Second Harmonic Generation
Definition and meaning of Second Harmonic Generation in chemistry.
Second harmonic generation (SHG) is a specialized nonlinear optical process in which two interacting photons within a nonlinear material combine to form a single new photon possessing exactly twice the energy and half the wavelength.
In more detail
This unique optical phenomenon only strictly occurs in chemical materials that completely lack inversion symmetry, such as certain non-centrosymmetric crystals or the distinct interfaces between two different isotropic media. Because SHG is completely forbidden in the bulk of centrosymmetric media, it is an exceptionally powerful and selective spectroscopic tool for probing surface properties and interfacial molecular chemistry. It is also routinely used in commercial laser technology to efficiently convert infrared laser emission into visible light.
Key facts
| Field | Physical Chemistry |
|---|---|
| Requirement | Non-centrosymmetric environment |
| Result | Frequency doubling |
A high-power Nd:YAG laser emitting infrared light at 1064 nm can be passed directly through a potassium titanyl phosphate (KTP) crystal to successfully produce a bright green beam at 532 nm via SHG.
Frequently asked questions
Why is SHG uniquely sensitive to surfaces and liquid interfaces?
In bulk isotropic materials with inversion symmetry, SHG is quantum mechanically forbidden, so the measured optical signal only originates from the narrow surface or interface where this symmetry is naturally broken.