Optical microresonators are structures which confine light to a small region in the range of one wavelength. The
radiation of a quantum emitter is coupled to cavity resonances which leads to an optical confinement of the broadband
fluorescence. A practical design for this single-mode microresonator is formed by two silver mirrors enclosing a
transparent dielectric medium with single quantum emitters. In our tunable microresonator, the resonator length can be
changed reversibly with piezoelectric elements to a distinct position corresponding to a specific emission wavelength.
The local mode structure of the electromagnetic field is changed at this position which results in a redistribution of the
fluorescence and a modification of the lifetime for the same single molecule. The radiative coupling of the emitter to the
electromagnetic field is also determined by the orientation of its transition dipole moment with respect to the cavity
normal. The doughnut laser modes used for illumination of the single molecule allow us by analyzing the excitation
patterns to determine its three-dimensional orientation in the microresonator. In addition, these modes provide an
excitation pattern which can be used to detect the longitudinal position of a fluorescent bead in the microresonator with
an accuracy of a few nanometers.
We present experimental results on changing the fluorescence spectrum of a single molecule by embedding it within a
tunable optical microresonator with subwavelength spacing. The cavity length is reversibly changed across the entire
visible range with nanometer precision by using a piezoelectric actuator. By varying its length, the local mode structure
of the electromagnetic field is changed together with the radiative coupling of the emitting molecule to the field. Since
mode structure and coupling are both frequency dependent, this leads to a renormalization of the emission spectrum of
the molecule. Moreover, we use doughnut laser modes in the tunable microcavity to determine the longitudinal position
of an isotropic emitter. By analyzing the excitation patterns resulting from the illumination of a single fluorescent bead in
the focus of a radially polarized doughnut mode laser beam we can determine the longitudinal position of this bead in the
microcavity with an accuracy of a few nanometers.