The Characteristics and Classification of Near-Field Scanning Optical Microscope
Near-field optic not only retains the advantages of ordinary microscopes, but also significantly increases resolution; some of these advantages can be simply summarized as the following:
- (1) the non-invasive, non-destructive scanning of sample surface, particularly suitable for biological sample, in-situ detection can be realized;
- (2) a wealth of method to improve or adjust image contrast by placing the samples through light absorption, reflection, scattering, polarization, phase and the spectral selection, allowing a full range of rich information for the researchers.
- (3) detection sensitivity can reach as high as 1photon/sec;
- (4) capable of achieving 10 nm optical resolution;
- (5) able to detect spectral information as well as lifetime of a single fluorescence molecule;
- (6) By point by point scanning near-field scanning optical microscope can obtain the morphology image of the sample along with 2D optical image at the same time.
Near-field optical microscopy is able to break through the diffraction limit through using a nanoscale optical probe to disturb the evanescent wave close to sample surface, which turns the evanescent wave into detectable propagation wave, so as to improve the resolution of the image. According to the type of probes used, near-field optical microscopes can be divided into two kinds: the probe allows transmitting light signals, and is connected to a light source or detector, usually made by stretching optical fiber or glass capillary into a conical tip, coated with a thin reflective metal layer, often aluminum. The end of the fiber is an uncoated small pinhole through which an evanescent light wave can pass. The diameter of the hole is often 50-100 nm. The transmission decreases very rapidly with diameter (approximately the 6th power), in industry this type of optical fiber based probe is also named as SNOM probe (scanning near-field optical microscope probe), usually fabricated by stretching an optical fiber under electric coil heating. Another kind probe is an opaque conical needle without optical aperture, only perturbing the near field with its sharp tip. During scan, intensity change in reflected irradiation laser beam is picked up and computer processed to produce image; those probes are mostly made of semiconductor (e.g. silicon) or metal, and often used with AFM simultaneously. Figure2.9 shows the imaging schemes using two types of probes.
As for near-field scanning optical microscopes with light-guided probes are further classified into two configurations that light source and detector are located: one kind is reflective near-field scanning optical microscope, the light source and detector are on the same side of the sample, and the probe can be either light source or detection device. It is suitable for the research of metal, thicker, opaque or samples of the insulation, while the other is a transmission near-field scanning optical microscope: the light source and detector on the opposite side of the sample. Probes are only used for illumination, suitable for transparent thin samples, where transmitted near-field signals are collected.
Recently near-field scanning optical microscopes have seen some new hybrids in practical applications, such as the combination of scanning near-field optical microscope with an atomic force microscope (AFM) to become a multifunctional system whose resolution reached 35nm, and a near-field scanning optical microscope on which a sensitive polarization detector is installed to detect local magneto-optic effect with a high lateral resolution.
Fig. 2.9 Two near-field imaging schemes: a solid metal tip probe, b SNOM probe