At the beginning of the 1990s, a new kind of near-field optical microscope was developed at the IBM Research Laboratory in Rueschlikon. It combined the "best of both worlds", the Scanning Near-Field Optical Microscopy (SNOM) and the Scanning Tunneling Optical Microscope (STOM) or Photon Scanning Tunneling Microscope (PSTM).
What is Forbidden Light?
Due to its size, a SNOM probe can be described as a spatial delta-function for the emitted light intensity. Therefore, the Fourier spectrum contains a wide variety of spatial frequencies. That is why a SNOM probe is a source not only for propagating light, but also for non-propagating (evanescent) light. The forbidden light, with its high spatial frequencies, contains most of the information on sample structures that are smaller than half of the wavelength.
How Forbidden Light is Achieved and What Benefits It Can Offer Researchers
When the tip approaches a sample surface and comes closer than the decay length for evanescent waves, the evanescent light is converted into propagating light. This light is radiated under supercritical angles, i.e. angles that are greater than the angle of total internal reflection for the dielectric sample. Collection of this forbidden light allows us to obtain additional information on the optical properties of a sample. Detection of the high spatial frequencies is favourable not only because of the resulting higher resolution, but also since these contain the k-vectors that can excite surface plasmons in thin metal films and excitons in semiconductors and isolators.
The Process for Viewing Forbidden Light and Allowed Light Using Scanning Near-Field Optical Microscopy (SNOM)
If a sample is prepared - as conveniently - on a glass slide, total internal reflection on the lower part of the slide occurs for light with an evanescent nature. Therefore, this light cannot be detected with a normal lens. If we use an oil immersion objective, the allowed light and parts of the forbidden light are detected simultaneously. That is why we use a different approach, where light with a different nature can be detected separately. In our instrument, the sample holder consists of a glass hemisphere onto which the sample is attached by means of immersion oil. The propagating allowed light is detected with a lens mounted directly under the glass hemisphere. Subsequently, the allowed light is guided via two perpendicularly oriented mirrors, through a polarization analyzer and a pinhole onto a photomultiplier tube.
Figure 1. A diagram showing the process for observing forbidden light with Scanning Near-Field Optical Microscopy (SNOM).
Collecting Forbidden Light with Elliptical Mirrors
The forbidden light is collected by an elliptical mirror with the tip being positioned in one of its focal points. In the second focus, another photomultiplier tube is placed. The forbidden light can be analyzed for all azimuth angles with an aperture mounted above the second focus.
What is Constant Height Mode Scanning?
In order to avoid topographical artefacts, that induce a "super-resolution" by the constant gap-width operation of the probe, we use constant height mode scanning. To do so, fractions of the xy-scan voltage are added to the z-voltage of the feedback loop to compensate for the sample slope during the scan. A constant offset voltage retracts the tip until it leaves of the shear force interaction range. This permits the tip to be scanned within an average height plane of a few nanometers above the sample. Since the feedback loop is still active, it can still retract the tip if it bounces into a feature that exceeds this average plane.
The Different Behaviour of Allowed Light and Forbidden Light
Upon approaching the tip to the sample, the allowed light and forbidden light show a distinctly different behaviour. The allowed light is governed by interference undulations. These occur since a part of the light is back-reflected on the sample surface, subsequently on the tip and is then superposed with the directly transmitted light. The forbidden light, due to its evanescent nature, shows an exponential increase when the tip is approached towards the sample. At a tip-sample separation of one wavelength, almost no forbidden light can be detected.
Figure 2. Chart showing the different behaviour of forbidden light and allowed light, with regards to light intensity and piezo position.
The Importance of the Tip-Sample System in Scanning Near-Field Optical Microscopy (SNOM)
In theoretic approaches on SNOM, it has to be kept in mind, that tip and sample are a system. They belong together and influence each other. This results in a complicated distribution of the electric field between tip and sample that can hardly be treated analytically and has to be studied numerically. One method for calculating the field distribution for a tip-sample system is the Multiple-Multipole Method (MMP) invented and applied at the Swiss Federal Institute of Technology (ETHZ) in Zurich. An analytical approach has been undertaken by van Labeke et al. in Besancon.
Research Goals for Studying Forbidden Light Using Scanning Near-Field Optical Microscopy (SNOM)
Our research with the forbidden light SNOM is directed towards the following interests:
• Better understanding of near-field optics,
• Interaction of sample features with the near-field,
• Excitation of surface plasmons and/or single molecule fluorescence,
• Gain maximum information on unknown samples,
• Compare computer simulations with experimental results.
Note: A complete set of references can be found by referring to the University of Basel website.