Surface profile measurements using opticaly trapped probe

One of the most precise technique how to detect the surface profile with nanometer precision is AFM (Atomic Force Microscopy). It uses a sharp tip which is attached to the soft cantilever. In the contact mode, the tip is just dragged across the sample and the deflection of the cantilever is monitored and consequently the surface profile restored. However, there has to be mechanical access to the surface.

Photonic Force Microscope uses optically trapped probe in a focused laser beam – optical trap – to study tiny force interaction in pN range. Therefore, this method is non-contact and usable to trap and manipulate probes behind transparent walls. The deviation of the probe from its equilibrium position can be detected using scattered light on quadrant photodiode or two-photon fluorescence. We chose two-photon fluorescence from dyed trapped polystyrene spheres. This method is direction insensitive and provides information about the distance of the probe from the beam focus  (see the figure). However, this method suffers by unwanted decay of the fluorescence caused by photobleaching. Therefore, we developed a special procedure how to elliminate this effect. It is based on the scanning of the surface in such a way that the perimeter points are measured before, during and after the surface scanning. Because we know the time between each event we can find the parameters for the exponential decay at each of these points and correct the photobleaching. 

First Similarly to AFM techniques, we use two modes of operation - contact and tapping modes. In the contact mode the probe is dragged along the surface and the variations of the two-photon fluorescence is recorded (see figure). In the tapping mode the probe was taken above the surface and then it approached the surface until the two-photon fluorescence decreased to predefine value. The distance of vertical piezo-stage movement then corresponds to the surface profile.

The axial resolution was obtained by vertical scanning of the piezo stage in nanometer steps over 25 nm (see figure). The vertical bars show the standard deviation of the probe position. This measurement shows that vertical resolution is better than 10 nm. The uncertainity of the probe position is due to the Brownian motion.

We used time-sharing of the trapping beam to get more optical traps and so also more trapped probe at the same time. We tested two probes and using the same method as above we scanned the surface profile.

Examples of the surface with attached polymer spheres of diameter 269 nm. The probes were red fluorescent probes with diameter 820 nm.

Examples of the surface with attached polymer spheres of diameter 60 nm. The probes were red fluorescent probes with diameter 820 nm.

Related publications:

M. Sery, P. Jakl, J. Jezek, A. Jonas, P. Zemanek, M. Liska: "The use of an optically trapped microprobe for scnning details of surface",
       Proceedings of SPIE 5259, 166-169, 2003,    ABSTRACT   DOWNLOAD

P. Jakl, M. Sery, J. Jezek, P. Zemanek: "Optical tweezers and its use in local probe microscopy",
       Proceedings of VII International Conference Optical methods of flow investigation, Moscow, 28-31, 2003.

M. Šerý, P. Jákl, J. Ježek, M. Liška, P. Zemánek: "Využití více opticky zachycených sond pro mìøení profilù nepøístupných prùhledných povrchù",
       Jemná mechanika a optika 48 (6), 170-173, 2003

P. Jakl, J. Jezek, M. Sery, A. Jonas, P. Zemanek: "The use of a microprobe held by a laser beam for the study of the surface reliefs",
       Proceedings of SPIE 4900, 777-783, 2002.

Send comments to webmaster
Last modification: 30 Mar 2007