Sichuan online reporter Lan Zhen
In the vast desert, on the cliff edge of the northern bank of the Weigan Valley, the thousands of years of frescoes are silent, and cutting-edge technology is quietly unveiling the veil of history. The Kizil Grottoes are the earliest large grottoes in China and the most western geographical location. The existing frescoes cover an area of about 10,000 square meters. Recently, the team of Min Hu, a professor at the University of Electronic Science and Technology of China (UESTC), and the team of Zhang Hui, a professor at Zhejiang University, are exploring the secrets that cannot be revealed by the naked eye in the grottoes. The key is terahertz imaging technology.
Inside the Kizil Grottoes, where the monks lived through years of fire, the walls and cave tops of the exquisite murals were originally painted and covered with a layer of ash and black smoke. “The terahertz imaging technology can penetrate the surface nondestructively, reopening the artifacts hidden in the historical smoke and revealing the secrets that began in the end of the Eastern Han Dynasty. Min Hu said.
What is it? The fascinating perspective is hailed as the “black technology” of the future.
located between microwaves and infrared waves, terahertz wave is the least studied and most developed band in the electromagnetic spectrum, and is known as one of the top ten technologies to change the future world.
It was not until the 1970s and 1980s that terahertz was formally defined as an independent band. Previously, it was known as far-infrared in the field of optics and submillimeter or ultramicrowave in the field of electronics.
Perspective is one of the most fascinating features of terahertz. Like microwave radiation, terahertz can penetrate many dielectric materials, non-polar liquids, and nonconductive materials such as clothing, paper, wood, masonry, plastics, ceramics, etc., and is strongly reflective of metallic objects. The energy of terahertz photons is very low, being one ten thousandth of that of X-rays, and does not destroy the detected substance due to ionization. Based on this, after studying terahertz for more than 20 years, Min Hu led the team in the field of cultural relic protection and archaeological research, opening up more new application scenarios.
According to Min Hu, terahertz is more sensitive to object stratification than X-rays. terahertz pulsed radiation has a picosecond pulse width and is effective for time-resolved research. In other words, without touching or cutting the cultural relics, the internal level information can be obtained according to the reflected signals between the material levels to achieve three-dimensional imaging.
On Min Hu’s computer, an outline of a portrait rendered using terahertz technology is clearly visible. “Terahertz archaeology” began to sprout as early as the 1990s, he said. In 2013, Spanish scientists used the terahertz time-domain spectroscopy system to scan and analyze “sacrifices to Vista” paintings that were “covered” with paint and lacquer, and found an early pencil trace under the surface of the painting. By contrast, this is the signature of the artist Goya. This discovery confirms that the painting is a work of Goya. This study is also considered a classic case of terahertz technology application.
In addition, many macromolecules have rotational vibration in the terahertz band, and their terahertz spectra contain rich molecular structure information, just as everyone’s fingerprint is unique. For terahertz waves, objects also have unique “fingerprints” – the absorption characteristics of different material molecules are also different, so they are called “terahertz fingerprint spectra”. Using this property, the research team can test lacquerware, murals, mud sculptures, etc., including analysis of lipid coatings, pigment composition.
Min Hu, for example, in the Kizil Grottoes murals, the red color is often cinnabar, lead pellet, or a mixture of the two. Using terahertz technology, it is possible to accurately know the use of pigments, which can be used as one of the bases for evaluating the period of the creation of cultural relics, and can also better restore the appearance of the portrait covered with soot.
It is reported that in 2007, Japan established the first open database of cultural relics materials in the terahertz band. A number of research groups around the world continue to test terahertz spectral data supplementing different artwork materials. The database has included more than 1,500 spectral data.
How to use? terahertz + archaeology reveals the secrets of the “hidden” artifacts
Advanced detection equipment is essential to achieve the application of terahertz technology in the study of cultural relics. In the laboratory of the terahertz research center, the reporter saw the terahertz non-destructive testing device. The core part of the device is the time-domain spectrometer and the robotic arm that controls the detection probe, and the internal program is independently developed by the Min Hu team and Miji Technology. Pointing to two devices the size of a thumb, Min Hu explains: “This is the key part for transmitting and receiving terahertz waves.” After the system starts, the robot arm will plan the path according to the algorithm, along the normal line of the cultural relics below, non-contact point-by-point scanning. During operation, the robot arm maintains the transmitter — the receiver head is perpendicular to the surface of the object, and the probe jitter transmits terahertz waves at high frequencies up and down.
At present, the terahertz technology has achieved results in many major archaeological projects. In 2019, Min Hu’s team collaborated with Zhang Hui’s team at Zhejiang University, the Sichuan Provincial Institute of Cultural Relics and Archaeology, and the Sanxingdui Museum to measure the rust layer of bronze ware unearthed. “This is the first time in the world that three-dimensional tomographic analysis of bronze artifacts has been performed by terahertz.” Min Hu said.
In December 2024, Min Hu’s team participated in the archaeological work of the Terracotta Army Pit 2 in Xi ‘an. The team innovatively uses the photoelectric fusion non-destructive testing method of visible light-terahertz to accurately detect and analyze relevant cultural relics. According to the feedback of the team members, the on-site experts gave a high evaluation to the detection effect, which not only clearly presented the detailed characteristics of the cultural relics, but also accurately identified the internal structure, composition and potential defects of the cultural relics, providing a new technical support for the protection of cultural relics.
Not long ago, “the Tanyuan Program2024” was officially released, and the team was selected for the Kizil Grottoes cultural scene in Xinjiang. The project uses terahertz time-domain spectroscopy technology to perform non-destructive testing, image recognition and virtual restoration of the Kizil Grottoes 161th cave smoke-covered mural.
Min Hu revealed that this test at the Kizil Grottoesterahertz involved a large area of grottoes and work may continue until summer. Especially based on the “short board” of low power terahertz waves, the team is struggling to solve the problem of high power radiation sources. “Low power signal penetration is weak, which limits the depth, accuracy, and imaging resolution of material analysis and affects imaging results.” Foreign traditional terahertz time-domain spectroscopic imaging technology can see objects up to 300 microns, and hair is difficult to identify. The Min Hu team introduced near-field technology, combined the terahertz system with atomic force microscopy, and gradually made the first terahertz scattering near-field microscopy system based on high-power radiation sources by building a vacuum radiation source, improving optical path parts and mirrors, etc., with an imaging level accuracy of 20 nanometers. “This will be of great benefit to the subsequent conservation work. Min Hu said.
What is the prospect? It has a huge application prospect in astronomy, communication, biological diagnosis and other fields.
At present, the terahertz technology is still in the exploration stage and has wide application prospects in astronomy, communication, biological diagnosis, etc. In 2019, scientists first observed black holes, which was realized under the terahertz astronomical telescope.
terahertz imaging achieves nanoscale resolution, providing a new method of identification for biological diagnostics. For example, oral caries detection and diagnosis can be non-destructively tested by terahertz, which can detect caries white spots under the tooth surface and the depth of caries without tooth extraction.
According to Min Hu, terahertz waves have higher frequencies than radio waves, which allows them to transmit more information. “It can be used as a powerful tool to transfer large amounts of data quickly in future networks. It is reported that the world’s first 6G white paper uses terahertz technology as a breakthrough in communication solutions.
However, the physical shortcomings of terahertz are equally prominent – it is quickly absorbed by water vapor in the air and is lost in commonly used electronic materials such as copper. Some scholars believe that with the development of technology, the challenges of terahertz waves in terms of atmospheric absorption and material loss can be effectively addressed by using low-loss materials, optimizing waveguide structure, and increasing transmission power.
In addition, terahertz radar, as a cutting-edge technology, also has great application potential.
In the military field, terahertz radar can “see through” the “invisibility cloak” of the aircraft surface and effectively detect the target; its high resolution can play a role in precision guidance, battlefield reconnaissance, etc. In anti-aircraft and anti-missile defense, it can accurately identify and track incoming targets to improve defense effectiveness.
In the field of aviation, terahertz radars can be used for navigation and collision avoidance of aircraft. Compared with traditional radar, it can provide more accurate position and speed information to improve flight safety.
“We are continuing to independently innovate and develop terahertz technology.” Min Hu is very confident about the future.
Source: Sichuan Online (Sichuan Guan News) View Original