Locally induced electro-optic activity in silicon nanophotonic devices

Summary form only given. Recently, the interest in silicon-based nanophotonic devices with electro-optic functionality has strongly increased. In Silicon a second-order electro-optic activity can be induced by breaking the inversion symmetry of the crystal lattice. Commonly this is done by straining the silicon lattice for example by applying a Silicon-Nitride layer to a nanophotonic waveguide [1,2]. Another innovative approach has been demonstrated, which is based on a chemical surface-activation in a CMOS-compatible plasma-supported dry etching process [3].In this work, we show that this simple and versatile "plasma-activation" process can be applied to endow an integrated nanophotonic Mach-Zehnder interferometer (cross-section is shown in the inset of Fig.1a)) with a second-order nonlinearity. Figure 1a) shows the transmission spectra of such a device for three applied static electric fields. In reference to the transmission function without applied static field the graph reveals a blue-shift of the spectrum for a positive static field and a red-shift for an inverted field. The analysis of field sweeps shows a strictly linear dependence of the shift on the applied field. This proofs the linear Pockels-effect and therefore the induced second-order nonlinearity to be responsible for the shift. A quantitative consideration reveals a value of χ(2) = 9 ± 1 pm/V for the induced nonlinearity. This value is of the same order of magnitude as reported values χ(2) induced by strain [1,2]. Further investigations show that this value can be scaled up to = 17 pm/V by reducing the thermal stress during the process steps. Using terahertz near-field probes [4] the efficiency and homogeneity of local χ(2)-induction can be monitored. Under ultra-short pulse excitation at 1560nm center-wavelength a χ(2)-based difference frequency generation (DFG) process becomes operative within the plasma-activated waveguide sections. This is demonstrated by- spatially and temporally resolved measurements of the DFG-based THz-radiation from the nonlinear active waveguide areas.Figure 1b) shows the THz intensity of a periodically plasma-activated waveguide accumulated over 7 ps of time. The blue lines indicate the waveguide dimension. One can clearly identify the distinct areas of THzemission and second-order nonlinear optical activity. The size of an active area is 50μm x 50μm followed by an inactive area of the same size. The graph indicates the high contrast between activated and non-activated areas. The possibility of building locally confined areas of nonlinear activity is an important advantage of the plasmaactivation in comparison to other methods and could for instance be applied for quasi-phase matching structures and other electro-optic periodic structures such as tuneable gratings.