Non-contact friction in near-field optomechanical transducers*

Nanophotonic devices offer excellent capabilities for probing neutral atoms, color centers, free electrons and nano-mechanical oscillators. In most applications, the near-field interaction, requires sub-wavelength gaps between the nanophotonic device and the system of interest. However, short-range interactions with the photonic structure can affect the delicate coherence properties of the quantum system. In this work, we study the interaction between a nano-mechanical resonator and a photonic crystal (PhC) cavity in a monolithic optomechanical system. A high-Q Si₃N₄ binary tree nanobeam resonator is suspended in parallel to the PhC with a few hundred nanometers gap. In the absence of the PhC, the fundamental mode of the nanobeam at 160 kHz has a quality factor of 70 million and a thermal-limited force sensitivity of 2 aN/sqrt(Hz) at room temperature. We observe a decrease in the Q of the fundamental mode with decreasing nanobeam-PhC gap––the Q decreases by more than an order of magnitude for gap of 200 nm. While the exact physical mechanism is still unclear, we present a systematic study of this phenomenon. The observed damping mechanism poses a challenge for realizing sensitive integrated optomechanical sensors.