High-sensitivity monitoring of nanomechanical motion using optical heterodyne detection

Summary form only given. Due to the prospect of transferring the optical quantum control achieved with ions and atoms also to macroscopic mechanical oscillators, cavity optomechanics [1,2] has been an increasingly active research field commencing with the first demonstration of radiation pressure dynamical backaction cooling more than half a decade ago. By optimizing the optical and mechanical quality factors and operating at cryogenic temperatures, quantum-coherent coupling could be achieved [3]. Simultaneously, it has been shown that nanomechanical resonators based on photonic crystals can be advantageous, as they offer low phonon occupancies (n̅ = k<;sub>B<;/sub>T/ħΩ) due to the high vacuum optomechanical coupling rates and high mechanical frequencies in the GHz domain [4]. To fully exploit the potential of these systems, sensitive measurement techniques need to be developed.Here we present a novel route using a heterodyne measurement technique to achieve significantly improved sensitivity. Based on an optical design presented in [5], we fabricate a suspended 1 D photonic crystal cavity surrounded by 2 D photonic crystals with a band gap at 1550 nm in silicon-on-insulator (Fig. 1 (a)). Together with a mesa structure defined by photolithography, it is possible to couple the cavity with a straight tapered fiber. The loaded optical Q-factor is measured as > 104. In direct detection, we observe the mechanical breathing mode around 5.7 GHz using the 12 GHz detector 1544-A by Newport, thus placing the optical cavity in the resolved sideband limit. The low quality factor of the mechanical oscillator <; 103 is explained by the domination of anchor and surface losses but can be improved by appropriate engineering.In the heterodyne experiment, we branch the light of a tunable diode laser before the device under test (DUT). One branch is coupled in and out of the cavity using a straight tapered fiber. A shifted local oscillator is created in the second branch, offset by a tunable frequency close to the mechanical oscillator mode. The shifted LO is then combined using free space optics with the signal coming from the device and carrying the mechanically induced sideband. Afterwards, the beam is split again and both branches are sent onto a balanced heterodyne detector. Fig. 1 (c) shows the resulting measurement. The local oscillator is detuned to proof that indeed the mixed signal of the nano-mechanical mode is measured and not any other components. The down-mixed signal exhibits a SNR of nearly 20 dB, greatly exceeding the SNR in direct detection. The shown data was carried out with an unfiltered signal. Further improvement is expected from suppressing the carrier by using a fiber loop cavity or Fabry-Perot filter. The elemental demonstration of this measurement scheme paves the way towards quantum limited mechanical measurements in the GHz domain.