Jacob Fage-Pedersen

Strained silicon as a new electro-optic material.

For decades, silicon has been the material of choice for mass fabrication of electronics. This is in contrast to photonics, where passive optical components in silicon have only recently been realized. The slow progress within silicon optoelectronics, where electronic and optical functionalities can be integrated into monolithic components based on the versatile silicon platform, is due to the limited active optical properties of silicon. Recently, however, a continuous-wave Raman silicon laser was demonstrated; if an effective modulator could also be realized in silicon, data processing and transmission could potentially be performed by all-silicon electronic and optical components. Here we have discovered that a significant linear electro-optic effect is induced in silicon by breaking the crystal symmetry. The symmetry is broken by depositing a straining layer on top of a silicon waveguide, and the induced nonlinear coefficient, χ(2) ≈ 15 pm V-1, makes it possible to realize a silicon electro-optic modulator. The strain-induced linear electro-optic effect may be used to remove a bottleneck in modern computers by replacing the electronic bus with a much faster optical alternative.

Photonic crystal waveguides with semi-slow light and tailored dispersion properties.

We demonstrate a concept for tailoring the group velocity and dispersion properties for light propagating in a planar photonic crystal waveguide. By perturbing the holes adjacent to the waveguide core it is possible to increase the useful bandwidth below the light-line and obtain a photonic crystal waveguide with either vanishing, positive, or negative group velocity dispersion and semi-slow light. We realize experimentally a silicon-on-insulator photonic crystal waveguide having nearly constant group velocity ~c(0)/34 in an 11-nm bandwidth below the silica-line.

Direct experimental and numerical determination of extremely high group indices in photonic crystal waveguides.

We report on time-of-flight experimental measurements and numerical calculations of the group-index dispersion in a photonic crystal waveguide realized in silicon-on-insulator material. Experimentally group indices higher than 230 has been observed. Numerical 2D and 3D time-domain simulations show excellent agreement with the measured data.

Advances in silica-based integrated optics

Recent advances within the realization of silica-based planar waveguide circuitry are presented. This ranges from the production methods for planar waveguides, including a novel method based on the utilization of focused UV-laser beams for direct waveguide imprinting, to the functionalities that are embedded into the glass materials and waveguide circuitry. Planar waveguide amplifiers, lasers, and the pursuit to obtain highly nonlinear materials to realize purely glass-based switches, modulators, and wavelength converters are also presented. Furthermore, microring resonators are discussed, and finally the latest results within 2-D photonic bandgap structures are reviewed.

Imprinted silicon-based nanophotonics

We demonstrate and optically characterize silicon-on-insulator based nanophotonic devices fabricated by nanoimprint lithography. In our demonstration, we have realized ordinary and topology-optimized photonic crystal waveguide structures. The topology-optimized structures require lateral pattern definition on a sub 30-nm scale in combination with a deep vertical silicon etch of the order of ~300 nm. The nanoimprint method offers a cost-efficient parallel fabrication process with state-of-the-art replication fidelity, comparable to direct electron beam writing.

High-resolution second-harmonic microscopy of poled silica waveguides

Abstract A second-harmonic scanning optical microscopy (SHSOM) apparatus operating in reflection is used for high-resolution imaging of second-order optical non-linearities (SONs) in electric-field poled silica-based waveguides. SHSOM of domain walls in a periodically poled KTiOPO 4 crystal is performed, and the spatial resolution at the pump wavelength of 790 nm is determined to be better than 0.7 μm. SHSOM images of positively poled silica waveguides were obtained for different polarization combinations of the incident pump beam and the detected second-harmonic radiation. Calibration of the SHSOM with a GaAs-sample indicates the presence of large (non-uniformly distributed) SONs of ∼10 pm/V in the area of UV-written waveguides.

Planar glass devices for efficient periodic poling

We demonstrate that frequency-converting devices of high quality can be realised with glass poling. The devices, made with silica-on-silicon technology, are poled with periodic, embedded electrodes, and used for second-harmonic generation. We obtain precise control of the quasi phase-matching wavelength and bandwidth, and a normalised conversion efficiency of 1.4x10-3 %/W/cm2 which, to our knowledge, is the highest obtained so far with periodic glass poling.

Poled-glass devices: influence of surfaces and interfaces

Devices in periodically poled glass must have a large periodic variation of the built-in field. We show that the periodic variation can be severely degraded by charge dynamics taking place at the external (glass-air) interface or at internal (glass-glass) interfaces if the interfaces have imperfections. The problem of the external interface can be solved by poling with periodic electrodes that are buried inside the glass, in many cases improving the poling efficiency dramatically. Internal interfaces can be addressed by the proper choice of waveguide design and processing. Without poling the device, one can reveal the existence of imperfect interfaces by use of electric field induced second-harmonic generation.

Topology-optimized and dispersion-tailored photonic crystal slow-light devices

We review our work done for topology optimization of passive photonic crystal component parts for broadband and wavelength dependent operations. We show examples of low-loss topology-optimized bends and splitters optimized for broadband transmission and demonstrate the applicability of topology optimization for designing slow-light and/or wavelength selective component parts. We also present how the dispersion of light in the slow-light regime of photonic crystal waveguides can be tailored to obtain filter functionalities in passive devices and/or to obtain semi-slow light having a group velocity in the range ~(c0/15 - c0/100); vanishing, positive, or negative group velocity dispersion (GVD); and low-loss propagation in a practical ~5-15 nm bandwidth.

Optical characterisation of photonic wire and photonic crystal waveguides fabricated using nanoimprint lithography

We have characterised photonic-crystal and photonic-wire waveguides fabricated by thermal nanoimprint lithography. The structures, with feature sizes down below 20 nm, are benchmarked against similar structures defined by direct electron beam lithography.