de René M. Ridder

Large area photonic crystal slabs for visible light with waveguiding defect structures: Fabrication with focused ion beam assisted laser interference lithography

Extended photonic crystal slabs with light-guiding defects have been created by a combination of laser interference lithography (LIL) and local focused ion beam (FIB) assisted deposition. Large area, highly uniform photonic crystal slabs for visible light are thus made possible. The Figure shows a freestanding Si3N4-air photonic crystal with a light- guiding defect line running along the center of the slab (total length = 1 mm).

Design and Application of Compact and Highly Tolerant Polarization-Independent Waveguides

In this paper, the design, fabrication, and application of a highly tolerant polarization-independent optical-waveguide structure suited for operation in the third communication window is presented. The waveguide structure has been optimized toward minimized sensitivity to technological tolerances and low fabrication complexity. The tolerance analysis has been based on the typical processing tolerances of the widely applied silicon-oxynitride technology, being plusmn3times10 -4 in refractive index, plusmn1% in thickness, and plusmn0.1 mum in channel width. The optimized waveguide design fulfills the criterion of a channel birefringence within 5times10-5, including processing tolerance. It also enables a fiber-to-chip coupling loss below 1 dB/facet and is suited for the realization of low-loss bends with a radius down to 600 mum. Based on this waveguide design, a passband-flattened optical wavelength filter with 50-GHz free spectral range has been realized and tested. The measured TE-TM shift of 0.03 nm confirms the polarization dependence of the optical waveguides being as low as 3times10-5

Modal fields calculation using the finite difference beam propagation method

A method is described to construct modal fields for an arbitrary one- or two-dimensional refractive index structure. An arbitrary starting field is propagated along a complex axis using the slowly varying envelope approximation (SVEA). By choosing suitable values for the step-size, one mode is maximally increased in amplitude on propagating, until convergence has been obtained. For the calculation of the next mode, the mode just found is filtered out, and the procedure starts again. The method is tested for one-dimensional refractive index structures, both for nonabsorbing and for absorbing structures, and is shown to give fast convergence. >

Micromechanically tuned ring resonator in silicon on insulator

Monolithic integration of a micromechanical cantilever with an optical ring resonator in silicon on insulator is demonstrated. The ring is tuned over a 120 pm wavelength range by applying 9 V, without affecting its Q-factor.

High-Resolution Measurement of Resonant Wave Patterns by Perturbing the Evanescent Field Using a Nanosized Probe in a Transmission Scanning Near-Field Optical Microscopy Configuration

In order to model transmission scanning near-field optical microscopy (T-SNOM) experiments, we study the interaction between a nanosized atomic-force-microscopy-type probe and the optical field in a microcavity (MC) at or near resonance. Using a 2-D cross-sectional model of an experimentally studied photonic crystal MC, we have simulated the T-SNOM method by scanning a probe over the surface while monitoring the transmitted and reflected power. The simulations were performed for two probe materials: silicon and silicon nitride. From the probe-induced change in the transmission and reflection spectra, a wavelength shift was extracted. A shift almost proportional to the local field intensity was found if the resonator was excited just below a resonance wavelength. However, at the spots of highest interaction, we observed that besides the desired resonance wavelength shift, there was an increase in scattering. Furthermore, by moving the probe at such a spot in the vertical direction to a height of approximately 0.5, a 5% increase in transmission can be established because the antiresonant condition is satisfied. Finally, a 2-D top view simulation is presented of the experimentally studied T-SNOM method, which shows a remarkably good correspondence in intensity profile, except for the exact location of the high-interaction spots.

Dual-Frequency Distributed Feedback Laser With Optical Frequency Locked Loop for Stable Microwave Signal Generation

We demonstrate the photonic generation of microwave signals by using a dual-frequency distributed feedback waveguide laser in ytterbium-doped aluminum oxide (Al2O3:Yb3+). An optical frequency locked loop (OFLL) was implemented to stabilize the center frequency of the microwave signal. This approach resulted in a microwave frequency at ~14 GHz with a phase noise of -75 dBc/Hz at 1 MHz offset from the center frequency. The frequency stability of the photonic microwave signal has an Allan deviation of more than 1 × 10-10 for an averaging time of 1000 s. The combination of the dual-frequency laser and the OFLL scheme holds great potential for the photonic generation and distribution of highly stable microwave or millimeter-wave signals.

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.

Excitation and Light Collection From Highly Scattering Media With Integrated Waveguides

We investigate excitation and light collection from a scattering medium by an integrated waveguide probe. A probe with one excitation and eight collector waveguides is fabricated in silicon oxynitride technology. Experiments are performed on light collection from a highly scattering water suspension of latex nanospheres. By use of a Monte Carlo model, the propagation of light through highly scattering media is simulated and good agreement with the experimental data is found.

Polarization-Independent Enhanced-Resolution Arrayed-Waveguide Grating Used in Spectral-Domain Optical Low-Coherence Reflectometry

The performance of an arrayed-waveguide grating (AWG) as an integrated spectrometer in spectral-domain optical low-coherence reflectometry (SD-OLCR) is significantly improved. By removing the output waveguides of the AWG, the depth range is enhanced from 1 to 3.3 mm at 800 nm and 4.6 mm at 1300 nm. Periodic signal fading, that was previously observed in the sensitivity roll-off curve in depth ranging measurements, is shown to be evoked by beat-frequency generation between the two polarizations of partially polarized signal light in a birefringent AWG. By carefully controlling the polarization state-of-light, the signal fading is eliminated. As a permanent solution to this problem, a polarization-independent AWG is demonstrated, which can reduce the size and cost of OCLR and optical coherence tomography systems further by eliminating the components for polarization control.

Micro-cantilever integrated 2D photonic crystal slab waveguide for enhanced dispersion tuning

This paper presents the fabrication technology for a novel class of photonic devices which integrates silicon 2D photonic crystal (PhC) waveguides and electrostatically actuated microelectromechanical systems. Bimorph cantilevers equipped with tips that are self-aligned relative to the holes of the PhC modulate the propagation properties of the slab PhC depending on the proximity of the tips to the holes. The integrated devices have been successfully fabricated by surface micromachining techniques. Preliminary experiments with these devices have shown 80% throughput modulation using a square-wave drive signal of 0–8 V at 1 kHz.