I would like to know why diffraction is limited by nano-world. I have read that plasmons can be used to overcome this diffraction limit in the nanometer world. I'm hoping some answers and some good references to understand the science behind this phenomenon.
Nano-optics deals with optical phenomena on nanometer scale, that is beyond the diffraction limit of light. The spatial confinement that can be achieved for photons is inversely proportional to the spread of wavevector components in the corresponding spatial direction. Such spread occurs in light field that converges towards the focus (behind the lens). Thus, one can obtain some finite spread in the wavevector components for highest possible NA that can be obtained with oil-based microscope objectives. If one would be able to to achieve a wider spread of wavevectors, then light could be focused to the spot smaller than the one dictated by the diffraction limit and one can achieve sub-wavelength resolution...
In 1928 Synge published an article that outlined the concepts of what is currently known as scanning near-field optical microscopy. He stated if one takes an opaque film with a sub-wavelength aperture in it and places it within a sub-wavelength distance from a sample, then the produced illumination spot will not be limited by diffraction but the size of the aperture!
In modern days this concept is the basis of the near-field scanning optical microscopy (NSOM) or scanning near-field optical microscopy (SNOM). Both abbreviations have been used. The tip can be made in several ways: metal coated tapered fiber that has an aperture 10-100 nm (achieved by so called shadow coating evaporation technique) or micro-fabricated probes (for example, based on standard AFM cantilever technology). These are just examples because the technology is evolving and so do the methods of making probes with sub-wave apertures.
So, in the nutshell this is a combination of AFM with optical probing. One brings such a probe to a close contact with a sample and scans it across the sample. Collection is done in far-field with a diffraction limited optics. As in optical microscopy, various modes of operation are possible: transmission, reflection etc. One can add non-linear effects as contrast mechanism (SHG, for example), fluorescence etc.
To get more info, do a literature search for NSOM, SNOM and you end up with excellent reviews and original articles on the subject. Alternatively, you may look at some books on the topic like L. Novotny and B. Hecht "Principles of Nano-Optics", Cambridge University Press (2006)
Hope it helps!
Nano-optics deals with optical phenomena on nanometer scale, that is beyond the diffraction limit of light. The spatial confinement that can be achieved for photons is inversely proportional to the spread of wavevector components in the corresponding spatial direction. Such spread occurs in light field that converges towards the focus (behind the lens). Thus, one can obtain some finite spread in the wavevector components for highest possible NA that can be obtained with oil-based microscope objectives. If one would be able to to achieve a wider spread of wavevectors, then light could be focused to the spot smaller than the one dictated by the diffraction limit and one can achieve sub-wavelength resolution...
In 1928 Synge published an article that outlined the concepts of what is currently known as scanning near-field optical microscopy. He stated if one takes an opaque film with a sub-wavelength aperture in it and places it within a sub-wavelength distance from a sample, then the produced illumination spot will not be limited by diffraction but the size of the aperture!
In modern days this concept is the basis of the near-field scanning optical microscopy (NSOM) or scanning near-field optical microscopy (SNOM). Both abbreviations have been used. The tip can be made in several ways: metal coated tapered fiber that has an aperture 10-100 nm (achieved by so called shadow coating evaporation technique) or micro-fabricated probes (for example, based on standard AFM cantilever technology). These are just examples because the technology is evolving and so do the methods of making probes with sub-wave apertures.
So, in the nutshell this is a combination of AFM with optical probing. One brings such a probe to a close contact with a sample and scans it across the sample. Collection is done in far-field with a diffraction limited optics. As in optical microscopy, various modes of operation are possible: transmission, reflection etc. One can add non-linear effects as contrast mechanism (SHG, for example), fluorescence etc.
To get more info, do a literature search for NSOM, SNOM and you end up with excellent reviews and original articles on the subject. Alternatively, you may look at some books on the topic like L. Novotny and B. Hecht "Principles of Nano-Optics", Cambridge University Press (2006)
Hope it helps!
Few more books that I'd recommend:
Eds. Anatoly Zayats and David Richards
"Nano-Optics and Near-Field Optical Microscopy"
Artech House, 2009
Ed. Satoshi Kawata
"Near-field Optics and Surface Plasmon Polaritons"
Springer, 1996
Dear Mr. Palappra:
A way to get insight into your question is the Fourier transform. Because the spatial domain and the momentum (wavevector) domain are related via the Fourier transform, if your electromagnetic field has to be very confined in space, it must have large components of momentum. This is related to the large spread concept in Prof. Grabtchak's comprehensive answer.
The thing is that propagating light of a given wavelength has a physical limit into how large any component of the momentum vector can be: The modulo square of such vector has to be equal to the square of the wavenumber k2 = (2pi/lambda)2, where lambda is the wavelength. Shorter wavelengths allow more confinement. You can check this in the approximated expression relating a lens numerical aperture, wavelength and focal spot size, for example here
http://en.wikipedia.org/wiki/Numerical_aperture
There is one way to increase some components of the momentum vector without decreasing the wavelength. When one or more of the components have an imaginary part, the others can have a squared value larger than k2 This is because when you compute the norm of such a vector (with complex components), you get negative contributions from the imaginary parts, then, other components can grow and the total k2 still be the same.
Waves with complex momentum components are typically called evanescent waves. Plasmons are evanescent waves. The price to pay for this is that they can no longer travel in all directions: Their amplitude decays exponentially in the direction of the complex component of the momentum. These kind of waves exists in the interfaces between two media (metal and dielectric for plasmons). The example of the SNOM given above can be seen from this point of view: The tip of the probe has evanescent plasmon modes, really confined to the metal. In order to interact with anything the tip needs to get very close to it. In this way, you are exciting your object of interest with light that has transverse components of the momentum vector (i.e. "parallel" to the surface of the tip) that are larger than k2. This is impossible using propagating light.
Kind Regards,
Ivan Fernandez-Corbaton
I will suggest you to go through "Introduction to Nanophotonics by S. V. Gaponenko". Its a good book to start. Hope that will help you.
Regards
Ankit
- Badges
- Science topic
- Similar topics
- Chemistry
- Nanotechnology
- Nanostructures