][ Florian Formanek - research@ESPCI ][

2001-2004:Physical Optics Laboratory - ESPCI (Paris, France)
Apertureless scanning near-field optical microscopy
with or without laser source

- F. Formanek et al., "Imaging subwavelength holes using an apertureless near-field scanning optical microscope"
J. Appl. Phys. 93, 9548 (2003). PDF
- F. Formanek et al., "Thermal radiation scanning tunneling microscopy" (熱放射走査型トンネル顕微鏡)
Nature 444, 740 (2006).

This work was realized as part of my PhD Thesis of the Pierre et Marie Curie University, Paris VI, at the Physical Optics Laboratory in ESPCI (Ecole Supérieure de Physique et de Chimie Industrielles) engineer school of Paris.
The subject was:

"Development of a near-field scanning optical microscope
working in the visible, in the infrared, with or without illumination"

The jury was composed of Prof. Claude FABRE (chair), Dr. Daniel COURJON (examiner), Prof. Jean-Jacques GREFFET (examiner), Dr. Bernard QUERLEUX, Prof. Claude BOCCARA (thesis supervisor) and Dr. Yannick DE WILDE (thesis supervisor).

The main part of my research concerned the development of a near-field optical microscope based on a quartz tuning-fork and an apertureless tungsten tip operating in tapping mode. This instrument is able to work in various configurations, especially in the infrared as sketched below:


In this configuration, the beam from a CO2 laser (λ = 10.6 µm) is focused at grazing incidence onto the tip apex by a ZnSe lens. The field scattered by the tip is collected by a Cassegrain objective (36X, NA = 0.5) made of two spherical gold mirrors and sent towards an HgCdTe nitrogen cooled detector connected to a lock-in amplifier operating at the fundamental oscillation frequency (Ω) of the tip or at its higher harmonics. The lock-in enables to extract the weak contribution of the near-field signal from the far-field background.
Using this setup we have investigated subwavelength holes (diameter ~300 nm) randomly distributed in a thin chromium film deposited over a glass substrate. The resolution in the near-field optical images (Fig.2a) is about 30 nm which corresponds to λ/300 at the illumination wavelength. The chromium regions appear brighter than the bottom of the holes made of glass.
In order to interpret this optical contrast, we have used a theoretical model (dipole model) based on a modified version of Mie's theory which takes into account the interaction of the probe dipole with its electrostatic image to calculate the scattering cross-section of the tip. The model can also account for the dependency of the optical signal that we observe on approach curves towards the z-direction (Fig.2b) for different demodulation frequencies. It indicates that at higher harmonics the interference patterns can be significantly suppressed and the resolution slightly improved.
Finally, our experiments demonstrate the possibility of infrared chemical mapping at the nanometer scale.
For details see: Ultramicroscopy 103, 133 (2005). Journal of Applied Physics 93, 9548 (2003).

Fig.2a Fig.2b

More recently, we have transformed the infrared configuration into a "thermal radiation scanning tunneling microscope" (TRSTM) to detect thermal infrared evanescent fields naturally emitted by a surface. With this new kind of near-field microscope operating without any external illumination, we were able to observe thermally excited surface plasmons and to demonstrate spatial coherence effects in near-field thermal emission.

Details to come…meanwhile see: Nature 444, 740 (2006).

- My article about dyed human hair, written in collaboration with people from L'Oréal, is now published in Journal of Microscopy.
- Our paper "Thermal radiation scanning tunneling microscopy" is now published in Nature as a letter !

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