| Summary: | Optomechanical nanocavities open a new hybrid platform such that the interaction between an optical cavity and mechanical oscillator can be achieved on a nanophotonic scale. Owing to attractive advantages such as ultrasmall mass, high optical quality, small mode volume and flexible mechanics, a pair of coupled photonic crystal nanobeam (PCN) cavities are utilized in this paper to establish an optomechanical nanosystem, thus enabling strong optomechanical coupling effects. In coupled PCN cavities, one nanobeam with a mass m<sub>eff</sub>~3 pg works as an in-plane movable mechanical oscillator at a fundamental frequency of <inline-formula>π<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi mathvariant="sans-serif">Ω</mi><mi mathvariant="normal">m</mi></msub><mrow><mo>/</mo><mn>2</mn><mi mathvariant="sans-serif">π</mi><mo>=</mo><mn>4.148</mn></mrow><mrow><mo> </mo><mi>MHz</mi></mrow></mrow></semantics></math></inline-formula>. The other nanobeam couples light to excite optical fundamental supermodes at <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>1542.858</mn></mrow></semantics></math></inline-formula> and 1554.464 nm with a <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi mathvariant="normal">Q</mi><mi mathvariant="normal">o</mi></msub></mrow></semantics></math></inline-formula> larger than 4 × 10<sup>4</sup>. Because of the optomechanical backaction arising from an optical force, abundant optomechanical phenomena in the unresolved sideband are observed in the movable nanobeam. Moreover, benefiting from the in-plane movement of the flexible nanobeam, we achieved a maximum displacement of the movable nanobeam as 1468 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mrow><mrow><mi>fm</mi><mo>/</mo><mi>Hz</mi></mrow></mrow><mrow><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></mrow></msup></mrow></semantics></math></inline-formula>. These characteristics indicate that this optomechanical nanocavity is capable of ultrasensitive motion measurements.
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