Near-field optical effect

GVD effect can be cancelled out, and the original pulse width can be recovered at the aperture of the near-field probe. In this way, high time resolution can be achieved while retaining the high spatial resolution of a near-field optical microscope. For time-resolved and non-linear near-field measurements, a mode-locked HiSapphire oscillator (TSL) is useful as a light source. Amplified lasers, which are frequently used for far-field time-resolved measurements, are not compatible with the aperture near-field probe because pulses with high power break the near-field tip easily by thermal effects. Moderate peak power is desirable for combination with aperture near-field probes. 

Figure 13 A shear-force scanning near-field optical microscope uses an optical fiba- probe, formed into a sharp tip, coated with A1 and with a subwavelength apertnre at its apex, to conduct nearfield excitation of a sample. Effectively, it provides a nondiffracted light source suitable for photolithography.

Near-field optical techniques are able to achieve resolutions beyond the diffraction limit (A/2) by exploiting the properties of evanescent waves, which are not restricted by the effects of diffraction. Evanescent waves are non-propagating components of electric fields, which decay rapidly with increasing distance from their source. 

Silva, T. J., A Scanning Near-Field Optical Microscope with Magneto-Optic Kerr Effect Contrast for the Imaging ofMagnetic Domains with 200 A Resolution. 1994, University of California, San Diego. 

It is difficult to design experiments to characterize the near field of the structures. Any probe such as the scanning near field optical microscope, which probes the near field directly, could end up altering the stmcture of the near field. This could have an effect of shifting the wavelength of the desired resonance. Experiments that account for the light radiated in the far field can also be designed. 

The theoretical framework for near-field imaging, on the other hand, is not as straightforward as that for the far-field optical measurements. This is primarily because the effects of perturbation from the near-field probe on the optical characteristics of the samples are not well known. Further developments in theoretical treatments and practical and precise simulation methods for realistic near-field measurement systems are desired. 

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