Total internal reflection fluorescence apparatus

Techniques are provided for illuminating a sample by using total internal reflection (TIR) from a diffraction limited focused annular illumination beam. The illumination forms an affected region and an overlapping confocal region that may have dimensions the below 1 mum. An adjustable diffractive optical element, for example, may create a second order Bessel profile laser beam that is focused on a sample using a high-numerical aperture objective under TIR. An evanescent field excites fluorescent biological material in the confocal region, and fluorescence from the material is analyzed in fluorescence correlation spectroscopy system.