Rene M. de Ridder

Focused ion beam scan routine, dwell time and dose optimization for submicrometre period planar photonic crystal components and stamps in silicon

Focused ion beam (FIB) milling is receiving increasing attention for nanostructuring in silicon (Si). These structures can for example be used for photonic crystal structures in a silicon-on-insulator (SOI) configuration or for moulds which can have various applications in combination with imprint technologies. However, FIB fabrication of submicrometre holes having perfectly vertical sidewalls is still challenging due to the redeposition effect in Si. In this study we show how the scan routine of the ion beam can be used as a sidewall optimization parameter. The experiments have been performed in Si and SOI. Furthermore, we show that sidewall angles as small as 1.5◦ are possible in Si membranes using a spiral scan method. We investigate the effect of the dose, loop number and dwell time on the sidewall angle,interhole milling and total milling depth by studying the milling of single and multiple holes into a crystal. We show that the sidewall angles can be as small as 5◦ in (bulk) Si and SOI when applying a larger dose. Finally, we found that a relatively large dwell time of 1 ms and a small loop number is favourable for obtaining vertical sidewalls. By comparing the results with those obtained by others, we conclude that the number of loops at a fixed dose per hole is the parameter that determines the sidewall angle and not the dwell time by itself.

Design and fabrication of electro-optic polymer modulators and switches

Electro-optic (EO) polymers are interesting materials for realising active functions in integrated optics devices, since they can have relatively high EO coefficients and they can be easily combined or integrated with several (passive) materials. EO interaction can be exploited for obtaining modulation or switching in polymer-based devices using several principles (e.g. Mach–Zehnder, Digital Optical Switch or tuned coupling to surface plasmons), which have been investigated and tested using a number of different materials. We compare these principles in view of several aspects of practical importance: realistic values of EO coefficients and wavelength-dependent attenuation, optical, electrical and chemical compatibility of substrate, guiding and cladding layers, channel definition by etching (inverted) ridge waveguides or by photo-bleaching, local and global poling methods, polarisation-dependence, and the design of efficient high-bandwidth travelling wave electrodes. In most of the investigated devices, we employ a waveguiding structure based on silicon and its oxynitrides, exploiting the fact that the refractive index of silicon oxynitride can be accurately adjusted over a wide range by adjusting its composition. We will discuss the practical difficulties encountered and show the obtained results with phase- and intensity modulators and switches.

Spectral domain optical coherence tomography imaging with an integrated optics spectrometer

We designed and fabricated an arrayed-waveguide grating (AWG) in silicon oxynitride as a spectrometer for spectral domain optical coherence tomography (SD-OCT). The AWG has a footprint of only 3.0 cm × 2.5 cm, operates at a center wavelength of 1300 nm, and has 78 nm free spectral range. OCT measurements are performed that demonstrate imaging up to a maximum depth of 1 mm with an axial resolution of 19 μm, both in agreement with the AWG design parameters. Using the AWG spectrometer combined with a fiber-based SD-OCT system, we demonstrate cross-sectional OCT imaging of a multilayered scattering phantom.

Integrated approach to laser delivery and confocal signal detection

We present an on-chip arrayed waveguide grating (AWG) sensor based on the confocal arrangement of two AWGs, one acting as focusing illuminator and one as signal collector. The chip can be close to, or in direct contact with, a sample, e.g., biological tissue, without the need of external optics. The collection efficiency of our device can be more than 1 order of magnitude higher than that of a standard AWG, in which light is collected by one input channel. Experimental results on the collection efficiency and volume are presented, together with a demonstration of multiwavelength imaging.

Laser interference lithography with highly accurate interferometric alignment

Three dimensional photonic crystals, e.g. for obtaining the so-called woodpile structure, can, among others, be fabricated by vertical stacking of multiple gratings. One of the requirements for obtaining a full photonic bandgap in such a photonic crystal is an accurate angular and lateral alignment of the successive gratings. Using laser interference lithography at 266 nm wavelength, we fabricated gratings in silicon with periods down to 300 nm. We present a method for aligning further grating exposures with respect to this grating with a 0.001 degree angular and a few nanometers lateral resolution.

Improved arrayed-waveguide-grating layout avoiding systematic phase errors.

We present a detailed description of an improved arrayed-waveguide-grating (AWG) layout for both, low and high diffraction orders. The novel layout presents identical bends across the entire array; in this way systematic phase errors arising from different bends that are inherent to conventional AWG designs are completely eliminated. In addition, for high-order AWGs our design results in more than 50% reduction of the occupied area on the wafer. We present an experimental characterization of a low-order device fabricated according to this geometry. The device has a resolution of 5.5 nm, low intrinsic losses (< 2 dB) in the wavelength region of interest for the application, and is polarization insensitive over a wide spectral range of 215 nm.

Focused ion beam nano-structuring of

In this work, we report on utilization and optimization of the focused-ion-beam technique for the fabrication of nanostructures on Al2O3 waveguides for applications in integrated photonic devices. In particular, the investigation of the effects of parameters such as ion-beam current, dwell time, and scanning strategy is addressed. As a result of optimizing these parameters, excellent quality gratings with smooth and uniform sidewalls are reported. The effects of redeposition are minimized and good control of the nanostructuring process is reported. The effect of Ga+ ion implantation during the milling process on the optical performance of the devices is discussed.

Al_2O_3

We propose a Y splitter in a two-dimensional photonic crystal with a triangular lattice of air holes suitable for mechano-optical switching. Full switching between the two output ports can be achieved at a certain wavelength owing to the presence of a resonant central cavity with well-chosen symmetry. Experimental results confirm the operation of the device and its potential as an ultracompact mechanical switch.

dielectric layers for photonic applications

Several illustrations are given of the applicability of the Finite-Difference Time-Domain (FDTD) method to photonic crystal structures. An intuitive method for calculating the band gap of a two-dimensional photonic crystal is demonstrated. For waveguides in such a crystal, the dispersion and the coupling efficiency to a conventional dielectric waveguide are determined. The time-evolution of the field distribution in systems consisting of a cavity coupled to a crystal waveguide, as well as the transfer function of such a system are calculated. Diffraction losses, which can severely affect quasi two-dimensional photonic crystal structures, are modeled using a quasi one-dimensional structure, a dielectric slab waveguide in which a deep grating (a quasi-one-dimensional photonic crystal) of finite length has been etched. After calculating the reflection and transmission spectra, the diffraction loss spectrum is easily determined. The simultaneous availability of both the electromagnetic field distribution thro...

Photonic Crystal Cavity-Based Y Splitter for Mechano-Optical Switching

We demonstrate a fabrication procedure for the direct integration of micro-ball lenses on planar integrated optical channel waveguide chips with the aim to reduce the divergence of light that arises from the waveguide in both horizontal and vertical directions. Fabrication of the lenses is based on photoresist reflow which is a procedure that allows for the use of photolithography for careful alignment of the lenses with respect to the waveguides and enables mass production. We present in detail the design and fabrication procedures. Optical characterization of the fabricated micro-ball lenses demonstrates a good performance in terms of beam-size reduction and beam shape. The beam half divergence angle of 1544 nm light is reduced from 12.4 ° to 1.85 °.