Reviewed by Lexie CornerMay 29 2024
In a recent study published in Light Science & Application, a team of scientists from the Photonics Research Institute, Gwangju Institute of Science and Technology, and the University of Maryland has created the world's strongest terahertz fields, measuring 260 MV/cm or equivalent peak intensity of 9 × 10³ W/cm2. This is the highest value achieved so far at terahertz frequencies (0.1~20 THz), including all times of terahertz sources using lasers, free electron lasers, accelerators, and vacuum electronics.
(a) A series of 15-THz beam profiles captured at various positions along the propagation when focused by a concave metallic mirror. The spot size is as small as 43 micrometers in full width at half maximum at the focus. (b) Plasma fluorescence emitted from a solid target, irradiated by an intense terahertz pulse and captured by a camera. (c) Microscope image of a semiconductor surface damaged by an ionizing terahertz pulse. Image Credit: Hyeongmun Kim, Chul Kang, Dogeun Jang, Yulan Roh, Sang Hwa Lee, Joong Wook Lee, Jae Hee Sung, Seong Ku Lee, and Ki-Yong Kim.
The terahertz (1 THz = 10¹² Hz) gap, which lies between the microwave and infrared regions of the electromagnetic spectrum, is rapidly closing due to the development of new terahertz sources and detectors. These sources have promising uses in communication, imaging, sensing, and spectroscopy. High-energy or high-average-power radiation delivered by terahertz sources is highly advantageous for these applications.
However, high-intensity or strong-field terahertz sources are required to observe or take advantage of new nonlinear terahertz-matter interactions, where the strengths of the electric and magnetic fields are crucial.
A team of scientists led by Dr. Chul Kang from Advanced Photonics Research Institute, Gwangju Institute of Science and Technology (GIST), Korea, and Professor Ki-Yong Kim from Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, MD, USA were involved in the study. The peak field strength or intensity, encompassing all terahertz sources, is the highest value attained to date at terahertz frequencies (0.1~20 THz).
The scientists employed a 150-terawatt-class Ti:sapphire laser to convert optical energy into terahertz radiation in lithium niobate (LiNbO₃), a crystal with strong nonlinearities and high damage thresholds. This process, known as optical rectification, produces high-energy terahertz pulses.
Specifically, they generated energy-up-scalable terahertz radiation using a large-diameter (75 mm) lithium niobate wafer doped with 5 % magnesium oxide (MgO).
Phase (or velocity) matching is another crucial component for an effective transition from optical to terahertz radiation.
The scientists explained, “If the optical laser pulse that generates terahertz radiation propagates at the same velocity with the generated terahertz waves in lithium niobate, then the output terahertz energy can continuously grow with the propagation distance.”
Scientists added, “Conventionally, a tilted pulse front method is used to satisfy phase matching in a prism-shaped lithium niobate. This method, however, produces mostly low-frequency terahertz radiation, typically peaked at less than 1 THz, which naturally leads to relatively large focal spot sizes (~mm), consequently limiting the peak terahertz field strength at the focus.”
They previously discovered a novel phase-matching condition in lithium niobate that does not require pulse front tilt.
Scientists noted, “The velocity of terahertz waves is generally frequency-dependent and varies so large between two phonon resonance frequencies that there exists a frequency at which both terahertz and laser pulses propagate at the same velocity. This occurs at approximately 15 THz for Ti:sapphire laser pulses having a central wavelength of 800 nm. This phase matching made it possible to produce millijoule-level terahertz waves. Moreover, the resulting 15-THz radiation can be tightly focused, potentially producing strong electromagnetic fields at the focus.”
Measurements of the terahertz energy, focal spot size, and pulse duration were made independently, and the scientists were able to carefully determine the peak electric and magnetic field strengths, which were 260 ± 20 MV/cm and 87 ± 7 T at the focus.
“Such an intense terahertz pulse, when focused into a gaseous or solid medium, can tunnel ionize the constituent atoms or molecules and convert the medium into a plasma. As proof of principle, we have demonstrated terahertz-driven ionization of various solid targets, including metals, semiconductors, and polymers,” the researchers emphasized.
Scientists added, “Our terahertz source uses a planar lithium niobate crystal and is promising for scaling up the output energy and field strength even further. This can generate super-strong (~GV/cm) terahertz fields.”
The researchers believe their work will create new avenues to investigate nonlinear effects in terahertz-produced plasmas. It could also open up opportunities to use terahertz-driven ponderomotive forces for different applications, such as multi-keV terahertz harmonic generation and investigation and even investigating relativistic effects by terahertz-accelerated electrons.
Journal Reference:
Kim, H., et al. (2024) Ionizing terahertz waves with 260 MV/cm from scalable optical rectification. Light: Science & Applications. doi.org/10.1038/s41377-024-01462-w.