Meeting the stringent requirements of optical design has been the task of material developers for centuries. Preventing simple solutions, however, are the fundamental limitations to the performance of optical materials following the Kramers-Kronig-relations of solid state physics. To achieve optimum optical performance, an entanglement between optical designers and material developers has occurred. This entanglement has led to suitable multi-lens designs that push optical materials to their limits. Herein is a contribution that reviews pioneering work in optical material development, the role of Otto Schott and the intricate entanglement of optical design and optical materials.

We present systematic experimental investigations on the effects of laser polarization and interface orientation in second and third harmonic generation microscopy. We find that the laser polarization has no measurable effect on signal strength and resolution in third harmonic microscopy, while the second harmonic strongly depends upon the polarization direction of the driving laser. Moreover, we observe a strong effect of the interface orientation with respect to the laser beam direction-both in second and third harmonic generation. This affects the signal strength, as well as the obtained transversal and longitudinal resolution in microscopic imaging. As an (on the first glance) surprising feature, also surfaces parallel to the optical axis of the laser beam yield strong harmonic signal. This enables applications of harmonic microscopy in specific geometries. As an example we monitor the flow of immiscible microfluids in lateral cut by third harmonic microscopy.

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We present experimental investigations, accompanied by numerical simulations on third harmonic generation microscopy at interfaces. In particular we study the variation of the emitted third harmonic intensity profile with the interface orientation. Our data confirm previous theoretical predications that only at interfaces perpendicular to the direction of the fundamental laser beam can the generated third harmonic profile exhibit a single spot in the forward direction. At interfaces parallel with the direction of the fundamental beam, the third harmonic intensity profile moves outside the forward direction and develops into a double-spot beam with a large opening angle. As an important consequence for implementations of harmonic generation microscopy, the numerical aperture of the double-spot third harmonic beam exceeds the numerical aperture of the fundamental beam.