Stimulated emission depletion (STED) has been used to
break the
diffraction limit in fluorescence microscopy. Inspired by this success,
similar methods were used to reduce the structure size in three-dimensional,
subdiffractional optical lithography. So far, only a very limited
number of radical polymerization starters proved to be suitable for
STED-inspired lithography. In this contribution, we introduce the
starter Michler’s ethyl ketone (MEK), which has not been used
so far for STED-inspired lithography. In contrast to the commonly
used 7-diethylamino-3-thenoylcoumarin (DETC), nanostructures written
with MEK show low autofluorescence in the visible range. Therefore,
MEK is promising for being used as a starter for protein or cell scaffolds
in physiological research because the autofluorescence of DETC so
far excluded the use of the green emission channel in multicolor fluorescence
or confocal microscopy. In turn, because of the weak transitions of
MEK in the visible spectrum, STED, in its original sense, cannot be
applied to deplete MEK in the outer rim of the point spread function.
However, a 660 nm laser can be used for depletion because this wavelength
is well within the absorption spectrum of transient states, possibly
of triplet states. We show that polymerization can be fully stopped
by applying transient state absorption at 660 nm and that structure
sizes down to approx. 40 nm in the lateral and axial directions can
be achieved, which means 1/20 of the optical wavelength used for writing.