Terahertz History What are there?

Terahertz History

In the early stages, Tera Hertz (THz) was known by different names in various fields. In optics, it was referred to as far-infrared, while in electronics, it was called sub-millimeter waves, ultra-microwaves, and so forth. Prior to the mid-1980s, the development of infrared and microwave technologies on either side of the THz band was relatively mature. However, understanding of the THz band itself remained very limited, leading to the formation of the so-called "THz Gap".

In 2004, the U.S. government recognized THz technology as one of the "Top Ten Technologies That Will Change the World". Japan, on January 8, 2005, placed THz technology at the top of its "Ten Key Strategic National Goals", mobilizing the entire country for research and development.

In November 2005, the Chinese government specifically convened the "Xiangshan Science Conference", inviting multiple influential academicians in the field of THz research to discuss the development direction of China's THz endeavors and formulate a development plan for China's THz technology. Currently, several research institutions in China are conducting research related to the THz field. Among them, Capital Normal University is one of the early starters with significant investments. It has made many groundbreaking contributions in the THz spectroscopy, imaging, and identification of drugs and explosives, as well as in the non-destructive testing of internal defects in non-polar aerospace materials using THz. Due to the unique advantages of THz rays in security inspections, the THz Laboratory at Capital Normal University is concentrating on developing security prototype equipment that can be used for real-world testing. Additionally, governments, institutions, enterprises, universities, and research institutions in many countries and regions, including the United States, Europe, Asia, and Australia, have actively engaged in the THz research boom. Dr. Zhang Xicheng, a renowned American scholar and one of the pioneers in the field of THz research, exclaimed, "Nextray, T-Ray!"

Terahertz characteristics

1. Quantum energy and blackbody temperature are very low, as shown in Table 1.
2. The vibrational and rotational frequencies of many biological macromolecules lie within the THz band, so utilizing THz waves can yield abundant information about biological organisms and their materials.
3. THz radiation can penetrate substances such as ceramics, fats, carbon plates, fabrics, and plastics with minimal attenuation.
4. The signal-to-noise ratio of THz time-domain spectroscopy is very high, making THz highly suitable for imaging applications.
5. The instantaneous bandwidth is very wide (0.1 to 10 THz), which is beneficial for high-speed communication, as shown in the figure on the right.

THz-TDS

Currently, commercial operations have already begun, and several companies worldwide, mainly from China, the United States, Europe, and Japan, have started producing commercial THz time-domain spectrometers. The basic principle of THz time-domain spectroscopy is to generate and detect time-resolved THz electric fields using femtosecond pulses, and then obtain the spectral information of the measured object through Fourier transform. Since the vibrational and rotational energy levels of large molecules are mostly in the THz band, and large molecules, especially biological and chemical macromolecules, are groups of substances with inherent physical properties, it is possible to analyze and identify the structure and properties of substances through characteristic frequencies. One important application could be in pharmaceutical quality control. Imagine installing a THz time-domain spectrometer on the production line of a pharmaceutical factory. Every tablet produced by the factory undergoes spectral measurement and is compared with the spectrum of the standard drug. Qualified tablets proceed to the next stage, while defective tablets are removed from the production line, avoiding quality differences between different tablets or batches and ensuring the quality of the medication.

THz imaging technology

Like other imaging techniques in different spectral bands, THz imaging technology also utilizes THz rays to irradiate the object being measured, obtaining information about the sample through transmission or reflection, and then forming an image. THz imaging technology can be divided into two methods: pulsed and continuous. The former possesses the characteristics of THz time-domain spectroscopy. At the same time, it can perform functional imaging of material groups, obtaining the distribution of refractive indices within the material. For example, with a sunflower seed, it is possible to easily obtain internal information about the seed. Figures 3-4 show the physical photograph of a sunflower seed sample and the corresponding THz transmission image reconstructed using the method, which can clearly distinguish the contour of the shell and the shape of the kernel hidden inside the shell, which is highly desirable. Similarly, if the sample is a human tooth, the normal parts of the tooth and the damaged or decayed parts can be easily distinguished, and there is no additional harm to the human body as X-rays are not required.