Integrated Grating-Nanoslot Probe Tip for Near-Field Subwavelength Light Confinement and Fluorescent Sensing

We demonstrate a near-field sub-wavelength light confinement probe tip comprised of compact embedded metallic focus grating (CEMFG) coupler and photonic crystal (PhC) based λ/4 nano-slot tip, in terms of its far-field radiation directivity and near-field sub-wavelength light enhancement. The embedded metallic grating coupler increases the free space coupling at tilted coupling angle of 25° with over 280 times light intensity enhancement for 10 μm coupler size. Further, 20 nm air slot embedded in single line defect PhC waveguide are designed, using the impedance matching concept of the λ/4 “air rod”, to form the TE mode light wave resonance right at the probe tip aperture opening. This leads to the light beam spot size reduction down to λ/20. The near-field center peak intensity is enhanced by 4.2 times from that of the rectangular waveguide input, with the total enhancement factor of 1185 from free space laser source intensity. The near-field fluorescence excitation and detection also demonstrate its single molecular enhanced fluorescence measurement capability.