Terahertz Wave Generation in Plasma Systems

Terahertz radiation occupies the spectral region between microwaves and infrared light, presenting unique opportunities for non-invasive imaging, broadband communications and the investigation of ultrafast carrier dynamics. Plasma systems have emerged as versatile platforms for terahertz generation, overcoming the power and bandwidth limitations of conventional solid-state sources. In such systems, intense femtosecond laser pulses ionise gases, liquids or solids to create transient plasmas in which free electrons act as broadband radiators. Key mechanisms include two-colour laser excitation, in which the interference of fundamental and second-harmonic fields produces a transient photocurrent, and laser-wakefield acceleration, which drives high-energy electron bunches that emit coherent radiation upon exiting the plasma. Advances in mid-infrared driver wavelengths and tailored plasma geometries have pushed conversion efficiencies towards the per-cent level, yielding multi-millijoule, sub-cycle pulses with field strengths exceeding tens of megavolts per centimetre. Such capabilities herald new horizons in nonlinear terahertz optics, deep-penetration spectroscopy and compact accelerator-based sources, underscoring the global relevance of plasma-based terahertz technologies across scientific and industrial domains.

Research from Nature Portfolio

Recent work has demonstrated that mid-infrared two-colour filamentation of femtosecond pulses can generate sub-cycle terahertz bursts with millijoule energies and conversion efficiencies up to 2 per cent, yielding field amplitudes above 100 MV cm−1. Numerical models indicate further upscaling potential through optimisation of pulse parameters and filament length, paving the way for tabletop extreme nonlinear terahertz experiments. Complementary studies have explored filamentation in liquid media, revealing that ultrashort pulses in water and organic liquids can produce broadband terahertz outputs an order of magnitude more energetic than air-based schemes. These findings challenge preconceived absorption limits in liquids and open alternative pathways for compact, high-field terahertz sources tailored to spectroscopic and imaging applications.

Research from all publishers

An optical plasma modulator has been introduced to control terahertz wave amplitude and phase ex situ, enabling sub-picosecond switching across the 0.1–2 THz band by adjusting the plasma dipole phase. This approach offers a dynamic tool for terahertz routing and sensing in integrated systems. In parallel, multi-millijoule terahertz emission has been observed from laser-wakefield-accelerated electrons in gas jets driven by 100 TW-class pulses, achieving conversion efficiencies near 0.15 per cent at frequencies below 10 THz. Analytical and particle-in-cell simulations corroborate a ponderomotive and wakefield-driven acceleration model for this emission. Seminal work from symmetry-broken laser fields has also refined the transient photocurrent formalism, establishing foundational principles for two-colour air plasma generation and guiding subsequent advancements in source design.

Terahertz Wave Generation in Plasma Systems publication trend

The graph below shows the total number of articles in terahertz wave generation in plasma systems across all publications each year (not limited to Nature Index journals).

Technical terms

Two-colour laser excitation: Simultaneous irradiation with fundamental and its second harmonic to induce an asymmetric electric field, driving a transient electron current that emits terahertz radiation.

Laser-wakefield acceleration: The generation of plasma density waves by an intense laser pulse, which trap and accelerate electrons to high energy, producing radiation upon de-coupling from the plasma.

Ponderomotive force: A nonlinear force experienced by charged particles in an oscillating electromagnetic field, driving electron motion that contributes to terahertz emission.

Sub-cycle pulses: Electromagnetic pulses whose duration is shorter than one oscillation period of the carrier frequency, enabling extreme field strengths and ultrabroadband spectra.

References

1. Observation of extremely efficient terahertz generation from mid-infrared two-color laser filaments. Nature Communications (2020).
2. Highly efficient broadband terahertz generation from ultrashort laser filamentation in liquids. Nature Communications (2017).
3. Ultrafast plasma-based terahertz modulator. Optica (2024).
4. Multi-millijoule terahertz emission from laser-wakefield-accelerated electrons. Light: Science & Applications (2023).
5. Terahertz emission from ultrafast ionizing air in symmetry-broken laser fields.. Optics Express (2007).