Detecting Single Gold Nanoparticles (1.8 nm) with Ultrahigh‐Q Air- Mode Photonic Crystal Nanobeam Cavities

The growing applications of nanoparticles in energy and healthcare demand new metrology techniques with improved sensitivity, lower sample concentration, and affordable instrument cost. Here we demonstrate the first air-mode photonic crystal nanobeam cavity with ultrahigh Qfactor (Q = 2.5 × 10) and ultrasmall mode volume (V = 0.01λ) at telecom wavelength. The air-mode cavity has strong field localization outside of its high-index material, thus significantly improving the sensitivity to detect nanoparticles. The strong field gradient attracts the nanoparticles to its field maximum, improving the detection efficiency. Combining these advantages, we report detecting and sizing single gold nanoparticles down to 1.8 nm in diameter (equivalently single polystyrene nanoparticle of 3 nm in diameter) with significantly reduced sample concentration (∼fM) than traditional optical techniques. In addition, the air-mode ultrahigh Q, ultrasmall V photonic crystal nanobeam cavity will be a useful platform to study strong light−matter interactions, nonlinear processes, and cavity quantum electrodynamics.