Nano-Optics Graz Methods

Near Field Optical Microscopy


Spatial resolution in conventional optical microscopy is limited by diffraction to about half the light wavelength. Nevertheless, light fields spatially modulated at much smaller scales can exist close to illuminated surfaces, nanoparticles or molecules. This so-called near field zone is probed by near field optical microscopes which rely on either a nanoscopic light source or a nanoscopic light detector placed close to the sample of interest.

PSTM setup

Here in Graz, we run a variant of a scanning near field optical microscope known as photon scanning tunneling microscope (PSTM). The sample is illuminated with a conventional light source and the optical near field zone of the sample is probed by an optical fiber tapered to a sharp tip. This tip (held in a few nanometers distance from the sample surface) scatters part of the optical near field into propagating modes in the fiber which in turn give rise to a signal at a photodetector attached to the far end of the fiber. Scanning the fiber tip over the sample and assigning the measured light intensity to the lateral tip position allows to acquire a non-diffraction limited optical image. To avoid the fiber tip from touching the surface, we apply a tuning fork based shear force feedback to maintain a constant tip-surface distance during scanning.

Near field probes

As near field probes we utilize wet-chemically etched tips of mono-mode optical fibers. No metal coating is applied to the tips, ensuring they can be considered non-invasive for a variety of samples, i.e., these tips disturb the near field only minimally compared to the undisturbed case. As a trade-off, however, these tips are sensitive not only to optical near fields, but also to propagating light modes that can couple via the tip taper to fiber modes. Scattered light levels therefore have to be kept low in the region of the fiber cone.

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Illumination modes

We apply two sample illumination schemes, termed global and local. In both cases the excitation is accomplished by a laser beam, incident from the substrate side (transparent substrates are thus evidently needed) with angels of incidence larger than the critical angle of total internal reflection. Therefore, light directly transmitted to the region of the fiber tip is minimized, a prerequisite for the application of purely dielectric fiber tips.
For global illumination, we use a collimated or only weakly focused laser beam, incident from the substrate side of the sample. For this purpose, the sample substrate is optically coupled to a glass prism by index matching oil. Over the areas probed by the fiber tip (maximum 100 µm x 100 µm) the excitation intensity can be considered as constant.

Local illumination is realized with the help of high numerical aperture immersion objective, which allows a relatively tight focusing of the exciting laser beam, again at angles larger than the critical angle of total internal reflection. With the setup sketched to the right, the mean angle of incidence can be adjusted by varying the distance d of the laser beam from the optical axis of the objective, and the focal diameter can be controlled in the range of ~1 to 5µm by changing laser beam diameter. When working on surface plasmons, this setup allows to apply leakage radiation microscopy simultaneously to PSTM imaging.

Example

Global illumination: Optical near fields of multipolar plasmon excitations on gold nano-rods.
A.Hohenau et al., Europhys.Lett. 69, 538 (2005)
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Gold nanorod topography
(image size 4 x 2.5 µm)
Optical near field image for resonant excitation
(light wavelength 830 nm)
Optical near field image for non-resonant excitation
(light wavelength 750 nm)
        Modified 5.5.2007