Photonic force microscopy

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.

PhFM of the membrane of living malaria-infected red blood cells presenting knobs on its surface. (a) Optical (left) and SEM (right) image of an infected red blood presenting knobs on its surface. A region scanned by PhFM is shown in the optical image. (b) Axial position-displacement curves obtained on the hard glass surface and on the cell membrane. The linear fits after contact are shown by dashed lines. Inset: corresponding indentation curves, where the curve corresponding to the glass surface is set vertical. (c) Image obtained from the axial displacement signal Sz in a first scan of an infected cell and (d) its high-pass filtered version. (e) Image obtained in a subsequent scan in the region highlighted in red and (f) its filtered version. In (d,f), the same protrusions ascribable to knobs are marked by circles, showing that their position is the same in the two scans. (g−i) Scan on a different infected cell, where the images are obtained from the signals Sz, Sx, Sy as indicated and high-pass filtered. Despite the limited axial resolution in Sz (g), the presence of the knobs is retrieved in the transverse signals (h,i).