Ultrafast Terahertz Modulations And Applications

Materials such as metal and semiconductor can produce photocarriers induced by ultrafast laser.This physical process can be applied to the generation,regulation and absorption of Terahertz(THz)radiation under certain conditions.With the development of THz technology,strong field THz photonics and THz regulatory device manufacturing have gradually become a hot research topic in basic physical science and applied technology of THz.During my doctoral period,series of reseaches on new materials of THz source,THz intensity modulation,interaction between THz wave and phase change materials,and fabrication of THz polarizor have been studied.The following achievements are listed:1.The plasmons and photoelectron motion of nano-metal under ultrafast laser irradiation have unique advantages in developing terahertz source devices with high conversion efficiency and high integration.Using femtosecond laser as the pump source,it is revealed that the optical rectification effect of nanoporous gold(NPG)films(thickness 100 nm,irregular nano aperture average 15 nm)is the main mechnism for the radiation of THz.The experimental results show that NPG combined with Ga As can induce electron migration to substrate driven by surface photoelectric field,which enhance emitted THz signal.The researches on photoelectrons motion in chapter 2 are conducive to design terahertz metamaterials with composite materials,such as nanoporous metals,semiconductors and graphene.2.Femtosecond filament technology is one of the methods to generate THz wave with strong field and wide spectrum,but the strong destruction characteristics of filament makes it hard to directly modulate the THz conversion efficiency.To explore the feasibility of THz wave modulation by filaments,a multi-filaments optical system is built.The pre-plasma generated by a short filament(1 mm)is used to enhance the plasma density of a small segment of another longer two-color filament(10 mm),resulting in the two-color filament being truned.By adjusting the interacted position and power of the short filament,intensity of THz wave can be attenuated by more than 90 %.This study enriches the means of intensity modulation of THz wave and provides help for remote control of THz conversion efficiency and optical filament transmission in high-energy laser ionization process.3.Phase change material Ge2Sb2Te5(GST)is one of the emerging materials for the development of THz high-speed modulator devices,which has a wide range of applications.Firstly,an optical pump THz probe(OPTP)system has been built to study the relaxation characteristics of GST(at different annealing temperatures),which can be used to identify the crystal types of GST materials.Using continuous THz wave as pump source,the experimental and simulated results of z-scan show that when the power density of THz reaches a higher value,THz can induce the phase transformation of amorphous GST,thus causing the change of refractive index and transmittance in THz frequency region.Finally,the saturated absorption phenomenon of GST is observed for the first time by using the intense field THz pulse as the pump source.The saturated absorption rate and other parameters are calculated by the saturated absorption model.This research provides a feasible scheme for the design of a new type of THz saturated absorber and is beneficial to develop the technology of THz mode-locked laser.4.In order to improve the speed of femtosecond laser machining and reduce the cost of THz polarizor,the method of plasma grating assisted chemical etching is proposed to prepare THz polarizor on photosensitive glass.The performance is close to that of commercial metal wire grating polarizor.Fabrication platform is built based on plasma gratings in glass induced by interference of two femtosecond laser.The phenomenon of the waveguide structure making the randomly distributed multi-filaments regular is studied and discussed.Combining the waveguidecoupling characteristics with the optical filament gratings formed by two beam interference,a splicing processing scheme for large-area gratings is proposed,which can improve the speed of laser processing periodic structures and reduce the mechanical accuracy requirements of equipment.