Photochemistry and Spectroscopy Department
Institute of Physical Chemistry, Polish Academy of Sciences
The immersion mirror objective was specially developed for working in liquid helium [J. Jasny and J. Sepioł, Chem. Phys. Lett. 273, (1997) 439-443]. It has the transverse magnification of 100 and the object field diameter of 120 microns. The numerical aperture is 0.722. It is a homocentric mirror objective, consisting of two Suprasil components. The smaller element is fabricated from a full silica glass sphere. Its polished planar surface serves as the specimen holder. A sample is placed between the holder and a cover slip and all elements resist the temperature of liquid helium. The objective has a very small depth of field which enables determining, within 1 micron, the position of the emitting species in the object plane. The annular aperture of the objective is very useful in this case. Only in the image plane (dz=0 plane, related to the selected object plane, see Figure) the image is a small spot. A molecule situated outside the selected object plane produces a fuzzy annularly shaped image. Consequently, a molecule detected as a spot in the dz=0 plane will be seen as annular zones at -dz or at + dz.
One can expect that the light intensity distribution in the annular image is rotationally symmetric only if the point object emits unpolarised light or if the polarisation direction lies parallel to the optical axis. On the other hand, the resulting intensity distribution of the ring will be irregular if the transition dipole of the sample molecule lies at a non-zero angle with respect to the optical axis.
