Complex Refractive Index Extraction for Ultrathin Molybdenum Oxides Using Micro-Photonic Integrated Circuit Chips

The emerging 2D nanomaterials with unique optical properties are promising for next-generation miniatured on-chip devices. One of the prerequisites is to precisely measure their optical parameters during their implementation. However, the inherent features of 2D layers, including limited lateral dimensions and ultra-small thicknesses, are not favorable to the conventional characterization techniques applied in the bulk system, especially for optical complex refractive indices measurement. Here, this work proposes a silicon photonics-enabled platform to evaluate the complex refractive indices of ultrathin 2D materials in a facile and reliable manner. Ultrathin molybdenum oxides (MoOx) with multiple stoichiometric states are selected as the target 2D material to provide sufficient complexity of the system for investigation. Upon the integration of ultrathin MoOx, the silicon photonic chip, in the form of a Mach-Zehnder interferometer, exhibits wavelength shifts which are used for calculating the optical complex refractive indices. Compared with the theoretical calculation, the deviation is as low as 1% and generally less than 5%. This work demonstrates a highly accurate and reliable approach for measuring the complex refractive index of 2D films, possibly assisting future advances in 2D materials-enabled optical and photonic applications.