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Near-Field Optical Imaging

1) The Photon Scanning Tunneling Microscope (PSTM)

Among the passive probe devices, we find the PSTM (Photon Scanning Tunneling Microscope) or STOM (Scanning Tunneling Optical Microscope).

PSTM exploit bare and sharply elongated optical fibres which may sometimes be coated with metals. In these experiments, the reduction of the tip-sample spacing below the tunneling decay length makes the energy transfer possible.

The numerical method based on the implementation of the Green Dyadic formalism is now sufficiently mature to faithfully reproduce different experimental processes at work in real experiments (imaging, local spectroscopy, optical binding forces, ...). In particular the possibility of including the 3D character of the devices simultaneously with a description of their photonic energy transfer is interesting for interpreting realistic experimental situatiuons.

Depending on the nature of the very tip this kind of microscope can detect either the magnetic or the electric field.
A complete theoretical analysis of this configuration is given in the following references:
(a) "The physics of the near--field'', Report on Progress in Physics, 63} 893-938 (2000);
(b) "Transmittance of subwavelength optical tunnel junctions'', Phys.Rev. B58 12551-12554 (1998);



Figure 1: Schematic description of a PSTM device

(c)"Local detection of the optical magnetic field in the near zone of dielectric samples'', Phys.Rev. B62, 10504-10514 (2000).


In the near futur, our main objective is the implementation of a numerical STOM/PSTM with spectropic and imaging capabilities of miscellaneous samples. Preliminary benchmarks performed on mesoscopic parallel computers indicate an excellent performance of our code when increasing the number of processors. This parallelization facility is inherent to the selfconsistent procedure that proceeds iteratively inside the source region (i.e. both the pointed tip and the supported nanostructures in our applications).

 

 

The main feature of all near-field optical microscopes is the nanometre-sized tip which is piezoelectrically driven to scan close to the sample surface. According to the experimental setup, the tip may be used as a local photon emitter (active probe) or as a local probe of the optical near-field (passive probe). From the 3D Green Dyadic Method we develop different numerical models of these microscopes.


(a) Schematic drawing of the experimental setup.
A molecular light source illuminates an object supported by a transparent sample. The transmitted signal used to record the image is collected under the sample in the solid angle W.
(b) Map of the normalized intensity integrated over the solid angle W=2p str

 
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