Single-molecule methods optical fibers

Fig. 2.8. Left (A) Optical configuration for exciting a single molecule with a nearfield fight source, an Al-coated pulled optical fiber. Application of a potential V to the A1 coating produces a highly anisotropic DC local electric field. (B) Spectra of pentacene in p-terphenyl molecules at various applied potentials, (a-b) saturation method to identify molecules close to the tip, (c-1) transverse dithering with Stark shift. For details, see [63]...

The experimental techniques for single molecule spectroscopy described in the previous chapters differ mainly in the method employed to reduce the excitation volume of the sample (combined with different fluorescence collection methods). This was achieved in four different ways (i) the laser was focused to a tiny spot on the sample by a lens immersed in liquid helium, (ii) the excitation light was coupled into an optical fiber carrying the sample at its end, (iii) the sample was mounted behind a small aperture (pinhole with typically 5 pm diameter). All these methods reduce the excitation area to a few pm. The near-field technique (iv) allows investigations beyond the classical diffraction limit the tapered tip used had a typical diameter in the order of 50-100 mn. 

The principle behind SFM is that the lateral or shear force between an oscillating probe tip and the sample increases as the distance decreases. The probe is usually mounted in a support such that several millimeters of the aperture end of the optical fiber extends beyond the clamping point. The probe thus forms a cantilever having one fixed and one free end. It is driven transversely at a so-called tip resonance , which indicates that the resonance is due to the cantilever rather than the support structure of the microscope, with an amphtude 5nm. Shear forces between the probe tip and sample surface damp the oscillation. The amplitude is measured and fed back to the sample height position so as to maintain constant oscillation amplitude and presumably constant tip-sample distance. The amplitude was measured, originally, with optical deflection methods. Recently, a number of electrical measurement schemes have been demonstrated that may prove to have a number of advantages in speed, sensitivity or ease-of-use [12]. In near-field single molecule experiments the bandwidth of the feedback is not an issue as scan rate is limited by... 

Of course, the achievable polarization anisotropy is limited by the degree of orientational order of polymer backbones (Equations (2.7a) and (2.7b)), and by the degree of alignment of the optical transition dipole moments (Equation (2.8)). For instance, polarized Raman microscopy is an especially powerful tool to evaluate the order of molecules within single nanofibers. By this method values of the orientational order parameter (Section 2.1.5) up to 0.75 have been found for poly(ethylene terephthalate) (PET) electrospun nanofibers. In addition, interesting correlations can be drawn between the orientational order of molecules and fiber fabrication parameters, used solvents, etc. The investigation of electrospun poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(l,4-benzo- 2,T-3 -thiadiazole)] nanofibers shows that more ordered molecular chains, without nanoscale aggregates, are obtained when the nanostructures are produced from THF solutions rather than from mixtures of THF and dimethyl sulfoxide (DMSO). ... 

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