G J Parker

Complete photonic bandgaps in 12-fold symmetric quasicrystals

Photonic crystals are attracting current interest for a variety of reasons, such as their ability to inhibit the spontaneous emission of light. This and related properties arise from the formation of photonic bandgaps, whereby multiple scattering of photons by lattices of periodically varying refractive indices acts to prevent the propagation of electromagnetic waves having certain wavelengths. One route to forming photonic crystals is to etch two-dimensional periodic lattices of vertical air holes into dielectric slab waveguides. Such structures can show complete photonic bandgaps, but only for large-diameter air holes in materials of high refractive index (such as gallium arsenide, n = 3.69), which unfortunately leads to significantly reduced optical transmission when combined with optical fibres of low refractive index. It has been suggested that quasicrystalline (rather than periodic) lattices can also possess photonic bandgaps. Here we demonstrate this concept experimentally and show that it enables complete photonic bandgaps—non-directional and for any polarization—to be realized with small air holes in silicon nitride (n = 2.02), and even glass (n = 1.45). These properties make photonic quasicrystals promising for application in a range of optical devices.

Guided mode analysis, and fabrication of a 2-dimensional visible photonic band structure confined within a planar semiconductor waveguide

Abstract In this paper, we present a graphical method for calculating the guided Bloch modes supported by a 2-dimensional photonic lattice etched into a planar waveguide structure. We combine a 3-dimensional plane wave analysis with standard waveguide theory to determine the broadband transmission characteristics of waveguiding PBG devices away from the primary band gap region, and demonstrate the suppression of normal waveguide modes within the band gap region. In addition, we demonstrate the successful fabrication of a waveguiding photonic band structure with a polarisation dependent band gap centred around 632.8 nm in the visible region of the spectrum. This nano-structure consists of a triangular array of air holes etched through the cladding and core of a silicon nitride waveguide.

Near-field optical microscopy of thin photonic crystal films

Near-field optical microscopy is used to image the light propagating in a Si3N4-on-silica film with a strip of deeply etched submicron holes in the center. Fringe patterns created by interference between incident and diffracted guided modes are observed in the smooth waveguide surrounding the large air-hole photonic film. Observation of the fields in the photonic crystal regions indicates that out-of-plane scattering at the smooth/periodic transitions and leakage caused by grating coupling play major roles. The effects of different air-hole sizes and lattice pitches are also explored.

High aspect ratio submicron silicon pillars fabricated by photoassisted electrochemical etching and oxidation

A technique for fabricating submicron free‐standing silicon pillars has been developed. The silicon pillars have a high packing density, and aspect ratios over 50:1 can easily be achieved. Photoassisted electrochemical etching in hydrofluoric acid is used to etch deep macropores in n‐type silicon wafers which have been patterned by standard photolithography. The regular macropores can be used for fabricating photonic band‐gap structures. The bulk silicon remaining between the close‐packed macropores is oxidized. Free‐standing pillars are then formed by subsequently wet etching the silicon dioxide. The pillars are the initial structures for forming quantum wires using further oxidation and etch steps.

Visible photonic band gap engineering in silicon nitride waveguides

We demonstrate experimentally the tuning of complete photonic band gaps in patterned silicon nitride waveguides. Transmission measurements were performed using an ultrabroadband high-brightness white light laser continuum, extracting extinction ratios as low as 10–4 in the gap regions. Angle-resolved measurements show the perfect alignment of the gap around the -J direction.

Experimental investigation of photonic crystal waveguide devices and line-defect waveguide bends

Photonic crystal waveguide devices incorporating line-defect waveguide bends have been fabricated. In this paper we present preliminary experimental analysis of these structures. Although evidence of photonic band-gap effects are observed in the spectra, transmission efficiency was found to be extremely low due to significant up-scattering losses from the holes. In order to quantify this loss mechanism, a detailed experimental and theoretical analysis of scattering effects in regular photonic crystal waveguide devices with band gaps at visible wavelength is presented. Field profiles in line defect structures are analysed using a FDTD (finite difference time domain) method.

Fabrication of high aspect ratio silicon microstructures by anodic etching

We describe a refinement of the anodization process commonly used for the formation of porous silicon, which allows the fabrication of arrays of very high aspect ratio sub-micron pores and free-standing pillars. These structures are shown to possess a wide photonic band gap in the near infra red.

Complete and absolute photonic bandgaps in highly symmetric photonic quasicrystals embedded in low refractive index materials

It is firmly established that periodic lattice structures can support photonic bandgaps (PBG). However, complete and absolute photonic bandgaps (CAPBG) have only been achieved in high dielectric constant mediums such as GaAs (e=13.6). An artificial quasiperiodic photonic crystal based on the random square-triangle tiling system was designed and fabricated. The photonic quasicrystal possesses 12-fold symmetry and was analysed using a finite difference time domain (FDTD) approach. High orders of symmetry in photonic quasicrystals have been shown to provide isotropic bandgaps across all the directions of propagation of light. As an outcome of these properties, this new class of photonic quasicrystal has been shown, for the first time, to possess a secondary non-directional CAPBG for a relatively low index material, silicon nitride (e=4.08). These materials are compatible with integrated optical technologies. This allows the fabrication of efficient integrated optical PBG devices such as WDM filters and multiplexers to become a real possibility.

High-Aspect ratio silicon pillars fabricated by electrochemical etching and oxidation of macroporous silicon

Abstract A technique for fabricating sub-micron free-standing silicon pillars has been developed. The silicon pillars have a high packing density, and aspect ratios over 50:1 can easily be achieved. Silicon wafers of 3–5 Ω cm resistivity is patterned with regular windows on a silicon nitride layer by conventional photolithography. Inverted pyramids are formed within the windows after orientation-dependent KOH etching. During photo-assisted electrochemical etching in 2.5% ethanoic hydrofluoric acid, the photo-generated holes are collected at the sharp tips of the inverted pyramids where the directional etching of macropores is localized. The bulk silicon left between the close-packed macropores is oxidised. Free-standing pillars are then formed by subsequent wet etching of the silicon dioxide.

Ultrabroadband transmission measurements on waveguides of silicon-rich silicon dioxide

We report ultrabroadband measurements on waveguides of photoluminescent silicon-rich silicon dioxide produced by plasma enhanced chemical vapor deposition. Material absorption below 700 nm and waveguide loss above 1300 nm leave a broad spectral region of good transmission properties, which overlaps with the photoluminescence spectrum of the core material. Proposed mechanisms for the material absorption and photoluminescence are discussed based on our findings.