Parametric Processes in Plasmas: From Instabilities to Spectral Broadening and Optical Amplification
Suming Weng¹, Zhiyu Lei¹, Hong Ai¹, Zhengming Sheng¹, Jie Zhang<sup>1

1</sup>Laboratory for Laser Plasmas, Shanghai Jiao Tong University, Shanghai, China
wengsuming@sjtu.edu.cn

Parametric processes are central to laser-plasma interactions. Their studies have been expanded from the usual instabilities to be mitigated in inertial confinement fusion (ICF)  to emerging plasma-based optical devices for different applications. In this talk, we report our progresses made on these topics.
Firstly, to mitigate parametric instabilities, such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), we proposed the idea of decoupled broadband lasers and derived the bandwidths required to suppress these instabilities [1]. Further studies revealed complex competition among different parametric instabilities driven by broadband lasers [2,3]. Therefore, simultaneous suppression of these parametric instabilities demands a considerably large bandwidth. To reduce the required bandwidth, we introduced the concept of “sunlight-like” lasers, i.e., broadband lasers with randomized polarization. Simulations show that a sunlight-like laser with only 1% bandwidth achieves SRS suppression comparable to a conventional broadband laser with 2% bandwidth [4].
Secondly, to increase laser bandwidth, we proposed spectral broadening schemes using parametric processes in plasmas. It is found that a signal laser pulse can be modulated via the forward Raman scattering in large-amplitude electron plasma wave excited by an intense drive laser pulse, producing extremely large bandwidths ~100% [5,6]. Recent studies show that when the signal laser initially has a small continuous bandwidth, the coherence time of the modulated pulse drops to 30 fs—about an order of magnitude shorter than the monochromatic case. This provides superior light sources for parametric instability suppression [7].
Finally, for parametric optical amplification, we recently proposed a plasma-based amplification scheme based on forward Raman scattering. Leveraging the group velocity difference between copropagating pump and seed pulses, efficient energy transfer from the pump to the seed is achieved via forward Raman scattering over extended interaction lengths. It is shown that a 1.8 μm seed pulse can be amplified by a factor of 10⁵ and self-compressed to near-single-cycle duration, with an energy conversion efficiency of approximately 30% [8]. This scheme offers a new route to generate petawatt-level, wavelength-tunable, few-cycle infrared pulses.
In conclusion, this series of works illustrates a paradigm shift in which plasmas transition from “unstable media” to “controllable optical devices.” In particular, parametric processes in plasmas provide new light sources and manipulation tools for different applications in inertial confinement fusion, high-field physics, and ultrafast science.
[1] Y. Zhao et al., Phys. Plasmas 24, 112102 (2017). 
[2] X. F. Li et al., Matter Radiat. Extremes 8, 065601 (2023). 
[3] Z. Liu et al., Nucl. Fusion 63, 126010 (2023). 
[4] H. H. Ma et al., Matter Radiat. Extremes 6, 055902 (2021). 
[5] L. L. Yu et al., Nat. Commun. 7, 11893 (2016). 
[6] Y. Zhao et al., Opt. Express 28, 15794 (2020). 
[7] H. Ai et al., Matter Radiat. Extremes 11, 047401 (2026). 
[8] Z. Y. Lei et al., Phys. Rev. Lett. 134, 255101 (2025).

inertial confinement fusion,,parametric instabilities,Spectral Broadening,Optical Amplification,forward Raman scattering