Towards Terawatt-Scale Spectrally Tunable Terahertz Pulses via Relativistic Laser-Foil Interactions

Abstract

An ever-increasing number of strong-field applications, such as ultrafast coherent control over matter and light, require driver light pulses that are both high power and spectrally tunable. The realization of such a source in the terahertz (THz) band has long been a formidable challenge. Here, we demonstrate, via experiment and theory, efficient production of terawatt (TW)-level THz pulses from high-intensity picosecond laser irradiation on a metal foil. It is shown that the THz spectrum can be manipulated effectively by tuning the laser pulse duration or target size. A general analytical framework for THz generation is developed, involving both the high-current electron emission and a time-varying electron sheath at the target rear, and the spectral tunability is found to stem from the change of the dominant THz generation mechanism. In addition to being an ultrabright source (brightness temperature of about1021K) for extreme THz science, the THz radiation presented here also enables a unique in situ laser-plasma diagnostic. Employing the THz radiation to quantify the escaping electrons and the transient sheath shows good agreement with experimental measurements.

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Plain Language Summary

Tunable high-power light sources are highly desired for exploring the unknown and revolutionizing technologies as near-infrared and x-ray lasers have demonstrated in recent decades. However, the realization of such a source at the long-wavelength terahertz (THz) band—located between the microwave and far-infrared regions of the electromagnetic spectrum—has long been a formidable challenge. Here, we realize a spectrally tunable broadband THz source delivering peak power of about1012W, more than 100 times as high as state-of-the-art accelerator- and crystal-based sources.

In our experiments, we focus a high-intensity picosecond laser pulse onto a metal foil and observe a high-brightness THz burst at the foil’s rear side. Further modeling and experiments allow us to clarify the physical scenarios for this THz generation: The laser pulse accelerates a large number of energetic electrons, some of which escape from the foil target while the rest are trapped around the target surface, accelerating ions. All of these processes can induce strong broadband THz radiation but with distinctly different spectra. This enables us to manipulate the global THz spectra by controlling the relative contributions of various radiation mechanisms, which we confirm experimentally by varying the laser pulse duration and target size. The tunable THz spectra also reflect some dynamical change in laser-plasma interactions.

The unprecedented THz source demonstrated in our work opens up a new avenue towards emerging extreme THz science, likely enabling a new paradigm of relativistic optics that is currently accessible only at infrared wavelengths. The THz radiation also offers a novel versatile laser-plasma diagnostic.