Feasibility of molecular-resolution fluorescence near-field microscopy using multi-photon absorption and field enhancement near a sharp tip

Aperture-based near-field microscopy suffers from fundamental limitations imposed by the electromagnetic skin depth of the aperture material and a rapidly decreasing throughput as the aperture is made smaller. Apertureless approaches without these limitations have been demonstrated for coherent imaging but are not easily applicable to incoherent processes such as fluorescence or Raman scattering and to photochemical surface modification. Using multi-photon processes in conjunction with the field enhancement that occurs at a sharp tip in close apposition to a substrate should permit substantial localization of absorption and excitation to a nm sized volume. The expected enhancement of the optical field at the tip edge is calculated here for various combinations of metallic and nonmetallic tip and substrate materials. It is estimated that when using 100 fs pulses repeating at 100 MHz average laser powers of about 10 mW should be sufficient to reach saturating field strengths for three- photon absorption. Steady state and instantaneous temperature rises at the tip are estimated and found likely not to be a limiting factor. Fluorescence quenching is expected to limit the resolution achievable with metallic tips to about 5 nm, but tips made from highly refracting insulators or semiconductors should allow truly molecular resolution. (C) 1999 American Institute of Physics. [S0021-8979(99)05303-7]