M.C. Netti

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.

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.

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.

Visible-wavelength super-refraction in photonic crystal superprisms

We demonstrate the fabrication of superprism devices in photonic crystal waveguides with excellent transmission through 600 rows of 160nm diameter holes. Broadband spectral and angular measurements allow mapping of the chromatic refractivity. This shows the ability of such devices to super-refract by more than 1°/nm close to the principal band gaps,10× more than equivalent gratings, and 100× more than equivalent prisms. Simple theories based on plane-wave models give excellent agreement with these results.

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.

Neodymium-doped tantalum pentoxide waveguide lasers

The fabrication, spectroscopic properties, and laser performance of Nd/sup 3+/-doped Ta/sub 2/O/sub 5/ channel waveguide lasers are described. Lasing is obtained at both 1.066 and 1.375 /spl mu/m with threshold pump powers as low as 2.7 mW. The rib waveguides are reactive-ion-etched into Nd:Ta/sub 2/O/sub 5/ layers formed by reactive magnetron sputtering. These high-index low-loss rare-earth-doped waveguides are fabricated on silicon substrates and offer the potential for integration with photonic crystal structures for compact optical circuits.

Separation of Photonic Crystal Waveguides Modes using Femtosecond Time-of-Flight

We demonstrate that ultrabroadband ultrashort-pulse white light supercontinua can be used to track the group velocity of photons in optical waveguides using a Kerr gate technique. Results on silicon nitride slab waveguides show both polarization birefringence and multimode dispersion, which vanish at critical wavelengths. When photonic crystals are embedded in the waveguides, the higher order modes are excited within the band-gap region, demonstrating the need to control their dispersion to make effective use of photonic crystal waveguide devices.

Self refractive non-linearities in chalcogenide based glasses

Abstract We report third order non-linear absorption and refraction measurements at 1.20 and 1.52 μm on selected gallium–Lanthanum sulfide-based glasses (Ga:La:S) showing negligible non-linear absorption and a refractive non-linearity close to one hundred times that of SiO 2 . Their potential use in telecommunication as base materials for all-optical switching practical devices is evaluated resulting in large figures of merit. The addition of a glass modifier to the Ga:La:S matrix has improved thermal and optical properties, resulting in ease of fibre drawing. The non-linear optical response of this new variant of the Ga:La:S family is studied.

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.

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.