Interrogation of a plasmonic nanoantenna with a phase-shifted Bragg grating waveguide configuration

Localized surface plasmon resonances (LSPR) in plasmonic nanostructures have shown high sensitivities for optical sensing and spectroscopy applications. Individual plasmonic nanoresonators are capable of detecting analytes in ultrasmall volumes. Light being diffraction limited results in weak coupling efficiency (approximately 1% extinction in free space) to the nanoresonator. Improving the signal to noise ratio (SNR) at the output requires a dark field microscope which makes the sensing system bulky. Light interaction with nanoantenna can be improved by using dielectric waveguides for interrogation. Besides this, compact and completely integrated on-chip biosensors can be realized with easier interrogation and multiplexed detection capabilities. Waveguide interrogated plasmonic nanoresonators have been investigated in the past for spectral shift based refractive index (RI) sensing modality. Such sensors require bulky spectrometers for obtaining the output spectrum. In contrast to it, intensity shift based RI sensors require photodetectors which are compact and easily integrable on chip. We propose a small footprint phase shifted Bragg grating (PSBG) waveguide made of silicon nitride loaded with a gold nanoresonator. Numerical investigation of the structure's optical response and its potential for intensity shift based refractometric sensing has been done in the visible wavelength range. The grating possesses a Fabry Perot cavity with a high Q-factor (1008) thus increasing the photon lifetime and interaction of light with the nanoantenna. Intensity shift based RI sensing results show sensitivity of 394%/RIU for PSBG excited nanoantenna compared to 57%/RIU observed in strip waveguide loaded with nanoantenna. Besides this, light coupling efficiency to plasmonic nanoresonator is 35% on excitation with PSBG waveguide in comparison to 12% observed in strip waveguide based excitation scheme. Strategies for further improvement in coupling efficiency are also reported.