Second Harmonic Generation

SHG is a second-order nonlinear optical process, which requires an environment without a centre of symmetry to produce signals. Two photons (ω_p) are mixed in the sample to generate a third photon (2ω_p) Several indigenous tissue components are known to exhibit SHG which can be used to generate label free images. Collagen fibres are a well-known source of SHG; the picture below shows an SHG image of collagen fibres within a rat tail tendon.

SHG deposits no energy into the sample due to the emitted SHG photon energy being equal to the total absorbed excitation photon energy.

Example

Collagen in a rat tail tendon viewed with SHG