I. Jahresfeier am 30. Mai 2015

everything looked brighter, because thinking about light microscopy took on a new
meaning.
So I decided to pursue the thesis work as initially requested, but what really
motivated me was the resolution problem. I knew of course that near-field optical
microscopy existed, but it seemed to me like a kind of scanning tunneling mi-
croscope. In contrast to that and notwithstanding the merits of near-field optical
microscopy, I wanted to come up with a light microscope that looks like a light
microscope and operates like a light microscope - but without the limits set by dif-
fraction. So I began to comb through my textbooks again searching for phenomena
suitable for breaking the diffraction barrier. I pondered all kind of options from
the Stark to the Zeeman effect. I even checked textbooks on nuclear physics. My
efforts weren’t initially met with success.
But one thing came up most naturally: Virtually isolated from the optics Com-
munity, I had figured out how to calculate the focal light field at large focusing
angles, and had written a Computer program to do so. I had solved the problem in
my own way and had a lot of fun playing around with the field calculations, which
worked beautifully. The largest focusing (i. e. aperture) angle of the best objec-
tive lenses at that time was around 71°. Of course, I also plugged the theoretically
largest value of 90° into my program, which corresponded to a converging hemi-
spherical wavefront and I also calculated what would happen for a complete sphere.
While the last two cases were interesting but impractical, far more realistic was to
calculate what would happen if one juxtaposed two lenses with a 71° aperture an-
gle and caused their wavefronts to add up constructively at a common focal point.
That the main diffraction peak would become three to four times sharper along
the optical axis (z) than with the best single lens was to be expected. However, less
obvious was the outcome that the secondary diffraction peaks along the axis were
small enough to be discriminated against in a potential image. So it seemed feasible
to improve the resolution along the optic axis by 3-4 fold, by using two counter
aligned —70° lenses in a coherent männer. That was the idea behind what was later
to be called the 4Pi microscope.
Back then I called it the double-lens microscope and presented the results
sometime in 1988 in Professor Hunklinger’s seminar series - as an addendum to
what I was actually supposed to do. The idea was perceived as interesting, but the
difficulties in aligning two lenses to focus at the same point and Controlling the
phase of the wavefronts were thought to be daunting. And, of course the concept
wasn‘t suitable for Silicon wafers - only for transparent specimens such as biolog-
ical cells. Actually, I set off to try it out, but Heidelberg Instruments disintegrated
into several subunits in 1989, and Prof. Hunklinger resigned from it. It is left to
be noted that the subunit dealing with confocal microscopy was purchased by the
Company Ernst Eeitz which later became Leica Microsystems GmbH, a leading
supplier of confocal microscopes.

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