Terahertz (THz) science is one of the technological frontier fields of significant research in the world, and THz technology has crucial applications in aerospace, national security, communication radar, quantum information, material science, biomedicine and other fields. The THz electromagnetic wave is located between microwave and infrared, and its spectrum width is about thirty times greater than that of microwave and millimeter waves. It is a strategic frequency resource that various countries are scrambling for, and enormous demands occur in both military and civil applications. However, this frequency band, which connects electronics and photonics, has not been fully exploited and utilized. The THz frequency band has many unique characteristics, such as the time-resolving ability with narrow pulse widths at the picosecond level, the ability to penetrate paper and clothing, the spectral properties of many matters, and the low photon energy. These unique properties give THz waves many important applications, such as nondestructive testing, communication radar, security checks and anti-terrorist, and biomedicine. However, the key factor which hinders the development of THz science and applications is the lack of high-performance THz sources, core devices, and system integration. Among them, the lack of high-efficiency, high-beam quality, and high-stability strong-field THz radiation sources is the focus and difficulty of the current solution.
The methods of generating THz radiation include mainly the electrical and optical methods. This paper mainly discusses the optical methods to generate THz radiation. Optical methods for generating THz radiation include femtosecond laser excitation of nonlinear crystals, photoconductive antennas, plasmas, and so on, which are widely used in sensing imaging and communication. However, the low efficiency and low energy of the current THz radiation source directly limit the nonlinear effects of THz-matter interaction, novel quantum matter state regulation, electron acceleration, biomedicine and other multifaceted frontier scientific and applied research. Therefore, researchers in related fields are working to improve the performance of THz sources and to further promote the development of THz technology.
After the presentation of the tilted pulse front technique, lithium niobate crystals with large nonlinear coefficients, mature manufacturing processes, and high destruction thresholds are expected to realize the generation of high-energy strong-field THz radiation through femtosecond laser action. Currently, pumping lithium niobate crystals by femtosecond laser based on the tilted pulse front technique is still one of the effective ways to generate high-energy strong-field THz radiation. Therefore, it is crucial and necessary to summarize the relevant research on lithium niobate strong-field THz sources to promote the development of this field.
In this paper, the study on lithium niobate strong-field THz sources is summarized as follows. Firstly, the development of strong-field THz generation based on femtosecond-laser-pumped lithium niobate crystals is reviewed in five stages (Fig. 1). Shen’s research group at the University of California generated the world’s first THz pulsed radiation from lithium niobate crystals by laser pulses. Hebling’s group at the University of Pécs, Hungary, proposed the tilted pulse front technique to solve the phase mismatch between near-infrared (NIR) light and THz in lithium niobate crystals. Then, the development of strong-field THz generation via tilted pulse front technique based on femtosecond-laser-pumped lithium niobate crystals has been initiated.
Secondly, the principle of lithium niobate tilted pulse front is described in four aspects: the history of the tilted pulse front theory of lithium niobate (Fig. 2), the key factors to be considered in the theoretical model (Fig. 3), the phase matching and pulse front tilt angle (Fig. 4), and the main methods of model calculation, respectively. Guidance is provided for generating high-energy strong-field THz sources in the future by summarizing the historical evolution of the theoretical model for generating strong-field THz based on the tilted pulse front technique and the mechanism of radiation efficiency saturation of lithium niobate THz strong sources.
Thirdly, the generation of lithium niobate single-period strong-field THz is described. A typical optical path diagram (Fig. 5) based on lithium niobate to generate a single-period strong-field THz and the composition of the tilted pulse front device are introduced.
Fourthly, the lithium niobate multi-period strong-field THz generation is described. Two methods to generate multi-period strong-field THz are introduced: the one based on the tilted pulse front technology of lithium niobate (Fig. 6) and the one based on the quasi-phase matching of periodically polarized lithium niobate (Fig. 7). Lithium niobate crystals are one of the most popular materials for generating strong-field THz, both for single-period and multi-period.
Fifthly, the applications of lithium niobate strong-field THz are discussed, which are presented in three aspects: strong-field THz-matter interactions (Fig. 9), strong-field THz electron acceleration and manipulation (Fig. 10), and strong-field THz biological effects (Fig. 11). These applications demonstrate the advantages of this type of strong-field THz sources and raise the need for higher strong-field THz sources.
Finally, the strong-field THz sources and applications at Beihang University are summarized and the results of Beihang University and its collaborative team in this field are introduced (Fig. 12). We are looking forward to the unprecedented new challenges and opportunities that extreme THz science and applications and their multidisciplinary intersection will bring in the future.
Pumping lithium niobate crystals by femtosecond laser via tilted pulse front technique is one of the effective ways to generate strong-field THz. In summary, lithium niobate crystals are one of the popular materials for generating strong-field THz sources, and lithium niobate strong-field THz sources have played an important role in the applications and studies such as the strong-field THz interactions with matter, electron acceleration and manipulation, and biomedicine. In order to promote better development in strong-field THz sources, the study of lithium niobate strong-field THz source still needs to be deeply explored from the aspects of theoretical basis, structure, and application scenarios.
