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In photonic force microscopy (PhFM), a laser trap raster scans the sample and the induced displacement of the trapped particle from its equilibrium position is measured by fast interferometry. PhFM has the recognized potential to apply low force (fN-pN) during the scan, therefore reducing the artifacts of other techniques like AFM on soft materials. However, the resolution of PhFM remained limited to hundreds of nanometers in two of the three dimensions. We have developed a novel nanofabrication protocol to produce optically trappable particles (60 × 10^6 per batch) which mimic the sharp tips of Atomic Force Microscopy. To allow stable trapping and to maximize the optical signals, the probe is designed as a large micron-sized truncated cone, holding a sharp feature (~35 nm radius) in the center of its top surface.
Tip fabrication. (a) Microfabrication process. Cylindrical particles are generated by laser interference lithography, etching a quartz substrate where a 800 nm thick SiO2 layer is deposited. A tuned acid thinning by hydrofluoric acid produces sharp tips in the SiO2 layer. The particles are then cleaved mechanically off the substrate. (b) Scanning electron microscope image of cleaved particles, where the contrast of one tip has been enhanced for clarity. The radius of curvature of the tip is 35 nm. (c) Schematic cartoon of the optical trap holding the particle and scanning the surface of the sample with the sharp tip.
AFM and PhFM imaging of rigid samples.
(a) z position-displacement curves for the two possible particle orientations. When the tip faces the surface (orange points), the contact point appears before (∼400 nm) than when the tip is in the opposite direction (blue points). The shape and relative values (in volts) of the two curves differ and allow systematic recognition of the tip orientation. The dashed lines correspond to a fit to a piece-wise polynomial function. (b) Image of 100 nm random features on glass obtained with the tip not facing the sample. (c) AFM image and (d) PhFM image (with tip facing the sample) of the same region. (e,g) AFM and (f,h) PhFM images of the same regions. Details of 80 nm can be resolved. The color bars of (d,f,h) are the same. All the scans cover an area of 1×1 μm^2.
R.Desgarceaux*, Z.Santybayeva*, E.Battistella, A.Nord, C.Braun-Breton, M.Abkarian, O.M.Maragò, B.Charlot*, F.Pedaci*. High resolution photonic force microscopy based on sharp nano-fabricated tips. Nano Letters (May 2020).
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