Stark-shift microscopy : interaction of a confined electric field with single emitters

In this thesis, a new technique, which we call Stark-shift microscopy, has been established
for the the manipulation and the detection of single molecules and their
optical properties. Stark-shift microscopy is performed combining fluorescence excitation
spectroscopy for single-molecule detection and the perturbation of the molecular
resonance by an externally controlled, inhomogeneous electric field of a sharp,
metallized tip.
The available setup of a confocal microscope and a scanning-tip unit for low
temperature application has been adapted for the use as Stark-shift microscope. Especially
important has been the selection of a suitable tip, the realization of a reliable
and good electrical contact to the tip and the establishment of a positioning control
of the tip by optical observation. Two different preparation techniques of the sample
system have been tried to optimize the properties of the sample for Stark-shift microscopy.
The experimental setup offers a large variety of control parameters for the fieldmolecule
interaction. An experimental protocol has been elaborated to run and to
control the imaging of single molecules by Stark-shift microscopy. The imaging protocol
is based on operating experiences from the experimental measurements and is
additionally reinforced by theoretical calculations.
A theoretical model for Stark-shift microscopy has been developed, which shade
light on the interaction of the electric field of the tip with single molecules. The
numerical calculation of Stark-shift patterns underlined the experimental results and
contributed to their understanding. Depending on the orientation of the molecule
and the Stark-shift coefficients (�~μ and ˜α), the Stark-shift patterns show circular or
elliptical features. The numerical calculations gave indications for further detectable
effects, such as dipole-dipole coupling between molecules or the flipping of a TLS in
the nearby environment. These effects are highly interesting to investigate and they
can be made visible by Stark-shift microscopy.
Single molecules have been imaged by Stark-shift microscopy at cryogenic temperatures.
Experimental measurements could be controlled by the tip voltage, by the
detuning and the gap-width. The influence of the experimental parameter settings
has been fully understood and could be controlled during the measurements. First
indications of effects, such as the the orientation of the dipole moment difference and
the coupling to a nearby TLS have been obtained. Several molecules, spectrally separated
by 170 to 420 MHz, have been resolved in a single tip scan. Hereby, the tip has
been scanned at a constant height of about 3 μm and the in-plane distance between
the molecule has been determined to be r = p(x − x0)2 + (y − y0)2 = 46 ± 11 nm.
The experimental setup and control has been established. Further improvements
of the experimental setup might be the implementation of a full-metal tip with a
smaller apex for a stronger electric field gradient. Full-metal tips are also less fragile.
For such a tip implementation, it is also necessary to develop a new mechanism
for the tip positioning. Future work might be directed towards the demonstration
of the simulated effects, such as the direct imaging of dipole-dipole interaction of
single molecules. Stark-shift microscopy might be a very powerful tool for fascinating
experiments to manipulate and tune the coupling or entanglement of single molecules
[15, 98, 99, 100, 101].