Generation of single-cycle relativistic infrared pulses at wavelengths above 20 µm from density-tailored plasmas

Xing-Long Zhu,Min Chen,Jie Zhang,Zheng-Ming Sheng,Wei-Yuan Liu,Su-Ming Weng

doi:10.1063/5.0068265

Xing-Long Zhu, Min Chen + Show 4 more

Open Access

https://doi.org/10.1063/5.0068265

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Abstract

Ultra-intense short-pulse light sources are powerful tools for a wide range of applications. However, relativistic short-pulse lasers are normally generated in the near-infrared regime. Here, we present a promising and efficient way to generate tunable relativistic ultrashort pulses with wavelengths above 20 µm in a density-tailored plasma. In this approach, in the first stage, an intense drive laser first excites a nonlinear wake in an underdense plasma, and its photon frequency is then downshifted via phase modulation as it propagates in the plasma wake. Subsequently, in the second stage, the drive pulse enters a lower-density plasma region so that the wake has a larger plasma cavity in which longer-wavelength infrared pulses can be produced. Numerical simulations show that the resulting near-single-cycle pulses cover a broad spectral range of 10–40 µm with a conversion efficiency of ∼2.1% (∼34 mJ pulse energy). This enables the investigation of nonlinear infrared optics in the relativistic regime and offers new possibilities for the investigation of ultrafast phenomena and physics in strong fields.

Highlights

In the first stage, an intense drive laser first excites a nonlinear wake in an underdense plasma, and its photon frequency is downshifted via phase modulation as it propagates in the plasma wake
It is still hard to expand these methods for obtaining single-cycle IR pulses at long wavelengths beyond 5 μm into the relativistic regime, since they normally suffer from optical breakdown damage and the low power-carrying capacity and limited bandwidth of optical materials
We examine the robustness of this IR radiation scheme with regard to the carrier-envelope phase (CEP), which is important for light–matter interactions with few-cycle pulses

Summary

INTRODUCTION

Ultrashort-duration high-energy light sources in the mid-infrared (mid-IR) spectral range play key roles in a number of areas of fundamental research, such as the scaling of strong-field interactions to mid-IR wavelengths, high-order harmonic generation, and supercontinuum generation. They are of particular interest for ultrafast molecular dynamics imaging, IR spectroscopy for biological and medical diagnostics, molecular fingerprinting, and the generation of optical frequency combs. The boosting of such IR light sources to relativistic intensity will open up a new realm of research, dealing, for instance, with the generation of bright hard x-ray or even gamma-ray sources, next-generation laser–plasma accelerators, and the study of relativistic light–matter interactions in the mid-IR domain. Most of these studies would benefit significantly from intense driving optical fields with ultrashort pulse durations of a few cycles, long carrier wavelength, multi-millijoule (multi-mJ) pulse energy, and high peak intensity reaching up to the relativistic level. The boosting of such IR light sources to relativistic intensity will open up a new realm of research, dealing, for instance, with the generation of bright hard x-ray or even gamma-ray sources, next-generation laser–plasma accelerators, and the study of relativistic light–matter interactions in the mid-IR domain.13 Most of these studies would benefit significantly from intense driving optical fields with ultrashort pulse durations of a few cycles, long carrier wavelength, multi-millijoule (multi-mJ) pulse energy, and high peak intensity reaching up to the relativistic level. We present a scheme to address this difficulty with the use of density-tailored plasmas as a new type of nonlinear optical medium These plasmas have a two-stage structure, with one stage having a relatively high density for efficient pulse modulation and photon deceleration, and the other stage having moderately low density for significant frequency down-conversion to the longerwavelength spectral domain. The drive laser pulse experiences strong frequency downshifting and continuous spectral broadening as it propagates in a plasma with such a two-stage structure, and it can be converted into near-single-cycle IR pulses of wavelengths above 20 μm with a few percent efficiency at relativistic intensities

THEORETICAL MODEL AND NUMERICAL SIMULATION

TUNABILITY OF IR PULSES

CONCLUSION