A Novel 2D Plasmonic MoO3 Driven pH Sensor on Silicon Photonics Platform

Silicon photonics has been demonstrated as a suitable platform for developing next generation on-chip biochemical micro- /nano-sensors due to its compactness, ultra-sensitivity, CMOS technology compatibility and extremely low power consumption, which other optical sensing platforms cannot provide as a combination. These sensors rely heavily on the change of ambient-refractive-index upon the immobilization of specific biomolecules via surface functionalization. However, if the immobilized biochemical species (e.g., ionic species) do not produce sufficient influence on the ambient-refractive-index, these sensors become "blind", hence hindering their wide applicability.In this paper, we attempt to divert the research interest by introducing the concept of functionalization-free silicon photonic biochemical sensors coupled with two-dimensional (2D) tunable plasmonic materials for ultra-sensitive ionic sensing. Here, 2D molybdenum oxide is chosen as its plasmon resonance wavelength is close to the operation wavelength of the silicon chip. At the same time, the induced scattering and absorption losses are kept low enough at the operation wavelength, maintaining relatively high optical performance of the silicon chip and hence sustaining excellent sensitivity. These important features make it the first successful report on the integration of 2D material onto the silicon photonic platform for sensing applications.Through the demonstration of a pH sensing model, the exposure of ionic analytes results in the modification of ion-driven plasmonic properties, hence changing the refractive index and plasmonic absorption properties of 2D molybdenum oxide. Consequently, the phase and amplitude of the light transmitted through the silicon photonic device are affected due to the evanescent interaction between the guided optical mode and the plasmonic material. Such a dual sensing output has never been successfully achieved by other optical sensing platform and results in the great enhancement of the sensor robustness. More importantly, this unique sensor demonstrates a up to 7 orders improvement on sensitivity per unit area over all other optical pH sensors, therefore generates new modalities for on-chip biochemical sensors in the micro/nano-scale. In addition, the possible combination with conventional biorecognition elements will potentially widen the applicability of silicon photonic based sensors and have a notorious impact in the development of more advanced and sophisticated biosensors.