Jul 5 2013
Third harmonic generation may be described as a process in which the frequency of a light source is tripled when it passes through a non-linear medium. Third harmonic generation is a non-linear process of the third order, which generates a photon having triple the frequency of the incident light source, and one-third the wavelength.
Third harmonic generation is a process that is used in laser applications. The frequency is tripled using an optical frequency multiplier. In this device a light source interacts with a non-linear optical material, the emerging photon has a higher frequency and a shorter wavelength.
Potassium dihydrogen phosphate (popularly known as KDP) is used in optical frequency multipliers for tripling the frequency. This article will elaborate on the experimental set up, working and applications of third harmonic generation.
Experimental Set Up
The experimental set up for achieving third harmonic generation consists of two thin type II KDP crystals of thickness 2.5 mm, one for frequency doubling and another for tripling. The input light source is a picosecond laser. This incident light is linearly polarized at 35o with respect to the ‘o’ direction of the doubling crystal. Thus, every ‘e’ photon (from incident beam) will combine with an ‘o’ photon to give an ‘e’ photon with doubled frequency. This photon passes through the second crystal to generate a photon with tripled frequency.
Working
The method of generating a photon with tripled frequency using two type II KDP crystals in a series is called polarization mismatching. One crystal acts as a frequency doubler, while the second one combines the second harmonic with an unconverted fundamental pulse by the process of sum frequency generation to generate a third harmonic pulse. In dispersive, non-linear birefringent medium optical waves with different polarizations will walk away from each other after propagating a certain distance in the crystal.
Applications
Some of the main areas of application of third harmonic generation are:
- Production of high power laser sources
- Non-linear microscopy
References