Optomechanics: a key towards next-generation experiments in atomic force microscopy?

Atomic Force Microscopy (AFM) is a versatile and ubiquitous technique based on a resonating tip probe that interacts with the sample to be analysed in terms of topography and mechanical properties. Next-generation investigations in molecular biophysics like protein folding/unfolding, receptor-ligand interaction and molecular diffusion on cell membrane, require tracking molecular forces at the nanosecond timescale in a non-perturbative manner [1]. Conventional AFM probes, made of micro-cantilevers vibrating in the MHz range with nanometer amplitudes and combined with an optical deflection detection system, are the current bottleneck to reach the required performances. Both aspects, a very low and non-perturbative vibration amplitude, time resolution, and measurement bandwidth, are impacted by the same chief parameter: the frequency f of the probe mechanical resonator. While a higher frequency unlocks the bandwidth and the time resolution, it also sets the Brownian motion low enough to provide exquisite signal-to-noise ratio and force resolution even for vibration amplitude in the picometer range, i.e. much lower than the molecular dimensions. Recent advances in optomechanical devices technology allow tackling the challenge by offering resonators at very high frequencies greater than the GHz and unprecedented motion sensitivity below 10-17 m.Hz-0.5. The talk will present our recent developments introducing a fully-optically operated resonating optomechanical AFM probe above 100 MHz of frequency, 2 decades above the fastest commercial cantilever probes, while Brownian motion 4 orders below [2]. Based on a silicon technology and operated at 1.55 µm wavelength, the probe shown in Fig. 1 demonstrates high-speed sensing of mechanical interactions with a sub-picometer resonantly driven motion, breaking open current locks for faster and finer force spectroscopy at the molecular level. Figure 1. Scanning electron microscopy image of the optomechanical atomic force microscopy probe. The ring-shaped whispering gallery mode resonator is 20 µm in diameter. A 4 µm-long tip apex protrudes from the ring, aiming at sensing near-field forces when interacting with a surface. [1] H. Yu et al., Science, 355, 945-950, (2017) [2] P.E. Allain et al., arXiv:1810.06209, (2018)