Optics 101

Sum Frequency Generation - Non-Linear Optics for Advanced Spectroscopy

Sum frequency generation (SFG) may be defined as a second order non-linear optical process in which a tunable infrared beam is combined with a visible beam, such that that the frequency of the output beam is the sum of the input frequencies. Two necessary conditions for SFG to occur are matter which interacts with light in the correct way, and a high intensity light source.

SFG does not alter the matter in any way; it is a purely parametric process. Second frequency generation is a type of sum frequency generation. SFG is a second order non-linear process that occurs at an interface where the inversion symmetry of the medium is broken. This datasheet will describe the experimental set up, working and applications of sum frequency generation.

Experimental Set Up

The set up for achieving SFG consists of a pulsed laser source. The surface that is to be analyzed is placed in front of the laser source. The laser source should be a picosecond or a femtosecond source with high repetition rates for effective SFG. The Nd:YAG mode locked with a mirror is generally used. The radiation source should be tunable from UV to mid-IR region.

The sample whose surface needs to be analyzed could be a liquid or a solid, which is buried in a number of interfaces. Polarizers are used for tuning the incident power. The detection system consists of photomultiplier tubes, mirrors and a monochromator. The output is used for photon counting or signal integration.


Using the above set up, the two incident beams IR and visible are made to fall on the sample medium. These two pulsed laser beams achieve a temporary spatial overlap at the surface of the sample interface, since they are traveling in the same space and at the same time. The sample interface responds by emitting a light beam whose frequency is the sum of the two incident laser beams. The output light falls in the visible range of the spectrum.


Some of the typical applications of sum frequency generation are listed below:

  • Ultra high resolution microscopy
  • Surface and interface analysis
  • Study of bimetallic catalysts
  • Biological molecular level studies.

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