Terahertz waves, typically known as non-ionizing radiation, have now been pushed to new limits where they can behave as ionizing radiation under specific conditions. A groundbreaking advancement by a collaborative team of scientists from Korea and the U.S. has led to the creation of the most intense terahertz pulses ever recorded. These pulses are powerful enough to instantaneously ionize atoms and molecules, transforming them into plasma.
This remarkable development was detailed in a recent study published in Light: Science & Applications, which delves into terahertz-driven tunneling ionization. The implications of this research pave the way for exploring extreme nonlinear and relativistic terahertz physics within plasmas.
The terahertz region of the electromagnetic spectrum, which lies between microwaves and infrared waves, has seen rapid advancements. These developments have spurred new applications in fields like spectroscopy, imaging, sensing, and communication. High-energy and high-power terahertz sources are particularly beneficial for these applications. Additionally, high-intensity terahertz sources are crucial for observing and utilizing novel nonlinear interactions between terahertz waves and matter, where the electric and magnetic field strengths are paramount.
The team, led by Dr. Chul Kang of the Advanced Photonics Research Institute at Gwangju Institute of Science and Technology (GIST) in Korea and Professor Ki-Yong Kim from the University of Maryland, has achieved an unprecedented feat by generating the strongest terahertz fields to date. These fields measure 260 megavolts per centimeter (MV/cm) or an equivalent peak intensity of 9 x 10¹³ watts per square centimeter (W/cm²). This achievement sets a new benchmark for terahertz frequencies (0.1~20 THz), surpassing all existing terahertz sources, including lasers, free electron lasers, accelerators, and vacuum electronics.
To achieve these high-energy terahertz pulses, the scientists employed a 150-terawatt-class Ti:sapphire laser. This laser converted optical energy into terahertz radiation through a process known as optical rectification in lithium niobate (LiNbO₃). Lithium niobate is a crystal renowned for its strong nonlinear properties and high damage thresholds. The researchers used a large-diameter (75 mm) lithium niobate wafer, doped with 5% magnesium oxide (MgO), to scale up the energy output of the terahertz radiation.
A critical aspect of this process is phase (or velocity) matching. For efficient conversion from optical to terahertz radiation, the optical laser pulse and the generated terahertz waves must propagate at the same velocity within the lithium niobate. This allows the terahertz energy to grow continuously along the propagation distance. Traditionally, a tilted pulse front method was used to achieve phase matching, but it produced primarily low-frequency terahertz radiation with larger focal spot sizes, limiting the peak terahertz field strength.
The team discovered a novel phase matching condition in lithium niobate that does not require pulse front tilting. They found that at approximately 15 THz, both terahertz and laser pulses propagate at the same velocity, especially for Ti:sapphire laser pulses with a central wavelength of 800 nm. This discovery enabled the production of millijoule-level terahertz waves that could be tightly focused, generating strong electromagnetic fields at the focus.
Careful measurements by the scientists determined the peak electric and magnetic field strengths at 260 ± 20 MV/cm and 87 ± 7 T, respectively. These measurements were obtained by separately analyzing the terahertz energy, focal spot size, and pulse duration.
The intense terahertz pulses can tunnel ionize atoms or molecules in a medium, converting it into plasma. As a proof of concept, the researchers demonstrated terahertz-driven ionization on various solid targets, including metals, semiconductors, and polymers.
The research team's approach, using a planar lithium niobate crystal, shows promise for further scaling up the output energy and field strength, potentially generating even stronger (~GV/cm) terahertz fields. They anticipate that their findings will unlock new opportunities for studying nonlinear effects in terahertz-produced plasmas. Moreover, the terahertz-driven ponderomotive forces could be harnessed for applications like multi-keV terahertz harmonic generation and studying relativistic effects through terahertz-accelerated electrons.
This breakthrough not only expands our understanding of terahertz physics but also opens up new frontiers in the interaction of intense electromagnetic fields with matter, paving the way for future technological advancements in a variety of scientific and industrial fields.
Source: Chinese Academy of Sciences