Terahertz (THz) radiation, with the frequency lying between microwave and infrared, has rich information content, high temporal-spatial coherence, low photon energy, strong penetration, and high bandwidth. Therefore, the THz radiation holds immense application value in fields such as national security, satellite communications, non-destructive material testing, and medical imaging. However, the current THz sources with the small energy cannot meet the demand of scientific and application research. The strong-field THz source, with peak electric fields larger than 1 MV/cm, low photon energy, and high temporal resolution, has become the research target of many scientists.
Up to now, the THz radiation output through photonics has attracted enough interests due to its ultra-fast resolution and ultra-wide spectrum. The lithium niobate (LiNbO3, LN) crystal, exhibiting weak THz absorption, a large second-order nonlinear coefficient, and stable physical and chemical properties, is one of the first materials to achieve THz pulse output. To be noted, since Hebling and co-workers proposed the tilted-pulse-front technique, the LN THz source has been greatly improved in terms of output power and conversion efficiency. Recently, Wu and co-workers achieved an ultra-strong LN-THz radiation with a single pulse energy of 13.9 mJ, a conversion efficiency of 1.2%, and a peak field strength of 7.5 MV/cm. Apart from the laser technique, the property of LN crystal is the main factor for the THz source. Moreover, the traditional LN crystal is a classical non-stochiometric crystal with high-concentration intrinsic defects, which greatly affects the crystal property. Correspondingly, some improvement methods have been exploited including magnesium-doped and near-stochiometric LN (SLN) crystals. Therefore, in order to promote the development of LN THz source, we analyze the performance requirements and development direction of LN crystals for strong-field THz sources.In 2008,Stepanov et al. used the large size MgO∶SLN crystal pumped by the pulsed laser with 6-mm-diameter spot, and got the single-period THz pulse with 30 μJ energy. Since then, the pump light with large-size spot with the small energy flow density became an effective method for strong-field THz radiation. Herein, the large-size and high-quality LN crystal is the key matrix material. In 2012, Fül?p and co-workers achieved the 0.4 mJ THz pulse from the MgO∶SLN crystal pumped by 186 mJ with the spot size of 8.1 mm×20 mm. The MgO∶SLN crystal was grown from Li-rich melt by the Czochralski method, which increased the growth difficulty of large-size crystal. Therefore, scientists started to use the Mg-doped congruent LN crystal (MgO∶CLN), and continued to explore the strong LN THz radiation. In 2021, Zhang et al. used a spliced MgO∶CLN crystal with the size of 64 mm×40 mm, and achieved a 1.4 mJ THz radiation with an energy conversion efficiency of 0.7% and a peak electromagnetic field of 6.3 MV/cm. Moreover, Wu and co-workers improved the technique and achieved an ultra-strong terahertz radiation with a single pulse energy of 13.9 mJ, a conversion efficiency of 1.2%, and a peak field strength of 7.5 MV/cm. This represents the highest values reported internationally using this method.
Though the LN crystal is one of the first materials to achieve THz pulse output, the LN THz radiation has not been improved due to the refractive index difference between the pump laser and the THz radiation in this crystal until the tilted-pulse-front technique. In 2002, Hebling and co-workers proposed the tilted-pulse-front technique for LN, allowing the propagation direction of the pump laser energy inside the crystal to phase-match with the group velocity of the THz waves, greatly improving the energy conversion efficiency from pulsed lasers to THz pulses. In 2003, Stepanov et al. used 2% Mg (molar fraction) doped MgO∶SLN crystal to obtain a THz pulse of 98 pJ under a 2.3 μJ, 200 kHz near infrared laser pump, and the energy conversion efficiency was only 0.0043%. The experimental results showed that the subpicosecond free space terahertz radiation can be generated by pumping LiNbO3 crystals using femtosecond laser pulses based on tilted wavefront technology.
Based on the tilted-pulse-front technique, the LN crystal pumped by femtosecond laser is a promising material for strong-field THz sources. Uniformly magnesium-doped LN, with a large laser damage threshold, can meet the demand of high-power pumping laser. The low-concentration magnesium doped near-stoichiometric LN crystal, with a larger nonlinear coefficient and lower THz wave absorption coefficient, is a promising material for high-beam-quality, high-efficiency, and high-stability strong-field THz sources. To lower the energy density of the pump laser and reduce damage to the crystal, a large-aperture crystal is necessary to obtain high-energy strong-field THz sources. In the future, the x-axis LN crystal with ultra-large diameter (300 mm) may support the ultra-large aperture sample exceeding 200 mm, which holds promise for achieving extremely high-energy strong-field THz sources.
