2JASCO International Co., LTD.,2-4-21, Sennin-cho, Hachioji Tokyo 193-0835 Japan.

Since the beginnings of observational science, the need to resolve objects smaller than those visible by eye alone has driven the development of microscopy and many related branches of the physical sciences. However, from the 19th Century and Abbe's seminal description of the role played by light waves and diffraction in image formation, the unwelcome but fundamental resolution limits in optical imaging due to diffraction have been understood, whether for light waves or electrons. In approximate but practical units, the wavelength used itself defines the minimum information limit, despite the heroic efforts of lens makers. Only by lowering the wavelength can the resolution be improved below a micron or so, as the recent (and costly) trends towards deep ultra-violet laser optics in semiconductor fabrication illustrate.

The gamut of light-optical analytical methods central to materials characterization studies: microscopy, polarization and spectrometry in many guises, suffer the diffraction-imposed resolving limit. Synge, (1) in 1928, described but was not able to demonstrate sub-wavelength imaging using "near-field" light. The near-field was first employed for microscopy in 1972 by Ash and Nicholls (using microwaves). Finally the invention and subsequent development of scanning probe "microscopy" (SPM) methods, from the original STM of Binnig and Rhrer in 1982 to the AFM and a host of other variations produced the necessary tools for an original step forward in optical methodology. Near-field light optical microscopes have been produced by many workers and conventionally employ the SPM's nanometre precision piezoelectric raster-scanning together with nanometrically-sharp probes to obtain light optical images exceeding the usual wavelength-limited resolution.

All the familiar "far-field" optical contrast formation mechanisms are applicable in near-field imaging but at much higher spatial resolution, the true limit for which is not yet known. The logical development of this "new" light microscopy is then to apply it to chemical and structural characterization by the addition of spectroscopic analysis. We describe here some recent and novel spectroscopic results obtained from a commercial SNOM equipped with an aperture fiber probe and a Raman / photoluminescence spectrometer with CCD detector.

Figure 1 shows the schematic illustration of our near-field micro-spectrometer. In the case of illumination-collection mode the exciting laser light is introduced into the fiber probe and irradiates the sample, then Raman scattering or photoluminescence is collected by the same fiber probe. On the other hand in the case of collection mode the laser irradiates the sample directly, then scattered light is collected by the fiber probe. The sample-probe separation is regulated by monitoring of a resonating probe to maintain the aperture of the probe in the near-field. The topographic image of the sample surface is then measured by monitoring variations in the position of the sample Z stage while XY scanning. The light from the probe is introduced to the monochromator and detected by a cooled CCD. A more detailed description of the system with illumination mode and optical fiber probe was reported elsewhere (2). Mapping of near-field micro spectroscopy at low temperature (Liquid He) can be performed in a cryostat.

A scanning electron microscope (SEM) photograph of the probe is shown in Figure 2. The tip was fabricated using the chemical etching method, which gives high reproducibility. Many different types of probe can be fabricated simply by changing the composition of the etching solution. The probe surface is coated by a metal such as gold or aluminum. The aperture of 50 to 1000nm sizes was prepared by pushing the probe against a flat substrate.

This work was funded by the Japan Science and Technology Corporation.

1. Synge E.H, 1928 Philos.Mag.6, pp 356-62
2. Marita Y. Tadokoro T, Ikeda T, Saiki T, Mononobe S and Ohtsu M. Appl.Spctrosc. 52(9) (1998) pp 1141-4