Two Photon Image

To evaluate the usefulness of two-photon NSOM (TP-NSOM) we imaged the fluorescence from isolated single Rhodamine B molecules in a thin layer of PMMA.  Uncoated probes were used to eliminate damage problems associated with metal coated probes due to the relatively high optical powers necessary for efficient two-photon excitation.

The Rhodamine B molecules were excited by light from a modelocked 100 fs Titanium:Sapphire laser operating at 800 nm with a repetition rate of 80 MHz.  A grating before the fiber coupler corrected for linear group velocity dispersion effects in the fiber and ensured that the pulse width at the fiber output remained 100 fs.  Typically, uncoated probes with apex diameters of 200 nm were used.

Figure 8 is an image of isolated single Rhodamine B molecules recorded by two-photon induced fluorescence using an average transmitted power of 150 mW.  Scans using different excitation intensities confirmed the expected dependence of the fluorescence on the square of the intensity (data not shown).

fig 8: Two-Photon SMD Image
 

Figure 8. Image of two-photon induced fluorescence from single Rhodamine B molecules.

TP-NSOM Resolution