Emerging Amorphous Chalcogenide Thin Films—From Bonding Mechanism to On-Chip Highly Non-Linear Photonic Devices

Chalcogenide glasses (ChGs) show a large transparency window in the infrared coupled with outstanding optical nonlinearities offering tremendous opportunities for achievement of innovative mid-infrared on-chip components. In this work, the amorphous structure and the nonlinear optical properties of As-free amorphous GeSb<i>_w</i>S<i>_x</i>Se<i>_y</i>Te<i>_z</i> chalcogenide thin films are studied. The nonlinear refractive Kerr indices (<i>n</i>₂) of the films were evaluated through modelling of spectroscopic ellipsometry data. The method has been then validated by means of experimental characterization of Kerr indices of a set of selected glassy chalcogenide alloys and SiN<i>_x</i> reference material using advanced nonlinear optical characterizations of waveguides (heterodyne interferometry setup). State-of-the-art and remarkably high <i>n</i>₂ values were obtained for some compositions of the amorphous chalcogenide compounds. Depending on the stoichiometry of the GeSb_wS_xSe_yTe_z ChGs, <i>n</i>₂ can vary of more than one order of magnitude. Besides, Fourier-Transform Infrared (FTIR), Raman and X-ray Absorption (XAS) spectroscopies analysis of the amorphous structure of prototypical chalcogenide alloys in relation with their nonlinear optical properties was used as a basis for <i>ab initio</i> molecular dynamics (AIMD) simulations. Thus, the intimal link between local atomic configurations recalling unique “metavalent” bonding (MVB) and optical nonlinearities is unveiled, giving unprecedented clues to control optical nonlinearities of chalcogenide materials.<br/>In this work we evaluated first the <i>n</i>₂ refractive indices of a wide range of GeSb_wS_xSe_yTe_z thin films by means of Sheik-Bahae model, a theoretical model already successfully applied to chalcogenide materials to predict the <i>n</i>₂. The amorphous as-deposited films exhibit a wide range of <i>n</i>₂ values that show strong dependence on the ChGs composition. The obtained results are well supported by previous study of similar GeSb<i>_x</i>Se<i>_y</i> compounds in the literature. In order to relate optical nonlinearities and structure, four prototypical compositions differing by their <i>n</i>₂ values were selected and finely described by means of advanced optical and structural characterizations. Therefore, the experimental nonlinear coefficients <i>γ</i> of the prototypical Ge-Se-Sb(-N) alloys and SiN<i>_x</i> reference material measured in waveguides using an experimental heterodyne detection setup at 1550 nm are in good agreement with those derived from calculated <i>n₂</i> Kerr indices using the Sheik-Bahae model. The obtained values are from one to two orders of magnitude higher than that of SiN reference sample. A dramatic change of Kerr index depending on the ChGs stoichiometry is observed and related to modifications of the amorphous structure. From the calculation of the bond polarizability, one could relate the origin of the local very high electronic polarizability, related to highly polarizable structural motifs in the amorphous network, to the enhanced optical nonlinearities of the material. These highly polarizable motifs are reminiscent of a unique bonding configuration that recalls the “metavalent” bonding, a unique bonding mechanism at the origin of the huge optical and electrical contrast between amorphous and crystalline state of Phase-Change Materials.<br/>To summarize, the outstanding linear and nonlinear optical properties of selected ChGs at origin of their promising potential of applications for MIR on-chip components is shown to result from the formation of peculiar structural motifs leading to specific bonding mechanism at the origin of their huge polarizability. In particular, some elements increase dramatically the local electronic polarizability by inducing structural motifs supporting bonds similar to the newly introduced “metavalent” bonding mechanism arising in some local crystal-like motifs. These results pave the way to control and improve further the unique optical properties of ChGs in thin films.