Optimising tip-enhanced optical microscopy

We discuss the range of tip-enhanced optical microscopies - namely mapping scattered light, fluorescence, Raman and coherent anti-Stokes Raman scattering (CARS) signals on the scale of 10-20 nm. Through the use of measurements and finite-element simulations, we explain what the optimum tip materials and sizes are and illumination wavelengths to use for all these modes of imaging. We also show that the observed limit of 50-100 mu W of illumination power is due to a temperature rise of tens of degrees beneath the tip apex, and is most likely linked to the evaporation of water from the tip apex region. This has two important implications - that the tip is surrounded by water even when imaging 'in air', and that the tip and sample are both heated when imaging. With emphasis on Raman and CARS, we discuss how both these types of tip-enhanced optical microscopy can be made about as fast as their diffraction-limited counterparts. Copyright (C) 2009 Her Majesty the Queen in right of Canada. Published by John Wiley & Sons. Ltd.