I. Jahresfeier am 30. Mai 2015

Why excite the molecules, why not de-excite them, i. e., keep them dark in order
to separate them from their neighbours. I was electrified by the thought and imme-
diately checkcd Fritz Schäfer‘s book on dye lasers to see what was reported about
the stimulated emission of fluorophores such as rhodamines. A quick assessment
showed that at least 30-35 nanometres could be achieved in the focal plane, i. e.
6-8 times beyond the diffraction barrier. That was amazing. It was also instant-
ly clear that the achievable resolution only depended on the intensity the sample
would tolerate, and in principle was unlimited. What also intrigued me was the fact
that the resolution could bc obtained without a priori assumptions about the dis-
tribution of features to be imaged. Mathematical processing was also not needed.
The concept was based just on the use of a basic state transition, i. e. just on physics.
I fmally had an example of the type of approach I had been seeking for. It was the
concept of STED microscopy.
But it wasn’t so easy to test this idca in Turku. I also thought that a tunable dye
laser would probably be needed to optimize for de-excitation. But there was no dye
laser to be had far and wide. I therefore feit I should publish the idea in theoretical
terms in such a way that it was as close as possible to reality and therefore hard
to challenge. Jan Wichmann, a Student from Heidelberg, whom I knew privately,
had expressed his desire to come to Turku for two weeks in December to work
with me as an intern after finishing his diploma work with Prof. Jürgen Wolfrum.
I explained the concept to him and we modelled it numerically to be sure that the
numbers are as close as possible to a real experiment. The paper proposing STED
microscopy eventually read like a recipe: it was full of numbers. I omitted anything
that could be interpreted as an exaggeration, because I was very much concerned
about a possible rejection. Still, I wrote it to convince the community that nanos-
cale far-field fluorescence microscopy is viable, as well as in the hope to get a job
and the funds to do it. Whether I would ever be able to realize it myselfwas indeed
doubtful at that time, because the Finnish Academy grant was gradually nearing
end. Yet, in retrospect, I must say that the time in Finland, and working with Pekka
Hänninen and Erkki Soini, were both exciting and important in my career.
I also realised that stimulated emission is not the only state transition that can
be used to the same end. After all, the basic idea was to ensure that a part of the
features illuminated by the excitation light remain briefly dark so that they can be
separated from other features residing within the diffraction ränge. So I had the
idea of parking the fluorophores in a dark metastable state, something dye laser
operators were trying to avoid at all costs. This also had the important benefit of
requiring less intense light. Since all my papers were published in specialised optics
journals (which didn’t make my CV look particularly impressive), I submitted this
idea to a more general physics journal. When I received no response after months,
I mustered all my courage and called the editor, who happened to be German.
He told me that he had doubts about whether the diffraction limit could actually

28