Hybrid Silicon-Polymer Nanophotonics
Optical intra chip communication becomes more and more interesting in microelectronics, since device footprint size decreases exponentially while the number of devices on the same chip is increasing, which makes simultaneous electronic intra chip communication more and more difficult due to increasing path length. Also, the ever growing data transfer in telecommunications and resultant data processing from electronic to optical signals and back lead to the need for integrated electronic and optical circuits. The silicon-on-insulator (SOI) platform is the most promising substrate for these integrated circuits. It is a layered material stack typically of a thin (few 100 nm thick) silicon layer on a buried oxide (BOX) layer with a thickness of up to 2 µm which is resting on a handling silicon wafer for mechanical stability reasons.
In current CMOS technology SOI is widely used, because the electrically insulating BOX layer leads to low parasitic capacitances and leakage currents. For similar reasons SOI can be used in optics at telecommunication wavelength as well. A standard single mode strip waveguide in this high refractive index contrast material system of silicon (n = 3.5) as core and SiO2 (n = 1.45) and air (n = 1) as cladding has a width of several 100 nm only and low loss bends can have radii of 5 µm or less. Thus, optical devices in SOI are relatively small and single chip integration of electronic and optical circuits becomes easy as both are processed using the same CMOS technology. For modulation and multiplexing of optical signals an optical response to electronic signals is needed. Therefore most optical SOI devices use Mach-Zehnder-Interferometers (MZI), arrayed waveguide gratings (AWG), resonant structures like cavities (link zu PhC cavities) or ring resonators.  Such devices are very sensitive to changes in propagation constant of the optical wave. The interference of the optical signal with itself coming from different optical paths of the device to the output port translates a weak shift in the propagation constant of certain regions of the device into a substantial change in signal output. To change the refractive index of silicon electrically the main access today is via charge carrier injection  or thermo optic effect . Though electro-optically modulation up to 40 GHz has been already reported , this is a result of sophisticated device layout with high effort in technology, because charge carrier injection increases optical absorption as well as charge carrier lifetime reduction is crucial for high speed operation. Heating electrically is also a relatively slow process.
The Pockels effect, which is a second order nonlinear effect, in poled EO-active polymers is an inherently faster effect then those mentioned above for refractive index shifts directly in silicon. The Pockels effect is not present in silicon but certain polymeric materials can be designed in their molecular structure to show large Pockels coefficients  and to change their index nearly instantaneously when an electric field is applied. Photonic silicon devices embedded in such polymer claddings  can be much simpler fabricated compared to the previously mentioned devices  and gain modulation frequencies on the same order of magnitude. The effective refractive index of a guided mode in a silicon waveguide is changed in this case. The evanescent field of a guided mode transfers changes in the refractive index of the surrounding cladding material into changes of the mode's effective refractive index.
Hybrid SOI/polymer nanophotonics research at the Institute of Optical and Electronic Materials aims to combine the advantageous optical properties of silicon and EO-active polymers. Silicon with its high refractive index acts as passive waveguide core and enables high scalability to very small device footprint sizes. Furthermore, monolithic integration with electronic circuits and use of mature CMOS technology is possible. Polymers allow to be designed in their molecular structure to have specific optical properties. Today's polymers possess Pockels coefficients of up to 300 pm/V  and allow very fast low voltage tuning of the refractive index. The combination of both materials results in simple SOI photonic devices within a polymer cladding that transfers its refractive index shifts into effective refractive index shifts of the guided mode.
The main goals of our research are the optimization of the waveguide geometry for the best translation of the cladding's refractive index change into the guided mode's effective index change (e.g. slot waveguides ) and the modification of relatively simple device layouts for low loss electrical contacting of the optical waveguides (e.g. segmented waveguides ).
The optical devices currently under research are race track and ring resonators with a straight waveguide evanescently coupled to the resonator (Fig. 1). Both, TE and TM modes, are considered for EO-modulation optimization.
|Fig. 1: SEM picture of a racetrack resonator with evanescently coupled waveguide and segmented part for optically insulated electric contact. Fabricated in collaboration with Xlith. |
In the case of the TM mode (Fig. 2a) the evanescent field is mainly located above and below the silicon strip waveguide and extends widely into the cladding. In this polarization the impact of sidewall roughness on the propagation loss is reduced. The volume of the evanescent field part of the guided TM mode is relatively large and so the sensitivity of the effective index to changes in the cladding material is strong. In this case the electric field for the EO-modulation is applied from the waveguide as a lower electrode to a gold electrode which is deposited on top of the EO-polymer cladding.
|Fig. 2a: TM mode in strip waveguide geometry. Evanescent field in top polymer cladding is widely extended.||Fig. 2b: TE mode in slotted waveguide geometry. Evanescent field is strongly enhanced in slot.|
In the case of the TE mode (Fig. 2b) a slot is introduced into the waveguide and filled with the surrounding EO-polymer. The optical wave is guided by the silicon rails (each 190 nm in width) but there is a strong enhancement of the evanescent field within the slot of 90 nm. The rails act as electrodes for EO-modulation. This geometry can be considered as a plate capacitor with the EO-polymer in the slot as a dielectric in between the rails. The volume of the evanescent field is smaller compared to the case of the TM mode but its field intensity is much higher. Less voltage is needed to gain large electric field strength in the slot. This geometry has a very sensitive mode to refractive index changes in the slot region, but the impact of sidewall roughness on the propagation loss is increased for the same reason. Also bends in a slot waveguide structure increase the loss strongly, because the TE mode profile of a slot waveguide is much more distorted than in standard strip waveguide bends, which leads to scattering due to mode conversion processes. For this reason we use slot waveguides in straight parts of optical devices only.
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