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.
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