Terahertz Active And Passive Modulation Techniques And Applications Based On Lithium Niobate Prism And Two-dimensional Materials

Terahertz(THz)waves are electromagnetic waves with a frequency range from0.1 THz to 10 THz.Compared with other frequency bands,THz wave has many unique properties,making it versatile.THz technology,which integrates the characteristics of electronics and photons,belong to a typical interdisciplinary field of science and technology.THz technology consists of three main aspects:THz emitters,THz wave detectors and THz devices.Optical rectification(OR)THz emitters have received much attentions as the simplest way to generate broadband THz waves.THz detection techniques based on the THz time domain spectroscopy(THz-TDS),which combine the advantages of fast,accurate and non-destructive detection,are also widely used in sample detection.THz functional devices can control the amplitude,polarization,pulse time and spectrum,and have been widely used in imaging,security scanning and communication.Among them,polarization modulator is one of the most important and widely used devices.In addition,it is an important method to control the device actively by using electric field,light field and other active fields.In this dissertation,some significant scientific advances in three main parts of the THz technology are outlined.Firstly,the generation efficiency and output characteristics of the THz emitters,based on the OR in lithium niobate(LN)crystal with the pulse front tilt(PTF)method,have been improved by optimizing the pumping conditions and the construction of the THz generation system.We give the novel proposals for the pump multiplexing method to increase THz generation efficiency and for the PTF realized by a thin prism set instead of the grating to reduce the angle dispersion,as well as improve efficiency and beam quality.Both the experimental verification is performed.Secondly,We report a single method for measuring the effective electro-optic(EO)coefficient only by spectral intensity modulation with a THz time-domain spectroscopy(THz-TDS)device.Compared with the conventional methods,the measuring device based on a THz-EOS(electro-optic sampling)system with probing the spectral intensity modulation is simple,rapid,and highly sensitive.It does not require special preparation process,electrode contact or applying high voltage on the sensor samples.The EO coefficients of four kinds of materials,namely Zn Te and Ga P crystals,DAST wafer and Ga Se film,are measured at 1040nm wavelength.The results showed good agreement with the literature data by the conventional methods.Thirdly,we demonstrate the passive Re Se₂nanofilm shows intrinsic THz polarization anisotropy.More importantly,the THz polarization properties of the active Re Se₂nanofilm by an external electric field applied on a selected directions are also demonstrated.The modulation depth of the THz transmission is up to 16%and the response time is on the order of picoseconds.In addition,a comparative experiment is performed on three kinds of THz polarizers.The results prove that the performance of the polarizer based on the active Re Se₂nanofilm is comparable with those of CNTs and the THz wire-gird polarizer.The polarization-sensitive Re Se₂nanofilm can find important applications in ultrafast switches,filters and modulation devices in THz domain.Based on femtosecond laser ablated processing,one-dimensional arrays with different line spacings were fabricated on Ti S₂nanosheet.Optical modulated measurements with these devices were demonstrated.Photo-induced complex conductivity changes of the Ti S₂nanosheet devices were studied by an optical-pump THz-probe(OPTP)measurement system.For the 700?m line spacing device,the optically introduced modulation depth reached up to 70%.Our experimental results have demonstrated the ultrafast dynamics and photoconductivity for the Ti S₂nanosheet devices in the THz frequency range.Ti S₂nanosheet based devices would be suitable as the photoelectric modulators of THz wave.Fourthly,direct ablation of large area graphene-like 2D materials,i.e.,Ta Se₂,Sn S₂and Ti S₂,by the back ablation method with the femtosecond laser of a repetition rate of 50 MHz and pulse width of 200 fs were studied for the first time.The ablation thresholds of the three kinds of materials have been discussed.In addition,the optimization and ablation of narrow grooves on the films were demonstrated.Our work had demonstrated the direct femtosecond laser ablation processing of the graphene-like 2D materials films and the potential of 2D material film-based devices.