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Optomechanical probe for very high speed sensing of atomic forces

In collaboration with Vmicro SAS, researchers from the CEA-Leti and the CNRS1,2,3 have developed a probe for an atomic force microscope, using optomechanical interactions for very high speed operation. With a vibrational frequency 100x higher than normal probes and an extreme motion sensitivity, the optomechanical probe paves the way for ultra-rapid AFM measurements. Our approach has been patented and the initial results have been published in the Nanoscale review.

Published on 21 February 2020
  • ​The atomic force microscope (AFM) is a versatile instrument which has contributed to the rise of nanosciences. Today, it is an omnipresent tool for scientific and technological research. Its operation is based on the analysis of vibrations from a very fine point moving across the surface of a sample. The most advanced applications, for example in biophysics, require ultra-high speed image capture and the measurement of molecular bonding forces, with a temporal resolution of close to a nanosecond. Such performance levels are currently beyond the capabilities of existing AFM probes, which are based on the concept of a vibrating mechanical lever that has evolved little since its invention.

  • Researchers from the CEA-Leti and the CNRS1,2,3, working jointly with the company Vmicro SAS and supported by the ANR OLYMPIA project4, are exploring an approach to counter this limitation by developing the concept of an optomechanical AFM probe. This probe is based on the use of a micrometric silicon ring, which couples the optical gallery mode with the mechanical breathing vibration modes in a single resonator, achieving high quality factor levels. The optical forces within the ring are used to set in motion a nano-tip, whereas the optomechanical interaction enables amplitude measurement of the mechanical vibrations induced, with an ultimate resolution.

​Finite element model of the mechanical and optical modes of the optomechanical resonator forming the core of the optomechanical AFM probe

Image from a scanning electron microscope of an optomechanical probe made of silicon, with its optical coupling guide and its nano-tip
  • To create the prototypes of the optomechanical AFM probe, the researchers combined their skills in optomechanics, microsystems, micro-manufacturing and instrumentation. The devices feature a single-piece optomechanical resonator fitted with a tip, which forms the heart of the probe, along with photonic guides and diffraction grids to allow the bonding of optical fibers. The full-optical operation of the optomechanical AFM probe was proven at mechanical frequencies above 100 MHz, 100x more than current AFM levers, whilst achieving a measurement sensitivity capable of detecting Brownian motion with an amplitude of a tenth of a femtometer. Furthermore, experiments on the mechanical interaction between the probe tip and another object have confirmed its sensitivity to external forces and to their gradient, in the standard operational configuration of an AFM microscope. All the ingredients are in place for higher-speed atomic force measurements using optomechanical probes.

  • The researchers are currently fitting optomechanical probes to the most advanced AFM microscopes. In particular, this involves experiments on biological systems in a liquid environment, to reveal dynamic phenomena within complex biomolecules on a nanosecond scale, which have been inaccessible up to now. The target market is that of the AFM and in particular, high resolution imaging at a video frame rate. One of the first applications will be to understand ultra-rapid, biochemical reactions at a molecular scale, which can currently be only accessed using simulations. It is important to understand the cellular mechanisms and changes in protein conformation, to enable the development of new antibiotics or medicines, for example.

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