Optimization of Coupling into Slow Light Modes of Planar Silicon Photonic Crystal Defect Waveguides in Aqueous Solutions

Full text: In planar 2D photonic crystal defect (PhC) waveguides the backscattering mechanism of light at each unit cell and omni directional reflection lead to the formation of slow light modes. We want to exploit these slow light modes for the development of a compact biophotonic sensor which is based on an integrated optical waveguide Mach Zehnder interferometer (MZI). This biosensor will be used for the detection of biomolecules such as proteins or DNA in aqueous solutions. The PhC sensor arm of the MZI is covered with a thin biosensitive layer to which only specific molecules can bind. This changes the phase velocity of the light, which can be sensitively measured by the MZI. The theoretic detection limits of such devices are as low as 0.1 pg/mm2 . The slow light mechanism leads to a significant enhancement of the interaction between light and the sensitive layer by up to a factor of >100, which will allow reducing the sensing length of typically 1 cm in conventional waveguide devices to ∼100μm. Achieving efficient coupling between a conventional wire waveguide mode and a slow light mode in a PhC defect waveguide represents a challenging task in the design of the sensor. A direct transition results in high losses. The major cause for these losses is the impedance missmatch between the conventional and PhC defect waveguide modes. For PhCs operated in air it has already been shown that a short intersection a PhC defect waveguide of a few microns supporting a faster mode can significantly enhance the coupling efficiency between the conventional and PhC defect waveguide. By modifying the lattice periods and the hole diameters of this intersection PhC defect waveguide the group velocities can be adapted such that the coupling losses are reduced significantly. Since the targeted compact biophotonic sensor operates in aqueous solutions we optimized the coupling efficiency by means of extensive 2D Finite Difference Time Method (FDTD) simulations employing the effective index approach. In order to design a compact biophotonic sensor we used 3D Plane Wave Expansion Method (PWE) simulations for calculating the PhC band structure and the defect modes. While the majority of PhC devices are dedicated to telecom applications at 1550 nm wavelength using an air cladding, we designed our PhCs for an operation wavelength of 1310 nm, where the absorption in aqueous solutions is much lower than at 1550 nm. For PhCs operated in water the coupling issue becomes even more critical because the higher refractive index of water compared to air considerably reduces the PhC working range. We investigated PhC intersection areas with constant period lengths and adiabatically reduced periods. Our simulation results explicitly indicate the increase of the coupling efficiency by the use of these concepts to reduce the impedance mismatch between the conventional and PhC defect waveguide modes. As a result, we obtained a new optimized PhC design for operation in aqueous solutions, which has a high potential to become a key component of compact biophotonic sensors. (author)