Stark Effect in the Optical Near-Field

Optical near-field microscopy (SNOM and NSOM [24,25]) is a novel technique with a spatial resolution power well beyond the diffraction limit. It was applied to image the fluorescence of single molecules at room temperature [26, 27] (see Chapter 2). Typical spectral linewidths reported were around 20 000 GHz [27], due to the room temperature operation. At liquid helium temperatures the linewidths are reduced to about 10-100 MHz, which allows study of the special effects on single molecular resonators. Moemer et al. [28, 29] combined the SNOM-technique with Single Mol- 

The optical apertures used in near-field microscopy are usually prepared by pulling a heated optical fibre until it breaks [25]. The sides of the tips are coated with aluminum. The typical diameters of the apertures produced by this technique are 60 10 nm, which is about one tenth of the optical wavelength. The transmission of such a tip ranges from 10 to 10 . The near-field tip is mounted on a xyz-piezo-electric (PZT) tube scanner to control the fine approach (2) to the surface and the lateral dithering (x,y) of the tip. The coarse z positioning was achieved by a coupled spring and steel plate comparable to the setup described in [30]. The sample was connected to a small glass hemisphere to minimize losses due to internal reflections and mounted in the focus of a paraboloid mirror with a numerical aperture of NA = 0.98. The whole setup, paraboloid mirror, sample, and PZT tube with the fibre tip, was then mounted inside a cryostat and immersed in superfluid Helium at 1.8 K. 

The approach of the tip to the sample surface was carefully monitored by observing the background fluorescence increase (BFI) [28] and by shear force detection. It was stopped at about 250 nm. The investigated molecules are not necessarily near the surface. Three techniques were used to determine the distance between the tip and the individual molecule (i) saturation measurements, (ii) static Stark effect, and (iii) static Stark effect with lateral dithering. Fig. 18 illustrates typical measurements using these techniques. 

Molecules near the tip are expected to saturate easily. Reducing the excitation power should decrease their signal strength less than in a linear way. The spectrum 

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