Creating the post powerful THz pulse ever measured

A 150TW Ti:sapphire laser was used to convert optical energy into THz radiation in a 75mm magnesium oxide doped lithium niobate crystal wafer – a material with strong optical non-linearity and high damage threshold.

A process called ‘phase-matched optical rectification’ was used.

“If the optical laser pulse that generates terahertz radiation propagates at the same velocity with the generated terahertz waves in lithium niobate, then the output terahertz energy can continuously grow with the propagation distance, according to the team, drawn from: GIST, POSTECH, Gwangju Institute for Basic Science, Chonnam National University and the University of Maryland.

They exploited the frequency-dependent velocity of THz waves, and the velocity difference between two phonon resonance frequencies, by finding a frequency at which both terahertz and laser pulses propagate at the same velocity – the laser can continue to optically rectify the THz pulse as they move along.

This is ~15THz for Ti:sapphire laser pulses centred around 800nm, a frequency which can be focused down to a 43μm (full-width half-max) spot.

Peak electric and magnetic fields in the spot were 260MV/cm and 87T.

“Such an intense terahertz pulse, when focused into a gaseous or solid medium, can tunnel ionize the constituent atoms or molecules, and convert the medium into a plasma,” said the researchers. “As proof of principle, we have demonstrated terahertz-driven ionization of various solid targets including metals, semiconductors and polymers.”

Scaling up is thought to be possible up to GV/cm fields.

The work is published in Light: Science & Applications as ‘Ionizing terahertz waves with 260 MV/cm from scalable optical rectification’.

Ionising radiation?

On the electromagnetic spectrum, visible light is non-ionising radiation’, while gamma rays are ‘ionising radiation’ – the latter being ‘radiation’ in the public definition, capable of generally stripping electrons from atoms rather than only doing it in special circumstances.

Image: 15THz beam profiles focused by a concave mirror. ‘b’ is plasma florescence from a solid target and ‘c’ is microscopic damage to the target cused by ionizing THz exposure. Image credit: Hyeongmun Kim, Chul Kang, Dogeun Jang, Yulan Roh, Sang Hwa Lee, Joong Wook Lee, Jae Hee Sung, Seong Ku Lee, and Ki-Yong Kim.