Stefan Prorok

40 GHz electro-optic modulation in hybrid silicon–organic slotted photonic crystal waveguides

In this Letter we demonstrate broadband electro-optic modulation with frequencies of up to 40 GHz in slotted photonic crystal waveguides based on silicon-on-insulator substrates covered and infiltrated with a nonlinear optical polymer. Two-dimensional photonic crystal waveguides in silicon enable integrated optical devices with an extremely small geometric footprint on the scale of micrometers. The slotted waveguide design optimizes the overlap of the optical and electric fields in the second-order nonlinear optical medium and, hence, the interaction of the optical and electric waves.

Optical spectroscopy of single Si nanocylinders with magnetic and electric resonances

Resonant electromagnetic properties of nanoparticles fabricated from high-index semiconductor or dielectric materials are very promising for the realization of novel nanoantennas and metamaterials. In this paper we study optical resonances of Si nanocylinders located on a silica substrate. Multipole analysis of the experimental scattering spectra, based on the decomposed discrete dipole approximation, confirms resonant excitation of electric and magnetic dipole modes in the Si nanocylinders. Influences of light polarization and incident angle on the scattering properties of the nanocylinders are studied. It is shown that the dependence of resonant excitation of the electric and magnetic modes in the nanocylinders on incident angle and polarization of light allows controlling and manipulating the scattered light in this system. The demonstrated properties of Si nanocylinders can be used for the realization of dielectric metasurfaces with different functional optical properties.

Trimming of high-Q-factor silicon ring resonators by electron beam bleaching

We demonstrate a novel position-resolved resonance trimming strategy for silicon ring resonators. Ring resonators are covered with a chromophore-doped guest host polymer cladding. Illumination of the polymer cladding with high-energy electrons causes a bleaching of the chromophore molecules. Bleaching of the chromophores induces a reduction of the polymer refractive index, which can be used to trim the resonance frequency of the ring resonators. A maximum refractive index change of 0.06 and a TM polarization resonance shift of 16.4 nm have been measured. A Q factor of 20,000 before bleaching remains unaltered after the electron beam exposure process.

Mechanism that governs the electro-optic response of second-order nonlinear polymers on silicon substrates

We use a modified Teng-Man technique to investigate the poling induced electro-optic activity of chromophore-doped organic polymers poled on silicon substrate in a thin film sample configuration. We reveal a fundamental difference between the poling processes on silicon substrate and ITO substrate. The electro-optic activity for polymers poled on silicon substrate is reduced which we ascribe to space charge formation at the silicon - organic interface that distorts the field distribution in the polymer film during high field poling, and therefore limits the effective induced polar order. We demonstrate that the electro-optic activity on silicon substrate can be improved by inserting a 5 nm thin dielectric layer of Al2O3 between the silicon substrate and the polymer which reduces the leak-through current during poling, thereby allowing for higher applicable poling voltages.

Broadband EBG Structures with Compact Unit Cell

The design of a hook-like EBG structure with a small unit cell size is presented in this paper. This hook-like structure has a high inductance of the equivalent electrical LC-circuit, which enables the realization of broadband band-gaps at very low center frequencies. Simulation and measurements show, that the novel structure improves the relative bandwidth by 77% and lattice size by 59% compared to the standard mushroom-like EBG structure. In addition, a second EBG structure with a relative bandwidth of 15% is presented, which is generated with the aid of a genetic algorithm (GA). Measurement results of the proposed designs show good agreement with simulation.

Configurable silicon photonic crystal waveguides

In this Letter, we demonstrate that the mode cut off of a photonic crystal waveguide can be trimmed with high accuracy by electron beam bleaching of a chromophore doped polymer cladding. Using this method, configurable waveguides are realized, which allow for spatially resolved changes of the photonic crystals effective lattice constant as small as 7.6 pm. We show three different examples how to take advantage of configurable photonic crystal waveguides: Shifting of the complete transmission spectrum, definition of cavities with high quality factor, and tuning of existing cavities.

Modification of a Teng-Man technique to measure both r33 and r13 electro-optic coefficients

In this paper, we present a modified Teng-Man method to measure both electro-optic coefficients in a single measurement. Using our method, we confirm a linear dependence between the applied poling field and the measured electro-optic coefficients. The ratio between the two electro-optic coefficients is close to three, which is theoretically expected from a weakly oriented polymer film. Since conductive silicon is used as substrate, no auxiliary layers of transparent oxide or metal are required on the substrate, which simplifies both the sample preparation and the evaluation of the results.

Photonic crystal cavity definition by electron beam bleaching of chromophore doped polymer cladding

We demonstrate a novel concept for the fabrication of high Q photonic crystal heterostructure cavities. First, photonic crystal waveguides without cavities are fabricated. The cavities are defined in a later fabrication step by spatially resolved bleaching of a chromophore doped polymer cladding. Bleaching of polymer films either by UV light or by electron beam illumination is well known to reduce the refractive index of the film. The reduction of the cladding refractive index leads to a reduction of the effective lattice constant of the photonic crystal waveguide. The maximum refractive index change was found to be 6•10-2 which corresponds to the effective lattice constant change of 12.2 nm. With this approach it is also possible to achieve very small effective lattice constant shifts of 0.02 nm which is not possible with state of the art lithography. Being able to precisely define the effective lattice constant at every point of the photonic crystal waveguide we are able to impose cavity mode profiles which closely resemble a Gaussian envelope. This leads to a dramatic increase of the Q-factor. In simulations we have obtained Q-factors as high as 3.0•106 for a vertically symmetric polymer cladding. First results for non-vertically symmetric structures are presented.

Hybrid silicon-organic racetrack resonator designs for electro-optical modulation

Racetrack resonators based on the silicon-on-insulator platform are proposed for electro-optical modulation. The resonators are functionalized by a cladding of a second order nonlinear optical polymer. Two different concepts for the racetrack design employing different waveguide geometries for quasi-TE and quasi-TM polarization operation are presented. In both resonator designs the electrical contact is established by fully etched segmented electrode sections to allow for an easy fabrication process. For quasi-TM polarization the width of the strip waveguide is optimized to 400 nm. The Q factor of 2000 is measured for a sample with segmented electrode. A loss of 0.4 dB per segmented waveguide is deducted. For the quasi-TE polarization the slot waveguide geometry is optimized to 470 nm total width including a vertical slot of 90 nm width. Only the straight parts of the racetrack are slotted, while the bends are built from strip waveguides. To convert the mode from strip to slot geometry stub like couplers of 100 nm length are employed. The measured Q factor is 550. The in device Pockels coefficient is measured to r33 = 1 pm/V. This small value indicates a very low poling induced polar order which needs to be improved. This is a topic of current investigation.

Electrical and electro-optic characterization of nonlinear polymer thin films on silicon substrate

In this paper we present electrical and electro-optical (EO) measurements of polymer thin films on silicon substrates. A method is presented on how to interpret ellipsometric measurements of the (EO) coefficient on silicon substrate by taking into account multiple reflections in each sample layer. The obtained EO coefficients on silicon substrate are compared to measurements for indium tin oxide (ITO) coated glass substrates. Electrical measurements are performed to analyze the conduction mechanisms inside the polymer film. Based on the presented experimental data different models are discussed in order to explain the differences in current density during poling between ITO coated glass substrates and silicon substrates.