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New Model for High-Power Terahertz Emissions from Laser Pulses

Due to a high increase in the development of new THz sources and detectors, the terahertz (THz) gap is being closed quickly.

The accelerating electrons radiate coherent THz emissions continuously along the laser propagation direction, resulting in broadband multi-mJ THz radiation in the far field. Image Credit: by Taegyu Pak, Mohammad Rezaei-Pandari, Sang Beom Kim, Geonwoo Lee, Dae Hee Wi, Calin Ioan Hojbota, Mohammad Mirzaie, Hyeongmun Kim, Jae Hee Sung, Seong Ku Lee, Chul Kang, and Ki-Yong Kim.

Laser-based THz sources are considered to be of great interest as a result of their capability of generating coherent, single-cycle-to-multicycle and broadband (or narrowband) radiation.

Also, such sources can offer natural synchronization with the driving laser, enabling ultrafast time-resolved spectroscopy and imaging. In recent times, high-power femtosecond lasers have been utilized to produce powerful THz radiation, as well as to examine novel THz-driven phenomena like harmonic generation, molecular alignment, and electron acceleration.

A new model for high-power terahertz emissions from laser pulses has been developed by a research group headed by Professor Ki-Yong Kim from the University of Maryland, College Park, also affiliated with Gwangju Institute of Science and Technology and the Institute for Basic Science, Korea.

The study has been reported in the journal Light Science & Application.

Laser-plasma-based ones are ideally suited for high-power THz generation among the several laser-based sources. Plasmas are ionized already and hence could sustain high electromagnetic fields, with minimal or no worry regarding material damage. At the same time, high-power laser pulses are concentrated into a small volume for energy-scalable THz generation.

Since the original work performed by Hamster et al., coherent THz generation from laser-produced gaseous and solid-density plasmas has been analyzed in an extensive manner.

In gases, single- or two-color laser-produced plasmas could produce coherent broadband THz radiation by ultrafast laser-driven currents. As far as two-color laser mixing is concerned, the laser-to-THz conversion efficiency went high up to the percent level by making use of mid-infrared laser drivers. Also, high-energy THz radiation was noted from laser-irradiated, high-density plasma targets depending on solids and liquids.

In recent times, tens of mJ of THz energy were noted from a metal foil that has been irradiated by high-energy (~60 J) picosecond laser pulses. Contrary to gas targets, high-density ones often pose target reloading and target debris problems, which further makes them unfavorable for use in constant or high-repetition-rate (>kHz) operation.

Laser-wakefield acceleration (LWFA), a gaseous plasma-based compact electron accelerator scheme, is one more source of broadband electromagnetic radiation. A relativistic electron bunch generated in LWFA could discharge THz radiation when it departs the plasma-vacuum boundary by coherent transition radiation (CTR).

This takes place when the bunch length size becomes compared to or below the wavelength of the emitted THz radiation, and the THz fields produced by separate electrons add up in the radiation direction in a coherent manner.

The research group noted multi-mJ THz emission from 100-TW-laser-driven LWFA with an energy conversion efficiency of 0.15%. The emitted THz radiation has been radially polarized and broadband, thereby possibly expanding beyond 10 THz.

The correlation happening between the electron beam properties (energy and charge) and THz output energy displays that high-energy (>150 MeV) electrons do not essentially yield high-power terahertz radiation. Rather, low-energy but high-charge electrons could generate much stronger terahertz radiation.

To describe this fascinating outcome collectively with multi-mJ THz generation, the research group has suggested a coherent radiation model, in which the electrons expedited by the laser ponderomotive force and consecutive plasma wakefields radiate broadband emission continuously together with the laser propagation direction, eventually leading to the phase-matched conical THz radiation in the far field.

However, this model requires to be confirmed or analyzed by more follow-up experiments and analytic or numerical studies to have complete knowledge of THz generation in LWFA, as well as to improve the source for high-power THz applications in the future.

Journal Reference:

Pak, T., et al. (2023) Multi-millijoule terahertz emission from laser-wakefield-accelerated electrons. Light Science & Applications.

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