Second harmonic generation is a phenomena observed in non-linear optics. It abbreviated as SHG and is also called frequency doubling. It is an important phenomenon in the field of non-linear optics. Non-linear optical phenomena are observed by using high-intensity incident beams, such as lasers. Through SHG, the frequency of the incident light can be doubled.
In brief, SHG is a phenomenon in which the photons interacting with a non-linear optical material are effectively combined to form new photons with double the frequency, and therefore double the energy. SHG was first demonstrated at the University of Michigan in 1961. This article will elaborate further on the working, experimental set up and application of generating second harmonics.
Combination laser rod and frequency doubling crystal. Image Credits: Laser Components
Experimental Set Up
The light source for second harmonics generation is a high-powered pulsed laser system. The laser light is linearly polarized and directed towards the non-linear optical medium (usually a crystal); a small part of the beam is directed towards a reference channel to remove any fluctuations. The generated second harmonic is separated from the light source by filters and a monochromator. The subsequent measurements are done by an electronic photon counting set up.
There are three types of SHGs, namely, type 0, type I and type II.
The principle behind second harmonics generation is the conversion of two photons of frequency ‘f’ into a single photon of frequency ‘2f’, in a non-centrosymmetric medium. The frequency doubling of a non-centrosymmetric (or non-linear) optic medium is brought about by a high intensity light beam. The light beam initiates non-linear polarization in the medium, which results in frequency doubling.
Some of the typical applications of second harmonics generation are listed below:
- Study of biological structures
- Lasers – generating different color laser beams
- Study of surface chemistry
Sources and Further Reading