Nano-photonic devices fabricated from chalcogenide glasses

Abstract

Chalcogenide glasses have been widely used for all-optical processing. Our team identified a particular chalcogenide glass, with composition Ge{u2081}{u2081}.{u2085}As{u2082}{u2084}Se{u2086}{u2084}.{u2085} (Ge{u2081}{u2081}.{u2085}), as having very the best properties for nonlinear waveguides. The material has a nonlinear refractive index about three times higher than ASZS3 and negligible two photon and free carrier absorptions. In particular, its optical properties have been shown to be thermally and optically stable unlike most chalcogenide glasses and thin films, and this is important, for example, if the photonic devices are to be stable enough to make structures that employ high-Q resonances as part of their optical response.

The main goal of the research described in this thesis was to establish the feasibility of producing high performance waveguide devices with extreme nonlinearity made from chalcogenide glass as a route to high-density photonic chips for all-optical signal processing.

Highly nonlinear dispersion-engineered nanowires and photonic crystal resonant cavities are examples of structures that can enhance the overall nonlinear response of the device and were both successfully fabricated in Geri.s glass films patterned using E-beam lithography (EBL) and inductively coupled plasma (lCP) etching. A record value of nonlinearity of 135,OOOW{u207B}{u2091}km{u207B}{u2091} was measured for Gen.s nanowires and this is the highest value ever reported for a glass waveguide and is more than one order higher than achieved using the ASZS3 rib waveguide platform commonly we commonly use for all-optical processing. The nonlinearity of these Gens nanowires is, in fact, comparable with that achieved from Si nanowires with the advantage that the chalcogenide glass has negligible two-photon absorption (TPA) and no free carrier absorption (FCA) allowing ultra-fast nonlinear processing without degradation due to those parasitic effects.

We also showed that polarization independent (P-I) nanowires could be produced with negligible differences in dispersion (and effective index) between the TM and TE modes. To achieve this square Gens nanowires were fully embedded in a silica cladding. In our experiments we showed that the four wave mixing (FWM) efficiency and supercontinuurn spectra generated using TE and TM polarizations were essentially identical. Any small differences could be attributed to the residual polarization dependent losses due to the larger scattering for the TE mode which interacts more strongly with the side-wall roughness induced during etching. In practice this is not, however, a serious limitation because the device length can be very small.

We studied competition between stimulated Raman scattering (SRS) on FWM in highly nonlinear waveguides experimentally using both in ASZS3 rib waveguide and Gens nanowires and analysed the observed phenomena systematically using numerical modelling. We identified the contributions from Raman scattering and FWM in terms of a modification to the gain and phase mismatching conditions. It was shown that SRS substantially enhances the gain close to the Stokes (and anti-Stokes via FWM) frequency but due to its affect on FWM phase matching it also results in a significant drop in FWM conversion at frequency shifts just beyond the Raman band. As a result FWM in these glasses will generally only be efficient when the frequency difference between the pump and signal waves is smaller than the Stokes shift. We were able to demonstrate this conclusively for Ge{u2081}{u2081}.{u2085} nanowires. Fortunately this still provides enough bandwidth for FWM to be used in telecommunications signal processing across the full S-Cand L-bands.

We showed that near-zero anomalous dispersion Ge{u2081}{u2081}.{u2085} nanowires could be used for the generation of high quality correlated photon pairs by spontaneous FWM at a very large frequency detuning from the pump so that the noise produced by stimulated Raman scattering would be absent. We also described an approach for fabricating the near-zero dispersion waveguides essential to implement the design.

High Q photonic crystal cavities were fabricated from air suspended AMTlR-l glass membrane by EBL. In collaboration with researchers at Sydney University, a post-tuning method utilizing the photosensitivity of glass was then used to create a microcavity. However, the bistable switching of these cavities was dominated by slow nonlinear processes such as thermal heating possibly with a contribution from TPA in this particular glass rather than the Kerr effect. In this respect the observation of bistability involved mechanisms similar to those present in silicon photonic crystal cavities.

To overcome this we designed and fabricated a photonic crystal cavity from Ge{u2081}{u2081}.{u2085} chalcogenide glass fully embedded in an index matched cladding to enhance the thermal conduction from the cavity. We used 3D FDTD simulations to show that narrowing the photonic crystal waveguide (to{u2248}WO.54) could lead to resonances with very high Q-factors in a PhC slab made from Ge{u2081}{u2081}.{u2085}, completely embedded in a cladding with the index of silica. According to our simulations a Q-factor of over 20 million could be achieved using such a design. We successfully fabricated PhC heterostructure resonators with graded reflectivity mirrors achieving an intrinsic Q factor of >750,000 which compares favourably with a value of 1,700,000 predicted from modeling of a completely ideal structure. This represents the highest Q-factor ever demonstrated in a chalcogenide PhC and is competitive with values achieved in air-clad silicon membranes with a much higher index contrast. Whilst clear bistable switching could be observed in these cavities probed with CW light, thermal nonlinearities still dominated in these experiments.