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Surface Imaging with optically trapped probes

Background: Photonic Force Microscopy (PFM) is a competing technique to Atomic Force Microscopy (AFM), where the thin tip of a mechanical cantilever is scanned across the surface to obtain a high-resolution height profile. With a much higher sensitivity and an easily adjustable trapping stiffness, the PFM can generate surface height profiles using an optically trapped probe.
 
Problem:The interferometric tracking signals of the probe's axial displacements are massively distorted by phase disturbances of the surface structure.
Approach: We use two optical laser foci that scan side by side across the structured surface: one with a 190nm small optically trapped probe and the other focus without a particle. By subtracting both interferometric signals Sz from each other and extracting the high energy fluctuations, we can recover the surface height profile with a resolution beyond the optical diffraction limit.
 
Scanning results. A polystyrene probe with 190 nm diameter is scanned across fixed silica beads with diameters of 280 to 500 nm. Top row: Sketches of a, bright field imaging (BF), b, empty scan (ES), and c, probe scan (PS). Middle row: d, BF image of the scanned region; e, axial signal Sz recorded during ES; f, axial signal Sz recorded during PS. Bottom row: g, scanning electron microscope image (SEM) of the scanned region; h, mean axial displacements (x,y) and i, reconstructed height profile h(x,y). Scalebars 1 µm.
 
 
SurfaceScanning.png
 Figures
  

100.000 probe positions per second (blue dots) are tracked in 3D at a 40nm raster with a trap scan speed of 4µm/s. An area of 1µm x 1µm (625 pixel) is scanned within 10s.

The resulting position histograms histograms are recorded convolved with the probe's shape function (sphere with D = 190nm) to obtain the lower contact surface shown in green.

Reconstructed height profile in red overlaid with sphere sizes  obtained from electron microscopy

 

 

 

 
 
 
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