Ideally, CMOS-compatible integrated silicon (Si) photonic circuits would contain microscopic analogues of all bulk-optic components such as emitters, receivers, and lenses. While many types of optical components have been re-created in microscopic 2D Si form, others are more difficult to convert. Nonlinear optical devices are troublesome because Si is centrosymmetric and thus cannot contribute a second-order optical nonlinearity. One way around this is to physically strain Si, breaking its centrosymmetry and allowing second-order susceptibility. However, the dependence of second-harmonic (SH) generation on strain in Si has not been well understood.
Researchers at Martin Luther University Halle-Wittenberg and the Fraunhofer-Institute for Mechanics of Materials (both in Halle, Germany) thermally oxidize the surface of Si to a thickness between 10 and 225 nm, creating strain that decays exponentially as a function of distance into the substrate. The curvature of the wafer was measured before and after oxidation, allowing strain calculation. A p-polarized beam from a Ti:sapphire laser was reflected from the wafer at 45º and the reflected SH signal measured (with surface-generated SH taken into account) for different rotations of the sample around its surface normal. Some measurements occurred at Brewster’s angle, eliminating effects of multiple reflections. The resulting data, taken over a stress range of -800 to -300 MPa, will help in the design of CMOS-compatible Si photonic circuits containing strain-based nonlinear elements.
Contact Clemens Schriever at [email protected].
