Editing Raman spectroscopy (section)

=== Non-linear Raman spectroscopy ===
Raman signal enhancements are achieved through non-linear optical effects, typically realized by mixing two or more wavelengths emitted by spatially and temporally synchronized pulsed lasers.
* ''Hyper Raman'' – A [[non-linear optics|non-linear]] effect in which the vibrational modes interact with the [[second harmonic generation|second harmonic]] of the excitation beam. This requires very high power, but allows the observation of vibrational modes that are normally "silent". It frequently relies on SERS-type enhancement to boost the sensitivity.<ref>{{cite journal | author = Kneipp K | author2 = Knelpp H| title=Surface-Enhanced Non-Linear Raman Scattering at the Single Molecule Level | journal = Chem. Phys. | volume = 247 | issue = 1| date = 1999 | pages = 155–162 | doi = 10.1016/S0301-0104(99)00165-2 |bibcode = 1999CP....247..155K | display-authors = 1 | last3 = Itzkan | first3 = Irving | last4 = Dasari | first4 = Ramachandra R. | last5 = Feld | first5 = Michael S. }}</ref>
* ''[[Stimulated Raman spectroscopy]]'' ''(SRS)'' – A [[Pump-probe spectroscopy|pump-probe]] technique, where a spatially coincident, two color pulse (with polarization either parallel or perpendicular) transfers the population from ground to a [[rotational–vibrational coupling|rovibrationally]] excited state. If the difference in energy corresponds to an allowed Raman transition, scattered light will correspond to loss or gain in the pump beam. 
* ''[[Inverse Raman effect|Inverse Raman spectroscopy]]'' – A synonym for stimulated Raman loss spectroscopy.
* ''[[Coherent anti-Stokes Raman spectroscopy]] (CARS)'' – Two laser beams are used to generate a coherent anti-Stokes frequency beam, which can be enhanced by resonance.