Nonlinear Optical Microscopy

Abstract

The constant evolution of optical microscopy over the past century has been driven by the desire to improve the spatial resolution and image contrast with the goal to achieve a better characterization of smaller specimens. Numerous techniques such as confocal, dark-field, phase-contrast, Brewster angle and polarization microscopies have emerged as improvement of conventional optical microscopy. Being a pure imaging tool, conventional optical microscopy suffers from its low physical and chemical specificity. This can be remedied by combining it with spectroscopic technique like fluorescence, infrared or Raman spectroscopy. Such microscopes have been successfully applied to the study of a wide range of materials with good spectral resolution. However their spatial resolution is restricted by the diffraction limit imposed by the wavelength of the probe light. In infrared microscopy, for instance, the lateral resolution is a few microns which is insufficient to resolve sub-micron structures. Conventional microscopy also does not provide microscopic information about the real surface structure of a specimen. Even in reflection geometry, they can only probe the structure of a surface layer averaged over a thickness of a reduced wavelength. Furthermore, they are insensitive to the polar organization of molecules in the layer although this could be important. In biophysics, for example, it is interesting to know the polar orientation of molecules adsorbed on a membrane and its influence on the membrane physiology.