2, Laboratoire Charles Fabry, Institut d'Optique Graduate School, CNRS, Université Paris Saclay, Palaiseau, , France
3, L-NESS, Dipartimento di Fisica, Politecnico di Milano, Polo di Como, Como, , Italy
Silicon photonics is a large volume and large scale integration platform for applications from long-haul optical telecommunications to intra-chip interconnects. Extension to the mid-IR wavelength range is now largely investigated as it could potentially lead to key advances in several disciplines including molecular sensing, early medical diagnosis or secure communications, among others. Among the materials used in silicon photonics, germanium (Ge) is particularly compelling as it has a broad transparency window up to 15 µm and a much higher third-order nonlinear coefficient than silicon which is promising for the demonstration of efficient non-linear optics based active devices. In that regard, we have demonstrated that graded-index Ge-rich SiGe alloys, with Ge concentrations larger than 70%, can be an ideal material choice to develop such mid IR photonic platform since they permit an accurate control of the optical properties by fine tuning the Ge concentration along the growth direction. Nevertheless, the material nonlinearities are very sensitive to any modification of the energy bands, so Si1-xGex alloys are particularly interesting for nonlinear device engineering. We report on the first third order nonlinear experimental characterization of Ge-rich Si1-xGex waveguides, with Ge concentrations x ranging from 0.7 to 0.9, where previous numerical modeling disagree as there is a change between direct-like and indirect-like bandgap approaches. To this end, we have developed a novel non-destructive single beam technique that includes reliable measurement of the injected power in the waveguide, called bidirectional top-hat D-Scan. The D-Scan method, a temporal analogue of the Z-Scan technique, consists in measuring the output spectral broadening of transmitted pulses by varying the dispersion coefficient φ(2) introduced to the input pulses. For Z-Scan, the spatial deformation of a laser beam is analyzed while the nonlinear bulk-material is displaced through the focused spot and for increasing power. The optical (or thermal) Kerr effect induces an intensity dependent spatial variation of the refractive index of the material, playing the role of a spatial lens that modifies the beam propagation. Similarly, the D-Scan consists in recording the spectral broadening behaviors experienced by linearly chirped pulses with increasing incident power in order to characterize the temporal Kerr lens introduced by the nonlinear material. There is an analogy between Z-Scan and D-Scan as they both rely on the interplay between linear terms, respectively diffraction and dispersion, and nonlinear Kerr effect. The characterization performed at 1580 nm is compared with theoretical models and a discussion about the prediction of the nonlinear properties in the mid-IR is introduced. These results will provide helpful insights to assist the design of nonlinear integrated optical based devices in both the near- and mid-IR wavelength ranges.