Since the project's start, we developed advanced force sensing protocols and a prototype where the vibrations of a suspended nanowire are optically readout while piezo-scanned above a sample. This allows 2D force field gradient measurements in quasi-real-time. We combined real-time calibration, stabilization loops, and live analysis of driven trajectories via phase lock or multi-frequency excitation.
We first applied the ultrasensitive force sensing of suspended silicon carbide nanowires to explore force fields above metallic nano-structures. This enabled real-time surface exploration and quantitative agreement between force and field gradient measurements. At short distances, electrostatic and Casimir forces dominate, which can be tuned with bias voltages. We mapped topography and residual electrostatic fields, compensating them with a multi-electrode scheme. We confirmed Casimir force repulsion above a sub-micron trench, matching theoretical predictions.
In two nano-optomechanical experiments, we used suspended nanowires in microcavities to increase optomechanical interaction. By scanning the nanowire within the cavity, we mapped the 2D optomechanical interaction, demonstrating sensitivity to intracavity photon number variations. This opens new possibilities in cavity optomechanics, where a single photon can displace an oscillator more than its zero-point fluctuations.
We observed an unexpected interaction when optical and electrostatic fields combined. Laser wavelength near the nanowire semiconductor gap generated electron-hole pairs that, under an electrostatic field, created Coulomb forces, moving the nanowire. A vertical electric field modified charge density at the nanowire's tip, offering a non-contact method to study charge transport. This mechanism, investigated with pump-probe techniques, enables charge state imaging and controls electrostatic effects on Casimir measurements.
We developed a cryogenic platform using a dilution fridge to operate the nanowires, with interferometric fiber objectives enabling motion readout at 32 mK. In this setup, we achieved a force sensitivity of 40zN in 1s and a 1fN/m sensitivity to force gradients. We also explored heat conduction and photothermal effects in the nanowires, providing a promising low-noise environment.
Collaborating with semiconductor experts, we investigated hybrid qubit-mechanical systems combining quantum dots in micromechanical oscillators. The oscillator's vibration modulated the quantum dot's optical resonance, and resonant pumping revealed the hybrid interaction's signature for the first time.
