How we are getting High Optical resolution up to nanometer level in the Far field excitation and Near field Collection configurations of NSOM probe??
case1. It is understood that if we are illuminating the Nanostructures with Near field excitation (NSOM probe) and Far field collection (through objective) configuration,then
the interaction of the Near field (evanescent field) with the sample results in scattered light (propagating wave): the signal is s eventually collected in the far-field detector......
case2. But If I am doing the same experiment in reverse way ie; illuminating the Nanostructures with Far field excitation (through normal objective) and Near field collection (NSOM probe) configuration,then
How I can get the high resolution image?? because at the vicinity of the sample there will be a presence of evanescent wave, but it will be non-propagating..then how we can detect it even though we are using the NSOM probe for the collection ??
Hi
simply because the NSOM tip, due this small size (in ne the nanometer range) is able to convert the evanescent field in a propagative one that you can then collect in the far field. A small structure is capable , at the same time, to generate evanescent waves under the illumination of a propagative wave, but also, in the presence of an avanescent wave, to scatter it in the far field.
kindly
seb
Dear Dr.Sébastien Ducourtieux
thanks for your answer...but I am still not clear about How this evanescent field is converted to propagating one in the nearfield collection configuration..the light from objective already fallen on the sample and may contain the evanescent field components also..But this evanescent wave will exponentially decaying waves , then how it will propagate through the circular aperture of the NSOM probe.. If this circular aperture of NSOM is converting evanescent wave to propagating wave, then how it is possible..may I know the theory or can you elaborate it ???
even though this is in french, you could have a look on my phd thesis, page 27.
When you light a sample, both evanescent and propagative wave are present, but evanescent decay very quickly when you move away from the sample surface, then in your microscope objective cannot collect them. Then you need something to convert evanescent wave to propagative wave (and sometime modulation to improve signal to noise ratio) and detect them in you objective : this is the rule of you tip !!
Thesis Apertureless near-field optical microscopy : instrument deve...
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