be produced at MPa pressures from silane without the use of plasma at temperatures as low as 345 °C. High pressure chemical vapor deposition may open a new way to low cost deposition of amorphous silicon solar cells and other thin film structures over very large areas in very compact, simple reactors.