May 6 2014
High harmonic generation may be described as an optical process of generating high harmonics of frequency from a fixed frequency laser light. For the generation of high harmonics a high power laser source is required.
The first high harmonic generation (HHG) was observed in the year 1977, during the interaction of CO2 laser pulses with plasma generated from solid targets.
HHG takes place in an atomic or molecular jet. The generated high harmonics are odd-integer multiples of the frequency of the laser light. A notable feature of HHG is that the frequency range extends beyond the ionization potential of the gas, but abruptly falls off at the cut-off potential.
This article elaborates on the experimental set up, working, and applications of high harmonic generation.
Experimental Set Up
The set up for generating high harmonics consists of a pump-probe for generating pulses. Generation of high harmonics is most suitable in a gaseous medium. Typically, a hollow core waveguide, which is filled with a noble gas, is used for HHG. A high pulsed laser, like a femtosecond laser, is focused into the waveguide, in order to generate high harmonics.
Working
High harmonics are generated when a high power pulsed laser is focused into a gaseous medium.
The laser source generates a strong electromagnetic field, which in turn ionizes the gaseous atoms. The electron is excited and moves at an accelerated speed. When the field is reversed, the accelerated electron recombines, and emits its accumulated kinetic energy in the form of short-bursts that correspond to a series of narrow frequency lines, thus generating high harmonics.
Applications
Some of the typical applications of high harmonic generation are mentioned below:
- Generation of soft X-ray
- Diffractive microscopy
- Molecular studies
Sources and Further Reading