Research On The Terahertz Wave Modulation Based On Barium Strontium Titanate/Lead Zirconate Titanate Ferroelectric Photonic Crystal With Optical Pump

With the rapid development of terahertz wave communication technology,the requirements for the performance of terahertz wave modulator are constantly increasing.The ferroelectric materials are widely used in microwave communication because of their excellent dielectric,piezoelectric,photoelectric and thermoelectric properties.The terahertz wave is just between microwave and infrared light,so people began to explore the application of ferroelectric materials in terahertz wave field.In this thesis,the optical and the electrical properties of the Pb_0.52Zr_0.48TiO₃?PZT?ultrathin film,the Ba_0.7Sr_0.3TiO₃/SrTiO₃?STO?bilayers and the Ba_0.5Sr_0.5TiO₃/Pb_0.52Zr_0.48TiO₃?BST/PZT?photonic crystal in terahertz range were studied.The annealing temperature,the microstructure and the material properties of the related ferroelectric thin films were also studied to improve the terahertz wave modulation properties by analyzing the micro-dynamic mechanism.The details of the work are shown as follows:?1?The terahertz wave modulation properties of the 532 nm laser-pumped ferroelectric thin films,such as the dielectric coefficient and the loss,were studied.In order to analyze the physical mechanism of the terahertz wave modulation in the optically pumped ferroelectric thin films,the dielectric spectra in terahertz range were fitted by two different dielectric response models of ferroelectric materials,and the accuracy of the fitting results was verified by the experimental data of Raman spectra.On this basis,we found that when the ferroelectric thin films were illuminated by the 532 nm laser,the electrons were excited and redistributed,leading to the formation of a built-in electric field,and promoted the hardening of the soft mode,along with the increasing oscillator strength and the damping coefficient.The effects of the oxygen vacancies and the sub-bandgap caused by the photonic crystal structure on the optical absorption of the ferroelectric materials were analyzed.It was also found that the conduction current would cause the loss increased with the increasing optical pump power.?2?The optical modulation properties of the PZT ultrathin film and the Ba_0.7Sr_0.3TiO₃/SrTiO₃ bilayer thin films in THz range were studied.The effects of the size and the polarization on the modulation properties of the ferroelectric thin films were observed.The PZT ultrathin films with the high terahertz modulation properties were obtained,and the transmission modulation reached 3.1%with 500 mW optical pump power.Comparing the Ba_0.7Sr_0.3TiO₃/SrTiO₃ bilayer thin films with the Ba_0.7Sr_0.3TiO₃ monolayer thin film,the polarization was found increasing by introducing the SrTiO₃ thin film which improved the optical modulation properties of the ferroelectric films.The transmission modulation increased from 4.0%to 5.0%with 400 mW optical pump power.?3?The effects of the annealing temperatures on the structure and the modulation properties of the BST/PZT photonic crystal were investigated.XPS results showed the increasing Ti³⁺/Ti⁴⁺ratio?7.79%,8.97%,10.93%,12.91%?,which indicated that the increase of the modulation performance of the photonic crystal with the increasing annealing temperature was due to the increasing oxygen vacancy concentration.In addition,the sub-band phenomenon caused by the photonic crystal structure and the shift of absorption peak with annealing temperature caused by the increasing oxygen vacancy concentration were observed by the ultraviolet-visible absorption spectroscopy,which explained that the 680?annealed photonic crystal had the highest modulation.At last,the increasing polarization and the increasing leakage current with the increasing annealing temperature were observed through the results of the hysteresis loop test,which explained that the loss increased with the increasing annealing temperature.?4?On this basis,to explore the application of the BST/PZT photonic crystal in terahertz wave modulation,we studied the modulation properties of the BST/PZT ferroelectric photonic crystal combined the 532 nm optical pump and the bias voltage in THz range.It was found that the different modulation order of the optical pump field and the bias electric field would cause the different distributions of the photo-generated carriers and the different polarization fields,which would change the interaction between them,resulting in two different results of the suppressed modulation and the promoted modulation.The conclusion was verified by 1064 nm hybrid optoelectronic modulation.For the 680?annealed photonic crystal,the transmission modulation decreased from 24.7%to 5.5%in the suppressed modulation and increased from 9.8%to 25.1%in the promoted modulation.