Toward all on chip optical detection in the few molecule regime

Integrated optics devices are one of the most promising technologies in many fields such as biosensing, optical monitoring, and portable devices. They provide several advantages such as unique sensitivity and the possibility of the well-established and developed silicon photonics technology. However some challenges still remain open, as the implementation of multiplex assay able to reach the single particle sensitivity. In this context, we propose a new design for a Si-based photonic structure that enables the realization of on chip sub-wavelength optical sources. The idea is based on the insertion of opportunely designed nanometric holes in the photonic circuit, which are available for analyte detection with high efficiency. We propose three different configurations in which both excitation and detection are obtained through the same waveguide thus simplifying the detection scheme and potentially enabling multiplexed detection. We proved the high confinement of the electromagnetic field in the holes both by theoretical modelling and spectroscopic measurements. We investigate the possibility of inserting an arbitrary number of optical sources by using a resonator and evaluate advantages and drawbacks of resonating and non-resonating solutions. Finally, we report the proof-of-concept experiment, where detection sensitivity down to single Quantum Dots is obtained by combining the novel design with fluorescence-based techniques. Importantly, the presented results are achieved by a simple modification of photonic sensing chips which are already on the market thus having an excellent translational perspective.