SNOM

Abstract

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Scanning Near Field Optical Microscopy (SNOM), when used in combination with NC-AFM, provides a powerful method for combining both high resolution in mechanical and fluorescence studies of living cells in physiological solution. Using a long L shaped force sensor fabricated from an etched optical fiber inside a “diving bell” arrangement, allows an enhancement on the probe Q factor.

We present a method for fabrication of Scanning Near-field Optical Microscopy (SNOM) aperture probes. We show these fiber probes are capable of attaining high resolution imaging in Non-Contact Atomic Force Microscopy (NC-AFM) mode when used in combination with a “diving bell”. They allow simultaneous AFM imaging and local illumination for fluorescence studies on delicate samples under physiological solution.

 

High Q optical fiber tips for NC-AFM in liquid

Dynamic Atomic Force Microscopy (AFM) has diversified from ultra high vacuum to liquid, making possible experiments in cells and bio-molecules using high resolution imaging and force spectroscopy. The interaction dynamics between the AFM tip and the sample, plus the ability and the precision of the instruments make AFM a potent technique for studies of mechanical properties of cells under physiological conditions. Working in Non Contact (NC) Mode allows the oscillating AFM force sensor to operate over the sample with a low applied force, thus increasing the resolution and the imaging speed and reducing the drag caused by lateral forces [1]. In order to operate in the  NC mode, a sharp resonance is necessary to achieve stability and low noise operation. When measurements are performed in physiological solutions, force sensors are immersed in water which strongly dampens oscillations and thus decreases the mechanical Q factor from 200 in air to 30 in liquid. This can lead to a substantial increase of noise. [2].

Our approach to overcome this problem is to use a long L shaped force sensor fabricated from an etched optical fibre. Using a “diving bell” arrangement which allows only a small portion of the tip to be submerged in the liquid then leads to Q factors in excess of 100.

Optical L shaped fiber tips fabrication:

1) Bending the fiber
2) Chemical etching to form a taper (tip)
3) Gluing the fiber to a Si chip
4) Coating with Al vapor deposition
5) FIB processing to form an aperture at the tip
6) Fiber probe mounted on a chip

Probe characterization:

Frequency:                                    Q factor: 

for a fiber with a k=20N/m and thickness=30m.m.

When submerged in water, the resonance peak of the fiber changes:

We can include the following as some of the liquid effects:

* Viscous damping of the cantilever in fluid. Higher drive is required to maintain a reasonable oscillation amplitude.

* Extreme  reduction  of  the  Q-factor  due  to  the  hydrodynamic  interaction between  the  cantilever  and  the  liquid.

* Increase in frequency noise

* Due to the acoustic excitation of the cantilever, other modes of oscillation ( spurious peaks related to the liquid cell eigen-frequencies) may appear [4].

*Poor resolution imaging


The diving bell

As we mentioned before, using a “diving bell” we allow only a small portion of the tip to be submerged in the liquid while the rest of the fiber oscillates in the air trapped in the bell. This imporvement prevents from some the undesired effects of the liquids and leads to Q factors in excess of 100. As we approach to the sample, the trapped volume of air keeps the greatest part of the L shaped probe dry and, as a result, when approaching to the sample, the Q factor is kept high. Stable spectroscopy measurements have been reported with this setup [3]. The force sensors can simultaneously be used as  Scanning Near Field Optical Microscopy (SNOM) probes, which, when used in combination with NC-AFM, provide a powerful method for combining both high resolution in mechanical and fluorescence studies of living cells in physiological solution.


Consists of a quartz ring which is sealed to the plastic Kel-F body of the tip holder. The airtight seal formed between the quartz ring and the tip holder allows the air to displace the liquid in the sample dish as the tip is lowered towards the sample

Successful imaging and force spectroscopy

Stable topographical and simultaneous topographical and fluorescence imaging has been demonstrated by imaging a clean glass surface (a) and a micro-contact pattern of protein circles in a glass surface (b-c).


References

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[1] TR Albercht, et al., “Frequency Modulation Detection using high Q Cantilevers for Enhanced ForceMicroscope Sensitivity”, J Appl Phys. 69 668 (1991).

[2] T Fukama, et al., “True atomic resolution in liquid by frequency-modulation atomic force microscopy”, Appl Phys Lett 86 193108 (2005).

[3] J M LeDue et al., “High Q Optical Fiber for NC-AFM in Liquid”, to be published (2009).

[4] A Maali, Dynamic AFM in Liquids: Viscous Damping and Applications to the Study of Confined Liquids. Ch 15. Springer V, 2009.

For more information on this project, the paper(s) can be found on the following address:

http://www.physics.mcgill.ca/~peter/publications/Qtip.pdf


Researchers: Jeff LeDue and Monserratt Lopez


People

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Researchers: Monserratt Lopez

Supervisors: Jeff LeDue, Peter Grutter (Physics)

Collaborators

 - Sébastien Ricoult, David Junker (BMED Department, McGill University)

 - Helene Bourque, Yoichi Miyahara (Physics)