In biological and medical science, the effect of second harmonic
generation is used for high-resolution optical microscopy. Because of
the non-zero second harmonic coefficient, only non-centrosymmetric
structures are capable of emitting SHG light. One such structure is
collagen, which is found in most load-bearing tissues. Using a
short-pulse laser such as a femtosecond laser
and a set of appropriate filters the excitation light can be easily
separated from the emitted, frequency-doubled SHG signal. This allows
for very high axial and lateral resolution comparable to that of confocal microscopy
without having to use pinholes. Most importantly, SHG signal is
generated without the need for staining samples with fluorescent labels.