In the Chini ultrafast lab, we use ultrafast lasers that produce electromagnetic waves with a certain wavelength. To obtain these wavelengths, we take the ultrashort pulses generated by the laser and put them through a nonlinear process called high harmonic generation. Put simply, high harmonic generation passes electromagnetic pulses with an initial frequency through a material that creates new frequencies that are harmonics of the initial frequency (i.e.; three times, five times, ect.). This results in a new electromagnetic wave being outputted with a different wavelength than it had to begin. Unlike other harmonic generation mechanisms, high harmonic generation produces very high harmonics – typically in the 10’s to even 100’s of orders! This high harmonic generation can be obtained by using either a gas or a solid as the nonlinear material.

First, high harmonic generation can occur in various gases. On an atomic level, high harmonic generation occurs when an electromagnetic pulse penetrates a gas. The pulse interacts with a gas atom by removing an electron from its valence shell. Then, the electron is accelerated while outside the atom, causing it to gain kinetic energy from the electric field generated by the passing electromagnetic pulse [3]. After being accelerated, the electron is forced back into the parent atom, resulting in a deionization of the gas atom. However, due to conservation of energy, the kinetic energy gained by the electron through its acceleration needs to be released somehow, and this is where the new harmonic pulse comes from.

Fig. 1. Visual representation of high harmonic generation three step process in a gas. Quantum tunneling is shown on the far left, followed by electron acceleration from an electric field and recombination of electron and parent ion [1].

Alternatively, high harmonic generation can occur through a solid target through a slightly different process. In a solid, an electron hole pair must be generated. This means that an electron gains energy and jumps to a higher electron shell than the valence shell, leaving a hole in the valence shell and an electron in the conduction band (electron is excited above band gap) [2]. The electron and hole are then rapidly driven further apart by the laser pulse, causing a gain in kinetic and electric potential energy. Then, similar to high harmonic generation in a gas, the electron and hole are accelerated toward each other and combine. When this occurs, a harmonic photon with energy equal to the energy difference between the electron and hole is released, creating a new harmonic.

In the Chini lab, we are using high harmonic generation in solids as a tool to both learn more about the properties of different solid-state materials and develop new laser light sources. I’ll post in the future about what it’s like to work on high harmonic generation experiments, including some of the difficulties that come with researching the relatively new field of solid nonlinear materials.

References:

1. Corkum, P. Phys. Rev. Lett. 71, 1994 (1993).
2. Marangos, J. “High Harmonic Generation Solid Progress” 7, 97-98 (2011).
3. Hecht, J. “High harmonic generation pushes spectroscopy to the cutting edge,” http://www.laserfocusworld.com/articles/print/volume-48/issue-02/features/high-harmonic-generation-pushes-spectroscopy-to-the-cutting-edge.html.