Wico C.L. Hopman

Nano-mechanical tuning and imaging of a photonic crystal micro-cavity resonance

We show that nano-mechanical interaction using atomic force microscopy (AFM) can be used to map out mode-patterns of an optical micro-resonator with high spatial accuracy. Furthermore we demonstrate how the Q-factor and center wavelength of such resonances can be sensitively modified by both horizontal and vertical displacement of an AFM tip consisting of either Si(3)N(4) or Si material. With a silicon tip we are able to tune the resonance wavelength by 2.3 nm, and to set Q between values of 615 and zero, by expedient positioning of the AFM tip. We find full on/off switching for less than 100 nm vertical, and for 500 nm lateral displacement at the strongest resonance antinode locations.

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.

Closure of the stop-band in photonic wire Bragg gratings.

Photonic Wire Bragg Gratings, made by periodic insertion of lateral rectangular recesses into photonic wires in silicon-on-insulator, can provide large reflectivity with short device lengths because of their large index contrast. This type of design shows a counter-intuitive behaviour, as we demonstrate - using experimental and numerical data - that it can have low or null reflectance, even for large indentation values. We provide physical insight into this phenomenon by developing a model based on Bloch mode theory, and are able to find an analytical expression for the frequency at which the grating does not sustain the stop-band. Finally we demonstrate that the stop-band closing effect is a general phenomenon that may occur in various types of periodic device that can be modeled as transmission line structures.

Fabrication and characterization of high-quality uniform and apodized Si/sub 3/N/sub 4/ waveguide gratings using laser interference lithography

A method is presented for fabricating high-quality ridge waveguide gratings by combining conventional mask lithography with laser interference lithography. The method, which allows for apodization functions modulating both amplitude and phase of the grating is demonstrated by fabricating a grating that is chirped by width-variation of the grated ridge waveguide. The structure was optically characterized using both an end-fire and an infrared camera setup to measure the transmission and to map and quantify the power scattered out of the grating, respectively. For a uniform grating, we found a Q value of ~8000 for the resonance peak near the lower wavelength band edge, which was almost completely suppressed after apodization

Modeling and Experimental Verification of the Dynamic Interaction of an AFM-Tip With a Photonic Crystal Microcavity

We present a transmission model for estimating the effect of the atomic-force microscopy tapping tip height on a photonic crystal microcavity (MC). This model uses a fit of the measured tip-height-dependent transmission above a ¿hot spot¿ in the MC. The predicted transmission versus average tapping height is in good agreement with the values obtained from tapping mode experiments. Furthermore, we show that for the existing, nonoptimized structure, the transmission coefficient can be tuned between 0.32 and 0.8 by varying the average tapping height from 26 to 265 nm. A transmission larger than that of the undisturbed cavity at resonance was observed at specific tip locations just outside the cavity-terminating holes.

Ultracompact Optical Sensors based on high index-contrast Photonic Structures

There is a strong parallelism between electronic integrated circuits (ICs) and integrated optics. In both cases microand increasingly nano-technology is applied resulting in devices for a broad spectrum of applications: communication, data processing, sensing and others. The most striking difference is the maturity and complexity. Electronic ICs have followed Moores law for about 40 years resulting in the currently more than 100 million transistors in a single chip, whereas in photonic circuitries 10 100 functional elements per chip represent state-of-the-art results. Photonics in this respect is clearly lagging behind and will do so also in hture, as the minimum dimensions of the fimctional elements will be always in the order of the wavelen th of light, i.e. at least some hundreds of nm. But even then, F a density of 10 to lo5 functional elements per optical chip is feasible, so that the tenn Very Large Scale Integrated (VLSI) photonics1 is not an exaggeration. Optical circuits will not simple mimic electronics, but will exploit in a complementary way the unique phenomena offered by light. For example THz bandwidth amplification is straightforward in optical structures with

Characterization techniques for planar optical microresonators

Optical microresonators that are coupled to optical waveguides often behave quite similar to Fabry-Perot resonators. After summarising key properties of such resonators, three characterization methods will be discussed. The first involves the analysis of transmission and reflection spectra, from which important parameters like (waveguide) loss and coupling or reflection coefficients can be extracted. The second method is called transmission-based scanning near-field optical microscopy (T-SNOM) which allows to map out the intensity distribution inside a high-Q resonator with subwavelength resolution. For such resonators conventional SNOM suffers from inaccuracies introduced by the disturbing effect of the presence of the probe on the field distribution. T-SNOM avoids this problem by exploiting this disturbing effect. The third method is most applicable to large-size (many wavelengths across) resonators. It involves simultaneous analysis of scattered light and transmission/reflection spectra in order to relate spectral features to large-scale field distributions inside the resonator. Together, these techniques form a convenient toolbox for characterizing many different planar optical microresonators.

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

We report our recent results on an optimization study of focused ion beam (FIB) nano-structuring of Bragg gratings in

Quasi 1-dimensional photonic crystals as building block for compact integrated optical sensors

Al_2O_3

Mapping the Field Distribution of a Photonic Crystal Resonator Using Transmission SNOM

channel waveguides. By optimizing FIB milling parameters such as ion current, dwell time, loop repetitions, scanning strategy, and applying a top metal layer for reducing charging effects and improving sidewall definition, reflection gratings with smooth and uniform sidewalls were achieved.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.