A schematic view of the pump-induced stimulated superradiant Smith-Purcell radiation (PIS-SPR) device, which consists of the electron pre-bunching, electron-compression and harmonic-emission section. Based on PIS-SPR, the free electron beam is well bunched and ultra-narrow spectral linewidth at THz frequency region has been observed. Credit: Yuechai Lin, Jinyu Li et al. CC BY.

A pump-induced stimulated superradiant Smith-Purcell radiation (PIS-SPR) with ultra-narrow linewidth has been developed by scientists in China.

Superradiant Smith-Purcell radiation (S-SPR) is a kind of free electron radiation with a train of free electron bunches passing over a periodic grating.

In theory, ultra-narrow spectral linewidth of S-SPR could be realised, which would be greatly beneficial to various applications such as imaging, sensing and communication.

However, in the free electron accelerators, customised setups and orotrons, the instability of electron kinetic energy, coulomb effect and the finite number of electron bunches worsened the radiation linewidth, and the large size of equipment limits the application scenarios.

A team of scientists, led by Professor Fang Liu and Yidong Huang from the Department of Electronic Engineering, Tsinghua University China, and co-workers, has developed the first compact S-SPR device with ultra-narrow and continuously tuneable spectral linewidth.

They proposed the new effect of pump-induced stimulated S-SPR (PIS-SPR) and observed successfully the PIS-SPR at the frequency of ~0.3 THz with radiation linewidth continuously tuned from 900 kHz to 0.3 kHz.

The device can even be easily held by one hand with a size of 22cm × 7cm × 6.5cm and a weight of only 1.68 kg.

High harmonic S-SPR

The compact device with PIS-SPR effect includes three sections: the electron pre-bunching, electron-compression and the harmonic-emission section.

The scientists summarised the operational principle of PIS-SPR: “The low-frequency and low-power THz pump wave excites the localised electromagnetic mode on the grating which pre-bunches the electron beam.

“Subsequently, these pre-bunched electrons emit S-SPR at the same frequency as the pump wave.

“With an F-P cavity, the localised electromagnetic mode and the electron bunching are enhanced alternatively and continuously, which leads to the stimulated S-SPR (namely PIS-SPR), as well as well-bunched periodic electrons.

“Finally, utilising the small period grating, the ideal electron bunches could generate high harmonic S-SPR.”

With PIS-SPR effect, the three factors worsening the radiation linewidth, namely instability of electron kinetic energy, coulomb effect and the finite number of electron bunches, had been overcome.

They tuned the emission spectrum linewidth from 900 kHz to 0.3 kHz, by varying the number of electron bunches from ~10⁵ to ~10⁹.

Compared with previous reported S-SPR generated by electron accelerators or other equipment, the linewidth could be shrunk by about two to six orders of magnitude, and the spectral linewidth could be tuned continuously in range of 0.3 kHz~900 kHz.

Backward waveguide mode

These scientists also found that the device can be operated in the model of backward waveguide mode (BWM) with no pump wave injected.

The electron bunches and 3^rd harmonic S-SPR was also observed, but the linewidth is wider than MHz.

The scientists explained: “The mode evolution of the device from BWM-induced S-SPR to PIS-SPR is observed experimentally by gradually increasing the power of pump wave from 0mW to 60mW.

“The BWM is gradually suppressed with increased pump power, and the PIS-SPR dominates the electron bunching process.” 

They added: “This work provides the possibilities of not only realising compact, narrow linewidth radiation sources in different frequency region which would promote the applications of S-SPR in different fields, but also generating frequency-locked/ -tuneable free electron bunches for interaction with different materials and micro-/ nano- structures.

“And the proposed stimulated effect to enhance the light field might also be considered for on-chip electron acceleration to achieve a higher electron acceleration gradient.”

The paper is published in eLight.