An Introduction to Near-Field Scanning Optical Spectrometry

Key Points:- Near-field: Light that does not propagate through space but is localised on the surface of objects. Propagating (normal) light is called far-field. Illuminated objects in general have both near-field and far-field light components but “conventional” imaging uses the far-field light only. Evanescent light A more poetical way of saying “near field”, meaning fleeting or scarcely perceptible. Also “forbidden light”. Scanning near-field optical microscope (NSOM): A microscope that uses either:- a) the interaction of a sharp probe tip with near-field light on a sample or b) a sub-wavelength aperture near-field light source and / or detector, in order to image the surface at sub-wavelength optical resolution. The spatial resolution is determined by the size and shape of the probe tip. Scanning Probe microscopes (SPM): Any one of a growing variety of high resolution surface microscopy techniques (including NSOM) that use a very sharp tip, maintained very close to or touching the sample surface and scanned X-Y style over the sample by using a combination of piezo-electric transducers and stepping motors.  Common acronyms used in near-field : AFM Atomic force microscopy CCD Charge-coupled device, usually implies a light detector device FF Far-field (normal light waves) NF Near-field NOM Near-field optical microscopy NSOM Near-field scanning optical microscopy (= NOM) PSTM Photon scanning tunneling microscope (= NOM) PZT Lead zirconate titanate: a perovskite piezo-electric material Pb(Zr,Ti)O3 SEM Scanning electron microscope SNOM Scanning near-field optical microscopy (= NOM) SPM Scanning probe microscopy (= AFM, NSOM, STM.. .) STM Scanning tunneling microscopy Background 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, since the 19th Century, the role played by light waves and diffraction in image formation and the correspondingly fundamental limits to resolving power have been an unwelcome but understood barrier to optical imaging, whether by light waves or electrons. In approximate but reasonably practical terms, 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 trends towards deep ultra-violet laser optics in semiconductor fabrication illustrate. The gamut of light-optical analytical methods central to materials characterisation studies: microscopy, polarisation and the various spectrometries, suffer the diffraction-imposed resolving limit.  Synge, in 1928, described but was not able to demonstrate sub-wavelength imaging using “near-field” light.

The near-field was fast 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 Binning and Rohrer in 1982 to the AFM and a host of other variations, have 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 nanometer precision piezoelectric raster-scanning together with nanometrically-sharp probes to obtain light optical images at rather better than the usual. All the familiar “far-field” optical contrast formation mechanisms are applicable in near-field imaging but at much higher spatial resolution, the ultimate limit for which is not yet known, The logical development of this “new” light microscopy is then to apply it to chemical and structural characterisation by the addition of spectroscopic analysis. We describe here some recent and novel spectroscopic results obtained from a commercial SNOM equipped with an aperture fibre probe and a  CCD detector and photoluminescence/luminescence spectrometer. What problems exist in present microscopy techniques? Optical systems of any kind that use lenses or mirrors to form an image are limited in their spatial resolution, even with the best designs. This limit is best known as the diffraction limit, or Abbé's limit, after the 19th century optical researcher who first described the optical imaging process in terms of the diffraction of light waves. Abbe showed that the smallest object or feature, that can be resolved by using a lens system, is about half the wavelength of the light used: d= 0.61l/n sinq n is the refractive index of the medium (usually air, n = 1.0) between the sample and objective lens; q is the acceptance angle of the lens (q = 1.0 if the angle is 90o - not actually possible!). dmin = 0.61 l n sin q is called the lens numerical aperture, NA. Reducing the wavelength (say, from red light to blue) can improve the resolution, as can using an oil immersion lens (n >1.0), but only so far. Using ultra-violet light is an expensive but increasingly popular approach to sub-micron microscopy. So optical microscopy is limited to roughly half a micron resolution, even with the best optics. d » so for green light of 532 nm, d » 266 nm. However, lens aberrations further limit this. A more reasonable figure is d » l This wavelength dependency is familiar to users of infrared imaging techniques, where spatial resolutions of several microns are normal. What are “Near-Field” Light and “Near-Field” Microscopy? The light field at the surface of an object actually contains more information - higher spatial frequencies - than we can image by using a “far-field” lens system. Only the spatial frequencies that reach the imaging lens (pass through the numerical aperture) are “seen”. These are the propagating, low-frequencies. Higher spatial frequencies exist at the sample