Intense Terahertz Radiation From Lithium Niobates Via Tilted Pulse Front Technique

Intense terahertz(THz)sources play an increasingly important role in the exploration of novel material properties,active regulation of matter,acceleration and manipulation of charged particles due to their low photon energy,strong field and unique frequency band.However,the lack of stable intense THz sources hinders the further development of THz science.Among many THz sources based on different principles,the source based on lithium niobate tilted pulse-front technique(TPFT)has become the first choice of many research teams studying strong THz interaction with matter due to its high efficiency,high beam quality,high stability and other characteristics,and this source has the potential to generate millijoule level or even larger pulse energy.In the process of high energy laser pumping lithium niobate,the main factors limiting the energy conversion efficiency include crystal damage caused by high pumping fluence,multi-photon absorption,material lattice absorption,nonlinear distortion effect and so on.These factors will seriously shorten the effective interaction length of THz radiation generated by pumping laser inside the crystal,reduce the THz radiation efficiency,and hinder the further improvement of the intense field THz radiation energy and the development of THz application experiment.This article aims at the bottleneck problem of THz generation efficiency saturation when high energy femtosecond laser pumping lithium niobate crystal,experimental studies have been carried out on the THz generation at the millijoule level.The world’s first lithium niobate strong THz source with single pulse energy greater than 1 m J is obtained.Moreover,for the problem of efficient generation and regulation of multicycle THz pulse,the echelon mirror combined with TPFT is used to shape the pumping laser pulse,and the multicycle strong field THz radiation with high efficiency and tunable frequency is realized.The research results and innovations obtained are summarized as follows:1.1.4 m J lithium niobate strong THz radiation source.The large size lithium niobate crystal is pumped by a 20 TW commercial titanium sapphire laser in the Institute of Physics,Chinese Academy of Sciences.The cooperative compensation mechanism is adopted,such as expanding pump pulse spot,optimizing pump pulse spectrum and pulse chirp,and cooling lithium niobate crystal,which overcomes the problem of low generation efficiency caused by the short intrinsic effective interaction length when the lithium niobate crystal is pumped by 30 fs ultra-short laser pulses,and greatly improves the energy conversion efficiency of 800 nm to THz radiation.Finally,under the pumping energy of 214 m J,the single pulse of 1.4 m J and the energy conversion efficiency of 0.7% were successfully achieved.This is by far the highest pulse energy of a THz source based on optical rectification technology.The THz timedomain waveform was characterized by the single-shot diagnostic technology.The THz pulse duration is about 1 ps,the peak frequency is 0.4 THz,and the spectrum width is0.1-2.0 THz.The peak electric field at the focused spot is calculated to be 6.3 MV/cm,and the peak magnetic field is 2.1 Tesla.The low frequency and strong field characteristics of the ultra-strong THz source are very suitable for the application of electron acceleration and manipulation of tabletop X-ray sources,extreme THz nonlinear science and THz biological effects.2.Multicycle lithium niobate strong field THz radiation.For the generation of multicycle narrow-band strong THz source,there are some difficulties such as low generation efficiency and poor tunable performance.In this paper,a new scheme combining TPFT and echelon mirror pulse train preparation is proposed.A multicycle THz pulse with a single pulse energy of 2.4 μJ,15 cycles,a peak frequency of 0.76 THz,a spectrum width of 0.1 THz,and a generation efficiency of about 0.1% was achieved at the pump energy of 2.6 m J femtosecond laser with a 1 k Hz repetition rate.By using this new method,the number of cycles and spectrum bandwidth of multicycle strong field THz radiation pulses can be adjusted flexibly,and the radiation efficiency is theoretically equivalent to that of single-cycle THz generation.This scheme has the advantages of simple operation,good tunability and high energy conversion efficiency,and is expected to be applied to the application scenarios of THz electron acceleration and control based on the dielectric tube,and non-equilibrium state control of condensed matter system.