Interleavers

The chapter describes principles, analysis, design, properties, and implementations of optical frequency (or wavelength) interleavers. The emphasis is on finite impulse response devices based on cascaded Mach-Zehnder-type filter elements with carefully designed coupling ratios, the so-called resonant couplers. Another important class that is discussed is the infinite impulse response type, based on e.g. Fabry-Perot, Gires-Tournois, or ring resonators

Design of waveguides, bends and splitters in photonic crystal slabs with hexagonal holes in a triangular lattice

Waveguides in photonic crystal slabs (PCS) can be obtained by omitting a row of holes (W1-waveguides). In general the propagation properties in such waveguides suffer from the unavoidable periodic sidewall corrugation caused by the remaining parts of the crystal. The corrugation acts as a Bragg reflector, causing the occurrence of so-called mini stopbands in the transmission of the waveguide. The effect is quite strong in PCS with circular holes, but it can be significantly reduced if correctly oriented hexagonal holes are used. This so-called hexagon-type PCS allows the design of waveguides, bends and splitters having a relatively high group velocity and a wide transmission window in the PCS stopband for modes having their magnetic field oriented mainly perpendicular to the slab

A hysteresis model for an orthogonal thin-film magnetometer

The operation of a ferromagnetic thin-film magnetometer using the anisotropic magnetoresistance effect in a permalloy film is discussed. Measurements showed the presence of a hysteresis effect not predicted by available models. It is shown that the sensitivity of the magnetometer is predicted by applying F.S. Greene and R.B. Yarbrough's (1970) orthogonal susceptibility model, and that the hysteresis can be explained by assuming dispersion in the magnitude of anisotropy. The orthogonal susceptibility model must be evaluated numerically, accounting for a finite driving field. The effect of an inhomogeneous demagnetizing field in the film is discussed in relation to magnitude dispersion of the anisotrop

Focused Ion Beam Nano-structuring for Applications in Photonics

To date, nano- and micro-structuring has commonly been implemented by a combination of specifically optimized processes of electron-beam lithography and reactive ion etching, thus limiting the range of materials that can be structured to only a few. In this talk we will introduce focused ion beam (FIB) milling as an emerging technology that enables fast, reliable and well-controlled nanometer-size feature definition. Since the method involves physical removal of material by a beam of ions, the technique can be adapted and optimized almost for any material system. We will introduce the technique and discuss the basic application areas. In particular, we have investigated the impact of parameters such as ion beam current, dwell time, scanning strategy, and dielectric charging. We will discuss strategies to optimize the nano-structuring processes that are strongly dependent on the geometry of the desired structure. Finally, we will report our recent results on utilization and optimization of the focused ion beam technique for fabrication of nano-structures in integrated photonic devices on several material platforms such as Si, Al2O3, Y2O3, Sc2O3, and KY(WO4)2

Focused ion beam milling strategy for sub-micrometre holes in silicon

Focused ion beam (FIB) milling can be used as a tool to fabricate structures with sub-micrometer details. The slab material can be silicon, for example, which can then be used as a mould for nano-imprint lithography, or in silicon on insulator (SOI) layer configuration suitable for photonic applications. In the latter, additional effort has to be taken to prevent high FIB induced losses, due to ion implantation and material crystal damage. Perfectly vertical sidewalls are, in principle, required for photonic crystal applications to guarantee low-loss propagation; sidewall angles of 5 degrees can already induce a 8 dB/mm propagation loss. We report on optimization of the sidewall angle (FIB) fabricated submicron diameter holes. Our best case results show that sidewall angles as small as 1.5 degree are possible in Si membranes and 5 degree for (bulk) Si and SOI by applying larger doses and using a spiral scan method

Focused-ion-beam processing for photonics

Although focused ion beam (FIB) processing is a well-developed technology for many applications in electronics and physics, it has found limited application to photonics. Due to its very high spatial resolution in the order of 10 nm, and its ability to mill almost any material, it seems to have a good potential for fabricating or modifying nanophotonic structures such as photonic crystals. The two main issues are FIB-induced optical loss, e.g., due to implantation of gallium ions, and the definition of vertical sidewalls, which is affected by redeposition effects. The severity of the loss problem was found to depend on the base material, silicon being rather sensitive to this effect. The optical loss can be significantly reduced by annealing the processed samples. Changing the scanning strategy for the ion beam can both reduce the impact of gallium implantation and the redeposition effect

Characterisation of slow light in a waveguide grating

A grating was defined in a silicon nitride waveguide, using a combination of both conventional lithography and laser interference lithography. The structure was optically characterized in the 1520 – 1560 nm wavelength range by combining transmission measurements with the analysis of local out-of-plane scattered light, using a high-resolution infrared camera. From the measured power enhancement of the first Bloch-mode resonance above the long-wavelength band edge we estimated a Q > 10^4 and a group velocity of < 0.1 c