Near-Field Scanning Optical Microscopy Imaging Technology
Near-fleld scanning optical microscope (also known as NSOM/SNOM) currently has the highest optical resolution. The literature reported its resolution can reach 10nm and normally the resolution can be better than 50nm. And previous optical microscope, even the confocal laser scanning microscope, the general spatial resolution of which can only up to 250 nm due to Ernst Abbe diffraction limit, namely d = 0.61/n sin в, where d is the smallest scale that an ordinary microscope can distinguish, is the wavelength of light, n is the refraction index of medium, and в is convergent angle of light beam [14, 17, 53, 54].
Working Principle of the Near Field
In regard to a sample surface, its optical information can be roughly divided into near field information (less than a light wavelength range) and the far field information (more than a light wavelength range). Near field information contains more of the high frequency components than that in the far field, which can reveal more fine surface structure. But the near field information decays exponentially with the increase of propagation distance, it is difficult to obtain this information by using the general methods. Near-field optical microscope is a tool to collect the high frequency information. Although the near-field scanning optical microscope is an optical microscope, it does not have a lens system. It uses an extremely sharp beam probe to collect optical information on the surface of the sample. Usually we use a microscope to obverse samples in a distance at least several wavelengths (Fig. 2.8a), which can only see the image of the far field. If we decrease the distance between collect lighting and the sample on the surface (^1), it is possible to acquire an image with high frequency information, namely high resolution (Fig. 2.8b).
Back in 1928, Synge proposed that if there is a hole in an opaque plate with backward lighting, with the hole scale being far less than wavelength, then the scale of the light through the plate depends on the size of the hole. If the plate is closely near the surface, the light through the hole can implement imaging of the sample surface, so as to realize the breakthrough of the image resolution restricted by diffraction limit. But at that time the hole production, lighting and sample controlling problems are hard to verify his idea, until 1972 Ash and Nichols using microwave confirmed his
Fig. 2.8 Comparison of focusing beam spread in imaging process: a far-field, b near-field
prediction. Since then, Winfried Denk, inspired by scanning tunneling microscope (STM), realized the sub-wavelength visible light detection in IBM laboratory at Zurich in 1982 for the first time. Aaron Lewis from Cornell University also realized near-field detection independently by using laser and a probe drawn by glass capillary to propagate light signals. By keeping the distance between the probe and the sample on the scale of a few nanometers, he scanned the sample surface point by point to recover the optical image of the sample. These creative work of near-field scanning optical microscope provided possibility in the application of scientific research and started a widely research of near-field scanning optical microscope since then.
Studies found that several conditions must be met in order to obtain satisfactory near-field optical images. First, using a laser as light source to supply sufficient incoming light intensity, as the near field optical signal is so weak that it is easy to drown in the background noise. For this reason, oftentimes lock-in amplification is utilized to resolve signal out of noise, too. Second, the scale of the probe needs to be at the nanometer level to effectively detect near-field optical signal, otherwise it is difficult to guarantee a high resolution exploration. Third, the distance between the probe and the sample should be controlled within a few nanometers.